LX1571MY [MICROSEMI]
AC Synchronous. Secondary-Side Controller ; 交流同步。二次侧控制器\n
| 型号： | LX1571MY |
| 厂家： | 
Microsemi |
| 描述： | AC Synchronous. Secondary-Side Controller
|
| 文件： | 总12页 (文件大小：259K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LIN DOC #: 1570
P A T E N T P E N D I N G
LX1570/1571
PHASE
MODULATED AC SYNCHRONOUS
S
ECONDARY-SIDE
C
ONTROLLER
T H E I N F I N I T E P O W E R O F I N N O V A T I O N
P R E L I M I N A R Y D A T A S H E E T
DESCRIPTION
KEY FEATURES
p REPLACES COSTLY MAG-AMP CORES WITH
The LX1570/71 series of controller ICs are
designed to provide all control functions in
a secondary-side regulator for isolated auxil-
iary 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 tele-
communications applications. Although they
can be used in all secondary output applica-
tions requiring precision regulation, they are
mainly optimized for outputs delivering more
than 3A current where standard three-termi-
nal regulators lack the desired efficiency. For
these applications, the Mag Amp regulators
have traditionally been used. However, Mag
Amps have several disadvantages. First, be-
cause 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, lim-
ited by the minimum delay and the mag-
netic 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 perfor-
mance and reducing overall system cost.
The LX1570/71 is a current mode control-
ler IC that controls the duty cycle of a switch
in series with the secondary AC output of
the power transformer in buck-derived ap-
plications, such as forward or bridge topolo-
gies. It offers features such as 100% duty
cycle operation for maximum energy trans-
fer, pulse-by-pulse and hiccup current limit-
ing with long off-time between the cycles
for reduced power dissipation, high-fre-
quency operation for smaller magnetics, soft-
start, and current mode control for excel-
lent dynamic response.
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
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
PRODUCT HIGHLIGHT
AVA I L A B L E OP T I O N S P E R PA R T #
Aux Output
12V/8A
C.L.
C.S.
Part #
Application
Threshold
Option
Output
Currents
< 4A
Resistive
Sensing
LX1570 -0.2V
Current
Output
LX1571
1V
Transformer Currents
Sensing
> 4A
OUT
C.S.
DRV
VCC
VFB
LX1571
COMP
S.S.
CT
GND
PACKAGE ORDER INFORMATION
Plastic DIP
8-pin
Plastic SOIC
8-pin
Ceramic DIP
Y
8-pin
TA (°C)
M
DM
0 to 70
-40 to 85
-55 to 125
LX157xCM
LX157xIM
—
LX157xCDM
LX157xIDM
—
—
—
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 © 1997
Rev. 0.9.3 1/97
1
11861 WESTERN AVENUE, GARDEN GROVE, CA. 92841
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
LX1570/1571
PHASE
MODULATED AC SYNCHRONOUS
SECONDARY-SIDE
C
ONTROLLER
P R E L I M I N A R Y D A T A S H E E T
ABSOLUTE MAXIMUM RATINGS (Note 1)
PACKAGE PIN OUTS
Supply Voltage (VCC) .................................................................................................... 40V
Digital Inputs.......................................................................................................-0.3 to 7V
Output Peak Current Source (500nS) ........................................................................... 1A
Output Peak Current Sink (500nS)................................................................................ 1A
S.S.
VFB
COMP
C.S.
1
2
3
4
8
7
6
5
CT
VCC
OUT DRV
GND
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.
M & Y PACKAGE
(Top View)
THERMAL DATA
M PACKAGE:
1
2
3
4
8
7
6
5
S.S.
VFB
COMP
C.S.
CT
VCC
OUT DRV
GND
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θJA
DM PACKAGE:
95°C/W
165°C/W
130°C/W
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θJA
Y PACKAGE:
DM PACKAGE
(Top View)
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θJA
Junction Temperature Calculation: TJ = T + (PD x θJA).
The θJA numbers are guidelines for the theArmal performance of the device/pc-board system.
All of the above assume no ambient airflow.
LX1571 BLOCK DIAGRAM
Minimum
Current Comp
2.5V
0.25V
PWM Latch
R
Q
6
OUT DRV
Error Amp
1
S.S.
