Datasheet LX1571MY, LX1571IM, LX1571IDM, LX1571CM, LX1571CDM Datasheet (Microsemi Corporation)
...Page 1
PACKAGE ORDER INFORMATION
T
A
(°C)
Plastic DIP
8-pin
0 to 70 LX157xCM LX157xCDM —
-40 to 85 LX157xIM LX157xIDM —
-55 to 125 — — LX157xMY
M
Plastic SOIC
8-pin
DM
Ceramic DIP
8-pin
Y
DESCRIPTION KEY FEATURES
p REPLACES COSTLY MAG-AMP CORES WITH
A LOW ON-RESISTANCE MOSFET
p LOOK-AHEAD SWITCHING
TM
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
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.
PRODUCT HIGHLIGHT
APPLICATIONS
■ SECONDARY-SIDE REGULATOR IN OFF-LINE
POWER SUPPLIES
■ COMPUTER POWER SUPPLIES, 3.3V OUTPUT
FOR NEW LOW-VOLTAGE PROCESSORS
AND MEMORIES
■ TELECOMMUNICATION AND MILITARY
DC/DC CONVERTERS
LX1571
V
CC
OUT
DRV
C.S.
V
FB
COMP
S.S.
GND
C
T
Aux Outpu
t
12V/8A
P
HASE MODULATED
AC S
YNCHRONOUS SECONDARY-SIDE CONTROLLER
P RELIMINARY DATA SHEET
THE INFINITE POWER OF INNOVATION
Copyright © 1997
Rev. 0.9.3 1/97
FOR FURTHER INFORMATION CALL (714) 898-8121
11861 WESTERN AVENUE, GARDEN GROVE, CA. 92841
LX1570/1571
LIN DOC #:
1570
1
P
ATENT PENDING
Note: All surface-mount packages are available in Tape & Reel.
Append the letter "T" to part number. (i.e. LX157xCDMT)
A VAILABLE OPTIONS PER PART #
Part #
C.L. C.S.
Application
Threshold
Option
Resistive
Output
LX1570 -0.2V
Sensing
Currents
< 4A
Current Output
LX1571 1V
Transformer
Currents
Sensing > 4A
Page 2
P
HASE MODULATED
AC S
YNCHRONOUS SECONDARY-SIDE CONTROLLER
LX1570/1571
PRODUCT DATABOOK 1996/1997
Copyright © 1997
Rev. 0.9.3 1/97
2
P
RELIMINARY DAT A SHEET
ABSOLUTE MAXIMUM RATINGS (Note 1)
Supply Voltage (VCC) .................................................................................................... 40V
Digital Inputs ....................................................................................................... -0.3 to 7V
Output Peak Current Source (500nS) ........................................................................... 1A
Output Peak Current Sink (500nS)................................................................................ 1A
PACKAGE PIN OUTS
C
T
V
CC
OUT DRV
GND
S.S.
V
FB
COMP
C.S.
1 8
27
36
45
M & Y PACKAGE
(Top View)
DM PACKAGE
(Top View)
C
T
V
CC
OUT DRV
GND
S.S.
V
FB
COMP
C.S.
1 8
27
36
45
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.
LX1571 BLOCK DIAGRAM
THERMAL DATA
M PACKAGE:
THERMAL RESISTANCE-JUNCTION TO AMBIENT,
θθ
θθ
θ
JA
95°C/W
DM PACKAGE:
THERMAL RESISTANCE-JUNCTION TO AMBIENT,
θθ
θθ
θ
JA
165°C/W
Y PACKAGE:
THERMAL RESISTANCE-JUNCTION TO AMBIENT,
θθ
θθ
θ
JA
130°C/W
Junction Temperature Calculation: T
J
= TA + (P
D
x θ
JA
).
The θ
JA
numbers are guidelines for the thermal performance of the device/pc-board system.
All of the above assume no ambient airflow.
Error Amp
Voltage Hiccup
Comp.
2.5V
Minimum
Current Comp
C.S. Comp
Current Mode
Hiccup Comp
RSQ
PWM Latch
7
6
5
GND
V
CC
OUT DRV
6V
RSQQ
Hiccup
Latch
2R
R
Internal
Bias
2.5V
REF
1V
5V
2.5V
16V
1.5V
0.25V
0.5V
4
3
2
1
S.S.
V
FB
C
OMP
C.S.
