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LD7552
2/21/2005
Green-Mode PWM Controller
General Description
The LD7552 is a low cost, low startup current, current mode PWM controller with green-mode power-saving operation. The integrated functions such as the leading-edge blanking of the current sensing, internal slope compensation and the small package provide the users a high efficiency, minimum external component counts, and low cost solution for AC/DC power applications. The special green-mode control does not only achieve low power consumption but also offer a non-audible-noise operation when the LD7552 operates under light load or no load condition. The LD7552 is designed for the switching adaptor with 30W~60W output. The LD7552 is offered in both SOP-8 and DIP-8 package. Patent pending
Features
High-Voltage CMOS Process with Excellent ESD Protection Very Low Startup Current (Typical 5A) Under Voltage Lockout (UVLO) Current Mode Control with Cycle-by-Cycle Peak Current Limiting Leading-Edge Blanking on CS Pin Programmable Switching Frequency Internal Slope Compensation Non-audible-noise Green Mode Control 500mA Driving Capability Proprietary Green-Mode Control for Power Saving
1
Applications
Switching AC/DC Adaptor and Battery Charger Open Frame Switching Power Supply 384X Replacement
1
Note: Please see Application Information
Typical Application
AC input
EMI Filter
VCC
40V 16.0V/ 11.4V UVLO
OUT
RT COMP
LD7552
OSC
Control Logic
photocoupler
Divider
CS GND
TL431
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LD7552
Pin Configuration
SOP-8 & DIP-8 (TOP VIEW) VCC OUT NC
5
8
7
CS
6
TOP MARK YYWWPP
1 2 3 4
YY: Year code (D: 2004, E: 2005.....) WW: week code PP: production code
GND
VCC
Ordering Information
Part number LD7552 IS LD7552 BS LD7552 IN LD7552 BN Package SOP-8 SOP-8 (PB Free) DIP-8 DIP-8 (PB Free) TOP MARK LD7552IS LD7552BS LD7552IN LD7552BN Shipping 2500 /tape & reel 2500 /tape & reel 3600 /tube /carton 3600 /tube /carton
Pin Descriptions
PIN 1 2 3 4 5 6 7 8 NAME GND COMP VCC RT NC CS VCC OUT Ground Voltage feedback pin (same as the COMP pin in UC384X), By connecting a photo-coupler to close the control loop and achieve the regulation. Supply voltage pin This pin is to program the switching frequency. By connecting a resistor to ground to set the switching frequency. Unconnected pin Current sense pin, connect to sense the MOSFET current Supply voltage pin Gate drive output to drive the external MOSFET FUNCTION
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COMP
RT
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LD7552
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 (SOP-8) Lead temperature (LD7552IS & LD7552IN, Soldering, 10sec) Lead temperature (LD7552BS & LD7552BN, Soldering, 10sec) 36V -0.3 ~7V 150C -65C to 150C 100C/W 160C/W 230C 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 implied.
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LD7552
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 9 160 60 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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LD7552
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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LD7552
Application Information
Operation Overview
The LD7552 is optimized to achieve power saving and minimize the external components counts. The device incorporated several functions to make it ideal to use in switching power supplies and switching adaptors. special circuit design, the maximum startup current of LD7552 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 included to detect the voltage on the Vcc pin to ensure the supply voltage enough to power on the LD7552 PWM controller and in addition to drive the power MOSFET. As shown in Fig. 6, a hysteresis is
AC input
EMI Filter Cbulk R1 D1
provided to prevent the shutdown from the voltage dip during startup. The turn-on and turn-off threshold level are set at 16V and 11.4V, respectively.
Vcc
UVLO(on) UVLO(off)
VCC
C1
OUT
LD7552
t I(Vcc)
operating current (~ mA)
GND CS
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 LD7552 detects the primary MOSFET current from the CS pin, which 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:
Fig. 6
Startup Current and Startup Circuit
The typical startup circuit as shown in Fig. 7 powers ups the LD7552. During the startup transient, the Vcc is lower than Therefore, the the UVLO threshold thus there is no gate pulse generated from LD7552 to drive power MOSFET. as charge the capacitor C1. current through R1 is to provide the startup current as well Whenever the Vcc voltage is higher enough to power on the LD7552 and further to deliver the gate drive signal, the supply current is provided from the auxiliary winding of the transformer. The lower
IPEAK (MAX) =
0.85 V RS
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).
