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ATENT
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LIN D O C #: 1570
LX1570/1571
PHASE MODULATED AC SYNCHRONOUS SECONDARY-SIDE CONTROLLER
T
HE
I
NFINITE
P
OWER
OF
I
N N O VAT I O N
P
RELIMINARY
D
ATA
S
HEET
DESCRIPTION
The LX1570/71 series of controller ICs are designed to provide all control functions in a secondary-side regulator for isolated auxiliary or secondary power supplies. Auxiliary or secondary-side controllers are used in a variety of applications including multiple output off-line power supplies, commonly found in desktop computers, as well as telecommunications applications. Although they can be used in all secondary output applications requiring precision regulation, they are mainly optimized for outputs delivering more than 3A current where standard three-terminal regulators lack the desired efficiency. For these applications, the Mag Amp regulators have traditionally been used. However, Mag Amps have several disadvantages. First, because they have to withstand the maximum input voltage during a short-circuit condition, they are "over designed", typically by 2 times, increasing the cost and size of the power supply. Second, Mag Amps are inherently leading edge modulators, so they can only approach a certain maximum duty cycle, limited by the minimum delay and the magnetic BH loop characteristic of the Mag Amp core. This forces an increase in the size of the main transformer as well as the output inductor, resulting in higher overall system cost. The LX1570/71 eliminates all the disadvantages of the Mag Amp approach as well as improving system performance and reducing overall system cost. The LX1570/71 is a current mode controller IC that controls the duty cycle of a switch in series with the secondary AC output of the power transformer in buck-derived applications, such as forward or bridge topologies. It offers features such as 100% duty cycle operation for maximum energy transfer, pulse-by-pulse and hiccup current limiting with long off-time between the cycles for reduced power dissipation, high-frequency operation for smaller magnetics, softstart, and current mode control for excellent dynamic response.
K E Y F E AT U R E S
p REPLACES COSTLY MAG-AMP CORES WITH A LOW ON-RESISTANCE MOSFET p LOOK-AHEAD SWITCHINGTM ENSURES SWITCH TURN ON BEFORE THE AC INPUT TO ACHIEVE 100% ENERGY TRANSFER p LOWER OVERALL SYSTEM COST p LOWER PEAK CURRENT STRESS ON THE PRIMARY SWITCH p ALLOWS HIGHER OPERATING FREQUENCY AND SMALLER OUTPUT INDUCTOR p EASY SHORT-CIRCUIT PROTECTION p CURRENT MODE APPROACH ACHIEVES EXCELLENT DYNAMIC RESPONSE
A P P L I C AT I O N S
s SECONDARY-SIDE REGULATOR IN OFF-LINE POWER SUPPLIES s COMPUTER POWER SUPPLIES, 3.3V OUTPUT FOR NEW LOW-VOLTAGE PROCESSORS AND MEMORIES s TELECOMMUNICATION AND MILITARY DC/DC CONVERTERS
PRODUCT HIGHLIGHT
Aux Output 12V/8A
A VA I L A B L E OP T I O N S
Part # LX1570 C.L. Threshold -0.2V
PER
PART #
Application Output Currents < 4A
C.S. Option Resistive Sensing
LX1571
1V
Current Output Transformer Currents Sensing > 4A
VCC
OUT DRV
C.S. VFB
LX1571
COMP CT GND S.S.
PA C K A G E O R D E R I N F O R M AT I O N TA (C) 0 to 70 -40 to 85 -55 to 125
M Plastic DIP 8-pin
LX157xCM LX157xIM --
DM Plastic SOIC 8-pin
LX157xCDM LX157xIDM --
Y Ceramic DIP 8-pin
-- -- LX157xMY
Note: All surface-mount packages are available in Tape & Reel. Append the letter "T" to part number. (i.e. LX157xCDMT)
F O R F U R T H E R I N F O R M AT I O N C A L L ( 7 1 4 ) 8 9 8 - 8 1 2 1
Copyright (c) 1997 Rev. 0.9.3 1/97
11861 WESTERN AVENUE, GARDEN GROVE, CA. 92841
1
PRODUCT DATABOOK 1996/1997
LX1570/1571
PHASE MODULATED AC SYNCHRONOUS SECONDARY-SIDE CONTROLLER
P
RELIMINARY
D
ATA
S
HEET
A B S O L U T E M A X I M U M R AT I N G S
(Note 1)
PACKAGE PIN OUTS
S.S. VFB COMP C.S.
