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LT1572 100kHz, 1.25A Switching Regulator with Catch Diode
FEATURES
s s s s s s s s s
DESCRIPTIO
Catch Diode Included in Package Wide Input Voltage Range: 3V to 30V Low Quiescent Current: 6mA Internal 1.25A Switch Very Few External Parts Required Self-Protected Against Overloads Operates in Nearly All Switching Topologies Shutdown Mode Draws Only 50A Typical Current Can Be Externally Synchronized
The LT (R)1572 is a 1.25A 100kHz monolithic switching regulator with on-board switch and catch diode included in one package. It combines an LT1172 with a 1A Schottky catch diode. The LT1572 can be operated in all standard switching configurations, including boost, buck, SEPIC, flyback, forward, inverting and "Cuk". All necessary control, oscillator and protection circuitry is included on the die with the high efficiency switch. This makes the part extremely easy to use and provides "bustproof" operation similar to that obtained with 3-pin linear regulators. The LT1572 operates with supply voltages from 3V to 30V and draws only 6mA quiescent current. It can deliver load power up to 15W with no external power devices. By utilizing a current mode switching technique, the LT1572 achieves excellent response to load and line transients. The LT1572 has many unique features not found on the more difficult to use control chips presently available. It uses adaptive anti-sat switch drive to allow very wide ranging load currents with no loss in efficiency. An externally activated shutdown mode reduces total supply current to 50A typical for standby operation. External synchronizing of switching frequency is possible, with a range of 120kHz to 160kHz.
, LTC and LT are registered trademarks of Linear Technology Corporation.
APPLICATI
s s s
S
s
3.3V-to-5V and 5V-to-12V Boost Converters Negative-to-Positive Converter SEPIC Converter (Input Can Be Greater or Less Than Output) Battery Charger
TYPICAL APPLICATI
VIN 4.5V TO 10V L1* 50H 9 VIN 11
5V-to-12V Boost Converter
100
Boost Converter Efficiency
BOOST CONVERTER VIN = 5V VOUT = 12V
2 15
EFFICIENCY (%)
VSW ANODE
*COILTRONICS CTX50-2 **AVX TPS OR SPRAGUE 593D ALWAYS CONNECT BOTH ANODE (2, 15) AND CATHODE (3, 14) PINS 3 R1 10.7k 1% 12V 0.25A
90
CATHODE 14
80
+
LT1572 C3 100F 10V FB E1 10 E2 12 GND 4, 13 VC 5 R3 1k C1 1F
70
+
R2 1.24k 1%
C2** 100F 16V
60
50
0
50
LT1572 * TA01
U
150 200 100 L0AD CURRENT (mA) 250
LT1572 * TA01
UO
UO
1
LT1572 ABSOLUTE AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW NC 1 ANODE* 2 CATHODE* 3 GND 4 VC 5 FB 6 NC 7 NC 8 16 NC 15 ANODE* 14 CATHODE* 13 GND 12 E2 11 VSW 10 E1 9 VIN
Supply Voltage (Note 4).......................................... 40V Switch Output Voltage (Note 4) .............................. 60V Feedback Pin Voltage (Transient, 1ms) ................ 15V Operating Junction Temperature Range Operating .............................................. 0C to 100C Short Circuit ......................................... 0C to 125C Storage Temperature Range ............... - 65C to 150C Lead Temperature (Soldering, 10 sec)................. 300C DIODE Average Forward Current .......................................... 1A Peak Repetitive Forward Current .............................. 2A Peak Non-Repetitive Forward Current....................... 3A Peak Repetitive Reverse Voltage............................. 20V Continuous (Average) Reverse Voltage .................. 15V Operating Junction Temperature ......................... 125C
Note 1: Minimum effective switch "on" time for the LT1572 (in current limit only) is 0.6s. This limits the maximum safe input voltage during an output shorted condition. Buck mode and inverting mode input voltage during an output shorted condition is limited to: R x IL + Vf VIN (max, output shorted) = 15V + txf buck and inverting mode R = Inductor DC resistance IL = 2.5A Vf = Output catch diode forward voltage at IL t = 0.6s, f = 100kHz switching frequency
ORDER PART NUMBER LT1572CS
S PACKAGE 16-LEAD PLASTIC SO *ALWAYS CONNECT BOTH ANODE AND BOTH CATHODE PINS TJMAX (REGULATOR) = 100C TJMAX (DIODE) = 125C SEE THERMAL MANAGEMENT SECTION FOR JA
Consult factory for Industrial and Military grade parts. Maximum input voltage can be increased by increasing R or Vf. External current limiting such as that shown in AN19, Figure 39, will provide protection up to the full supply voltage rating. C1 in Figure 39 should be reduced to 200pF. Transformer designs will tolerate much higher input voltages because leakage inductance limits rate of rise of current in the switch. These designs must be evaluated individually to assure that current limit is well controlled up to maximum input voltage. Boost mode designs are never protected against output shorts because the external catch diode and inductor connect input to output.
