AN8353UB Datasheet, PDF (3/4 Pages) Panasonic Semiconductor – High Efficiency Car Dashboard Dimmer IC

s Application Circuit
• Bipolar Transister Output
• MOS FET Output
AN8353UB
1 2345678 9
180Ω (3W)
RT
0.1µF
CT
0.1µF
AN8353UB
1 2345678 9
RT
0.1µF
CT
0.1µF
0.056µF
±
180kΩ VR
+
22V
33µF
20kΩ
10kΩ
1/fOSC=1.705RTCT
s Supplementary Description
• System Operational Principle
The following describes the operational principle of the
system using the AN8353UB.
As shown in the block diagram in Fig. 1, a battery voltage
is divided by the VR and input to the input Pin3 in accord-
ance with rotation amount. The voltage at the output Pin1 is
controlled by the AN8353UB so that the duty of the ON/OFF
period of the external output transistor will be proportional to
the input voltage, thus controlling a current flowing to the
lamps of the dashboard, etc. to adjust their brightness. Since
the output transistors are saturated at ON time and no current
flows at OFF time, power consumption is low.
The PWM method is used to control the output transistors.
This method, as shown in Fig. 2 I/O Characteristic Chart
(III), generates the triangular wave V6 as a reference signal
0.1µF
C1
4
Control
3
voltage
converting
circuit
PWM
compa-
rator
Over
voltage
protection
9
10kΩ
1
Tranguian
waveform
generator
+
–
2
6
5
78
RT
0.1µF
CT
C2
STB
Fig. 1 AN8353UB Block Diagram
Battery
±
0.056µF
±
180kΩ VR
+
22V
33µF
10kΩ
2kΩ
20kΩ
1/fOSC=1.705RTCT
to generate pulses and input them to one end (Pin6) of the
PWM comparator. The triangular wave frequency fOSC can be
freely set from 50Hz to 10kHz, depending on the resistance
value RT connected between the square wave output Pin5 and
triangular wave output Pin6, and capacity value CT connected
between the triangular wave output Pin6 and ground Pin2.
The approximating expression for the then PWM frequency
fOSC is ;
1/fOSC=1.705CTRT ······················································(1)
For your refence, Fig. 3 shows the relations among CT, RT,
and oscillation frequency fOSC. The voltage V4, whose voltage
level is made matching the amplitude of the triangular wave
by the control voltage converter, is given to the other input
(Pin4) of the PWM comparator. That is, in Fig. 2 (II), the
input voltage V3 is linearly converted into V4 by the control
voltage converter so that the amplitude of the triangular wave
will be about 20% to 80% of the input voltage input range
(axis of abscissas in Fig. 2 (II)).
Then, a current is supplied from the output Pin1 to turn on
the output transistors during the period (TON) when the
inverted input voltage is larger than the triangular wave. (Fig.
2 (II), (IV)) To the contrary, while the converted input
voltage is smaller than the triangular wave, no current is
supplied from the output Pin1 and the output transistors are
turned off. The output pulse duty is expressed as follows.
Duty=TON · fOSC ······················································(2)
For the duty control characteristic of the output pulses to the
input voltage V3, the duty of the output pulses is controlled
from 0% to 100% at high-precision linearity while the “input
voltage V3/supply voltage V9” is between about