LD7550-B

01/03/2005

Green-Mode PWM Controller

General Description Features

The LD7550-B is a low cost, low startup current, current High-Voltage CMOS Process with Excellent ESD
mode PWM controller with green-mode power-saving protection
operation. The integrated functions such as the Very Low Startup Current (<20µA)
leading-edge blanking of the current sensing, internal slope Under Voltage Lockout (UVLO)
compensation and the small SOT-26 package provide the Current Mode Control with Cycle-by-Cycle Peak
users a high efficiency, low external component counts, and Current Limiting
low cost solution for AC/DC power applications. Leading-Edge Blanking on CS Pin
Programmable Switching Frequency
Compared with LD7550, LD7550-B has different parameters Internal Slope Compensation
or functions on the following characteristics --- Proprietary Green-Mode Control for Power Saving
Non-audible-noise Green Mode Control
Enlarger the hysteresis range of UVLO by lowering
300mA Driving Capability
down the UVLO(off) from 11.4V to 10V.
OVP (Over Voltage Protection) on Vcc Pin
Add OVP (Over Voltage Protection) function on Vcc
Pin. Applications
Switching AC/DC Adaptor and Battery Charger
Shorten the OCP delay time to 100nS to reduce the
Open Frame Switching Power Supply
difference of OCP trip level under high line & low line.
384X Replacement

Typical Application

AC EMI
input Filter

VCC

UVLO OUT
16.0V/
10.0V

LD7550-B
RT
OSC

Control
COMP Logic
photocoupler
Divider

CS TL431

GND

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 LD7550-B
Pin Configuration
DIP-8 (TOP VIEW) SOT-26 (TOP VIEW)

COMP
GND

NC

RT
OUT VCC CS
8 7 6 5

6 5 4
TOP MARK
YWP 50B
YYWW## 1 2 3

1 2 3 4
GND COMP RT
VCC
NC
CS
OUT

YY, Y : Year code

WW, W: Week code
P : LD75..
(Product family code)
## : Production code

Ordering Information
Part number Package TOP MARK Shipping
LD7550-B IL SOT-26 YWP/50B 3000 /tape & reel
LD7550-B IN DIP-8 LD7550BIN 3600 /tube
( )
LD7550-B BL SOT-26 (PB free) * YWP/50B 3000 /tape & reel
LD7550-B BN DIP-8 (PB free) LD7550BBN 3600 /tube
( )
* Identified in different laser ink

Pin Descriptions

PIN PIN
NAME FUNCTION
(DIP-8) (SOT-26)
1 6 OUT Gate drive output to drive the external MOSFET
2 5 VCC Supply voltage pin
3 NC Unconnected pin
4 4 CS Current sense pin, connect to sense the MOSFET current
This pin is to program the switching frequency. By connecting a resistor
5 3 RT
to ground to set the switching frequency.
6 NC Unconnected pin
Voltage feedback pin (same as the COMP pin in UC384X), By connecting
7 2 COMP
a photo-coupler to close the control loop and achieve the regulation.
8 1 GND Ground

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Block Diagram
VCC

UVLO
internal bias
16.0V/ & Vref
10.0V

29V
EN OVP
RT OSC Vref OK

EN

OUT

Green-Mode
Oscillator

COMP
2R R Q
R
PWM
Comparator

Leading
+ + Ramp from
CS Edge ∑ Oscillator
Blanking

GND

Absolute Maximum Ratings

Supply Voltage VCC 36V
COMP, RT, CS -0.3 ~7V
Junction Temperature 150°C
Operating Ambient Temperature -40°C to 85°C
Storage Temperature Range -65°C to 150°C
Package Thermal Resistance (SOT-26) 250°C/W
Package Thermal Resistance (DIP-8) 100°C/W
Power Dissipation (SOT-26, at Ambient Temperature = 85°C) 250mW
Power Dissipation (DIP-8, at Ambient Temperature = 85°C) 650mW
Lead temperature (SOT-26 & DIP-8, Soldering, 10sec) 230°C
Lead temperature (All PB Free Packages, Soldering, 10sec) 260°C
ESD Voltage Protection, Human Body Model 3.5KV
ESD Voltage Protection, Machine Model 300V
Gate Output Current 300mA

Caution:
Stresses beyond the ratings specified in “Absolute Maximum Ratings” may cause permanent damage to the device. This is a stress only
rating and operation of the device at these or any other conditions above those indicated in the operational sections of this specification is not
limited.

