IR3623MPBF_1 IRF [International Rectifier], IR3623MPBF_1 Datasheet - Page 12

• Manufacturer Part Number: IR3623MPBF_1
• Description: HIGH FREQUENCY 2-PHASE, SINGLE OR DUAL OUTPUT SYNCHRONOUS STEP DOWN CONTROLLER WITH OUTPUT TRACKING AND SEQUENCING
• Manufacturer: IRF [International Rectifier]

Soft-Start (cont.)
During power up, the converter output starts at
zero and thus the voltage at Fb is about 0V. A
current (64uA) injects into the Fb pin and
generates a voltage about 1.6V (64ux25K)
across the negative input of error amplifier, see
figure 7.
The magnitude of this current is inversely
proportional to the voltage at soft-start pin. The
28uA current source starts to charge up the
external capacitor. In the mean time, the soft-
start voltage ramps up, the current flowing into
Fb pin starts to decrease linearly and so does the
voltage at negative input of error amplifier.
When the soft-start capacitor is around 1V, the
voltage at the negative input of the error amplifier
is approximately 0.8V.
As the soft-start capacitor voltage charges up,
the current flowing into the Fb pin keeps
decreasing.
The feedback voltage increases linearly as the
injecting current goes down. The injecting current
drops to zero when soft-start voltage is around
1.8V and the output voltage goes into steady
state. Figure 8 shows the theoretical operational
waveforms during soft-start.
The output 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. The start-
up time can be estimated by:
For a given start up time, the soft-start capacitor
(nF) can be estimated as:
For normal start up the Seq pin should be pulled
high (usually can be connected to Vout3).
www.irf.com
C
SS
≅
20
28
μA
(
μ
A
∗
0
T
* )
C
.
start
8
ss
T
(
V
=
start
1
)
.
8
(
V
ms
−
1
)
V
- -
- -
5 (
)
Voltage at negative input
SS1/SD1
SS2/SD2
Seq
Track
VP1
Fb1
VP2
Fb2
Fig. 8: Theoretical operation waveforms
Fig. 7: Soft-Start circuit for IR3623
Voltage at Fb pin
Current flowing
Output of POR
of Error Amp
into Fb pin
Soft-Start
Voltage
64uA
POR
64uA
POR
during soft-start
64uA
≅ 1.6V
0V
0V
E/A1
IR3623MPbF
E/A2
3V
3V
I
SS2 = 28uA
I
SS1 = 28uA
I
hiccup2 = 3uA
0uA
0.8V
I
0.8V
hiccup1 = 3uA
≅ 1.8V
OCP2
≅ 1V
3V
OCP1
12