1 2 3 4 8 7 6 5
Supply Voltage (VCC) .................................................................................................... 40V Digital Inputs ....................................................................................................... -0.3 to 7V Output Peak Current Source (500nS) ........................................................................... 1A Output Peak Current Sink (500nS) ................................................................................ 1A
Note 1. Exceeding these ratings could cause damage to the device. All voltages are with respect to Ground. Currents are positive into, negative out of the specified terminal.
CT VCC OUT DRV GND
M & Y PACKAGE (Top View)
T H E R MAL DATA
M PACKAGE: THERMAL RESISTANCE-JUNCTION TO AMBIENT, JA DM PACKAGE: THERMAL RESISTANCE-JUNCTION TO AMBIENT, JA Y PACKAGE: THERMAL RESISTANCE-JUNCTION TO AMBIENT, JA 130C/W 165C/W 95C/W
S.S. VFB COMP C.S.
1 2 3 4 8 7 6 5
CT VCC OUT DRV GND
DM PACKAGE (Top View)
Junction Temperature Calculation: TJ = TA + (PD x JA). The JA numbers are guidelines for the thermal performance of the device/pc-board system. All of the above assume no ambient airflow.
LX1571 BLOCK DIAGRAM
2.5V 0.25V Minimum Current Comp PWM Latch R S.S. 1 VFB 2 COMP 3 C.S. 4 0.5V Voltage Hiccup Comp. Voltage Mode Hiccup 5V Internal Bias
CHG CONTROL QUICK CHG CONTROL
Q
Error Amp 2R 1V R Current Mode Hiccup Comp C.S. Comp S
6 OUT DRV
1.5V
2.5V
2.5V REF
LATCH RESET CONTROL
7 VCC
CT 8
Timing / Duty Cycle Control
DISCH CONTROL VALLEY THRESHOLD CONTROL LATCH SET CONTROL
6V 16V
5 GND
Q Q
R S
Hiccup Latch
2
Copyright (c) 1997 Rev. 0.9.3 1/97
PRODUCT DATABOOK 1996/1997
LX1570/1571
PHASE MODULATED AC SYNCHRONOUS SECONDARY-SIDE CONTROLLER
P
RELIMINARY
D
ATA
S
HEET
ELECTRICAL CHARACTERISTICS
(Unless otherwise specified, these specifications apply over the ranges T A = -55 to 125C for the LX1570M/1571M, TA = -40 to 85C for the LX1570I/1571I, and TA = 0 to 70C for LX1570C/1571C. VCC = 15V. Typ. number represents TA = 25C value.)
Parameter Reference Section
Initial Accuracy Line Regulation Temp Stability
Symbol
Test Conditions
LX1570/1571 Min. Typ. Max.
2.475 2.500 2.525 1 1.5 110 115 1
Units
VRI VRL VRT fO f OL ICHG IDISCH I LK VRPP
TA = 25C, measured at F.B pin 11V < VCC < 25 Note 2 CT = , TJ = 25C, measured at pin 6 Over Temp, measured at pin 6
V % % kHz kHz % mA mA A V V A dB A A V V V/Sec ms/F mA V V A A V/V V/V mV mV ns V V V V V
Timing Section
Initial Accuracy Line Voltage Stability Charging Current Discharging Current Leakage Current Ramp PK to PK 90 85 100 100 3 3.5 4 0.6 6 0.005 0.1 70 400 400 5.1 1 35 50 TBD 65
C.S.INPUT = 1.5V C.S.INPUT = 0V C.S.INPUT = 1.5V (1571), C.S.INPUT = -0.4V (1570)
Error Amp / Soft Start Comp Section
Transconductance Input Bias Current Open Loop Gain Output Sink Current Output Source Current Output HI Voltage Output LO Voltage Slew Rate gm IB AVOL IEA(SINK) I EA(SOURCE) VCOMP-HI VCOMP-LO S KSS ISS-DIS LX1570 LX1571 LX1570 LX1571 LX1570 LX1571 LX1570 LX1571 LX1570 LX1571 LX1570 LX1571 VCSI -0.3 I CSB ACS VCSMIN 10% Overdrive VCLP VCLH VHCCP -0.18 0.9 -13.5 2.7 -15 3 -50 250 100 -0.2 1 -0.3 1.5 2 VFB = 2.6V VFB = 2.4V 1
60 200 200
0.8
Soft-Start Section
Soft Start Timing Factor Soft Start Discharge Current
Current Sense Section
Input Range Input Current C.S. Amplifier Gain Minimum Current Threshold Voltage C.S. Delay to Driver Output C.L. Pulse-By-Pulse Threshold Voltage C.L. Hiccup Threshold Voltage Voltage Hiccup Threshold -0.8 6 25 1 -16.5 3.3
200 -0.22 1.1
Note 2. Although this parameter is guaranteed, it is not 100% tested in production.