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Electrical Characteristics
(TA = +25oC unless otherwise stated, VCC=15.0V)
PARAMETER CONDITIONS MIN TYP MAX UNITS
Supply Voltage (Vcc Pin)
Startup Current 8 20 µA
VCOMP=0V 3 4 mA
Operating Current VCOMP=3V 2 mA
VCOMP=open 0.7 mA
UVLO (off) 9.0 10.0 11.0 V
UVLO (on) 15.0 16.0 17.0 V
OVP Level 27 29 32 V
Voltage Feedback (Comp Pin)
Short Circuit Current VCOMP=0V 2.2 3.0 mA
Open Loop Voltage COMP pin open 5.0 V
Green Mode Threshold VCOMP 2.35 V
Current Sensing (CS Pin)
Maximum Input Voltage, Vcs(off) 0.80 0.85 0.90 V
Leading Edge Blanking Time 350 nS
Input impedance 50 KΩ
Delay to Output 100 nS
Oscillator (RT pin)
Frequency RT=100KΩ 60.0 65.0 70.0 KHz
Green Mode Frequency Fs=65.0KHz 20 KHz
Temp. Stability (-40°C ~105°C) 3 %
Voltage Stability (VCC=11V-25V) 1 %
Gate Drive Output (OUT Pin)
Output Low Level VCC=15V, Io=20mA 1 V
Output High Level VCC=15V, Io=20mA 8 V
Rising Time Load Capacitance=1000pF 50 200 nS
Falling Time Load Capacitance=1000pF 30 100 nS

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Typical Performance Characteristics
12 5.6

10
5.4

8
Istartup (µA)

5.2

VCOMP (V)
6

5.0
4

4.8
2

0 4.6
-40 0 50 100 150 -40 0 50 100 150
Temperature (°C) Temperature (°C)
Fig. 1 Startup Current (Istartup) vs. Temperature Fig. 2 VCOMP open loop voltage v.s. Temperature

18 12

17 11
UVLO (On) (V)

UVLO (Off) (V)

16 10

15 9

8
14
-40 0 50 100 150 -40 0 50 100 150
Temperature (°C) Temperature (°C)
Fig. 3 UVLO (On) vs. Temperature Fig. 4 UVLO Off v.s. Temperature

70 25
Green Mode Frequency (KHz)

68 23
Frequency (KHz)

66 21

64 19

62 17

60 15
-40 0 50 100 150
-40 0 50 100 150
Temperature (°C) Temperature (°C)
Fig. 5 Frequency v.s. Temperature Fig. 6 Green Mode Frequency v.s. Temperature

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25
70

Green mode frequency (KHz)

68 23
Frequency (KHz)

66
21

64
19

62 17

60 15
11 12 14 16 18 20 22 24 25 11 12 14 16 18 20 22 24 25
Vcc (V) Vcc (V)
Fig. 7 Frequency v.s. Vcc Fig. 8 Green mode frequency v.s. Vcc

76.0 35

30

25
75.0
Max Duty (%)

VCC OVP (V)

20

15

74.5
10

74.0 0
-40 0 50 100 150 0 50 100 150
Temperature (°C) Temperature (°C)
Fig. 9 Max Duty v.s. Temperature Fig. 10 VCC OVP v.s. Temperature

0.87

0.86

0.85
(V)

