cs52845 ON Semiconductor, cs52845 Datasheet - Page 4

• Manufacturer Part Number: cs52845
• Description: Current Mode Pwm Control Circuit
• Manufacturer: ON Semiconductor

Figure 1: Startup voltage for the CS52845.
V
CC
Error Amp
Adjust
4.7k
4.7k
<15mA
1k
<1mA
I
CC
Adjust
100k
Sense
2N2222
5k
V
V
ON
OFF
V
ON
= 8.4V
= 7.6V
V
Test Circuit Open Loop Laboratory Test Fixture
OFF
R
ON/OFF Command
T
to reset of IC
COMP
V
Sense
OSC
FB
Circuit Description
V
CC
C
T
4
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.
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
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
V
Gnd
V
REF
OUT
CC
Undervoltage Lockout
CC
0.1 F
0.1 F
PWM Waveform
causes the inductor current slope to
A
1k
1W
V
V
V
Gnd
REF
CC
OUT