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER VREF IB gm Reference Voltage Feedback Input Current Error Amplifier Transconductance Error Amplifier Source or Sink Current Error Amplifier Clamp Voltage Reference Voltage Line Regulation AV IQ Error Amplifier Voltage Gain Minimum Input Voltage (Note 3) Supply Current CONDITIONS
VIN = 15V, VC = 0.5V, VFB = VREF, output pin open, unless otherwise noted.
MIN
q q
TYP 1.244 1.244 350
MAX 1.264 1.274 750 1100 6000 7000 350 400 2.30 0.52 0.03
UNITS V V nA nA mho mho A A V V %/V V/V
Measured at Feedback Pin VC = 0.8V VFB = VREF IC = 25A
1.224 1.214
q
3000 2400 150 120 1.80 0.25
4400 200
VC = 1.5V
q
Hi Clamp, VFB = 1V Lo Clamp, VFB = 1.5V 3V VIN 40V VC = 0.8V 0.9V VC 1.4V
q q
0.38
500
800 2.6 6 3.0 9
3V VIN 40V, VC = 0.6V
2
U
V mA
W
U
U
WW
W
LT1572
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER Control Pin Threshold Normal/Flyback Threshold on Feedback Pin VFB Flyback Reference Voltage (Note 3) Change in Flyback Reference Voltage Flyback Reference Voltage Line Regulation (Note 3) Flyback Amplifier Transconductance (gm) Flyback Amplifier Source and Sink Current BV VSAT Output Switch Breakdown Voltage (Note 4) Output Switch "On" Resistance (Note 1) Control Voltage to Switch Current Transconductance ILIM Switch Current Limit IFB = 50A CONDITIONS Duty Cycle = 0
VIN = 15V, VC = 0.5V, VFB = VREF, output pin open, unless otherwise noted.
MIN
q
TYP 0.9 0.45 16.3 6.8 0.01
MAX 1.08 1.25 0.54 17.6 18.0 9 0.03 500 70 70
UNITS V V V V V V %/V mho A A V
0.8 0.6 0.4 15.0 14.0 4.5
q
0.05 IFB 1mA IFB = 50A 7V VIN VMAX IC = 10A VC = 0.6V, Source IFB = 50A, Sink 3V VIN 40V, ISW = 1.5mA
q q q q
150 15 25 60
300 32 40 80 0.60 2
1.00
A/V
Duty Cycle = 50%, TJ 25C Duty Cycle = 50%, TJ < 25C Duty Cycle = 80% (Note 2)
q q q
1.25 1.25 1.00 25 88 85 80 100 90 100 100 50 150 1.5
3.0 3.5 2.5 35 112 115 95 250 250 300
A A A mA/A kHz kHz % A mV mV s
IIN ISW f DCMAX
Supply Current Increase During Switch On-Time Switching Frequency
q
Maximum Switch Duty Cycle Shutdown Mode Supply Current Shutdown Mode Threshold Voltage Flyback Sense Delay Time (Note 3) 3V VIN 40V VC = 0.05V 3V VIN 40V
q
q
DIODE
PARAMETER Forward Voltage (Note 5) CONDITIONS If = 200mA If = 500mA If = 1A VR = 5V, TJ = 25C VR = 5V, TJ = 75C VR = 20V, TJ = 25C VR = 20V, TJ = 75C Diode Thermal Resistance (Note 6)
q q q
MIN
TYP 0.45 0.52 0.55 1 25 3 70 90
MAX 0.57 0.65 0.70 5 100 15 300
UNITS V V V A A A A C/W
Reverse Leakage (Note 5)
3
LT1572
ELECTRICAL CHARACTERISTICS
VIN = 15V, VC = 0.5V, VFB = VREF, output pin open, unless otherwise noted.