0.84
Vcs(off)

0.83

0.82

0.81

0.80
0 50 100 150
Temperature (°C)
Fig. 11 Vcs (off) v.s. Temperature

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Application Information

further to deliver the gate drive signal, the supply current is

Operation Overview
provided from the auxiliary winding of the transformer. The
The LD7550B meets the green power requirement and is
lower startup current requirement on the PWM controller will
intended for use in those switching power suppliers and
help to increase the R1 value and then reduce the power
switching adaptors, demanding higher power efficiency and
consumption on R1. By using CMOS process and the
power saving. It integrated more functions to reduce the
special circuit design, the maximum startup current of
external components counts and the size. Its major features
LD7550-B is only 20µA.
are described as below.
If a higher resistance value of the R1 is picked, it will usually
Under Voltage Lockout (UVLO) take more time to startup. To carefully select the value for
An UVLO comparator is implemented to detect the voltage R1 and C1 will optimize the power consumption and startup
on the VCC pin to ensure the supply voltage is enough to time.
power on the LD7550-B PWM controller and further to drive
the power MOSFET. As shown in Fig. 12, a hysteresis is
implemented to prevent the shutdown from the voltage dip AC EMI
during startup. The turn-on and turn-off threshold level are input Filter

set at 16V and 10.0V, respectively. R1

Cbulk
D1

Vcc
C1

UVLO(on)
VCC
UVLO(off) OUT

LD7550-B
CS
t GND

I(Vcc) operating current

(~ mA)

Fig. 13
startup current
(~uA)

t Current Sensing and Leading-edge Blanking

Fig. 12 The typical current mode PWM controller feedbacks both

current signal and voltage signal to close the control loop
and achieve regulation. As shown in Fig. 14, the LD7550-B
Startup Current and Startup Circuit
detects the primary MOSFET current from the CS pin, which
The typical startup circuit to power up the LD7550-B is is not only for the peak current mode control but also for the
shown in Fig. 13. During the startup transient, the Vcc is pulse-by-pulse current limit. The maximum voltage
lower than the UVLO threshold thus there is no gate pulse threshold of the current sensing pin is set at 0.85V. From
generated from LD7550-B to drive power MOSFET. above, the MOSFET peak current can be calculated as:
Therefore, the current through R1 will provide the startup 0.85 V
IPEAK(MAX) =
current as well as charge the capacitor C1. Whenever the RS
Vcc voltage is higher enough to power on the LD7550-B and

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Vin Oscillator and Switching Frequency
Connecting a resistor from RT pin to GND according to the
R1
Cbulk equation can program the normal switching frequency:
D1
65.0
fSW = × 100(KHz )
RT(KΩ )
C1

VCC
The suggested operating frequency range of LD7550-B is
OUT
within 50KHz to 130KHz.
LD7550-B
Comp CS
GND Rs Voltage Feedback Loop
The voltage feedback signal is provided from the TL431 in
the secondary side through the photo-coupler to the COMP
Fig. 14
pin of LD7550-B. The input stage of LD7550-B, like the
UC384X, is with 2 diodes voltage offset then to feed the
A 350nS leading-edge blanking time is included in the input voltage divider with 1/3 ratio, that is,
of CS pin to prevent the false-trigger caused by the current
1
spike and further to eliminate the need of R-C filter which is V+ (PWM COMPARATOR ) = × ( VCOMP − 2VF )
3
usually needed in the typical UC384X application (Fig. 15).
A pull-high resistor is embedded internally, it can therefore
be eliminated from the external circuit.

Internal Slope Compensation

In conventional application, the problem for the stability is a
critical issue for current mode controlling, when it operates
350ns
blanking more than 50% of the duty-cycle. As UC384X, It takes slope
VCC
time compensation from injecting the ramp signal from the RT/CT
OUT
pin through a coupling capacitor. On the other hand, there is
no extra external component needed for the LD7550-B
LD7550-B since it has integrated this function with it already.

CS
GND On/Off Control
The LD7550-B can be turnned off by pulling COMP pin
lower than 1.2V. The gate output pin of the LD7550-B will
be disabled immediately under such condition. The
remove
off-mode can be released when the pull-low signal is
Fig. 15 removed.

