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19-0191; Rev 1; 3/94
TION KIT EVALUA LE AVAILAB
-5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-DC Controllers
_______________General Description
The MAX774/MAX775/MAX776 inverting switching regulators deliver high efficiency over three decades of load current. A unique current-limited, pulsefrequency-modulated (PFM) control scheme provides the benefits of pulse-width modulation (high efficiency with heavy loads), while using less than 100A of supply current (vs. 2mA to 10mA for PWM converters). The result is high efficiency over a wide range of loads. These ICs also use tiny external components; their high switching frequency (up to 300kHz) allows for less than 5mm diameter surface-mount magnetics. The MAX774/MAX775/MAX776 accept input voltages from 3V to 16.5V, and have preset output voltages of -5V, -12V, and -15V, respectively. Or, the output voltage can be user-adjusted with two resistors. Maximum VIN - VOUT differential voltage is limited only by the breakdown voltage of the chosen external switch transistor. These inverters use external P-channel MOSFET switches, allowing them to power loads up to 5W. If less power is required, use the MAX764/MAX765/MAX766 inverting switching regulators with on-board MOSFETs.
____________________________Features
o 85% Efficiency for 5mA to 1A Load Currents o Up to 5W Output Power o 100A Max Supply Current o 5A Max Shutdown Current o 3V to 16.5V Input Range o -5V (MAX774), -12V (MAX775), -15V (MAX776), or Adjustable Output Voltage o Current-Limited PFM Control Scheme o 300kHz Switching Frequency
MAX774/MAX775/MAX776
______________Ordering Information
PART MAX774CPA MAX774CSA MAX774C/D MAX774EPA MAX774ESA MAX774MJA TEMP. RANGE 0C to +70C 0C to +70C 0C to +70C -40C to +85C -40C to +85C -55C to +125C PIN-PACKAGE 8 Plastic DIP 8 SO Dice* 8 Plastic DIP 8 SO 8 CERDIP
________________________Applications
LCD-Bias Generators High-Efficiency DC-DC Converters Battery-Powered Applications Data Communicators
Ordering Information continued on last page. * Contact factory for dice specifications.
__________Typical Operating Circuit
INPUT 3V TO 16V
V+
__________________Pin Configuration
TOP VIEW
MAX774
ON/OFF SHDN
CS EXT P
OUT FB
1 2 3 4
8
GND EXT CS V+
OUTPUT -5V
SHDN REF
FB REF GND OUT
MAX774 MAX775 MAX776
7 6 5
DIP/SO
________________________________________________________________ Maxim Integrated Products
1
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-5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-DC Controllers MAX774/MAX775/MAX776
ABSOLUTE MAXIMUM RATINGS
Supply Voltages V+ to OUT ...........................................................................21V V+ to GND ..............................................................-0.3V, +17V OUT to GND ........................................................-0.3V, to -17V REF, SHDN, FB, CS...................................-0.3V to (V+ + 0.3V) EXT ...............................................(VOUT - 0.3V) to (V+ + 0.3V) Continuous Power Dissipation (TA = +70C) Plastic DIP (derate 9.09mW/C above +70C) .............727mW SO (derate 5.88mW/C above +70C) ..........................471mW CERDIP (derate 8.00mW/C above +70C) ..................640mW Operating Temperature Ranges: MAX77_C_ _ .........................................................0C to +70C MAX77_E_ _ ......................................................-40C to +85C MAX77_MJA ...................................................-55C to +125C Maximum Junction Temperatures: MAX77_C_ _/E_ _ ...........................................................+150C MAX77_MJA..................................................................+175C Storage Temperature Range .............................-65C to +160C Lead Temperature (soldering, 10sec) .............................+300C