Note 5: See graphs for guaranteed forward voltage and reverse leakage current over temperature. Parameters are 100% tested at 25C and guaranteed at other temperatures by design and QA sampling. Note 6: Package soldered to FR4 board with 1oz copper and an internal or backside plane underneath the package to aid thermal transfer. Diode is partly thermally coupled to regulator section. See Application Information section for details on thermal calculations.
The q denotes the specifications which apply over the full operating temperature range, 0C to 100C for the regulator chip and 0C to 125C for the diode. Note 1: Measured with VC in hi clamp, VFB = 0.8V. ISW = 1A. Note 2: For duty cycles (DC) between 50% and 80%, minimum guaranteed switch current is given by ILIM = 0.833 (2 - DC). Note 3: Minimum input voltage for isolated flyback mode is 7V. Note 4: Because the catch diode has a peak repetitive reverse voltage of 20V, diode breakdown may be the limiting factor on input voltage or switch voltage in many applications.
TYPICAL PERFOR A CE CHARACTERISTICS
Switch Current Limit vs Duty Cycle
4
2.9
SWITCH SATURATION VOLTAGE (V)
MINIMUM INPUT VOLTAGE (V)
SWITCH CURRENT (A)
3 25C
2
-55C 125C
1
0 0 10 20 30 40 50 60 70 80 90 100 DUTY CYCLE (%)
1572 G01
Line Regulation
5 REFERENCE VOLTAGE CHANGE (mV) 4
REFERENCE VOLTAGE (V)
3 2 1 0 -1 -2 -3 -4 -5 0 10 TJ = -55C
TJ = 150C
FEEDBACK BIAS CURRENT (nA)
TJ = 25C
30 40 20 INPUT VOLTAGE (V)
4
UW
50 60
1572 G04
Minimum Input Voltage
1.6
Switch Saturation Voltage
1.4 150C 1.2 1.0 0.8 0.6 0.4 0.2 0 0 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 SWITCH CURRENT (A)
1572 G03
SWITCH CURRENT = 1.25A 2.8 2.7 2.6 SWITCH CURRENT = 0A 2.5 2.4 2.3 -75 -50 -25
100C 25C -55C
0 25 50 75 100 125 150 TEMPERATURE (C)
1572 G02
Reference Voltage vs Temperature
1.250 1.248 1.246 1.244 1.242 1.240 1.238 1.236 1.234 -75 -50 -25 800 700 600 500 400 300 200 100
Feedback Bias Current vs Temperature
25 50 75 100 125 150 TEMPERATURE (C) 0
1572 G05
0 -75 -50 -25
0 25 50 75 100 125 150 TEMPERATURE (C)
1572 G06
LT1572
TYPICAL PERFOR A CE CHARACTERISTICS
Supply Current vs Supply Voltage (Shutdown Mode)
160 140 TJ = 25C
35
SUPPLY CURRENT (mA)
DRIVER CURRENT (mA)
SUPPLY CURRENT (A)
120 100 80 60 40 20 0 0 10 20 30 SUPPLY VOLTAGE (V) 40
1572 G07
VC = 50mV
VC = 0V
Shutdown Mode Supply Current
200 180 160
SUPPLY CURRENT (A)
TRANSCONDUCTANCE (mho)
VC PIN CURRENT (A)
140 120 100 80 60 40 20 0 0 10 20 30 40 50 60 70 80 90 100 VC PIN VOLTAGE (mV)
1572 G10
TJ = 150C
-55C TJ 125C
Idle Supply Current vs Temperature
11 10 VC = 0.6V 500 450
IDLE SUPPLY CURRENT (mA)
FEEDBACK VOLTAGE (mV)
9 8 7 6 5 4 3 2 1 -75 -50 -25 0 25 50 75 100 125 150 TEMPERATURE (C)
1572 G13
SWITCH CURRENT (A)
VSUPPLY = 60V VSUPPLY = 3V
UW
Driver Current* vs Switch Current
40
Supply Current vs Input Voltage*
15 14 13 TJ = 25C NOTE THAT THIS CURRENT DOES NOT INCLUDE DRIVER CURRENT, WHICH IS A FUNCTION OF LOAD CURRENT AND DUTY CYCLE. 90% DUTY CYCLE 50% DUTY CYCLE 10% DUTY CYCLE 0% DUTY CYCLE 0 10 30 40 20 INPUT VOLTAGE (V) 50 60
1572 G09
30 25 20 15 10 5 0 0 0.25 0.50 0.75 1.00 SWITCH CURRENT (A) 1.25
1572 G08
12 11 10 9 8 7 6 5
TJ = -55C
TJ = 25C
* AVERAGE POWER SUPPLY CURRENT IS FOUND BY MULTIPLYING DRIVER CURRENT BY DUTY CYCLE, THEN ADDING QUIESCENT CURRENT.