Output Stage and Maximum Duty-Cycle Dual-Oscillator Green-Mode Operation

An output stage of a CMOS buffer, with typical 300mA There are many difference topologies has been
driving capability, is incorporated to drive a power MOSFET implemented in different chips for the green-mode or power
directly. And the maximum duty-cycle of LD7550-B is saving requirements such as “burst-mode control”,
limited to 75% to avoid the transformer saturation. “skipping-cycle Mode”, “variable off-time control “…etc. The
basic operation theory of all these approaches intended to

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 LD7550-B
reduce the switching cycles under light-load or no-load The green-mode oscillator detects the signal of COMP pin
condition either by skip some switching pulses or reduce the to determine if it is within the green-mode operation. When
switching frequency. the detected signal V+ is lower than the green-mode
What LD7550-B used to implement the power-saving threshold VGREEN, the green-mode oscillator is on. The
operation is Leadtrend Technology’s own IP. In such green-mode oscillator, implemented by a VCO (voltage
approaching, as shown in the block diagram, there are 2 controlled oscillator), is a variable frequency oscillator.
oscillators are implemented in LD7550-B. The first oscillator By using this dual-oscillator control, the green-mode
is to provide the normal switching frequency, which can be frequency can be well controlled and further to avoid the
set by the RT pin through an external resistor. Under this generation of audible noise.
operation mode, as shown in Fig. 16, the 2nd oscillation
(green-mode oscillator) is not activated. Therefore, the
OVP (Over Voltage Protection) on Vcc
rising-time and the falling-time of the internal ramp will be
Most of the VGS ratings of the modern power MOSFETs are
constant to achieve good stability over all temperature range.
30V maximum. To prevent the VGS from the fault condition,
Under the normal operation, this oscillator will dominate the
LD7550-B also integrates an OVP function on Vcc.
switching frequency.
Whenever the Vcc voltage is higher than the threshold
voltage, the output gate drive circuit will be shut down
Green-Mode Osc is not activated
under normal operation.
simultaneously to stop the switching of the power MOSFET.
OSC EN=1 (from UVLO)
Whenever the Vcc level gets back to lower level, the output
will automatically return to the normal operation.
EN
OUT

Fault Protection
Green-Mode
Osc

Set S
Several curtail protection features have been integrated in
V+ the LD7550-B to protect the power supply or adapter from
COMP Reset
2R
V- R Q being damaged. Those damages usually come from open or
R
PWM
Comparator short condition on the pins of LD7550-B. Under the
+ + Ramp from conditions listed below, the gate output will active off
CS LEB ∑ Oscillator
immediately to protect the power circuit ---
RT pin short to ground
Fig. 16 RT pin floating
CS pin floating

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F1 FL1 R51B
R51A C51
D1A~D1D
R1A CX1 L51
AC T1
input
R1B Z1 R2A
R4A CR51
C1 C4 C52 C54
NTC1 R2B
R4B R56A ZD51 R56B
Reference Application Circuit #1 --- 10W (5V/2A) Adapter

R6 D4
D2
C2
VCC IC1
5
OUT Q1
6
LD7550-B R7
RT 4

10
3 1
2 CS
GND RS2 RS1
RT COMP
R54 R52
IC2
C5
photocoupler
C55

Leadtrend Technology Corporation

R55
CY1

LD7550-B-DS-P3 January 2005

IC51 R53

Schematic
 LD7550-B
Reference Application Circuit #1 --- 10W (5V/2A) Adapter

BOM

P/N Component Value Original P/N Component Value Note

R1A N/A C1 22µF, 400V L-tec
R1B N/A C2 10µF, 50V L-tec
R2A 750KΩ, 1206 C4 1000pF, 1000V, 1206 Holystone
R2B 750KΩ, 1206 C5 0.01µF, 16V, 0805
R4A 39KΩ, 1206 C51 1000pF, 50V, 0805
R4B 39KΩ, 1206 C52 1000µF, 10V L-tec
R6 10Ω, 1206 C54 470µF, 10V L-tec
R7 10Ω, 1206 C55 0.01µF, 16V, 0805
RS1 2.74Ω, 1206, 1% CX1 0.1µF X-cap
RS2 2.74Ω, 1206, 1% CY1 2200pF Y-cap
RT 100KΩ, 0805, 1% D1A 1N4007
R51A 100Ω, 1206 D1B 1N4007
R51B 100Ω, 1206 D1C 1N4007
R52 2.49KΩ, 0805, 1% D1D 1N4007
R53 2.49KΩ, 0805, 1% D2 PS102R
R54 220Ω, 0805 D4 1N4007
R55 10KΩ, 0805 Q1 2N60B 600V/2A
R56A 510Ω, 1206 CR51 SB540
R56B N/A ZD51 6V2C
NTC1 5Ω, 3A 08SP005 IC1 LD7550-B IL SOT-26
FL1 20mH UU9.8 IC2 EL817B
T1 EI-22 IC51 TL431 1%
L51 2.7µH F1 250V, 1A
Z1 N/A

