AN-6077 Datasheet, PDF (1/8 Pages) Fairchild Semiconductor – Highly Integrated Green-Mode PWM Controller

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AN-6077
SG6742 — Highly Integrated Green-Mode PWM Controller
Abstract
This application note describes a detailed design strategy for
a high-efficiency, compact flyback converter. Design
considerations, mathematical equations, and guidelines for a
Printed-Circuit-Board (PCB) layout are presented.
Features
ƒ High-Voltage Startup
ƒ Low Operating Current: 2.7mA
ƒ Linearly Decreasing PWM Frequency to 22KHz
ƒ Frequency Hopping to Reduce EMI Emission
ƒ Fixed PWM Frequency:
ML and MR are 65KHz.
HL and HR are 100KHz.
ƒ Peak-Current-Mode Control
ƒ Cycle-by-Cycle Current Limiting
ƒ Leading-Edge Blanking (LEB)
ƒ Synchronized Slope Compensation
ƒ Internal Open-Loop Protection
ƒ Gate Output Maximum Voltage Clamp: 18V
ƒ V_DD Under-Voltage Lockout (UVLO)
ƒ V_DD Over-Voltage Protection (OVP)
ƒ Programmable Over-Temperature Protection (OTP)
ƒ Internal Latch Circuit (OVP, OTP)
ƒ Internal Sense Short-Circuit Protection
ƒ Built-In Soft-Start Function
ML and MR are 5ms.
HL and HR are 6ms.
ƒ Constant Power Limit (Full AC Input Range)
ƒ Internal OTP Sensor with Hysteresis
Applications
General-purpose, switch-mode power supplies and flyback
power converters, including:
ƒ Power Adapters
ƒ Open-Frame Switch-Mode Power Supply (SMPS)
Introduction
The highly integrated SG6742 series of PWM controllers
provides several features to enhance the performance of
flyback converters.
To minimize standby power consumption, a proprietary
green-mode function provides off-time modulation to
linearly decrease the switching frequency at light-load
conditions. To avoid acoustic-noise problems, the minimum
PWM frequency is set above 22KHz. This green-mode
function enables the power supply to meet international
power conservation requirements. With the internal high-
voltage startup circuitry, the power loss due to bleeding
resistors is also eliminated. To further reduce power
consumption, SG6742 is manufactured using the BiCMOS
process, which allows an operating current of only 2.7mA.
Built-in synchronized slope compensation achieves stable
peak-current-mode control. The proprietary external line
compensation ensures a constant output-power limit over a
wide AC input voltage range, from 90V_AC to 264V_AC.
SG6742 provides many protection functions. In addition to
cycle-by-cycle current limiting, the internal open-loop
protection circuit ensures safety should an open-loop or
output short-circuit failure occur.
SG6742ML
SG6742MR
SG6742HL
SG6742HR
OVP
(VDD)
Latch
Latch
Latch
Latch
OLP (FB)
Latch
Auto Restart
Latch
Auto Restart
OTP
(RT)
Latch
Latch
Latch
Latch
SCP
(SENSE)
Frequency
Auto Restart
Auto Restart
Auto Restart
Auto Restart
65KHz
65KHz
100KHz
100KHz
SOP-8
GND
1
8
GATE
FB
2
7
VDD
NC
3
6
SENSE
HV
4
5
RT
Figure 1. Pin Configuration (Top View)
© 2008 Fairchild Semiconductor Corporation
Rev. 1.0.2 • 12/1/08
1
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