SG6105A データシートの表示（PDF） - Fairchild Semiconductor

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SG6105A

Power Supply Supervisor + Regulator + PWM

Fairchild Semiconductor 

Application Information

OP1

Introduction

The application guide shows the key features of

SG6105A and illustrates how to design in an ATX

switching mode power supply (SMPS). SG6105A is

suitable for half-bridge, push-pull topology and

incorporates with a four-channel supervisor, including

5V-standby. The PWM section of SG6105A comprises

a built-in 65kHz oscillator and high-immunity circuits,

which protect the system from noise interference and

provide more noise margins for improper PCB layout.

SG6105A has OVP and UVP for 12V, 5V, and 3.3V.

NVP is used for negative voltage protection, such as –

12V and/or –5V. The UVAC (AC fails detection) is applied

to detect AC line condition. Two built-in internal

precision TL431 shunt regulators can be used for 3.3V

or 5V auxiliary standby power.

AC Fails Detection

Through a resistor divider, UVAC is connected to the

secondary power transformer for detecting the AC line

condition. Once the voltage of UVAC is lower than 0.7V

for a period of time, such as 200µs, the PG (power-

good) signal is pulled low to indicate an AC line power-

down condition. The voltage amplitude of the PWM

switching signal in the secondary power transformer is

proportional to the AC line voltage. Adjust the ratio of

resistor divider to decide the threshold of power-down

warning. A small capacitor is connected from UVAC to

ground for filtering the switching noise.

VS

VO

VDD

R1

R2

OP2

OPP

C1 R3

Figure 18. OPP Protection Circuit

Negative Voltage Protection (NVP)

The NVP provides an under-voltage protection (UVP)

for negative voltage outputs. An under-voltage

represents the phenomenal of the overload condition in

negative voltage output. For example, the –12V output

may drop to –10V during the overload situation. A

resistor determining the threshold of the protection is

connected from pin NVP to the negative voltage output.

Via this resistor, NVP outputs a 64µA constant current

to the negative voltage output. When the NVP voltage is

over 2.1V and the situation kept for longer than 7ms,

SG6105A locks the power outputs off:

VNVP=64µA × (R1 + R2) – V-12V

(1)

The power outputs are locked off when VNVP ≥ 2.1V.

R1

0.7V

R2

C1

UVAC

Figure 17. AC Detection Circuit

Over-Power Protection (OPP)

The OPP is used for detecting over-power and/or short-

circuit conditions. When OPPS voltage (p.4, Note-1) is

higher than 2.1V and this situation exists for longer than

7ms, SG6105A pulls the PG low and locks off the power

outputs.

Figure 19. NVP Protection Circuit

© 2008 Fairchild Semiconductor Corporation

SG6105A • Rev. 1.0.2

12

www.fairchildsemi.com

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