* UNDER VERY LOW OUTPUT CURRENT CONDITIONS, DUTY CYCLE FOR MOST CIRCUITS WILL APPROACH 10% OR LESS.
Error Amplifier Transconductance
5000 4500 4000 3500 3000 2500 2000 1500 1000 500 0 -75 -50 -25 0 25 50 75 100 125 150 TEMPERATURE (C)
1572 G11
VC Pin Characteristics
300 200 100 0 TJ = 25C -100 -200 VFB = 0.8V (CURRENT OUT OF VC PIN) -300 -400 0 0.5 1.5 2.0 1.0 VC PIN VOLTAGE (V) 2.5
1572 G12
gm =
I (VC PIN) V (FB PIN)
VFB = 1.5V (CURRENT INTO VC PIN)
Feedback Pin Clamp Voltage
1000 900 800 -55C 25C 150C 700 600
Switch "Off" Characteristics
400 350 300 250 200 150 100 50 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 FEEDBACK CURRENT (mA)
1572 G14
500 VSUPPLY = 3V 400 300 200 100 0 0 10 20 30 40 50 60 70 80 90 100 SWITCH VOLTAGE (V)
1572 G15
VSUPPLY = 15V
VSUPPLY = 40V
5
LT1572
TYPICAL PERFOR A CE CHARACTERISTICS
Shutdown Thresholds
400 350 CURRENT (OUT OF VC PIN) - 400 - 350 - 300 - 250 - 200 VOLTAGE 150 100 50 VC VOLTAGE IS REDUCED UNTIL REGULATOR CURRENT DROPS BELOW 300A 0 - 150 -100 - 50
2.2 2.0 1.8
FLYBACK VOLTAGE (V)
VC PIN VOLTAGE (mV)
300 250 200
TIME (s)
0 -75 -50 -25
0 25 50 75 100 125 150 TEMPERATURE (C)
1572 G16
Transconductance of Error Amplifier
7000 6000
TRANSCONDUCTANCE (mho)
5000 4000 3000 2000 1000 0 gm
30 60 90 120 150 180 1k 10k 1M 100k FREQUENCY (Hz) 210 10M
1572 G19
FEEDBACK PIN VOLTAGE (mV)
-1000
6
UW
Flyback Blanking Time
23 22 21 20 19 18 17 16
Isolated Mode Flyback Reference Voltage
RFB = 500
VC PIN CURRENT (A)
1.6 1.4 1.2 1.0 -75 -50 -25 0 25 50 75 100 125 150 JUNCTION TEMPERATURE (C)
1572 G17
RFB = 1k
RFB = 10k
15 -75 -50 -25
0 25 50 75 100 125 150 TEMPERATURE (C)
1572 G18
Normal/Flyback Mode Threshold on Feedback Pin
-30 0 500 490 480 470 460 450 440 430 420 410 400 -50 -25 0 FEEDBACK PIN CURRENT (AT THRESHOLD) FEEDBACK PIN VOLTAGE (AT THRESHOLD) -24 -22
FEEDBACK PIN CURRENT (A)
-20 -18 -16 -14 -12 -10 -8 -6 -4 25 50 75 100 125 150 TEMPERATURE (C)
1572 G20
PHASE (DEG)
LT1572
BLOCK DIAGRA
FB
OPERATIO
The LT1572 is a current mode switcher. This means that switch duty cycle is directly controlled by switch current rather than by output voltage. Referring to the block diagram, the switch is turned "on" at the start of each oscillator cycle. It is turned "off" when switch current reaches a predetermined level. Control of output voltage is obtained by using the output of a voltage sensing error amplifier to set current trip level. This technique has several advantages. First, it has immediate response to input voltage variations, unlike ordinary switchers which have notoriously poor line transient response. Second, it reduces the 90 phase shift at mid-frequencies in the energy storage inductor. This greatly simplifies closedloop frequency compensation under widely varying input voltage or output load conditions. Finally, it allows simple pulse-by-pulse current limiting to provide maximum switch protection under output overload or short conditions.
