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LD7550
Block Diagram
VCC
40V 16.0V/ 11.4V
UVLO
internal bias & Vref
RT
OSC
EN
Vref OK
EN
OUT
Green-Mode Oscillator
S
COMP
2R R PWM Comparator + +
R
Q
CS
Leading Edge Blanking
Ramp from Oscillator
GND
Absolute Maximum Ratings
Supply Voltage VCC COMP, RT, CS Operating Junction Temperature Storage Temperature Range Package thermal resistance (DIP-8) Package thermal resistance (SOT-26) Lead temperature (DIP-8, Soldering, 10sec) Lead temperature (SOT-26, Soldering, 10sec) 36V -0.3 ~7V 150C -65C to 150C 100C/W 250C/W 260C 260C
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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Leadtrend Technology Corporation
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LD7550
Electrical Characteristics
(TA = +25 C unless otherwise stated, VCC=15.0V) PARAMETER Supply Voltage (Vcc Pin) Startup Current VCOMP=0V Operating Current UVLO (off) UVLO (on) Voltage Feedback (Comp Pin) Short Circuit Current Open Loop Voltage Green Mode Threshold VCOMP Current Sensing (CS Pin) Maximum Input Voltage Leading Edge Blanking Time Input impedance Delay to Output Oscillator (RT pin) Frequency Green Mode Frequency Temp. Stability Voltage Stability Gate Drive Output (OUT Pin) Output Low Level Output High Level Rising Time Falling Time VCC=15V, Io=20mA VCC=15V, Io=20mA Load Capacitance=1000pF Load Capacitance=1000pF 8 50 30 200 125 1 V V nS nS RT=100K Fs=66.5KHz (-30C ~85C) (VCC=12V-30V) 61.5 66.5 20 5 2 71.5 KHz KHz % % 0.80 0.85 250 50 300 0.90 V nS K nS VCOMP=0V COMP pin open 2.2 5.0 2.35 3.0 mA V V VCOMP=3V VCOMP=open 10.4 14.8 5 3 2 0.7 11.4 16.0 12.4 17.5 25 4 A mA mA mA V V CONDITIONS MIN TYP MAX UNITS
o
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Leadtrend Technology Corporation
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LD7550
Typical Performance Characteristics
12.2 17.0 16.8 12.0 16.4
UVLO (On) (V)
UVLO (Off) (V)
11.8
16.0 15.6 15.2 14.8
11.6
11.4
11.2
14.4 14.0 -40
11.0 -40 -20 0 20 40 60 80 100 120
-20
0
20
40
60
80
100
120
Temperature (C) Fig. 1
72.0
UVLO (Off) vs. Temperature
18.2
Fig. 2
Temperature (C) UVLO (On) vs. Temperature
71.0
18.0
Frequency (KHz)
Frequency (KHz)
70.0
17.8
69.0
17.6
68.0
17.4
67.0
17.2
66.0 -40
-20
0
20
40
60
80
100
120
17.0 -40
-20
0
20
40
60
80
100
120
Temperature (C) Fig. 3 Frequency vs. Temperature Fig. 4
Temperature (C) Green-Mode Frequency vs. Temperature
75.9
75.6
Max. Duty-Cycle (%)
75.3
75.0
74.7
74.4
-40
-20
0
20
40
60
80
100
120
Temperature (C) Fig. 5 Duty-Cycle (max.) vs. Temperature
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Leadtrend Technology Corporation
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LD7550
Application Information
Operation Overview
As long as the green power requirement becomes a trend and the power saving is getting more and more important for the switching power supplies and switching adaptors, the traditional PWM controllers are not able to support such new requirements. Furthermore, the cost and size limitation force the PWM controllers need to be powerful to integrate more functions to reduce the external part counts. The LD7550 is targeted on such application to provide an easy and cost effective solution; its detail features are described as below: deliver the gate drive signal, the supply current is provided from the auxiliary winding of the transformer. The lower startup current requirement on the PWM controller will help to increase the R1 value and then reduce the power consumption on R1. By using CMOS process and the special circuit design, the maximum startup current of LD7550 is only 25A. Theoretically, R1 can be very high resistance value. However, higher R1 will cause longer startup time. By properly select the value of R1 and C1; it can be optimized under the consideration of R1 power consumption and the startup time.
Under Voltage Lockout (UVLO)
An UVLO comparator is implemented to detect the voltage on the Vcc pin to ensure the supply voltage is enough to power on the LD7550 PWM controller and further to drive the power MOSFET. during startup. As shown in Fig. 6, a hysteresis is implemented to prevent the shutdown from the voltage dip The turn-on and turn-off threshold level are set at 16V and 11.4V, respectively.
Vcc
UVLO(on) UVLO(off)
AC input
EMI Filter Cbulk R1 D1
C1
VCC OUT
LD7550
GND CS
t I(Vcc)
operating current (~ mA)
Fig. 7
startup current (~uA)
Current Sensing and Leading-edge Blanking
t
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. 8, the LD7550 detects the primary MOSFET current from the CS pin, which
Fig. 6
Startup Current and Startup Circuit
The typical startup circuit to power up the LD7550 is shown in Fig. 7. During the startup transient, the Vcc is lower than Therefore, the the UVLO threshold thus there is no gate pulse generated from LD7550 to drive power MOSFET. current through R1 is to provide the startup current as well as charge the capacitor C1. Whenever the Vcc voltage is higher enough to power on the LD7550 and further to
is not only for the peak current mode control but also for the pulse-by-pulse current limit. The maximum voltage threshold of the current sensing pin is set as 0.85V. Thus the MOSFET peak current can be calculated as:
IPEAK(MAX) = 0.85 V RS
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Leadtrend Technology Corporation
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LD7550
Vin
Oscillator and Switching Frequency
Cbulk R1 D1
Connecting a resistor from RT pin to GND according to the equation can program the normal switching frequency:
fSW = 66.5 x 100(KHz ) RT(K )
C1
VCC OUT
The suggested operating frequency range of LD7550 is within 50KHz to 130KHz.
