SG6105A Fairchild Semiconductor, SG6105A Datasheet - Page 12

• Manufacturer Part Number: SG6105A
• Description: SG6105A controller is designed for switching-mode power supplies for desktop PCs
• Manufacturer: Fairchild Semiconductor

© 2008 Fairchild Semiconductor Corporation
SG6105A • Rev. 1.0.2
Application Information
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
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 U
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, U
secondary power transformer for detecting the AC line
condition. Once the voltage of U
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 U
ground for filtering the switching noise.
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.
for
Figure 17. AC Detection Circuit
half-bridge,
R
R
VAC
1
2
V
S
(AC fails detection) is applied
push-pull
VAC
C
VAC
1
0.7V
is connected to the
is lower than 0.7V
topology
UVAC
VAC
V
and
O
to
12
Negative Voltage Protection (NVP)
The NVP provides an under-voltage protection (UVP)
for
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:
V
The power outputs are locked off when VNVP
NVP
=64µA × (R1 + R2) – V-12V
negative
Figure 18. OPP Protection Circuit
Figure 19. NVP Protection Circuit
voltage
V
DD
R
outputs.
1
C
R
1
2
An
R
3
OP1
OP2
OPP
under-voltage
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≥
2.1V.
(1)