W
VIN 16V SWITCH OUT ANODE 2.3V REG CATHODE FLYBACK ERROR AMP LT1172 5A, 75V SWITCH DRIVER 100kHz OSC LOGIC MODE SELECT COMP ANTISAT
- +
ERROR AMP VC
+
SHUTDOWN CIRCUIT 1.24V REF 0.15V CURRENT AMP GAIN 6 0.16
-
E1* E2
*ALWAYS CONNECT E1 TO GROUND
1572 BD
U
A low dropout internal regulator provides a 2.3V supply for all internal circuitry on the LT1572. This low dropout design allows input voltage to vary from 3V to 40V with virtually no change in device performance. A 100kHz oscillator is the basic clock for all internal timing. It turns "on" the output switch via the logic and driver circuitry. Special adaptive anti-sat circuitry detects onset of saturation in the power switch and adjusts driver current instantaneously to limit switch saturation. This minimizes driver dissipation and provides very rapid turn-off of the switch. A 1.2V bandgap reference biases the positive input of the error amplifier. The negative input is brought out for output voltage sensing. This feedback pin has a second function; when pulled low with an external resistor, it programs the LT1572 to disconnect the main error amplifier output and connects the output of the flyback amplifier
7
LT1572 OPERATIO
to the comparator input. The LT1572 will then regulate the value of the flyback pulse with respect to the supply voltage.1 This flyback pulse is directly proportional to output voltage in the traditional transformer coupled flyback topology regulator. By regulating the amplitude of the flyback pulse, the output voltage can be regulated with no direct connection between input and output. The output is fully floating up to the breakdown voltage of the transformer windings. Multiple floating outputs are easily obtained with additional windings. A special delay network inside the LT1572 ignores the leakage inductance spike at the leading edge of the flyback pulse to improve output regulation. The error signal developed at the comparator input is brought out externally. This pin (VC) has four different functions. It is used for frequency compensation, current limit adjustment, soft starting, and total regulator shutdown. During normal regulator operation this pin sits at a voltage between 0.9V (low output current) and 2.0V (high output current). The error amplifiers are current output (gm) types, so this voltage can be externally clamped for adjusting current limit. Likewise, a capacitor coupled external clamp will provide soft start. Switch duty cycle goes to zero if the VC pin is pulled to ground through a diode, placing the LT1572 in an idle mode. Pulling the VC pin below 0.15V causes total regulator shutdown, with only 50A supply current for shutdown circuitry biasing. See AN19 for full application details. E1 and E2 Pins The LT1572 has the emitters of the power transistor brought out separately from the ground pin. This eliminates errors due to ground pin voltage drops and allows the user to reduce switch current limit 2:1 by leaving the second emitter (E2) disconnected. The first emitter (E1) should always be connected to the ground pin. Note that switch "on" resistance doubles when E2 is left open, so efficiency will suffer somewhat when switch currents exceed 300mA. Also, note that chip dissipation will actually increase with E2 open during normal load operation, even though dissipation in current limit mode will decrease.
1See note under block diagram.
8
U
Other Application Help More circuits and application help for the LT1572 can be found in the LT1172 data sheet, both in loose form and in the 1994 Linear Databook Volume III. Extensive additional help is contained in Application Note 19. All application circuits using the LT1172 can also use the LT1572 as long as the 20V maximum reverse voltage of the diode is not exceeded. A CAD program called SwitcherCAD is also available. This program can be used with the LT1572 by simply treating the LT1572 as an LT1172 and ignoring the predicted die temperature results obtained from SwitcherCAD itself. Thermal Management Thermal management is particularly important with the LT1572 because both switch and diode power dissipation increase rapidly at low input voltage when using the popular boost topology. Regulator and diode die temperature must be calculated separately because they are not connected to an isothermal plane inside the package. Diode plus regulator thermal resistance is approximately 70C/W when the LT1572 is soldered to 1oz copper traces over an internal or backside copper plane using FR4 board material. However, individual calculation of die temperature must take thermal coupling into account. To accomplish this, thermal resistance is broken into two sections, a common (coupled) section and a second uncoupled section. Die temperatures are calculated from: TREG = TA + PREG (90C/W) + PDIODE (45C/W) TDIODE = TA + PDIODE (90C/W) + PREG (45C/W) TA = ambient temperature TREG = regulator die temperature TDIODE = diode die temperature PREG = total regulator power dissipation PDIODE = diode power dissipation The following formulas can be used as a rough guide to calculate LT1572 power dissipation. For more details, the reader is referred to Application Note 19 (AN19), "Efficiency Calculations" section.
