HA17431H Series
Rev.0, Sep. 2001, page 7 of 15
Application Hints
(cont.)
No.
Application Example
Description
3
Single power supply inverting
comparator circuit
K
Vout
GND
Vin
GND
A
REF
V
CC
R
L
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
to several tens of k
.
R
is the load resistance, selected so that the cathode
current I
K
≥
0.2 mA when Vout is low.
Condition
C1
C2
2.5 V or more
Approx. 2 V (V
OL
)
Vin
Less then 2.5 V
Vout
V
CC
(V
OH
)
IC
OFF
ON
4
AC amplifier circuit
GND
K
Vout
Vin
REF
A
V
CC
R
L
R
1
R
2
R
3
Cin
Cf
Gain G =
(DC gain)
R
1
R
2
// R
3
Cutoff frequency fc =
1
2
π
Cf (R
1
// R
2
// R
3
)
This is an AC amplifier with voltage gain G = –R
/
(R
//R
). The input is cut by capacitance Cin, so that
the REF pin is driven by the AC input signal, centered
on 2.5 V
DC
.
R
also functions as a resistance that determines the
DC cathode potential when there is no input, but if the
input level is low and there is no risk of Vout clipping to
V
CC
, this can be omitted.
To change the frequency characteristic, Cf should be
connected as indicated by the dotted line.
5
Switching power supply error
amplification circuit
+
–
V+
V
–
R
1
R
2
R
3
R
4
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
×
(R
1
+ R
2
)
R
2
In this circuit, the gain with respect to the Vout error is
as follows:
G =
×
R
2
(R
1
+ R
2
)
HA17431H open
loop gain
×
photocoupler
total gain
As stated earlier, the HA17431H open-loop gain is 50
to 60 dB.