Copyright (c) 1997 Rev. 0.9.3 1/97
3
PRODUCT DATABOOK 1996/1997
LX1570/1571
PHASE MODULATED AC SYNCHRONOUS SECONDARY-SIDE CONTROLLER
P
RELIMINARY
D
ATA
S
HEET
ELECTRICAL CHARACTERISTICS Parameter PWM Section
E.A. Output to PWM Drive Offset Fixed Duty Cycle VOFS D tR / tF VDH VDL VDPD VST VOFF VH IQd I ST Out Freq = 100kHz, CL = 0 CL = 1000pF ISOURCE = 200mA, VCS = 0V, VFB = 2.3V ISINK = 200mA, VCS = 1.2V, VFB = 2.3V VCC = 0V, IPULL UP = 2mA
(Con't.)
Symbol
Test Conditions
LX1570/1571 Min. Typ. Max.
1.7 52 2.0 54 50 13.5 0.8 1 15 9 5.5 16 10 6 18 150 17 11 6.5 30 250 2.4 56
Units
V % ns V V V V V V mA A
Output Drive Section
Rise / Fall Time Output HI Output LO Output Pull Down
UVLO Section
Start-Up Threshold Turn Off Threshold Hysterises
Supply Current Section
Dynamic Operating Current Start-Up Current
FUNCTIONAL PIN DESCRIPTION
Pin
S.S.
#
1
Description
This pin acts as the soft-start pin. A capacitor connected from this pin to GND allows slow ramp up of the NI input resulting in output soft start during start up. This pin is clamped to the internal voltage reference during the normal operation and sets the reference for the feedback regulator. This pin is the inverting input of the Error Amplifier. It is normally connected to the switching power supply output through a resistor divider to program the power supply voltage. This pin instead of the NI pin is internally trimed to 1% tolerance to include the offset voltage error of the error amp. This pin is the Error Amplifier output and is made available for loop compensation. Typically a series R&C network is connected from this pin to GND. A voltage proportional to the inductor current is sensed by an external sense resistor (1570) or current transformer (1571) in series with the return line and is connected to this pin. The output drive is terminated and latched off when this voltage amplified by the internal gain (see option table) exceeds the voltage set by the E.A output voltage. The maximum allowable voltage at this pin during normal operation is -0.8V typ for LX1570 and 6V typ for LX1571. This pin is combined control circuitry and power GND. All other pins must be positive with respect to this pin, except for C.S pin. This pin drives a gate drive transformer which drives the power mosfet. A Schottky diode such as 1N5817 must be connected from this pin to GND in order to prevent the substrate diode conduction. This pin is the positive supply voltage for the control IC. A high frequency capacitor must be closely placed and connected from this pin to GND to provide the turn-on and turn-off peak currents required for fast switching of the power Mosfet. The free running oscillator frequency is programmed by connecting a capacitor from this pin to GND.
VFB
2
COMP C.S.
3 4
GND OUT DRV VCC
5 6 7
CT
8
4
Copyright (c) 1997 Rev. 0.9.3 1/97
PRODUCT DATABOOK 1996/1997
LX1570/1571
PHASE MODULATED AC SYNCHRONOUS SECONDARY-SIDE CONTROLLER
P
RELIMINARY
D
ATA
S
HEET
A P P L I C AT I O N I N F O R M AT I O N
R2 300, 2W C1 0.1F, 50V D1 1/2 MBR2545CT L1 10H (PE53700)
100kHz - 150kHz
AC(+)
D8 1N4937 Q1 IRLZ44
See Note 2
VOUT (+)
20V-30V
20-30V, 100-150kHz Secondary Transformer
T2 See Note 1
D1 2/2 MBR2545CT C9 1500F
C10 1500F C11 1500F
C12 1500F
3.3V / 7A
AC(-)
R5 0.02, 5W C2 0.1F
VOUT (-)
D4 1N4148
VIN (17 to 20V)
