HA17431 Series

REJ03D0678-0300 Rev.3.00 Apr 03, 2007

Page 13 of 19

Application Hints

(cont.)

No.

Application Example

Description

3

Single power supply inverting

comparator circuit

K

Vout

GND

Vin

GND

A

REF

Rin

This is an inverting type comparator with an input threshold voltage

of approximately 2.5 V. Rin is the REF pin protection resistance,

with a value of several k

Ω to several tens of kΩ.

mA when Vout is low.

Condition

C1

C2

Vin

Less then 2.5 V

2.5 V or more

Vout

IC

OFF

ON

4

AC amplifier circuit

GND

K

Vout

Vin

REF

A

Cin

Cf

Gain G =

(DC gain)

Cutoff frequency fc =

1

2

π Cf (R

input is cut by capacitance Cin, so that the REF pin is driven by the

potential when there is no input, but if the input level is low and

To change the frequency characteristic, Cf should be connected as

indicated by the dotted line.

5

Switching power supply error

amplification circuit

+

–

V

+

V

–

LED

Cf

Secondary

side GND

(Note)

Note: LED

R3

R4

: Light emitting diode in photocoupler

: Bypass resistor to feed IK(>Imin)

when LED current vanishes

: LED protection resistance

This circuit performs control on the secondary side of a transformer,

and is often used with a switching power supply that employs a

photocoupler for offlining.

The output voltage (between V+ and V–) is given by the following

formula:

Vout

≅ 2.5 V ×

In this circuit, the gain with respect to the Vout error is as follows:

G =

×

HA17431 open

loop gain

× photocoupler

total gain

As stated earlier, the HA17431 open-loop gain is 50 to 60 dB.