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(V+ = 5V, ILOAD = 0mA, CREF = 0.1F, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER V+ Input Voltage Range SYMBOL V+ V+ = 16.5V, SHDN 0.4V (operating) Supply Current V+ = 10V, SHDN 1.6V (shutdown) V+ = 16.5V, SHDN 1.6V (shutdown) FB Trip Point 3V V+ 16.5V MAX77_C FB Input Current IFB MAX77_E MAX77_M MAX774 Output Voltage VOUT MAX775 MAX776 MAX77_C Reference Voltage VREF IREF = 0A MAX77_E MAX77_M REF Load Regulation REF Line Regulation Output Voltage Line Regulation (Circuit of Figure 2-- Bootstrapped) Output Voltage Load Regulation (Circuit of Figure 2-- Bootstrapped) 0A IREF 100A 3V V+ 16.5V MAX774, 4V V+ 15V, ILOAD = 0.5A MAX775, 4V V+ 8V, ILOAD = 0.2A MAX776, 4V V+ 6V, ILOAD = 0.1A MAX774, 0A ILOAD 1A, V+ = 5V MAX775, 0mA ILOAD 500mA, V+ = 5V MAX776, 0mA ILOAD 400mA, V+ = 5V MAX77_C/E MAX77_M -4.80 -11.52 -14.40 1.4700 1.4625 1.4550 -5 -12 -15 1.5 1.5 1.5 4 4 40 0.035 0.088 0.137 1.5 1.5 1.0 mV/A mV/V -10 2 4 10 50 70 90 -5.20 -12.48 -15.60 1.5300 1.5375 1.5450 10 15 100 mV V/V V V nA mV CONDITIONS MIN 3.0 TYP MAX 16.5 100 5 A UNITS V
2
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-5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-DC Controllers
ELECTRICAL CHARACTERISTICS (continued)
PARAMETER Efficiency (Circuit of Figure 2-- Bootstrapped) SHDN Input Current SHDN Input Voltage High SHDN Input Voltage Low Current-Limit Trip Level (V+ - CS) CS Input Current Switch Maximum On-Time Switch Minimum Off-Time EXT Rise Time EXT Fall Time tON (max) tOFF (max) V+ = 12V V+ = 12V CEXT = 1nF, V+ = 12V CEXT = 1nF, V+ = 12V 12 1.8 16 2.3 50 50 VIH VIL VCS SYMBOL CONDITIONS MAX774, V+ = 5V, ILOAD = 1A MAX775, V+ = 5V, ILOAD = 500mA MAX776, V+ = 5V, ILOAD = 400mA V+ = 16.5V, SHDN = 0V or V+ 3V V+ 16.5V 3V V+ 16.5V MAX77_C/E 3V V+ 16.5V MAX77_M MIN TYP 82 88 87 1 1.6 180 160 210 210 0.4 240 260 1 20 2.8 MAX UNITS % A V V mV A s s ns ns
MAX774/MAX775/MAX776
3
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-5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-DC Controllers MAX774/MAX775/MAX776
__________________________________________Typical Operating Characteristics
(TA = +25C, unless otherwise noted.)
MAX774 EFFICIENCY vs. LOAD CURRENT VOUT = -5V (BOOTSTRAPPED)
MAX1774/5/6-01a
MAX774 EFFICIENCY vs. LOAD CURRENT VOUT = -5V (NON-BOOTSTRAPPED)
MA774/5/6--1b
MAX774 EFFICIENCY vs. TEMPERATURE
ILOAD = 100mA 80 EFFICIENCY (%)
MAX774/5/6-2
90 VIN = 5V 80 EFFICIENCY (%) VIN = 3V VIN = 15V 70
90 VIN = 5V 80 EFFICIENCY (%) VIN = 4V 70 VIN = 15V
90
ILOAD = 600mA ILOAD = 1A 70
60
60
60 VIN = 5V BOOTSTRAPPED
50 1 10 100 1000 LOAD CURRENT (mA)
50 1 10 100 1000 LOAD CURRENT (mA)
50 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)
MAX776 EFFICIENCY vs. LOAD CURRENT VOUT = -15V (BOOTSTRAPPED)
MA774/5/6-1c
MAX776 EFFICIENCY vs. LOAD CURRENT VOUT = -15V (NON-BOOTSTRAPPED)
MA774/5/6-1d
MAX775 EFFICIENCY vs. OUTPUT CURRENT VOUT = -12V (BOOTSTRAPPED)
MA774/5/6--1e
90
VIN = 6V
90
90
80 EFFICIENCY (%)
VIN = 5V EFFICIENCY (%) VIN = 4V VIN = 3V
80 VIN = 15V 70
VIN = 6V VIN = 4V
70
EFFICIENCY (%)
VIN = 5V
80 VIN = 4V 70
VIN = 5V VIN = 8V
60
60
60
50 1 10 100 1000 LOAD CURRENT (mA)
50 1 10 100 1000 LOAD CURRENT (mA)
50 1 10 100 1000 OUTPUT CURRENT (mA)
MAX774/MAX775/MAX776 EFFICIENCY vs. LOAD CURRENT VOUT = -24V (NON-BOOTSTRAPPED)
MA774/5/6--1f
MAX774/MAX775/MAX776 EFFICIENCY vs. LOAD CURRENT VOUT = -24V OUTPUT (ZENER CONNECTION)
MA774/5/6--1g
MAX774 EFFICIENCY vs. INPUT VOLTAGE VOUT = -5V AT 100mA
86 EFFICIENCY (%) 84 82 80 NON-BOOTSTRAPPED 78 76 VOUT = -5V AT 100mA
MAX774/5/6-3
90 VIN = 6V 80 EFFICIENCY (%) VIN = 4V 70 VIN = 5V
90 VIN = 6V 80 EFFICIENCY (%) VIN = 4V 70 VIN = 5V
88
BOOTSTRAPPED
60
60
50 1 10 100 1000 LOAD CURRENT (mA)
50 1 10 100 1000 LOAD CURRENT (mA)
74 2 4 6 8 10 12 14 16 INPUT VOLTAGE (V)
4
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-5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-DC Controllers
_____________________________Typical Operating Characteristics (continued)
(TA = +25C, unless otherwise noted.)