LD7550
Comp GND CS
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. 8 A 250nS leading-edge blanking time is included in the input of CS pin to prevent the false-trigger caused by the current spike and further to eliminate the need of R-C filter which is usually needed in the typical UC384X application (Fig. 9).
pin of LD7550.
The input stage of LD7550, like the
UC384X, is with 2 diodes voltage offset then feeding into the voltage divider with 1/3 ratio, that is,
V+ (PWM COMPARATOR ) = 1 x ( VCOMP - 2VF ) 3
A pull-high resistor is embedded internally thus can be eliminated on the external circuit.
Internal Slope Compensation
A fundamental issue of current mode control is the stability problem when its duty-cycle is operated more than 50%. To
VCC OUT 250ns blanking time
stabilize the control loop, the slope compensation is needed in the traditional UC384X design by injecting the ramp signal from the RT/CT pin through a coupling capacitor. In LD7550, the internal slope compensation circuit has been implemented to simplify the external circuit design.
LD7550
CS GND
On/Off Control
The LD7550 can be controlled to turn off by pulling COMP pin to lower than 1.2V. The gate output pin of LD7550 will be disabled immediately under such condition. The off mode can be released when the pull-low signal is removed. Fig. 9
remove
Dual-Oscillator Green-Mode Operation Output Stage and Maximum Duty-Cycle
An output stage of a CMOS buffer, with typical 300mA driving capability, is incorporated to drive a power MOSFET directly. And the maximum duty-cycle of LD7550 is limited to 75% to avoid the transformer saturation. There saving are many difference such as topologies has been implemented in different chips for the green-mode or power requirements "burst-mode control", "skipping-cycle Mode", "variable off-time control "...etc. The basic operation theory of all these approaches intended to
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Leadtrend Technology Corporation
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LD7550
reduce the switching cycles under light-load or no-load condition either by skip some switching pulses or reduce the switching frequency. What LD7550 used to implement the power-saving operation is Leadtrend Technology's own IP .
2
(voltage controlled oscillator), is a variable frequency oscillator. The rising time of the VCO is proportional to (VGREEN-V+), thus the lower voltage on V+ will generate longer rising time on VCO as well as lower frequency on VCO. By using this dual-oscillator control, the green-mode frequency can be well controlled and further to avoid the generation of audible noise.
from OSC "Set" Signal (to OSC & Nor gate)
In such
approach, as shown in the block diagram, 2 oscillators are implemented in LD7550. The first oscillator is to take care the normal switching frequency, which can be set by the RT pin through an external resistor. Under this operation mode, as shown in Fig. 10, the 2
nd
oscillation (green-mode
Green-Mode Oscillator VCO Level-detector & Counter
oscillator) is not activated. Therefore, the rising-time and the falling-time of the internal ramp will be constant to achieve good stability over all temperature range. frequency.
V+
Under the
Vgreen (V+ -Vgreen) >=0, VCO disabled (V+ -Vgreen) <0, VCO activated
normal operation, this oscillator is dominated the switching
V+
Green-Mode Osc is not activated under normal operation. OSC
EN=1
Normal Mode VGreen Mode
Vgreen
EN
OUT
Green-Mode Osc
Ramp of OSC
Green-Mode Oscillator activated
Set V+ Reset 2R R VPWM Comparator + + Ramp from Oscillator
S
COMP
R
Q
Ramp of VCO
CS
LEB
Fig. 11 Fig. 10
2 Note: Patent pending
As shown in Fig. 11, the green-mode oscillator detects the Comp pin signal to determine if it is within the green-mode operation. When the detected signal V+ is lower than the green-mode threshold VGREEN, the green-mode oscillator is on. The green-mode oscillator, implemented by a VCO
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Leadtrend Technology Corporation
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LD7550
Package Information
SOT-26
A
J
M B
D C F I
Dimensions in Inches Min 0.106 0.055 ------0.012 0.033 0.002 0.102 0.012 0 Max 0.122 0.071 0.058 0.022 0.010 0.006 0.118 0.024 10
o
Dimension in Millimeters Symbol Min A B C D F I J M c 2.692 1.397 ------0.300 0.838 0.050 2.600 0.300 0 Max 3.099 1.803 1.450 0.550 1.041 0.150 3.000 0.600 10
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Leadtrend Technology Corporation
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LD7550
DIP-8
A
B
E C L I D F
J
Dimension in Millimeters Symbol Min A B C D E F I J L 9.017 6.096 ----0.356 1.143 2.337 2.921 7.366 0.381 Max 10.160 7.112 5.334 0.584 1.778 2.743 3.556 8.255 ------
Dimensions in Inches Min 0.355 0.240 -----0.014 0.045 0.092 0.115 0.29 0.015 Max 0.400 0.280 0.210 0.023 0.070 0.108 0.14 0.325 --------
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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Leadtrend Technology Corporation
LD7550-DS-00 August 2004

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