LT1572
OPERATIO
Average supply current (including driver current) is: IIN 6mA + ISW (0.004 + DC/40) ISW = switch current DC = switch duty cycle Switch power dissipation is given by: PSW = (ISW)2 x RSW x DC RSW = LT1572 switch "on" resistance (1 maximum) Total power dissipation is the sum of supply current times input voltage plus switch power: PREG = IIN x VIN + PSW In a typical example, using a boost converter to generate 12V at 0.12A from a 5V input, duty cycle is approximately 60%, and switch current is about 0.65A, yielding: IIN = 6mA + 0.65(0.004 + DC/40) = 18mA PSW = (0.65)2 x 1 x 0.6 = 0.25W PREG = 5V x 0.018A + 0.25 = 0.34W Approximate diode power dissipation for boost and buck converters is shown below. For other topologies or more accurate results, see Application Note 19 or use SwitcherCAD. Boost: PDIODE = IOUT x Vf Buck: PDIODE = IOUT x Vf x (VIN - VOUT)/VIN Vf = diode forward voltage at a current equal to IOUT for a buck converter and IOUT x VOUT/VIN for a boost converter. In most applications, full load current is used to calculate die temperature. However, if overload conditions must also be accounted for, three approaches are possible. First, if loss of regulated output is acceptable under overload conditions, the internal thermal limit of the LT1572 will protect the die in most applications by shutting off switch current. Thermal limit is not a tested parameter, however, and should be considered only for noncritical applications with temporary overloads. The second approach for lower current applications is to leave the second switch emitter (E2) open. This increases
U
switch "on" resistance by 2:1, but reduces switch current limit by 2:1 also, resulting in a net 2:1 reduction in I2R switch dissipation under current limit conditions. The third approach is to clamp the VC pin to a voltage less than its internal clamp level of 2V. The LT1172 switch current limit is zero at approximately 1V on the VC pin and 2A at 2V on the VC pin. Peak switch current can be externally clamped between these two levels with a diode. See AN19 for details. Diode Characteristics The catch diode used in the LT1572 is a power Schottky diode with a very low storage time and low forward voltage. This gives good efficiency in switching regulator applications, but some thought must be given to maximum operating voltage and high temperature reverse leakage. Peak repetitive reverse voltage rating on the diode is 20V. In a boost converter, maximum diode reverse voltage is equal to regulated output voltage, so this limits maximum output voltage to 20V. In a negative-to-positive converter, maximum diode voltage will be equal to the sum of output voltage plus input voltage. Use the equations in Application Note 19 or SwitcherCAD or calculate maximum diode voltage for other topologies. Diode reverse leakage increases rapidly with temperature. This leakage is not high enough to significantly impact efficiency or diode power dissipation, but it can be of concern in shutdown mode if the diode is connected in such a way that the leakage adds to regulator shutdown current. Use the graphs of diode leakage versus voltage and temperature to ensure proper high temperature system performance. The LT1572 diode is internally bonded to more than two package pins to reduce internal bond wire currents. All pins must be used to prevent excessive current in the individual internal bond wires. This is important in low load current applications because the LT1572 will draw high surge currents during start-up (to charge the output capacitor) even with no output load current.