START-UP VOLTAGE vs. LOAD CURRENT (BOOTSTRAPPED)
MA744/5/6-14
MAX774/MAX775/MAX776
START-UP VOLTAGE vs. LOAD CURRENT (NON-BOOTSTRAPPED)
MA744/5/6-15
MAX774 MAXIMUM LOAD vs. INPUT VOLTAGE
VOUT = -5V 2000 LOAD CURRENT (mA) 1800 BOOTSTRAPPED 1600 1400 1200 NON-BOOTSTRAPPED 1000 800
MAX774/5/6-16
5.0 VOUT = -15V 4.5 START-UP VOLTAGE (V) VOUT = -12V
5.0 VOUT = -12V 4.5 START-UP VOLTAGE (V) VOUT = -15V
2200
4.0
4.0
VOUT = -24V
3.5 VOUT = -5V 3.0
3.5
3.0
VOUT = -5V
2.5 1 10 100 1000 LOAD CURRENT (mA)
2.5 0.1 1 10 100 1000 LOAD CURRENT (mA)
2
4
6
8
10
12
14
16
INPUT VOLTAGE (V)
EXT RISE AND FALL TIMES vs. TEMPERATURE
120 110 100 tRISE & tFALL (ns) 90 80 70 60 50 40 30 20 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C) 12V RISE 12V FALL 5V RISE 5V FALL CEXT = 1nF
MAX774/5/6-9
EXT RISE AND FALL TIMES vs. TEMPERATURE
CEXT = 5nF
MAX774/5/6-10
SWITCH ON-TIME vs. TEMPERATURE
V+ = 5V
MAX761-13
130
500 450 400 tRISE & tFALL (ns) 350 300 250 200 150 100 50
17
5V RISE ton (s) 5V FALL 12V RISE 12V FALL 15 -60 -40 -20 0 20 40 60 80 100 120 140 -60 0 60 120 TEMPERATURE (C) TEMPERATURE (C)
16
SWITCH OFF-TIME vs. TEMPERATURE
MAX761-13
SWITCH ON-TIME/OFF-TIME RATIO
7.8 7.6 tON/tOFF RATIO (s/s) 7.4 ICC (A) 7.2 7.0 6.8 6.6 6.4 6.2 0.5 V+ = 5V
MAX774/5/6-6
SHUTDOWN CURRENT vs. TEMPERATURE
3.5 3.0 2.5 2.0 1.5 1.0 V+ = 4V 0 V+ = 8V V+ = 15V
MAX774/5/6-7
2.5 V+ = 5V
8.0
4.0
tOFF (s)
2.0
1.5 -60 0 60 120 TEMPERATURE (C)
6.0 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C)
-60 -40 -20
0
20
40 60
80 100 120 140
TEMPERATURE (C)
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5
-5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-DC Controllers MAX774/MAX775/MAX776
_____________________________Typical Operating Characteristics (continued)
(TA = +25C, unless otherwise noted.)
OPERATING SUPPLY CURRENT vs. TEMPERATURE
MAX774/5/6-8
REFERENCE TEMPERATURE COEFFICIENT
MAX774/5/6-12
80 V+ = 16.5V 78 76 V+ = 10V ICC (A) 74 72 70 68 66 -60 -40 -20 0 20 40 60 V+ = 3V
1.506 1.504 REFERENCE OUTPUT (V) 1.502 1.500 1.498 1.496 1.494 1.492
80 100 120 140
-60 -40 -20
0
20
40 60
80 100 120 140
TEMPERATURE (C)
TEMPERATURE (C)
CS TRIP LEVEL
MAX774/5/6-11
REFERENCE OUTPUT RESISTANCE
REFERENCE OUTPUT RESISTANCE ()
MAX774/5/6-13
235 230 225 CS TRIP LEVEL (mV) 220 215 210 205 200 195 190 185 -60 -40 -20 0 20 40 60
250
200 IREF = 10A 150 IREF = 50A 100
50 IREF = 100A 0 -60 -40 -20 0 20 40 60 80 100 120 140
80 100 120 140
TEMPERATURE (C)
TEMPERATURE (C)
6
_______________________________________________________________________________________
-5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-DC Controllers
__________________________________________Typical Operating Characteristics
(TA = +25C, unless otherwise noted.)