2R
1V
C.S. Comp
S
VFB
COMP
C.S.
2
3
R
Current Mode
Hiccup Comp
2.5V
5V
4
1.5V
0.5V
Voltage Hiccup
Comp.
2.5V
REF
Voltage Mode Hiccup
Internal
Bias
QUICK
CHG
CONTROL
CHG
CONTROL
LATCH RESET
CONTROL
Timing / Duty Cycle
Control
8
7
5
VCC
CT
6V
LATCH
SET CONTROL
16V
DISCH
CONTROL
VALLEY
THRESHOLD
CONTROL
GND
Q
Q
R
S
Hiccup
Latch
Copyright © 1997
Rev. 0.9.3 1/97
2
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
LX1570/1571
PHASE
MODULATED AC SYNCHRONOUS
S
ECONDARY-SIDE
C
ONTROLLER
P R E L I M I N A R Y D A T A S H E E T
ELECTRICAL CHARACTERISTICS
(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 TA = 0 to 70ºC for LX1570C/1571C. VCC = 15V. Typ. number represents TA = 25ºC value.)
LX1570/1571
Parameter
Reference Section
Symbol
Test Conditions
Units
Min. Typ.
Max.
Initial Accuracy
Line Regulation
Temp Stability
VRI
∆VRL
∆VRT
TA = 25ºC, measured at F.B pin
11V < VCC < 25
2.475 2.500 2.525
V
%
%
1
Note 2
1.5
Timing Section
Initial Accuracy
fO
CT = , TJ = 25°C, measured at pin 6
Over Temp, measured at pin 6
90
85
100
100
110
115
1
kHz
kHz
%
mA
mA
µA
V
Line Voltage Stability
Charging Current
Discharging Current
Leakage Current
Ramp PK to PK
∆fOL
ICHG
IDISCH
ILK
3
3.5
4
C.S.INPUT = 1.5V
VRPP
C.S.INPUT = 0V
C.S.INPUT = 1.5V (1571), C.S.INPUT = -0.4V (1570)
0.6
6
V
Error Amp / Soft Start Comp Section
Transconductance
gm
IB
AVOL
0.005
0.1
70
µΩ
µA
dB
Input Bias Current
Open Loop Gain
1
60
200
200
Output Sink Current
Output Source Current
Output HI Voltage
IEA(SINK) VFB = 2.6V
400
400
5.1
µA
µA
V
IEA(SOURCE) VFB = 2.4V
VCOMP-HI
VCOMP-LO
S
0.8
65
V
Output LO Voltage
1
V/µSec
Slew Rate
Soft-Start Section
Soft Start Timing Factor
Soft Start Discharge Current
KSS
35
50
ms/µF
mA
ISS-DIS
TBD
Current Sense Section
Input Range
LX1570
VCSI
ICSB
-0.8
6
25
V
V
LX1571
LX1570
LX1571
LX1570
LX1571
LX1570
LX1571
-0.3
µA
µA
V/V
V/V
mV
mV
ns
V
Input Current
1
-13.5
2.7
-15
3
-16.5
3.3
C.S. Amplifier Gain
Minimum Current Threshold Voltage
ACS
-50
250
100
-0.2
1
VCSMIN
200
-0.22
1.1
C.S. Delay to Driver Output
10% Overdrive
-0.18
0.9
C.L. Pulse-By-Pulse Threshold Voltage
LX1570
LX1571
LX1570
LX1571
VCLP
VCLH
V
-0.3
1.5
2
V
V
C.L. Hiccup Threshold Voltage
V
Voltage Hiccup Threshold
VHCCP
Note 2. Although this parameter is guaranteed, it is not 100% tested in production.
Copyright © 1997
Rev. 0.9.3 1/97
3
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
LX1570/1571
PHASE
MODULATED AC SYNCHRONOUS
SECONDARY-SIDE
C
ONTROLLER
P R E L I M I N A R Y D A T A S H E E T
ELECTRICAL CHARACTERISTICS (Con't.)
LX1570/1571
Min. Typ. Max.