R15 1M
R11 1.1k
U1 LX1570
1 S.S. 2 VFB
R4 47
R6 324, 1%
CT 8 VCC 7 OUT DRV 6 GND 5
0.047F
0.56F
R7 1k, 1%
C6
3 COMP 4 C.S.
C5 1F
C4 0.047F
C7
C8 1000pF
R10 5k 1%
Note 1. T2 Core = RM4Z Np = 25T #28AWG Ns = 25T #28AWG 2. For further information on PE53700 and PE64978, contact Pulse Engineering at (619) 674-8100. FIGURE 1 -- THE LX1570 IN A TYPICAL 3.3V / 7A SECONDARY-SIDE POWER SUPPLY APPLICATION
Copyright (c) 1997 Rev. 0.9.3 1/97
5
PRODUCT DATABOOK 1996/1997
LX1570/1571
PHASE MODULATED AC SYNCHRONOUS SECONDARY-SIDE CONTROLLER
P
RELIMINARY
D
ATA
S
HEET
A P P L I C AT I O N I N F O R M AT I O N
L1
(+)
Q1 Secondary Transformer Voltage 1/2 D1
VOUT (+)
T2
1/2 D1
C9
(-)
D4 D2 D3 R5 R3 R4 T1
VOUT (-)
C2
(Note A)
C3
8 CT 7 VCC 6 5 OUT GND DRV
C5
D6 D5
C4
U1, 1571
S.S. 1 VFB COMP C.S. 2 3 4
Pwr Gnd Signal Gnd D7
C6
R9
C10
R10 R8
R6
C7
C8 R7
FIGURE 2 -- THE LX1571 IN A TYPICAL SECONDARY-SIDE POWER SUPPLY APPLICATION
6
Copyright (c) 1997 Rev. 0.9.3 1/97
PRODUCT DATABOOK 1996/1997
LX1570/1571
PHASE MODULATED AC SYNCHRONOUS SECONDARY-SIDE CONTROLLER
P
RELIMINARY
D
ATA
S
HEET
IC DESCRIPTION STEADY-STATE OPERATION Steady-state operation is best described by referring to the main block diagram and the typical application circuit shown in Figure 2. The output drive turns the external power MOSFET on and current ramps up in the inductor. Inductor current is sensed with an external resistor (or in the case of LX1571 with a current transformer) and is compared to the threshold at the inverting input of the current sense (C.S.) comparator. This threshold is set by the voltage feedback loop, which is controlled by the error amplifier. Exceeding this threshold resets the PWM latch and turns the MOSFET off. The Output drive goes low, turning the CT charging current off and the discharging current on, causing the CT voltage to ramp down. When this voltage goes below 1.5V, it sets the PWM latch and turns the output drive back on prior to the next rising edge of the transformer voltage, and the cycle repeats. The Steady-State Operation Timing Diagram - Normal Mode (Figure 4A) shows typical waveforms in the steady-state condi2.5V PWM Latch R S.S. 1 VFB 2 COMP 3 2.5V
2.5V REF CHG CONTROL
tion. Notice that when the current sense signal turns the MOSFET off, it also synchronizes the output drive to the transformer voltage (see discussion under heading Timing Section). In addition, the energy transfer occurs only when both transformer voltage and OUT DRV pin are "HI" at the same time, establishing the effective on-time of the converter. This shows that the regulation of this converter is achieved by modulating the trailing edge of the output drive with respect to the leading edge of the AC voltage, while maintaining a fixed output drive duty cycle. In other words, the converter duty cycle seen by L1 is controlled by varying the phase between the AC voltage and the output driver signal (phase modulation). Maximum converter duty cycle is achieved when both signals are in phase, as shown in Figure 4B. The LX1570/71 output drive always maintains a fixed duty cycle (54%), since both charge and discharge currents are almost equal as shown in Figures 4A and 4B.