MAX774/MAX775/MAX776
OPERATING WAVEFORMS
INDUCTOR CURRENT NEAR FULL LOAD
A 1A/div B
0A C
10s/div CIRCUIT OF FIGURE 2 V+ = 6.5V, ILOAD = 1A, VOUT = -5V A: OUTPUT RIPPLE, 200mV/div B: EXT WAVEFORM, 10V/div C: INDUCTOR CURRENT, 2A/div CIRCUIT OF FIGURE 2 VOUT = -5V, V+ = 4.7V ILOAD = 1.05A (1A/div)
20s/div
CONTINUOUS CONDUCTION AT ONE-HALF CURRENT LIMIT
ENTRY/EXIT FROM SHUTDOWN
A
1A/div 0A
B
20s/div CIRCUIT OF FIGURE 2 ILOAD = 300mA, VOUT = -5V V+ = 8V, L = 22H
2ms/div CIRCUIT OF FIGURE 2 V+ = 6V, ILOAD = 1A, VOUT = -5V A: SHUTDOWN PULSE, 0V TO V+, 5V/div B: VOUT, 2V/div
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7
-5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-DC Controllers MAX774/MAX775/MAX776
____________________________Typical Operating Characteristics (continued)
(TA = +25C, unless otherwise noted.) LOAD-TRANSIENT RESPONSE LINE-TRANSIENT RESPONSE
A
A
B B
100s/div CIRCUIT OF FIGURE 2 V+ = 6V, VOUT = -5V A: ILOAD, 30mA TO 1A, 1A/div B: VOUT, 100mV/div, AC-COUPLED
2ms/div CIRCUIT OF FIGURE 2 VOUT = -5V, ILOAD = 1A A: V+, 3V TO 8V, 5V/div B: VOUT, 100mV/div, AC-COUPLED
______________________________________________________________Pin Description
PIN 1 NAME OUT FUNCTION The sense input for fixed-output operation (VFB = VREF). OUT is connected to the internal voltage divider, and it is the negative supply input for the EXT driver. Feedback input. When VFB = VREF, the output will be the factory preset value. For adjustable operation, use an external voltage divider, as described in the Adjustable Output section. Active-high shutdown input. With SHDN high, the part is in shutdown mode and the supply current is less than 5A. Connect to GND for normal operation. 1.5V reference output that can source 100A. Bypass to ground with 0.1F. Positive power-supply input Noninverting input to the current-sense comparator. Typical trip level is 210mV (relative to V+). The gate-drive output for an external P-channel power MOSFET. EXT swings from OUT to V+. Ground
2
FB
3 4 5 6 7 8
SHDN REF V+ CS EXT GND
8
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-5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-DC Controllers MAX774/MAX775/MAX776
V+ FB MODE COMPARATOR REF 50mV
SHDN
ERROR COMPARATOR
MAX774 MAX775 MAX776
OUT
N 1.5V REFERENCE Q TRIG FROM V+ S TRIG Q Q FROM OUT CURRENT COMPARATOR CS EXT
ONE-SHOT
R
ONE-SHOT
CURRENT CONTROL CIRCUITS
0.2V (FULL CURRENT)
0.1V (HALF CURRENT) FROM V+
GND
Figure 1. Block Diagram
_______________Detailed Description
The MAX774/MAX775/MAX776 are negative-output, inverting power controllers that can be configured to drive an external P-channel MOSFET. The output voltages are preset to -5V (MAX774), -12V (MAX775), or -15V (MAX776). Additionally, all three parts can be set to any desired output voltage using an external resistor divider. The MAX774/MAX775/MAX776 have a unique control scheme (Figure 1) that combines the advantage of pulse-skipping, pulse-frequency-modulation (PFM) converters (ultra-low supply current) with the advantage of pulse-width-modulation (PWM) converters (high efficiency with heavy loads). This control scheme allows the devices to achieve 85% efficiency with loads from 5mA to 1A.
As with traditional PFM converters, the external P-channel MOSFET power transistor is turned on when the voltage comparator senses that the output is below the reference voltage. However, unlike traditional PFM converters, switching is controlled by the combination of a switch current limit (210mV/R SENSE ) and on-time/off-time limits set by one-shots. Once turned on, the MOSFET stays on until: 1) the 16s maximum on-time limit is reached or 2) the switch current reaches its limit (as set by the current-sense resistor). Once off, the switch is typically held off for a minimum of 2.3s. It will stay off until the output drops below the level determined by VREF and the feedback divider network.
