Test Circuit Open Loop Laboratory Test Fixture
VREF
RT
2N2222
VCC
A
100kΩ
COMP
VFB
VREF
4.7kΩ
0.1µF
0.1µF
1kΩ
Error Amp
Adjust
VCC
1kΩ
1W
5kΩ
Sense
Adjust
4.7kΩ
VOUT
VOUT
Sense
OSC
Gnd
Gnd
CT
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.
VCC
ON/OFF Command
to reset of IC
CSX844 CSX845
PWM Waveform
VON
VOFF
16V
10V
8.4V
7.6V
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 VCC 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.
ICC
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.
<15mA
<1mA
VCC
VON VOFF
Figure 1: Startup voltage for CSX844 and CSX845.
4
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