Error Amp C.S. Comp 1V R S
Q
6 OUT DRV
C.S. 4
5V Internal Bias
CT 8
Timing / Duty Cycle Control
DISCH CONTROL
7 VCC
LATCH SET CONTROL
5 GND
FIGURE 3 -- STEADY-STATE OPERATION BLOCK DIAGRAM
Copyright (c) 1997 Rev. 0.9.3 1/97
7
PRODUCT DATABOOK 1996/1997
LX1570/1571
PHASE MODULATED AC SYNCHRONOUS SECONDARY-SIDE CONTROLLER
P
RELIMINARY
D
ATA
S
HEET
IC DESCRIPTION
Transformer Voltage Transformer Voltage
LX157x OUT DRV
LX157x OUT DRV
LX1571 C.S. Signal
LX1571 C.S. Signal
CT Voltage
CT Voltage
2s / Div.
FIGURE 4A -- STEADY-STATE OPERATION TIMING DIAGRAM (NORMAL MODE)
2s / Div.
FIGURE 4B -- STEADY-STATE OPERATION TIMING DIAGRAM (MAXIMUM DUTY CYCLE)
START-UP OPERATION Using the main Block Diagram and the LX157x VCC Start-Up Voltage Timing Diagram (Figure 5) as a reference, when the VCC voltage passes the UVLO threshold (16V typ.), the output of the UVLO comparator changes to the "HI" state, which causes the following: a) provides biasing for internal circuitry, and b) enables the output drive and the HICCUP latch. This signal sets the "Q" output of the HICCUP latch "LO", allowing the soft-start (S.S.) capacitor voltage to ramp up, forcing the regulator output to follow this voltage. Since the IC provides a constant current source for charging the S.S. capacitor, the resulting waveform is a smooth linear ramp, which provides lower in-rush current during start up. The Start-Up Timing Diagram (Figure 6) shows the output voltage and the S.S. capacitor during start up. Notice that the output voltage does not respond to the S.S. capacitor until this voltage goes above 0.65 volts, allowing this pin to be used as an external shutdown pin. The value of the soft start capacitor must be selected such that its ramp up time (tRAMP) is always greater than the start up time of the converter, so that the converter is able to follow the soft-start capacitor. It is recommended that the soft start capacitor is always selected such that its ramp up time (tRAMP) be at least 4 times greater than the converter's minimum start-up time. Equations 1 and 2 show how to select this capacitor. tRAMP = 4 * CO * VO IO Equation 1 where CSS is in F and tRAMP is in ms. Example: If CO = 1600F, VO = 12V, IO = 4A tRAMP = 4 * 1600 * 10-6 * 12 = 19.2ms 4 19.2 = 0.55F CSS = 35
The LX1570/71 series also features micropower start-up current that allows these controllers to be powered off the transformer voltage via a low-power resistor and a start-up capacitor. After the IC starts operating, the output of the converter can be used to power the IC. In applications where the output is less than the minimum operating voltage of the IC, an extra winding on the inductor can be used to perform the same function. The start-up capacitor must also be selected so that it can supply the power to the IC long enough for the output of the converter to ramp up beyond the start-up threshold of the IC. Equation 3 shows how to select the start-up capacitor. I *t CST = 2 Q ST VH where: IQ tST VHYST Equation 3
Dynamic operating current of the IC Time for the bootstrap voltage to go above the minimum operating voltage (10V typ.) Minimum hysteresis voltage of the IC
Once tRAMP is known, the soft-start capacitor can then be calculated as follows: t Equation 2 C SS = RAMP 35
Example: If IQ = 30mA, tST = 19ms, VHYST = 5.5V 30 * 10-3 * 19 * 10-3 CST = 2 = 207F 5.5
8
Copyright (c) 1997 Rev. 0.9.3 1/97
PRODUCT DATABOOK 1996/1997
LX1570/1571
PHASE MODULATED AC SYNCHRONOUS SECONDARY-SIDE CONTROLLER
P
RELIMINARY
D
ATA
S
HEET
IC DESCRIPTION
16V
VO
VSTART UP Cap VCAP 10V VO
Output Voltage - 5V / Div. tRAMP
Soft-Start Voltage - 1V / Div.
tST
COMP Pin
OUT DRV
L1 Current
FIGURE 5 -- LX157x VCC START-UP VOLTAGE TIMING DIAGRAM
1ms / Div.
