IR3629 International Rectifier Corp., IR3629 Datasheet - Page 9

• Manufacturer Part Number: IR3629
• Description: High Frequency Synchronous Pwm Buck Controller With Power Good Output
• Manufacturer: International Rectifier Corp.

For a given start-up time, the soft-start capacitor
(nF) can be estimated as:
Soft-Start
The IR3629/29A has a programmable soft-start
to control the output voltage rise and limit the
inrush current during start-up.
To
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 (35uA)
into the Fb pin and generates a voltage about
0.84V (35ux24K) 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 the negative input of error amplifier.
When the soft-start capacitor voltage is around
1V, the voltage at the positive input of the error
amplifier is approximately 0.6V.
The output of the 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
waveforms during soft-start.
The output voltage start-up time is the time
period
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:
11/29/2007
ensure
C
6
when
SS
20
≅
shows
20
μA
correct
μ
∗
A
soft-start
T
C
*
start
ss
T
start
the
=
(
2
start-up,
ms
V
−
theoretical
)
1
V
capacitor
--(
) 1
the
operating
soft-start
voltage
Voltage at negative input
SS/SD
Comp
Voltage at Fb pin
Output of UVLO
Current flowing
0.6V
Fig. 6: Theoretical operation waveforms
Fig. 5: Soft-Start circuit for IR3629/29A
Fb
IR3629/IR3629A MPbF
of Error Amp
into Fb pin
POR
Soft-Start
20uA
Voltage
POR
≅ 0.96V
during soft-start
24K
24K
40uA
35uA
0V
0V
0.84V
3V
40uA
Error Amp
35uA
0uA
0.6V
0.6V
≅ 1V
≅ 2V
3V
9