Test Circuit Open Loop Laboratory Test Fixture
V
REF
R
T
2N2222
100kΩ
4.7kΩ
1kΩ
Error Amp
Adjust
4.7kΩ
5kΩ
A
COMP
V
REF
0.1µF
V
CC
V
FB
V
CC
0.1µF
1kΩ
1W
Sense
Adjust
Sense
V
OUT
V
OUT
OSC
Gnd
Gnd
C
T
Circuit Description
Undervoltage Lockout
During Undervoltage Lockout (Figure 1), the output driv-
er is biased to sink minor amounts of current. The output
should be shunted to ground with a resistor to prevent
activating the power switch with extraneous leakage cur-
rents.
PWM Waveform
To generate the PWM waveform, the control voltage from
the error amplifier is compared to a current sense signal
which represents the peak output inductor current (Figure
2). An increase in V
CC
causes the inductor current slope to
increase, thus reducing the duty cycle. This is an inherent
feed-forward characteristic of current mode control, since
the control voltage does not have to change during
changes of input supply voltage.
When the power supply sees a sudden large output cur-
rent increase, the control voltage will increase allowing
the duty cycle to momentarily increase. Since the duty
cycle tends to exceed the maximum allowed to prevent
transformer saturation in some power supplies, the inter-
nal oscillator waveform provides the maximum duty cycle
clamp as programmed by the selection of OSC compo-
nents.
V
CC
ON/OFF Command
to reset of IC
V
ON
= 8.4V
V
OFF
= 7.6V
I
CC
<15mA
<1mA
V
ON
V
OFF
V
CC
Figure 1: Startup voltage for the CS52845.
4
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
