User Manual
11/04/08
9
PD-60357
IR3802AMPbF
Soft-Start
The IR3802A has programmable soft-start to
control the output voltage rise and limit the inrush
current during start-up.
To ensure correct start-up, the soft-start
sequence initiates when Vcc and Vc rise above
their threshold and generate the Power On
Ready (POR) signal. The soft-start function
operates by sourcing current to charge an
external capacitor to about 3V.
Initially, the soft-start function clamps the output
of error amplifier by injecting a current (40uA)
into the Fb pin and generates a voltage about
0.96V (40ux24K) across the negative input of
error amplifier (see figure 5).
The magnitude of the injected current is inversely
proportional to the voltage at the soft-start pin. As
the soft-start voltage ramps up, the injected
current decreases linearly and so does the
voltage at negative input of error amplifier.
When the soft-start capacitor is around 1V, the
voltage at the positive input of the error amplifier
is approximately 0.6V.
The output of error amplifier will start increasing
and generating the first PWM signal. As the soft-
start capacitor voltage continues to rise up, the
current flowing into the Fb pin will keep
decreasing.
The feedback voltage increases linearly as the
soft start voltage ramps up. When soft-start
voltage is around 2V, the output voltage reaches
the steady state and the injected current is zero.
Figure 6 shows the theoretical operating
waveforms during soft-start.
The output voltage start-up time is the time
period when soft-start capacitor voltage
increases from 1V to 2V.
The start-up time will be dependent on the size of
the external soft-start capacitor and can be
estimated by:
Fig. 5: Soft-Start circuit for IR3802A
Fig. 6: Theoretical operation waveforms
during soft-start
20uA
40uA
POR
Error Amp
SS/SD
Fb
Comp
24K
0.6V
24K
3V
Soft-Start
Voltage
Voltage at negative input
of Error Amp
Voltage at Fb pin
Current flowing
into Fb pin
40uA
0uA
0V
0.6V
≅
0.96V
0.6V
0V
3V
≅
2V
≅
1V
Output of UVLO
POR
For a given start-up time, the soft-start capacitor
can be estimated as:
V1V2
C
T
μA20
ss
start
−=∗
)1 --(msTA20C
startSS
)(*
μ
≅