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
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LD7552
Vin R1 D1
Oscillator and Switching Frequency
Connecting a resistor from RT pin to GND according to the
Cbulk
equation can program the normal switching frequency:
C1
fSW =
66.5 x 100(KHz ) RT(K )
VCC OUT
The suggested operating frequency range of LD7552 is within 50KHz to 130KHz.
LD7552
Comp GND CS
Rs
Voltage Feedback Loop
The voltage feedback signal is provided from the TL431 in the secondary side through the photocoupler to the COMP
Fig. 8
pin of LD7552.
The input stage of LD7552, like the
UC384X, is with 2 diodes voltage offset then feeding into the voltage divider with 1/3 ratio, that is,
V+(PWMCOMPARATOR ) = 1 x ( VCOMP - 2VF ) 3
A pull-high resistor is embedded internally thus can be eliminated on the external circuit.
VCC OUT 250ns blanking time
Internal Slope Compensation
A fundamental issue of current mode control is the stability problem when its duty-cycle is operated more than 50%. To stabilize the control loop, the slope compensation is needed
LD7552
CS GND
in the traditional UC384X design by injecting the ramp signal from the RT/CT pin through a coupling capacitor. In LD7552, the internal slope compensation circuit has been implemented to simplify the external circuit design.
remove
On/Off Control
Fig. 9
The LD7552 can be controlled to turn off by pulling COMP pin to lower than 1.2V. The gate output pin of LD7552 will be disabled immediately under such condition. The off mode can be released when the pull-low signal is removed.
Output Stage and Maximum Duty-Cycle
An output stage of a CMOS buffer, with typical 500mA driving capability, is incorporated to drive a power MOSFET directly. And the maximum duty-cycle of LD7552 is limited
Dual-Oscillator Green-Mode Operation
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
to 75% to avoid the transformer saturation.
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LD7552
basic operation theory of all these approaches intended to reduce the switching cycles under light-load or no-load condition either by skip some switching pulses or reduce the switching frequency. What LD7552 used to implement the power-saving operation is Leadtrend Technology's own IP .
2
from OSC
By using this dual-oscillator control, the green-mode frequency can be well controlled and further to avoid the generation of audible noise.
In such
VCO
"Set" Signal (to OSC & Nor gate)
approach, as shown in the block diagram, 2 oscillators are implemented in LD7552. 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 2nd oscillation (green-mode 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.
Ramp of OSC V+ Normal Mode VGreen Mode V+
Green-Mode Oscillator
Level-detector & Counter
Vgreen (V+ -Vgreen) >=0, VCO disabled (V+ -Vgreen) <0, VCO activated
Under the
Vgreen
normal operation, this oscillator is dominated the switching
Green-Mode Osc is not activated under normal operation. OSC
EN=1
Green-Mode Oscillator activated
EN
OUT
Green-Mode Osc
Ramp of VCO
Set V+ Reset 2R R VPWM Comparator + + Ramp from Oscillator
S
COMP
R
Q
Fig. 11
CS
LEB
2 Note: Patent pending
Fig. 10
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 (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.
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LD7552
Package Information
SOP-8
A H M
J
B
F
C I D
Dimensions in Millimeters Symbols MIN MAX
Dimensions in Inch MIN MAX
A B C D F H I J M
4.801 3.810 1.346 0.330 1.194 0.178 0.102 5.791 0.406 0
5.004 3.988 1.753 0.508 1.346 0.229 0.254 6.198 1.270 8
0.189 0.150 0.053 0.013 0.047 0.007 0.004 0.228 0.016 0
0.197 0.157 0.069 0.020 0.053 0.009 0.010 0.244 0.050 8
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LD7552
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
Dimensions in Inches Min
0.355 0.240 -----0.014 0.045 0.092 0.115 0.29 0.015
Max
10.160 7.112 5.334 0.584 1.778 2.743 3.556 8.255 ------
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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