8
C
T
Voltage Mode Hiccup
Timing / Duty Cycle
Control
CHG
CONTROL
QUICK
CHG
CONTROL
LATCH RESET
CONTROL
LATCH
SET CONTROL
VALLEY
THRESHOLD
CONTROL
DISCH
CONTROL
Page 3
P
HASE MODULATED
AC S
YNCHRONOUS SECONDARY-SIDE CONTROLLER
LX1570/1571
PRODUCT DATABOOK 1996/1997
3
Copyright © 1997
Rev. 0.9.3 1/97
P RELIMINARY DATA SHEET
Input Range LX1570 V
CSI
LX1571
Input Current LX1570 I
CSB
LX1571
C.S. Amplifier Gain LX1570 A
CS
LX1571
Minimum Current Threshold Voltage LX1570 V
CSMIN
LX1571
C.S. Delay to Driver Output 10% Overdrive
C.L. Pulse-By-Pulse Threshold Voltage LX1570 V
CLP
LX1571
C.L. Hiccup Threshold Voltage LX1570 V
CLH
LX1571
Voltage Hiccup Threshold V
HCCP
Initial Accuracy fOCT = , TJ = 25°C, measured at pin 6
Over Temp, measured at pin 6
Line Voltage Stability ∆f
OL
Charging Current I
CHG
Discharging Current I
DISCH
Leakage Current I
LK
C.S.
INPUT
= 1.5V
Ramp PK to PK V
RPP
C.S.
INPUT
= 0V
C.S.
INPUT
= 1.5V (1571), C.S.
INPUT
= -0.4V (1570)
ELECTRICAL CHARACTERISTICS
Reference Section
Parameter
Symbol
Test Conditions
Initial Accuracy VRITA = 25ºC, measured at F.B pin
Line Regulation ∆V
RL
11V < VCC < 25
Temp Stability ∆V
RT
Note 2
Units
LX1570/1571
Min. Typ. Max.
2.475 2.500 2.525 V
±1 %
±1.5 %
Timing Section
90 100 110 kHz
85 100 115 kHz
±1 %
3mA
3.5 mA
4µA
0.6 V
6V
Error Amp / Soft Start Comp Section
Transconductance g
m
Input Bias Current I
B
Open Loop Gain A
VOL
Output Sink Current I
EA(SINK)VFB
= 2.6V
Output Source Current I
EA(SOURCE)VFB
= 2.4V
Output HI Voltage V
COMP-HI
Output LO Voltage V
COMP-LO
Slew Rate S
0.005 µΩ
0.1 1 µ A
60 70 dB
200 400 µA
200 400 µA
5.1 V
0.8 V
1 V/µSec
(Unless otherwise specified, these specifications apply over the ranges TA = -55 to 125ºC for the LX1570M/1571M, TA = -40 to 85ºC for the
LX1570I/1571I, and T
A
= 0 to 70ºC for LX1570C/1571C. VCC = 15V. Typ. number represents TA = 25ºC value.)
Soft-Start Section
Soft Start Timing Factor K
SS
Soft Start Discharge Current I
SS-DIS
35 50 65 ms/µF
TBD mA
Current Sense Section
-0.8 V
-0.3 6 V
25 µA
1µA
-13.5 -15 -16.5 V/V
2.7 3 3.3 V/V
-50 mV
250 mV
100 200 ns
-0.18 -0.2 -0.22 V
0.9 1 1.1 V
-0.3 V
1.5 V
2V
Note 2. Although this parameter is guaranteed, it is not 100% tested in production.
Page 4
P
HASE MODULATED
AC S
YNCHRONOUS SECONDARY-SIDE CONTROLLER
LX1570/1571
PRODUCT DATABOOK 1996/1997
Copyright © 1997
Rev. 0.9.3 1/97
4
P
RELIMINARY DAT A SHEET
S.S. 1
V
FB
2
COMP 3
C.S. 4
GND 5
OUT 6
DRV
V
CC
7
C
T
8
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.
Pin # Description
FUNCTIONAL PIN DESCRIPTIONFUNCTIONAL PIN DESCRIPTION
FUNCTIONAL PIN DESCRIPTIONFUNCTIONAL PIN DESCRIPTION
FUNCTIONAL PIN DESCRIPTION
Rise / Fall Time tR / tFCL = 1000pF
Output HI V
DHISOURCE
= 200mA, VCS = 0V, VFB = 2.3V
Output LO V
DLISINK
= 200mA, VCS = 1.2V, VFB = 2.3V
Output Pull Down V
DPDVCC
= 0V, I
PULL UP
= 2mA
ELECTRICAL CHARACTERISTICS (Con't.)
