I
NTEGRATED
C
IRCUITS
D
IVISION
IX9915
R01 www.ixysic.com 7
2. Functional Description
IX9915 is the functional equivalent of a 4-terminal 431
type precision shunt regulator and a high voltage
Darlington transistor in the same package.
The typical application for IX9915 is shown in
Figure 5.
Figure 5 IX9915 Application Diagram
This is a simplified application circuit that shows how
the IX9915 can be used in an LED lamp control circuit.
The high voltage Darlington transistor will bleed the
current when V
LINE
is lower than the predetermined
voltage. The 4-terminal 431 type shunt regulator is
used to monitor V
LINE
voltage and control the
Darlington transistor bleeding the current (ON) or not
(OFF). When V
LINE
reaches the predetermined
voltage, the shunt regulator starts to regulate to drive
V
OC
going low, and turns off the Darlington transistor
to make sure this bleeder circuitry only burns a little
power at the higher V
LINE
voltage. Maximum bleeding
current I
H
can be controlled by properly choosing R
E
and V
REG
.
Regulation of V
REG
is made possible by applying a
scaled sample of its voltage to pin FB, the error
amplifier's non-inverting input. The error amplifier
compares this scaled voltage against an internal high
accuracy reference voltage and generates an output
current which in turn regulate V
REG
through the
resistor R
3
.
As V
REG
increases, the error amplifier's input voltage
V
FB
will also increase. Ramping of V
FB
beyond the
internal reference voltage causes the error amplifier to
sink more I
OC
, which in turn decreases V
REG
.
Likewise, a reduction of V
REG
results in a lessoning of
I
OC
causing V
REG
to increase.
2.1 Regulation Voltage
When connected as shown in the application circuit
above and properly configured, the IX9915 will
regulate V
REG
such that V
FB
is equal to V
REF
(1.299V).
To achieve this, the values of the voltage divider
resistors, R
1
and R
2
, must be set in the following
manner:
Because V
REG
regulation occurs when V
FB
=V
REF
any
change in bias current through R
2
at the desired
regulated voltage level will cause a regulation error. As
shown in the Electrical Characteristics table the error
amplifier input at pin FB has an input bias current (I
IB
)
specification that reduces the current into R
2
. (I
IB
is
+
_
+
_
OC
FB
V
CC
C
E
B
V
SS
I
OC
I
Q
R
2
R
1
R
E
I
1
I
H
V
REG
R
3
I
3
R
0
V
REF
+V
LINE
Rectifier
Dimmer Switch
AC Supply
-
R
1
R
2
------
V
REG
V
REF
-------------
1–=