9
LT1572
OPERATIO
Synchronizing
The LT1572 can be externally synchronized in the frequency range of 120kHz to 160kHz. This is accomplished as shown in the accompanying figures. Synchronizing occurs when the VC pin is pulled to ground with an external transistor. To avoid disturbing the DC characteristics of the internal error amplifier, the width of the synchronizing pulse should be under 0.3s. C2 sets the pulse width at 0.2s. The effect of a synchronizing pulse on the LT1572 amplifier offset can be calculated from: KT VC q tS fS IC + R3 VOS = IC
( )( )
KT = 26mV at 25C q tS = pulse width fS = pulse frequency IC = VC source current ( 200A) VC = operating VC voltage (1V to 2V) R3 = resistor used to set mid-frequency "zero" in frequency compensation network. With tS = 0.2s, fS = 150kHz, VC = 1.5V, and R3 = 2k, offset voltage shift is 3.8mV. This is not particularly bothersome, but note that high offsets could result if R3 were reduced to a much lower value. Also, the synchronizing transistor must sink higher currents with low values of R3, so larger drives may have to be used. The transistor must be capable of pulling the VC pin to within 200mV of ground to ensure synchronizing.
10
U
Synchronizing with Bipolar Transistor
VIN LT1572 GND VC C2 39pF R3 C1 2N2369 R2 2.2k FROM 5V LOGIC
1572 OP01
R1 3k
Synchronizing with MOS Transistor
VIN
LT1572
GND
VC D1 1N4158 VN2222* R2 2.2k D2 1N4158 FROM 5V LOGIC C2 100pF
R3 C1
*SILICONIX OR EQUIVALENT
1572 OP02
LT1572
TYPICAL APPLICATI
C3* 100F
VIN -7V TO -20V
INPUT VOLTAGE 4.5V TO 20V L2*** 1k L1** 300H Q1* CATHODE 10F TANT E2 VIN VSW 0.02F Q2* B E1 GND VC FB C6 1F * Q1,Q2 = BCP56 OR MPS650/561 ** COILTRONICS CTX300-4 *** SUMIDA 6345-020 OR COILTRONICS 110092-1 A MODIFICATION WILL ALLOW OPERATION DOWN TO 4.5V. CONSULT FACTORY.
1572 TA04
+
LT1572 ANODE
2F
Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
+
* REQUIRED IF INPUT LEADS 2" ** PULSE ENGINEERING 92114 COILTRONICS 50-2-52
CATHODE
VIN VSW
L1** 50H
E1 E2 GND
LT1572 ANODE FB VC C1 R3 R2 1.24k
Backlight CCFL Supply (see AN55 for details)
A
+
+
UO
S
Negative Buck Converter
C2 200F
R1 4.64k R4 12k
LOAD
-5.2V 0.75A
Q1 2N3906
1572 TA03
33pF 3kV
LAMP
D1 1N914
D2 1N914
50k INTENSITY ADJUST
R3 10k
R1 560
11
LT1572
PACKAGE DESCRIPTIO U
Dimensions in inches (millimeters) unless otherwise noted. S Package 16-Lead Plastic SOIC
0.386 - 0.394* (9.804 - 10.008) 16 15 14 13 12 11 10 9
0.228 - 0.244 (5.791 - 6.197)
0.150 - 0.157* (3.810 - 3.988)
1 0.010 - 0.020 x 45 (0.254 - 0.508) 0.008 - 0.010 (0.203 - 0.254) 0 - 8 TYP 0.053 - 0.069 (1.346 - 1.752)
2
3
4
5
6
7
8
0.004 - 0.010 (0.101 - 0.254)
0.016 - 0.050 0.406 - 1.270
0.014 - 0.019 (0.355 - 0.483)
0.050 (1.270) TYP
SO16 0893
*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006 INCH (0.15mm).
RELATED PARTS
PART NUMBER LT1172 LT1173 LT1372 LTC1574 DESCRIPTION 100kHz, 1.25A High Efficiency Switching Regulator Micropower DC/DC Converter Adjustable and Fixed 5V, 12V 500kHz High Efficiency 1.5A Step-Up Switching Regulator High Efficiency Step-Down DC/DC Converter with Internal Schottky Diode COMMENTS LT1572 Without Diode Operates Down to 2V Input Latest Technology, Uses Tiny Inductors LTC1174 with Diode
12
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7487
(408) 432-1900 q FAX: (408) 434-0507 q TELEX: 499-3977
LT/GP 0595 10K * PRINTED IN USA
(c) LINEAR TECHNOLOGY CORPORATION 1995

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Price & Availability of LT1572

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