9
_______________________________________________________________________________________
-5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-DC Controllers MAX774/MAX775/MAX776
VIN
1 OUT V+ 5 R1 0.07 Q1 Si9435 P 1N5822/ MBR340 C1 150F R2
VIN
1 3 2 R1 4 C3 0.1F REF GND 8 OUT SHDN MAX774 FB V+ 5 R3 0.07 Q1 Si9435 P 1N5822/ MBR340 C4*
C1 150F
C2 0.1F
3 2 4 C3 0.1F
SHDN MAX775 CS 6
MAX774
MAX776
FB REF GND 8 EXT
7
C2 0.1F
MAX775 CS 6 MAX776 7
EXT
VOUT
C4*
VOUT
L1 22H
L1 22H
* MAX774 = 330F, 10V
MAX775, MAX776 = 120F, 20V PRODUCT MAX774 MAX775 MAX776 OUTPUT VOLTAGE (V) -5 -12 -15 INPUT VOLTAGE (V) 3 to 15 3 to 8 3 to 5 OUTPUT CURRENT (A) 1 0.5 0.4
* MAX774 = 330F, 10V
MAX775, MAX776 = 120F, 20V
Figure 4. Non-Bootstrapped Operation (VIN > 4.5V)
NOTE: Si9435 HAS VGS OF 20V MAX
Figure 2. Bootstrapped Connection Using Fixed Output Voltages
VIN
1 OUT R2 C1 150F C2 0.1F R1 3 SHDN MAX774 2 FB V+ 5 R3 0.07 Q1 Si9435 P 1N5822/ MBR340 C4*
MAX775 CS 6
EXT 7
MAX776
4
VOUT
REF
C3 0.1F
GND 8
L1 22H
* MAX774 = 330F, 10V
MAX775, MAX776 = 120F, 20V
Figure 3. Bootstrapped Connection Using External Feedback Resistors
the MOSFET off, and the current limit remains at one-half the peak current limit. If the output voltage is out of regulation after two consecutive pulses, the current limit for the next pulse will equal the full current limit. With heavy loads, the MOSFET first switches twice at one-half the peak current value. Subsequently, it stays on until the switch current reaches the full current limit, and then turns off. After it is off for 2.3s, the MOSFET switches on once more, and remains on until the switch current again reaches its limit. This cycle repeats until the output is in regulation. A benefit of this control scheme is that it is highly efficient over a wide range of input/output ratios and load currents. Additionally, PFM converters do not operate with constant-frequency switching, and have relaxed stability criterion (unlike PWM converters). As a result, their external components require smaller values. With PFM converters, the output voltage ripple is not concentrated at the oscillator frequency (as it is with PWM converters). So for applications where the ripple frequency is important, the PWM control scheme must be used. However, for many other applications, the smaller capacitors and lower supply current of the PFM control scheme make it the better choice. The output voltage ripple with the MAX774/MAX775/MAX776 can be held quite low. For example, using the circuit of Figure 2, only 100mV of output ripple is produced when generating a -5V at 1A output from a +5V input.
With light loads, the MOSFET switches on for one or more cycles and then switches off, much like in traditional PFM converters. To increase light-load efficiency, the current limit for the first two pulses is set to one-half the peak current limit. If those pulses bring the output voltage into regulation, the voltage comparator keeps
10
Bootstrapped vs. Non-Bootstrapped Operation
Figures 2 and 3 are the standard application circuits for bootstrapped mode, and Figure 4 is the circuit for nonbootstrapped mode. Since EXT is powered by OUT,
______________________________________________________________________________________
-5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-DC Controllers
0.1F RZ
VOUT
tions, you should see a short current pulse at half the peak current approximately every 100ms (the exact period depends on actual circuit leakages).
MAX774/MAX775/MAX776
EXT Drive Voltages
1 R2 OUT GND 8
2 R1 4 0.1F
FB
MAX774 MAX775 MAX776
REF
6V VZ + VIN 10V VOUT - VZ > IZ RZ IZ = ZENER BREAKDOWN CURRENT VZ = ZENER BREAKDOWN VOLTAGE VIN = INPUT SUPPLY VOLTAGE
EXT swings from OUT to V+ and provides the drive output for an external power MOSFET. When using the onchip feedback resistors for the preset output voltages, the voltage at OUT equals the output voltage. When using external feedback resistors, OUT may be tied to GND or some other potential between VOUT and GND. Always observe the V+ to OUT absolute maximum rating of 21V. For V+ to output differentials greater than 21V, OUT must be tied to a potential more positive than the output and, therefore, the output voltage must be set with an external resistor divider. In non-bootstrapped operation with low input voltages (<4V), tie OUT to a negative voltage to fully enhance the external MOSFET. Accomplish this by creating an intermediate voltage for VOUT with a zener diode (Figure 5).
Figure 5. Connection Using Zener Diode to Boost Base Drive
__________________Design Procedure
Setting the Output Voltage
The MAX774/MAX775/MAX776 are preset for -5V, -12V, and -15V output voltages, respectively; however, they may also be adjusted to other values with an external voltage divider. For the preset output voltage, connect FB to REF and connect OUT to the output (Figure 3). In this case, the output voltage is sensed by OUT. For an adjustable output (Figures 3 and 4), connect an external resistor divider from the output voltage to FB, and from FB to REF. In this case, the divided-down output voltage is sensed via the FB pin. There are three reasons to use the external resistor divider: 1) You desire an output voltage other than a preset value 2) The input-to-output differential exceeds 21V or 3) The output voltage (VOUT to GND) exceeds -15V. For adjustable operation, refer to Figures 3 and 4. The impedance of the feedback network should be low enough that the input bias current of FB is not a factor. For best efficiency and precision, allow 10A to flow through the network. Calculate (V REF - VFB) / R1 = 10A. Since VREF = 1.5V and VFB = 0V, R1 becomes 150k. Then calculate R2 as follows: R2 VOUT ___ = _______ R1 VREF (or, ______= 10A) VOUT R2
11
using bootstrapped or non-bootstrapped mode will directly affect the gate drive to the FET. EXT swings from V+ to VOUT. In bootstrapped operation, OUT is connected to the output voltage (-5V, -12V, -15V). In non-bootstrapped operation, OUT is connected to ground, and EXT now swings from V+ to ground. At high input-to-output differentials, it may be necessary to use non-bootstrapped mode to avoid the 21V V+ to VOUT maximum rating. Also, observe the V GS maximum rating of the external transistor. At intermediate voltages and currents, the advantages of bootstrapped vs. non-bootstrapped operation are slight. When input voltages are less than about 4V, always use the bootstrapped circuit.