Parameter
Symbol
Test Conditions
Units
PWM Section
E.A. Output to PWM Drive Offset
Fixed Duty Cycle
VOFS
D
1.7
52
2.0
54
2.4
56
V
%
Output Drive Section
Rise / Fall Time
Output HI
tR / tF CL = 1000pF
50
13.5
0.8
1
ns
V
V
VDH
VDL
ISOURCE = 200mA, VCS = 0V, VFB = 2.3V
ISINK = 200mA, VCS = 1.2V, VFB = 2.3V
VCC = 0V, IPULL UP = 2mA
Output LO
Output Pull Down
VDPD
V
UVLO Section
Start-Up Threshold
Turn Off Threshold
Hysterises
VST
VOFF
VH
15
9
16
10
6
17
11
V
V
V
5.5
6.5
Supply Current Section
Dynamic Operating Current
Start-Up Current
IQd
IST
Out Freq = 100kHz, CL = 0
18
30
mA
µA
150
250
FUNCTIONAL PIN DESCRIPTION
Pin
#
Description
S.S.
1
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.
VFB
2
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.
COMP
C.S.
3
4
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.
Avoltageproportionaltotheinductorcurrentissensedbyanexternalsenseresistor(1570) orcurrenttransformer(1571)
inserieswiththereturnlineandisconnectedtothispin. Theoutputdriveisterminatedandlatchedoffwhenthisvoltage
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.
GND
5
6
7
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.
OUT
DRV
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.
VCC
This pin is the positive supply voltage for the control IC. A high frequency capacitor must be closely placed and
connectedfromthispintoGNDtoprovidetheturn-onandturn-offpeakcurrentsrequiredforfastswitchingofthepower
Mosfet.
CT
8
The free running oscillator frequency is programmed by connecting a capacitor from this pin to GND.
Copyright © 1997
Rev. 0.9.3 1/97
4
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
LX1570/1571
PHASE
MODULATED AC SYNCHRONOUS
S
ECONDARY-SIDE
C
ONTROLLER
P R E L I M I N A R Y D A T A S H E E T
APPLICATION INFORMATION
R2
C1
0.1µF, 50V
300, 2W
L1
10µH (PE53700)
D8
1N4937
VOUT
(+)
AC(+)
D1 1/2
See Note 2
100kHz - 150kHz
Q1
IRLZ44
MBR2545CT
D1 2/2
MBR2545CT
20-30V, 100-150kHz
Secondary Transformer
C10
1500µF
C12
1500µF
20V-30V
T2
See Note 1
3.3V / 7A
C9
1500µF
C11
1500µF
AC(-)
VOUT
(-)
R5
0.02, 5W
C2
0.1µF
D4
1N4148
VIN (17 to 20V)
R15
1M
R11
1.1k
R4
47
U1
LX1570
R6
324, 1%
1
2
3
4
S.S.
CT
8
7
6
5
VFB
VCC
C5
1µF
C4
0.047µF
COMP
C.S.
OUT DRV
GND
R7
1k, 1%
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 © 1997
Rev. 0.9.3 1/97
5
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
LX1570/1571
PHASE
MODULATED AC SYNCHRONOUS
SECONDARY-SIDE
C
ONTROLLER
P R E L I M I N A R Y D A T A S H E E T
APPLICATION INFORMATION
L1
(+)
VOUT (+)
Q1
1/2 D1
Secondary
Transformer
Voltage
1/2 D1
C9
T2
(-)
VOUT (-)
D4
T1
R5
C2
(Note A)
D2
R3
D3
R4
C5
D6
D5
C3
8
CT
7
6
5
C4
VCC OUT GND
Pwr
Gnd
DRV
U1, 1571
Signal Gnd
S.S. VFB COMP C.S.
1
2
3
4
D7
R6
R7
R10
C8
C6
R9
C10
R8
C7
FIGURE 2 — THE LX1571 IN A TYPICAL SECONDARY-SIDE POWER SUPPLY APPLICATION
Copyright © 1997
Rev. 0.9.3 1/97
6
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
LX1570/1571
PHASE
MODULATED AC SYNCHRONOUS
S
ECONDARY-SIDE
C
ONTROLLER
P R E L I M I N A R Y D A T A S H E E T
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 Fig-
ure 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, caus-
ing 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.
tion. Notice that when the current sense signal turns the MOSFET
off, it also synchronizes the output drive to the transformer volt-
age (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 regula-
tion 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.