Parameter
Symbol
Test Conditions Units
LX1570/1571
Min. Typ. Max.
1.7 2.0 2.4 V
52 54 56 %
E.A. Output to PWM Drive Offset V
OFS
Fixed Duty Cycle D
UVLO Section
15 16 17 V
91011V
5.5 6 6.5 V
Start-Up Threshold V
ST
Turn Off Threshold V
OFF
Hysterises V
H
Supply Current Section
Dynamic Operating Current I
Qd
Out Freq = 100kHz, CL = 0
Start-Up Current I
ST
18 30 mA
150 250 µA
50 ns
13.5 V
0.8 V
1V
PWM Section
Output Drive Section
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P
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AC S
YNCHRONOUS SECONDARY-SIDE CONTROLLER
LX1570/1571
PRODUCT DATABOOK 1996/1997
5
Copyright © 1997
Rev. 0.9.3 1/97
P RELIMINARY DATA SHEET
APPLICATION INFORMATION
V
OUT (-)
V
OUT (+)
C9
1500µF
L1
10µH (PE53700)
D1 1/2
D1 2/2
MBR2545CT
C2
0.1µF
D4
1N4148
R4
47
AC(+)
AC(-)
20-30V, 100-150kHz
Secondary Transformer
Q1
IRLZ44
T2
See Note 1
OUT DRV
U1
LX1570
8
C7
C8
1000pF
C5
1µF
C10
1500µF
C11
1500µF
C12
1500µF
MBR2545CT
R5
0.02, 5W
3.3V / 7A
See Note 2
C1
0.1µF, 50V
D8
1N4937
300, 2W
R2
100kHz - 150kHz
20V-30V
1
72
63
54
V
CC
C
T
GNDC.S.
COMP
V
FB
S.S.
C4
0.047µF
R11
1.1k
R15
1M
VIN (17 to 20V)
R6
324, 1%
R7
1k, 1%
0.047µF
0.56µF
C6
R10
5k
1%
FIGURE 1 — THE LX1570 IN A TYPICAL 3.3V / 7A SECONDARY-SIDE POWER SUPPLY APPLICATION
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.
Page 6
P
HASE MODULATED
AC S
YNCHRONOUS SECONDARY-SIDE CONTROLLER
LX1570/1571
PRODUCT DATABOOK 1996/1997
Copyright © 1997
Rev. 0.9.3 1/97
6
P
RELIMINARY DAT A SHEET
APPLICATION INFORMATION
V
OUT (-)
V
OUT (+)
C9
L1
1/2 D1
1/2 D1
(Note A)
T1
R5
C2
D4
D3
D2
R3
R4
(+)
(-)
Secondary
Transformer
Voltage
Q1
T2
C3
CTVCCOUT
DRV
GND
C.S.
COMP
V
FB
S.S.
U1, 1571
1234
8765
D6
D5
C5
C6
C7
R9 C10
C8
R10
D7
R8
R6
R7
C4
Signal Gnd
Pwr
Gnd
FIGURE 2 — THE LX1571 IN A TYPICAL SECONDARY-SIDE POWER SUPPLY APPLICATION
Page 7
P
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AC S
YNCHRONOUS SECONDARY-SIDE CONTROLLER
LX1570/1571
PRODUCT DATABOOK 1996/1997
7
Copyright © 1997
Rev. 0.9.3 1/97
P RELIMINARY DATA SHEET
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 C
T
charging current off and the discharging current on, caus-
ing the C
T
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 condi-
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
2.5V
C.S. Comp
R
S
Q
PWM Latch
7
6
5
GND
V
CC
OUT DR
V
R
Internal
Bias
2.5V
REF
1V
5V
2.5V
4
3
2
1
S.S.
V
FB
COMP
C.S.