FIGURE 6 -- START-UP TIMING DIAGRAM
TIMING SECTION A capacitor connected from the CT pin to ground performs several functions. First, it sets the OUT DRV duty cycle to a constant 54% (regardless of the CT value) in order to: a) provide the gate drive for an N-channel MOSFET, utilizing a simple gate drive transformer, and b) insure reliable operation with a transformer duty cycle within a 0 to 50% range. Second, it sets the freerunning frequency of the converter in order to insure the continuous operation during non-steady state conditions, such as start up, load transient and current limiting operations. The value of the timing capacitor is selected so that the free-running frequency is always 20% below the minimum operating frequency of the secondary transformer voltage, insuring proper operation. Equation 4 shows how to select the timing capacitor CT. 1 Equation 4 1 1 VRPP fS + ICHG IDISCH where: VRPP Peak to peak voltage of CT (0.6V typ.) fS Free-running frequency of the converter. Selected to be 80% of the minimum freq. of the seconday side transformer voltage. ICHG CT charging current (3mA typ.) IDISCH CT discharge current (3.5mA typ.) CT = Example: Assuming the transformer frequency is at 100kHz, VRPP = 0.6V, ICHG = 3mA, IDISCH = 3.5mA. CT = 1 = 0.033F 1 1 3 0.6 80 10 + -3 3.5 10 -3 3 10
CURRENT LIMITING Using the main Block Diagram as a reference and the typical application circuit of Figure 2, note that current limiting is performed by sensing the current in the return line using a current transformer in series with the switch. The voltage at C.S. pin is then amplified and compared with an internal threshold. Exceeding this threshold turns the output drive off and latches it off until the set input of the PWM latch goes high again. However, if the current keeps rising such that it exceeds the HICCUP comparator threshold, or if the output of the converter drops by 20% from its regulated point, two things will happen. First, the HICCUP comparator pulls CT pin to 6V, which keeps the output drive off and causes CT charging current to be disconnected. Second, it sets the HICCUP latch, causing the discharge current to be turned off until the CT capacitor voltage goes below 0.3V. Since both charge and discharge currents are disconnected from the capacitor, the only discharge path for CT is the internal 2A current source. When this happens, a very slow discharge occurs, resulting in a long delay time between current limit cycles which greatly reduces power MOSFET dissipation under short circuit conditions.
Copyright (c) 1997 Rev. 0.9.3 1/97
9
PRODUCT DATABOOK 1996/1997
LX1570/1571
PHASE MODULATED AC SYNCHRONOUS SECONDARY-SIDE CONTROLLER
P
RELIMINARY
D
ATA
S
HEET
IC DESCRIPTION MINIMUM CURRENT COMPARATOR One of the main advantages of replacing a Magnetic Amplifier with a MOSFET, is the MOSFET's ability to respond quickly to large changes in load requirements. Because the LX1570/71 relies on the C.S. signal for synchronization, special circuitry had to be added to keep the output drive synchronized to the transformer voltage during such load transient conditions. This condition is best explained by referring to Figure 7. In Figure 7, it can be seen that the load current is stepped from 0.4A to 4A, causing the COMP pin to slew faster than the inductor current, starting with the second switching cycle after the load transient has occured. This condition eliminates the normal means of resetting the PWM latch through the C.S. comparator path. To compensate for this condition, a second comparator is ORed with the C.S. comparator, which resets the latch on the falling edge of the C.S. signal caused by the falling edge of the transformer voltage. In other words, the function of the minimum C.S. comparator is to turn OUT DRV off on the falling edge of the C.S. signal, if it is not already off. This assures that the output drive is on before the start of the next AC input cycle (Look-Ahead SwitchingTM), allowing maximum converter duty cycle. ERROR AMPLIFIER The function of the error amplifier is to set a threshold voltage for inductor peak current and to control the converter duty cycle, such that power supply output voltage is closely regulated. Regulation is done by sensing the output voltage and comparing it to the internal 2.5V reference. A compensation network based on the application is placed from the output of the amplifier to GND for closed loop stability purposes as well as providing high DC gain for tight regulation. The function of "3VBE" offset is to keep output drive off without requiring the error amplifier output to swing to ground level. The transfer function between error amp output (VCOMP) and peak inductor current is therefore given by: VCOMP - 3VBE = IP * G where: IP = inductor peak current, G = resistor divider gain, (-15 for LX1570, 3 for LX1571) VBE = diode forward voltage (0.65V typ)
Transformer Voltage 100V / Div. LX157x OUT DRV 20V / Div.
LX157x COMP PIN 2V / Div.
Output Current & Inductor Current 2A / Div.