Shutdown and Quiescent Current
The MAX774/MAX775/MAX776 are designed to save power in battery-powered applications. A TTL/CMOS logic-level shutdown input (SHDN) has been provided for the lowest-power applications. When shut down (SHDN = V+), most internal bias current sources and the reference are turned off so that less than 5A of current is drawn. In normal operation, the quiescent current will be less than 100A. However, this current is measured by forcing the external switch transistor off. Even with no load, in an actual application, additional current will be drawn to supply the feedback resistors' and the diode's and capacitor's leakage current. Under no-load condi-
______________________________________________________________________________________
-5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-DC Controllers MAX774/MAX775/MAX776
MAX775-fig6
RSENSE = 0.05 MAXIMUM OUTPUT CURRENT (mA) 2000 RSENSE = 0.06
VOUT = -12V MAXIMUM OUTPUT CURRENT (mA) 800 RSENSE = 0.05 RSENSE = 0.06 RSENSE = 0.07
1500
600
1000
RSENSE = 0.07 RSENSE = 0.08 RSENSE = 0.09 VOUT = -5V 34 56 7 8 9 10 11 12 13 14 15 INPUT VOLTAGE (V)
400 RSENSE = 0.08 RSENSE = 0.09
500
200
0
0 3 4 5 6 7 INPUT VOLTAGE (V) 8 9
Figure 6. MAX774 Maximum Output Current vs. Input Voltage (VOUT = -5V)
Figure 7. MAX775 Maximum Output Current vs. Input Voltage (VOUT = -12V)
Choosing an Inductor
Practical inductor values range from 10H to 50H. The maximum inductor value is not particularly critical. For highest current at high VOUT to V+ ratios, the inductor should not be so large that the peak current never reaches the current limit. That is: L(max) _______________________________ ILIM(max) This is only important if VIN 1 t OFF(min) = ___________ VOUT 6 t ON(max) More important is that the inductor not be so small that the current rises much faster than the current-limit comparator can respond. This would be wasteful and reduce efficiency. Calculate the minimum inductor value as follows:
minimize radiated noise, use a torroid, pot-core, or shielded-bobbin inductor. Inductors with a ferrite core or equivalent are recommended. Make sure that the inductor's saturation current rating is greater than ILIM(max).
Diode Selection
The ICs' high switching frequencies demand a highspeed rectifier. Schottky diodes such as the 1N5817 to 1N5822 families are recommended. Choose a diode with an average current rating approximately equal to or greater than ILIM (max) and a voltage rating higher than VIN(max) + VOUT. For high-temperature applications, Schottky diodes may be inadequate due to their high leakage currents; instead, high-speed silicon diodes may be used. At heavy loads and high temperature, the benefits of a Schottky diode's low forward voltage may outweigh the disadvantages of its high leakage current.
[V+(min) - VSW(max)] x 12s
_______ < --
[V+(max) - VSW(min)] x 0.3s L(min) _______________________________ (I) x ILIM(min) Where L is in H, 0.3s is an ample time for the comparator response, ILIM is the current limit (see CurrentSense Resistor section), and (I) is the allowable percentage of overshoot. As an example, Figure 2's circuit uses a 3A peak current. If we allow a 15% overshoot and 15V is the maximum input voltage, then L(min) is 16H. The actual value of L above this limit has minimal effect on this circuit's operation. For highest efficiency, use a coil with low DC resistance. Coils with 30m or lower resistance are available. To
Current-Sense Resistor
The current-sense resistor limits the peak switch current to 210mV/RSENSE, where RSENSE is the value of the current-sense resistor, and 210mV is the currentsense comparator threshold (see Current-Limit Trip Level in the Electrical Characteristics). To maximize efficiency and reduce the size and cost of external components, minimize the peak current. However, since the output current is a function of the peak current, do not set the limit too low. Refer to Figures 6-9 to determine the sense resistor (and, therefore, peak current) for the required load current.