8
C
T
Timing / Duty Cycle
Control
CHG
CONTROL
LATCH
SET CONTROL
DISCH
CONTROL
FIGURE 3 — STEADY-STATE OPERATION BLOCK DIAGRAM
Page 8
P
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AC S
YNCHRONOUS SECONDARY-SIDE CONTROLLER
LX1570/1571
PRODUCT DATABOOK 1996/1997
Copyright © 1997
Rev. 0.9.3 1/97
8
P
RELIMINARY DAT A SHEET
START-UP OPERATION
Using the main Block Diagram and the LX157x V
CC
Start-Up
Voltage Timing Diagram (Figure 5) as a reference, when the V
CC
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 (t
RAMP
) 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 (t
RAMP
) be at least 4 times greater than
the converter's minimum start-up time. Equations 1 and 2 show
how to select this capacitor.
t
RAMP
= 4
*
Equation 1
Once t
RAMP
is known, the soft-start capacitor can then be
calculated as follows:
C
SS
= Equation 2
where C
SS
is in µF and t
RAMP
is in ms.
Example: If C
O
= 1600µF, VO = 12V, IO = 4A
t
RAMP
= 4 * = 19.2ms
C
SS
= = 0.55µF
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.
C
ST
= 2 Equation 3
where: I
Q
≡ Dynamic operating current of the IC
t
ST
≡ Time for the bootstrap voltage to go above
the minimum operating voltage (10V typ.)
V
HYST
≡ Minimum hysteresis voltage of the IC
Example: If I
Q
= 30mA, tST = 19ms, V
HYST
= 5.5V
C
ST
= 2 = 207µF
IC DESCRIPTION
2µs / Div.
Transformer
Voltage
LX157x
OUT DRV
LX1571
C.S. Signal
C
T
Voltage
2µs / Div.
Transformer
Voltage
LX157x
OUT DRV
LX1571
C.S. Signal
C
T
Voltage
FIGURE 4A — STEADY-STATE OPERATION TIMING DIAGRAM
(NORMAL MODE)
FIGURE 4B — STEADY-STATE OPERATION TIMING DIAGRAM
(MAXIMUM DUTY CYCLE)
I
Q
*
t
ST
V
H
19.2
35
1600 * 10-6 * 12
4
t
RAMP
35
CO * V
O
I
O
30 * 10
-3
* 19 * 10
-3
5.5
Page 9
P
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AC S
YNCHRONOUS SECONDARY-SIDE CONTROLLER
LX1570/1571
PRODUCT DATABOOK 1996/1997
9
Copyright © 1997
Rev. 0.9.3 1/97
P RELIMINARY DATA SHEET
IC DESCRIPTION
C
.
F
T
=
∗∗∗
∗
+
∗
=
−−
1
06 80 10
1
310135 10
0033
3
33
.
.µ
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 C
T
pin to 6V, which keeps the output
drive off and causes C
T
charging current to be disconnected.
Second, it sets the HICCUP latch, causing the discharge current
to be turned off until the C
T
capacitor voltage goes below 0.3V.
Since both charge and discharge currents are disconnected from
the capacitor, the only discharge path for C
T
is the internal 2µA
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.
V
START UP
Cap
16V
t
ST
10
V
V
O
V
O
OUT DRV
V
CAP
FIGURE 5 — LX157x VCC START-UP VOLTAGE TIMING DIAGRAM
1ms / Div.
Output
Voltage - 5V / Div.
Soft-Start
V
oltage - 1V / Div.
COMP Pin
L1 Current
t
RAMP
FIGURE 6 — START-UP TIMING DIAGRAM
TIMING SECTION
A capacitor connected from the C
T
pin to ground performs several functions. First, it sets the OUT DRV duty cycle to a constant
54% (regardless of the C
T
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.
Equation 4
where: V
RPP
≡ Peak to peak voltage of C
T
(0.6V typ.)
f
S
≡ Free-running frequency of the converter.
Selected to be 80% of the minimum freq.
of the seconday side transformer voltage.
I
CHG
≡ C
T
charging current (3mA typ.)
I
DISCH
≡ C
T
discharge current (3.5mA typ.)
Example: Assuming the transformer frequency is at 100kHz,
V
RPP
= 0.6V, I
CHG
= 3mA, I
DISCH
= 3.5mA.
C
Vf
II
T
RPP S
CHG DISCH
=
∗∗ +
1
11
Page 10
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AC S
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LX1570/1571
PRODUCT DATABOOK 1996/1997
Copyright © 1997
Rev. 0.9.3 1/97
10
P
RELIMINARY DAT A SHEET
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 Switching™),
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 "3V
BE
" 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 (V
COMP
) and peak inductor current is therefore given
by:
V
COMP
- 3VBE = IP * G where:
I
P
= inductor peak current,
G = resistor divider gain,
(-15 for LX1570, 3 for LX1571)
V
BE
= diode forward voltage
(0.65V typ)
IC DESCRIPTION
FIGURE 7 — MINIMUM CURRENT COMPARATOR EFFECT
DURING LOAD TRANSIENT
Transformer
Voltage
100V / Div.
