IR3651SPBF International Rectifier, IR3651SPBF Datasheet - Page 10

• Manufacturer Part Number: IR3651SPBF
• Description: IC CTLR SYNC PWM BUCK 14-SOIC
• Manufacturer: International Rectifier

Specifications of IR3651SPBF

• Pwm Type: Voltage Mode
• Number Of Outputs: 1
• Frequency - Max: 460kHz
• Duty Cycle: 80%
• Voltage - Supply: 4.5 V ~ 13.2 V
• Buck: Yes
• Boost: No
• Flyback: No
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: No
• Operating Temperature: 0°C ~ 125°C
• Package / Case: 14-SOIC (3.9mm Width), 14-SOL
• Frequency-max: 460kHz
• Package: 14-Pin SOIC (NB)
• Circuit: Sync PWM Controller
• Vcc (min): 4.5
• Vcc (max): 13.2
• Iout (a): 25
• Switch Freq (khz): programmable to 400kHz
• Pbf: PbF Option Available
• For Use With: IRDC3651 - KIT REF DESIGN SYNCH BUCK REG
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

For a given start-up time, the soft-start capacitor
(nF) can be estimated as:
Soft-Start
The IR3651 has programmable soft-start to
control the output voltage rise and limit the inrush
current during start-up.
To
sequence initiates when Vcc rises above its
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 (64uA)
into the Fb pin and generates a voltage about
1.6V (64ux25K) across the negative input of error
amplifier (see figure 4).
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 1.25V.
The output of error amplifier will start increasing
and generating the first PWM signal. As the soft-
start capacitor voltage continues to go 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 is
reached the steady state and the injected current
is zero.
Figure 5 shows the theoretical operational
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
estimate by:
12/07/2010
ensure
C
when
SS
20
≅
20
μA
correct
μ
∗
A
soft-start
T
C
*
start
ss
T
start
=
(
2
start-up,
ms
V
−
)
1
V
capacitor
--(
) 1
the
soft-start
voltage
Voltage at negative input
SS/SD
Comp
Voltage at Fb pin
1.25V
Output of UVLO
Current flowing
Fig. 5: Theoretical operation waveforms
Fig. 4: Soft-Start circuit for IR3651
Fb
of Error Amp
POR
into Fb pin
Soft-Start
20uA
Voltage
POR
during soft-start
25K
25K
64uA
≅ 1.6V
0V
0V
IR3651SPBF
3V
64uA
Error Amp
1.25V
1.25V
0uA
≅ 1V
≅ 2V
3V
10