AS2316D datasheet(8/10 Pages) ETC

8 / 10 page

AS23xx

Secondary Side Housekeeping Circuit

ASTEC Semiconductor

Theory of Operation

the outputs of the PSU. Usually, just one or the

other output is used depending on the PSU’s cost

and system definition. Both methods are in-

tended to protect the customer’s system, and the

customer, as the first priority.

1.2 Undervoltage Faults: POK and FAULT

An undervoltage condition is sometimes not con-

sidered a catastrophic or dangerous condition,

but always one which the customer should be

warned about. The POK signal is a logic line to

the customer’s system that is specified in most

PC type power supply systems. The AS23xx will

pull the POK signal low when a UV fault is

detected. A UV fault may or may not require the

system to shut down, so an undervoltage blank-

ing pin is provided (UVB). Grounding this pin will

prevent UV faults from propogating to the FAULT

pin. CBD does not react to UV faults.

1.3 Input Undervoltage: AC and HYST

In addition, there is a special undervoltage detec-

tion input for sensing the input voltage to the

power supply, designated as the AC pin. This pin

will cause the POK pin to go low if there is

insufficient voltage to run the PSU outputs. Since

power supplies must maintain high voltage isola-

tion between the primary and secondary sides of

the system, the AC pin is usually tied to a divided

down and filtered representation of the second-

ary side switching waveform. Hysteresis for this

function, to provide immunity from line ripple, is

configured by the PSU designer and is imple-

mented with the HYST pin, which is an open

collector output of the AC comparator.

Section 2 - PSU Start-up Sequences

2.0 System Start-up Sequence

When the power supply starts up, the AS23xx

must not erroneously report a FAULT. In addi-

tion, most PC type power supply specifications

The AS23xx performs housekeeping functions

for power supplies, especially switching power

supplies for personal computers. The chip re-

sides on the secondary side of the power supply

(PSU), and it performs three primary functions:

1) monitors the output voltages and reports

faults

2) sequences the start-up of the PSU

3) sequences the shutdown of the PSU

Section 1 - Output Voltages and Faults

1.0 Output Voltage Monitoring

The AS23xx monitors the standard voltage out-

puts for PC type power supplies. It has inputs for

+12 V, +5 V, +3.3 V, -5 V and -12 V. These inputs

are tied directly to the outputs of the PSU, and

therefore do not require external dividers to set

the error thresholds. These pins are monitored

for both overvoltage (OV) and undervoltage (UV)

conditions. The spec’s for these thresholds are

listed in the data sheet.

1.1 Overvoltage Faults: FAULT and CBD

An overvoltage condition in a power supply is

considered to be a catastrophic and dangerous

condition which must result in a safe, complete

and near-instantaneous shutdown of the sys-

tem. Overvoltages most often result from a

break in the system feedback and control cir-

cuitry or from a short between outputs. When the

AS23xx detects an overvoltage, the fault is

latched internally, and the FAULT and CBD pins

go high. The FAULT pin is an open collector

NPN output which is intended to drive an

optocoupler LED for feedback to the primary

side controller of the PSU. The CBD pin is an

NPN Darlington output which is intended to drive

an SCR crowbar circuit which will short circuit

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