LX157x
OUT DRV
20V / Div.
LX157x
COMP PIN
2V / Div.
Output Current &
Inductor Current
2A / Div.
Page 11
P
HASE MODULATED
AC S
YNCHRONOUS SECONDARY-SIDE CONTROLLER
LX1570/1571
PRODUCT DATABOOK 1996/1997
11
Copyright © 1997
Rev. 0.9.3 1/97
P RELIMINARY DATA SHEET
12V/8A SCHEMATIC
V
OUT (-)
V
OUT (+)
C10
820µF
16V
C9
820µF
16V
L1
140µH
1/2 D1
MUR1620
1/2 D1
MUR1620
T1
PE64978
R5
475
W
1%
1
43
2
C2
D4
1N4148
D3
1N4001
D2
1N4935
R4
(+)
(-)
Secondary
Transformer
Voltage
D8, 1N4937
R2, 2W
C1, 0.1µF
250V
Q1
IRF530
13
4
6
V
IN
C3
220µF
25V
CTVCCOUT
DRV
GND
C.S.
COMP
V
FB
S.S.
U1, LX1571
1234
8765
D6
1N5819
D5
1N5819
C5
1µF
C6
0.56µF
C7
R10
C13
C8
R11
D7
1N4148
R8
SHORT
R9
4.99
W
1%
R6
3.83k
W
1%
R7
1k
W
1%
C4
0.047µF
Signal Gnd
Pwr
Gnd
4.7k
N
S
N
P
0.1
(Note 1)
47
W
2.7k
1/2W
20k,1%
22pF
1000pF
4700pF
80V
f = 100 to 150kHz
T2
R3
100
W
12V/8A
R12
1M
W
Note 2
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.
Core = RM4Z
N
P
= 20T #32AWG
NS = 60T #32AWG
T2
FIGURE 8 — THE LX1571 IN A 12V/8A SECONDARY-SIDE POWER SUPPLY APPLICATION
Note: Linfinity provides a complete and
tested evaluation board. For further
information contact factory.
Page 12
P
HASE MODULATED
AC S
YNCHRONOUS SECONDARY-SIDE CONTROLLER
LX1570/1571
PRODUCT DATABOOK 1996/1997
Copyright © 1997
Rev. 0.9.3 1/97
12
P
RELIMINARY DAT A SHEET
3.3V/10A SCHEMATIC
V
OUT (-)
V
OUT (+)
C12
1500
µF
6.3V
C11
1500
µF
6.3V
C10
1500
µF
6.3V
C9
1500
µF
6.3V
L1
10µH
PE53700
1/2 D1
MBR2545CT
1/2 D1
MBR2545CT
T1
PE64978
R5
475
W
1%
1
43
2
C2
D4
1N4148
R4
(+)
(-)
Secondary
Transformer
Voltage
D8, 1N4937
R2, 2W
C1, 0.1µF
50V
Q1
IRLZ44
13
4
6
V
IN
(17 to 20V)
C3
22µF
25V
CTVCCOUT
DRV
GND
C.S.
COMP
V
FB
S.S.
U1, LX1571
1234
8765
D6
1N5819
D5
1N5819
C5
1µF
C6
0.56µF
C7
R10
C13
C8
R11
D7
1N4148
R8
SHORT
R9
3.3
W
1%
R6
324
W
1%
R7
1k
W
1%
C4
0.047µF
Signal Gnd
Pwr
Gnd
300
W
N
S
N
P
0.1
(Note 1)
47
W
5.49k, 1%
22pF
1000pF
0.047µF
V
P
f = 100kHz to 150kHz
T2
100
W
3.3V/10
A
VP (10 to 30V)
(Note 1)
1M
Note2
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.
Core = RM4Z
N
P
= 25T #28AWG
NS = 25T #28AWG
T2
FIGURE 9 — THE LX1571 IN A 3.3V/10A SECONDARY-SIDE POWER SUPPLY APPLICATION
Note: Linfinity provides a complete and
tested evaluation board. For further
information contact factory.
Look-ahead SwitchingTM is a trademark of Linfinity Microelectronics Inc.
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