The Steady-State Operation Timing Diagram - Normal Mode
(Figure 4A) shows typical waveforms in the steady-state condi-
2.5V
PWM Latch
R
Q
6
OUT DRV
Error Amp
1
S.S.
C.S. Comp
S
VFB
2
3
R
1V
COMP
2.5V
4
C.S.
2.5V
REF
CHG
CONTROL
5V
Internal
Bias
Timing / Duty Cycle
Control
8
CT
DISCH
CONTROL
7
5
VCC
LATCH
SET CONTROL
GND
FIGURE 3 — STEADY-STATE OPERATION BLOCK DIAGRAM
Copyright © 1997
Rev. 0.9.3 1/97
7
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
LX1570/1571
PHASE
MODULATED AC SYNCHRONOUS
SECONDARY-SIDE
C
ONTROLLER
P R E L I M I N A R Y D A T A S H E E T
IC DESCRIPTION
Transformer
Voltage
Transformer
Voltage
LX157x
LX157x
OUT DRV
OUT DRV
LX1571
LX1571
C.S. Signal
C.S. Signal
CT Voltage
CT Voltage
2µs / Div.
2µs / Div.
FIGURE 4A — STEADY-STATE OPERATION TIMING DIAGRAM
FIGURE 4B — STEADY-STATE OPERATION TIMING DIAGRAM
(NORMAL MODE)
(MAXIMUM DUTY CYCLE)
START-UP OPERATION
where CSS is in µF and tRAMP is in ms.
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.
Example: If CO = 1600µF, VO = 12V, IO = 4A
1600 10-6 12
*
*
tRAMP = 4 *
= 19.2ms
4
19.2
35
CSS =
= 0.55µF
The LX1570/71 series also features micropower start-up current
that allows these controllers to be powered off the transformer
voltageviaalow-powerresistorandastart-upcapacitor. Afterthe
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.
IQ
VH
t
ST
*
CST = 2
Equation 3
Itisrecommendedthatthesoftstartcapacitorisalwaysselected
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.
where: IQ
tST
≡ Dynamic operating current of the IC
≡ Time for the bootstrap voltage to go above
the minimum operating voltage (10V typ.)
CO
V
O
*
IO
VHYST ≡ Minimum hysteresis voltage of the IC
tRAMP = 4
Equation 1
*
Example: If IQ = 30mA, tST = 19ms, VHYST = 5.5V
30 10-3 19 10-3
Once tRAMP is known, the soft-start capacitor can then be
calculated as follows:
tRAMP
35
*
*
*
CST = 2
= 207µF
CSS =
Equation 2
5.5
Copyright © 1997
Rev. 0.9.3 1/97
8
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
LX1570/1571
PHASE
MODULATED AC SYNCHRONOUS
S
ECONDARY-SIDE
C
ONTROLLER
P R E L I M I N A R Y D A T A S H E E T
IC DESCRIPTION
16V
VO
VSTART UP Cap
VCAP
10V
Output
Voltage - 5V / Div.
VO
tRAMP
Soft-Start
Voltage - 1V / Div.
tST
COMP Pin
OUT DRV
L1 Current
1ms / Div.
FIGURE 5 — LX157x VCC START-UP VOLTAGE TIMING DIAGRAM
FIGURE 6 — START-UP TIMING DIAGRAM
TIMING SECTION
A capacitor connected from the CT pin to ground performs sev-
eral 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 free-
running frequency of the converter in order to insure the con-
tinuous 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 fre-
quency 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.
Example: Assuming the transformer frequency is at 100kHz,
RPP = 0.6V, ICHG = 3mA, IDISCH = 3.5mA.
V
1
CT =
= 0.033µF
1
1
0.6 80 103
+
3 10−3 3.5 10−3
CURRENT LIMITING
Using the main Block Diagram as a reference and the typical
application circuit of Figure 2, note that current limiting is per-
formed 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. Ex-
ceeding 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 com-
parator 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 2µA
current source. When this happens, a very slow discharge oc-
curs, resulting in a long delay time between current limit cycles
which greatly reduces power MOSFET dissipation under short
circuit conditions.
1
CT =
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.)