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Reference Application Circuit #2 --- 10W Adapter with 2-Stage Startup Circuit
Pin < 0.25W when Pout = 0W

Schematic

input
AC
NTC1

F1
R1B

R1A
Z1

CX1
RT
RT

FL1
3
COMP

LD7550-B
5

VCC
2
GND

IC1

C1

D1A~D1D
1

6
4

OUT
CS

C3

R2B

R2A
D3
R7

2-stage Startup
C2

Circuit
D2 R6

R4B

R4A
C5

RS2

D4

C4
photocoupler
CY1

RS1

Q1

T1
IC2

CR51

R51A
R51B
C51
C52
R56A ZD51
R54
IC51

R55

C55

L51
R56B
R52
R53

C54

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Reference Application Circuit #2 --- 10W Adapter with 2-Stage Startup Circuit

BOM

P/N Component Value Original P/N Component Value Note

R1A N/A C1 22µF, 400V L-tec
R1B N/A C2 10µF, 50V L-tec
R2A 2.2MΩ, 1206 C3 2.2µF, 50V
R2B 2.2MΩ, 1206 C4 1000pF, 1000V, 1206 Holystone
R4A 39KΩ, 1206 C5 0.01µF, 16V, 0805
R4B 39KΩ, 1206 C51 1000pF, 50V, 0805
R6 2.2Ω, 1206 C52 1000µF, 10V L-tec
R7 10Ω, 1206 C54 470µF, 10V L-tec
RS1 2.74Ω, 1206, 1% C55 0.01µF, 16V, 0805
RS2 2.74Ω, 1206, 1% CX1 0.1µF X-cap
RT 100KΩ, 0805, 1% CY1 2200pF Y-cap
R51A 100Ω, 1206 D1A 1N4007
R51B 100Ω, 1206 D1B 1N4007
R52 2.49KΩ, 0805, 1% D1C 1N4007
R53 2.49KΩ, 0805, 1% D1D 1N4007
R54 220Ω, 0805 D2 PS102R
R55 10KΩ, 0805 D3 1N4148
R56A 1KΩ, 1206 D4 1N4007
R56B N/A Q1 2N60B 600V/2A
NTC1 5Ω, 3A 08SP005 CR51 SB540
FL1 20mH UU9.8 ZD51 6V2C
T1 EI-22 IC1 LD7550-B IL SOT-26
L51 2.7µH IC2 EL817B
IC51 TL431 1%
F1 250V, 1A
Z1 N/A

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Package Information
SOT-26

A
θ

M
J B

F I

Dimension in Millimeters Dimensions in Inches

Symbol
Min Max Min Max

A 2.692 3.099 0.106 0.122

B 1.397 1.803 0.055 0.071

C ------- 1.450 ------- 0.058

D 0.300 0.550 0.012 0.022

F 0.838 1.041 0.033 0.041

I 0.050 0.150 0.002 0.006

J 2.600 3.000 0.102 0.118

M 0.300 0.600 0.012 0.024

° °
θ 0 10 0 10

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Package Information
DIP-8

E
J

C
L
I

D F

Dimension in Millimeters Dimensions in Inches

Symbol
Min Max Min Max

A 9.017 10.160 0.355 0.400

B 6.096 7.112 0.240 0.280

C ----- 5.334 ------ 0.210

D 0.356 0.584 0.014 0.023

E 1.143 1.778 0.045 0.070

F 2.337 2.743 0.092 0.108

I 2.921 3.556 0.115 0.140

J 7.366 8.255 0.290 0.325

L 0.381 ------ 0.015 --------

Important Notice
Leadtrend Technology Corp. reserves the right to make changes or corrections to its products at any time without notice. Customers should

verify the datasheets are current and complete before placing order.

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