12
______________________________________________________________________________________
MAX775-FIG07
2500
1000
-5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-DC Controllers MAX774/MAX775/MAX776
MAX776-FIG08
VOUT = -15V RSENSE = 0.05 RSENSE = 0.06 RSENSE = 0.07
VOUT = -24V RSENSE = 0.05 RSENSE = 0.06 RSENSE = 0.07
MAXIMUM OUTPUT CURRENT (mA)
600 500 400 300 200 100 3
MAXIMUM OUTPUT CURRENT (mA)
600
400
200 RSENSE = 0.08 RSENSE = 0.09 0 34 56 7 8 9 10 11 12 13 14 15 INPUT VOLTAGE (V)
RSENSE = 0.08 RSENSE = 0.09 4 5 6 INPUT VOLTAGE (V) 7
Figure 8. MAX776 Maximum Output Current vs. Input Voltage (VOUT = -15V)
Figure 9. MAX774/MAX775/MAX776 Maximum Output Current vs. Input Voltage (VOUT = -24V)
To choose the proper current-sense resistor, simply follow the two-step procedure outlined below. 1) Determine: * Input voltage range, V+ * Maximum (absolute) output voltage, VOUT * Maximum output current, ILOAD For example, let V+ range from 4V to 6V, and choose VOUT = -24V and IOUT = 150mA. 2) Next, referring to Figure 9, find the curve with the lowest current limit whose output current (with the lowest input voltage) meets your requirements. In our example, we want a curve where IOUT is >150mA with a 4V input and a -24V output. The RSENSE = 80m (shown in Figure 9) shows only approximately 125mA of output current with a 4V input, so we look next at the RSENSE = 70m line. It shows IOUT >150mA for V+ = 4V and VOUT = -24V. The current limit will be 0.210V / 0.070 = 3A. These curves take into account worst-case inductor (10%) and current-sense trip levels, but not sense-resistor tolerance. The switch on resistance is 70m. Standard wire-wound and metal-film resistors have an inductance high enough to degrade performance. Metal-film resistors are usually deposited on a ceramic rod in a spiral, making their inductances relatively high. Surface-mount (or chip) resistors have very little inductance and are well suited for use as current-sense
resistors. If you want to use through-hole resistors, IRC has a wire resistor that is simply a band of metal shaped as a "U" so that inductance is less than 10nH (an order of magnitude less than metal-film resistors). These are available in resistance values between 5m and 0.1.
External Switching Transistor
The MAX774/MAX775/MAX776 are capable of driving P-channel enhancement-mode MOSFET transistors only. The choice of power transistor is dictated by input and output voltage, peak current rating, on resistance, gatesource threshold, and gate capacitance. The drain-tosource rating must be greater than the V+ - V OUT input-to-output voltage differential. The gate-to-source rating must be greater than V+ (the source voltage) plus the absolute value of the most negative swing of EXT. For bootstrapped operation, the most negative swing of EXT is VOUT. In non-bootstrapped operation, this may be ground or some other negative voltage. Gate capacitance is not normally a limiting factor, but values should be less than 1nF for best efficiency. For maximum efficiency, the MOSFET should have a very low on resistance at the peak current and be capable of handling that current. The transistor chosen for the typical operating circuit has a 30V drain-source voltage limit and a 0.07 drain-source on resistance at VGS = -10V. Table 1 lists suppliers of switching transistors suitable for use with the MAX774/MAX775/MAX776.
______________________________________________________________________________________
MAX776-FIG09
700
800
13
-5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-DC Controllers MAX774/MAX775/MAX776
Table 1. Component Suppliers
SUPPLIER INDUCTORS Coiltronics Gowanda Sumida USA Sumida Japan CAPACITORS Kemet Matsuo Nichicon Sanyo USA Sanyo Japan Sprague United Chemi-Con DIODES Motorola Nihon USA Nihon Japan POWER MOSFETS Harris International Rectifier Siliconix IRC (407) 724-3729 (310) 322-3331 (408) 988-8000 (704) 264-8861 (407) 724-3937 (310) 322-3332 (408) 970-3950 (704) 264-8866 (800) 521-6274 (805) 867-2555 81-3-3494-7411 (602) 952-4190 (805) 867-2556 81-3-3494-7414 (803) 963-6300 (714) 969-2491 (708) 843-7500 (619) 661-6835 81-7-2070-6306 (603) 224-1961 (714) 255-9500 (803) 963-6322 (714) 960-6492 (708) 843-2798 (619) 661-1055 81-7-2070-1174 (603) 224-1430 (714) 255-9400 (407) 241-7876 (716) 532-2234 (708) 956-0666 81-3-3607-5111 (407) 241-9339 (716) 532-2702 (708) 956-0702 81-3-3607-5144 PHONE FAX
typically maintains 120mV p-p output ripple when generating -5V at 1A from a 5V input. Smaller capacitors are acceptable for lighter loads or in applications that can tolerate higher output ripple. The value of C4 is chosen such that it acquires as small a charge as possible during the switch on-time. The amount of ripple as a function of capacitance is give by: VOUT x IOUT x ESR IOUT x tOFF(min) Vp-p = _____________________ + _________________ C VIN When evaluating this equation, be sure to use the capacitance value at the switching frequency. At 200kHz, the 330F tantalum capacitor of Figures 2, 3, or 4 may degrade by a factor of ten, which will significantly alter the ripple voltage calculation. The ESR of both the bypass and filter capacitors also affects efficiency. Best performance is obtained by doubling up on the filter capacitors or using low-ESR capacitors. Capacitors must have a ripple current rating equal to the peak current. The smallest low-ESR SMT capacitors currently available are the Sprague 595D series. Sanyo OS-CON organic semiconductor through-hole capacitors also exhibit low ESR and are especially effective at low temperatures. Table 1 lists the phone numbers of these and other manufacturers.