Copyright © 1997
Rev. 0.9.3 1/97
9
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
LX1570/1571
PHASE
MODULATED AC SYNCHRONOUS
SECONDARY-SIDE
C
ONTROLLER
P R E L I M I N A R Y D A T A S H E E T
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 re-
lies on the C.S. signal for synchronization, special circuitry had to
be added to keep the output drive synchronized to the trans-
former voltage during such load transient conditions. This con-
dition 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 trans-
former voltage.
Transformer
Voltage
100V / Div.
LX157x
OUT DRV
20V / Div.
LX157x
COMP PIN
2V / Div.
Output Current &
Inductor Current
2A / Div.
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.
FIGURE 7 — MINIMUM CURRENT COMPARATOR EFFECT
DURING LOAD TRANSIENT
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)
Copyright © 1997
Rev. 0.9.3 1/97
10
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
LX1570/1571
PHASE
MODULATED AC SYNCHRONOUS
S
ECONDARY-SIDE
C
ONTROLLER
P R E L I M I N A R Y D A T A S H E E T
12V/8A SCHEMATIC
Note: Linfinity provides a complete and
tested evaluation board. For further
information contact factory.
80V
R2, 2W
4.7k
C1, 0.1µF
250V
L1
140µH
f = 100 to 150kHz
D8, 1N4937
(+)
VOUT (+)
Q1
IRF530
1/2 D1
MUR1620
NS
NP
1
6
3
Secondary
Transformer
Voltage
T2
12V/8A
VOUT (-)
4
1/2 D1
MUR1620
C9
820µF
16V
C10
820µF
16V
4
3
2
(-)
1
D4
1N4148
T1
PE64978
C2 0.1
(Note 1)
D2
D3
1N4001
1N4935
R5
475
W
1%
2.7k
1/2W
R4 47W
R3
VIN
C3
220µF
25V
D6
C5
1µF
1N5819
D5
1N5819
8
CT
7
6
5
C4
0.047µF
VCC OUT GND
Pwr
Gnd
DRV
R8
SHORT
U1, LX1571
Signal Gnd
100
R12
1M
Note 2
S.S. VFB COMP C.S.
W
1
2
3
4
W
D7
1N4148
R6
3.83k
W
C6
0.56µF
R11
C13
1%
R9
4.99
W
1%
R7
C8
1k
W
1%
Core = RM4Z
C7
1000pF
NP =
NS =
20T #32AWG
60T #32AWG
T2
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 © 1997
Rev. 0.9.3 1/97
11
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
LX1570/1571
PHASE
MODULATED AC SYNCHRONOUS
SECONDARY-SIDE
C
ONTROLLER
P R E L I M I N A R Y D A T A S H E E T
3.3V/10A SCHEMATIC
Note: Linfinity provides a complete and
tested evaluation board. For further
information contact factory.
VP (10 to 30V)
VP
R2, 2W
W
L1
300
C1, 0.1µF
50V
10µH
PE53700
f = 100kHz to 150kHz
(+)
D8, 1N4937
VOUT
(+)
Q1
IRLZ44
(Note 1)
1/2 D1
MBR2545CT
NS
NP
1
6
3
Secondary
Transformer
Voltage
T2
1/2 D1
MBR2545CT
3.3V/10A
4
C9
1500 1500 1500 1500
µF µF µF µF
6.3V 6.3V 6.3V 6.3V
C10
C11
C12
4
3
2
(-)
VOUT
(-)
1
D4
1N4148
T1
PE64978
C2 0.1
(Note 1)
R5
475
W
1%
R4 47
W
VIN
(17 to 20V)
D6
C5
1µF
C3
1N5819
22µF
25V
D5
1N5819
8
CT
7
6
5
C4
VCC OUT GND
Pwr
Gnd
0.047µF
DRV
R8
SHORT
U1, LX1571
Signal Gnd
100
S.S. VFB COMP C.S.
1M
Note2
1
2
3
4
W
D7
1N4148
R6
324
W
C6
0.56µF
R11
C13
1%
R9
3.3
W
1%
C7
R7
C8
1k
W
1%
1000pF
Core = RM4Z
NP =
NS =
25T #28AWG
25T #28AWG
T2
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.
Copyright © 1997
Rev. 0.9.3 1/97
12
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