FIGURE 7 -- MINIMUM CURRENT COMPARATOR EFFECT DURING LOAD TRANSIENT
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Copyright (c) 1997 Rev. 0.9.3 1/97
PRODUCT DATABOOK 1996/1997
LX1570/1571
PHASE MODULATED AC SYNCHRONOUS SECONDARY-SIDE CONTROLLER
P
RELIMINARY
D
ATA
S
HEET
12V/8A SCHEMATIC
80V 4.7k f = 100 to 150kHz D8, 1N4937 Q1 IRF530 Secondary Transformer Voltage 1 6 1/2 D1 MUR1620 3 T2 NP 4 1/2 D1 MUR1620 4 D4 1N4148 D2 1N4935 R3 2.7k 1/2W D3 1N4001 R4 47W 1 C2 0.1 3 2 C9 820F 16V C10 820F 16V R2, 2W C1, 0.1F 250V L1 140H
Note: Linfinity provides a complete and tested evaluation board. For further information contact factory.
(+)
VOUT (+)
NS
12V/8A
(-)
VOUT (-)
T1 PE64978 (Note 1) R5 475W 1% D6 1N5819 D5 1N5819 Pwr Gnd
VIN
C3 220F 25V C4 0.047F R12 1MW Note 2 C6 0.56F
8 CT 7 VCC 6 5 OUT GND DRV
C5 1F
U1, LX1571
S.S. 1 VFB COMP C.S. 2 3 4
Signal Gnd 100W R11 R9 4.99W 1%
R8 SHORT
20k,1%
C13
D7 1N4148
R6 3.83kW 1%
R10
22pF
T2

C8 1000pF
4700pF
Core = RM4Z 20T #32AWG NP = NS = 60T #32AWG
C7
R7 1kW 1%
FIGURE 8 -- THE LX1571 IN A 12V/8A SECONDARY-SIDE POWER SUPPLY APPLICATION Unless otherwise noted all resistors are 1/4W, 5%. Note 1: For further information on PE64978 contact Pulse Engineering at 619-674-8100. Note 2: A high value resistor must be coupled back to "COMP" pin to insure proper operation under light load conditions.
Copyright (c) 1997 Rev. 0.9.3 1/97
11
PRODUCT DATABOOK 1996/1997
LX1570/1571
PHASE MODULATED AC SYNCHRONOUS SECONDARY-SIDE CONTROLLER
P
RELIMINARY
D
ATA
S
HEET
3 . 3 V / 1 0 A S C H E M AT I C
VP (10 to 30V) 300W f = 100kHz to 150kHz D8, 1N4937 Q1 IRLZ44 Secondary Transformer Voltage 1 6 1/2 D1 MBR2545CT 3 T2 NP 4 1/2 D1 MBR2545CT (Note 1)
VP
R2, 2W C1, 0.1F 50V
L1 10H PE53700
Note: Linfinity provides a complete and tested evaluation board. For further information contact factory.
(+)
VOUT (+)
NS
3.3V/10A
C9 1500 F 6.3V C10 1500 F 6.3V C11 1500 F 6.3V C12 1500 F 6.3V
(-)
D4 1N4148
4 1 C2 0.1
3 2
VOUT (-)
T1 PE64978 (Note 1) R5 475W 1% D6 1N5819 D5 1N5819 Pwr Gnd
R4
47W
VIN (17 to 20V)
C3 22F 25V C4 0.047F 1M Note2 C6 0.56F
8 CT 7 VCC 6 5 OUT GND DRV
C5 1F
U1, LX1571
S.S. 1 VFB COMP C.S. 2 3 4
Signal Gnd 100W R11 R9 3.3W 1%
R8 SHORT
5.49k, 1%
C13
D7 1N4148
R6 324W 1%
R10
C7
22pF
C8 1000pF
T2
Core = N = N =
P S
RM4Z 25T #28AWG 25T #28AWG
FIGURE 9 -- THE LX1571 IN A 3.3V/10A SECONDARY-SIDE POWER SUPPLY APPLICATION Unless otherwise noted all resistors are 1/4W, 5%. Note 1: For further information on PE53700 and PE64978 contact Pulse Engineering at 619-674-8100. Note 2: A high value resistor must be coupled back to "COMP" pin to insure proper operation under light load conditions.
Look-ahead SwitchingTM is a trademark of Linfinity Microelectronics Inc.
0.047F
R7 1kW 1%
12
Copyright (c) 1997 Rev. 0.9.3 1/97

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