CURRENT-SENSE RESISTORS
PC Layout and Grounding
Due to high current levels and fast switching waveforms, proper PC board layout is essential. Use a star ground configuration; connect the ground lead of the input bypass capacitor, the output capacitor, the inductor, and the GND pin of the MAX774/MAX775/MAX776 at a common point very close to the device. Additionally, input capacitor C2 (Figures 3 and 4) should be placed extremely close to the device. If an external resistor divider is used (Figures 3 and 4), the trace from FB to the resistors must be extremely short.
Capacitors
Choose the output capacitor (C4 of Figures 2, 3, and 4) to be consistent with your size, ripple, and output voltage requirements. Place capacitors in parallel if the size you want is unobtainable. You will not only increase the capacitance, but also decrease the capacitor's ESR (a major contributor of ripple). A 330F tantalum output filter capacitor with 0.07 ESR
14
______________________________________________________________________________________
-5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-DC Controllers
_Ordering Information (continued)
PART MAX775CPA MAX775CSA MAX775C/D MAX775EPA MAX775ESA MAX775MJA MAX776CPA MAX776CSA MAX776C/D MAX776EPA MAX776ESA MAX776MJA TEMP. RANGE 0C to +70C 0C to +70C 0C to +70C -40C to +85C -40C to +85C -55C to +125C 0C to +70C 0C to +70C 0C to +70C -40C to +85C -40C to +85C -55C to +125C PIN-PACKAGE 8 Plastic DIP 8 SO Dice* 8 Plastic DIP 8 SO 8 CERDIP 8 Plastic DIP 8 SO Dice* 8 Plastic DIP 8 SO 8 CERDIP
___________________Chip Topography
OUT GND
MAX774/MAX775/MAX776
EXT FB .109" (2.769mm) CS SHDN
* Contact factory for dice specifications.
REF V+ 0.080 (2.032mm)
TRANSISTOR COUNT: 442; SUBSTRATE CONNECTED TO V+.
________________________________________________________Package Information
DIM A A1 B C D E e H h L INCHES MAX MIN 0.069 0.053 0.010 0.004 0.019 0.014 0.010 0.007 0.197 0.189 0.157 0.150 0.050 BSC 0.244 0.228 0.020 0.010 0.050 0.016 8 0 MILLIMETERS MIN MAX 1.35 1.75 0.10 0.25 0.35 0.49 0.19 0.25 4.80 5.00 3.80 4.00 1.27 BSC 5.80 6.20 0.25 0.50 0.40 1.27 0 8
21-325A
E
H
D A e B
0.127mm 0.004in.
h x 45
A1
C
L
8-PIN PLASTIC SMALL-OUTLINE PACKAGE
______________________________________________________________________________________
15
-5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-DC Controllers MAX774/MAX775/MAX776
________________________________________________________Package Information
D1
DIM A A1 A2 A3 B B1 C D D1 E E1 e eA eB L INCHES MAX MIN 0.200 - - 0.015 0.175 0.125 0.080 0.055 0.022 0.016 0.065 0.050 0.012 0.008 0.390 0.348 0.035 0.005 0.325 0.300 0.280 0.240 0.100 BSC 0.300 BSC 0.400 - 0.150 0.115 15 0 MILLIMETERS MIN MAX - 5.08 0.38 - 3.18 4.45 1.40 2.03 0.41 0.56 1.27 1.65 0.20 0.30 8.84 9.91 0.13 0.89 7.62 8.26 6.10 7.11 2.54 BSC 7.62 BSC - 10.16 2.92 3.81 0 15
21-324A
E D A3 A A2 E1
L
A1 e B
C B1 eA eB
8-PIN PLASTIC DUAL-IN-LINE PACKAGE
DIM
S1
S
D A B2
E1 E
A B B1 B2 C D E E1 e L L1 Q S S1
INCHES MAX MIN 0.200 - 0.023 0.014 0.065 0.038 0.045 0.023 0.015 0.008 0.405 - 0.310 0.220 0.320 0.290 0.100 BSC 0.200 0.125 - 0.150 0.060 0.015 0.055 - - 0.005 15 0
MILLIMETERS MIN MAX - 5.08 0.36 0.58 0.97 1.65 0.58 1.14 0.20 0.38 - 10.29 5.59 7.87 7.37 8.13 2.54 BSC 3.18 5.08 3.81 - 0.38 1.52 - 1.40 0.13 - 0 15
21-326D
Q L e B L1 B1
C
8-PIN CERAMIC DUAL-IN-LINE PACKAGE
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
16 __________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 (408) 737-7600 (c) 1994 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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