IR3621F International Rectifier, IR3621F Datasheet - Page 8

• Manufacturer Part Number: IR3621F
• Description: IC PWM DUAL SYNC PREBIAS 28TSSOP
• Manufacturer: International Rectifier

Specifications of IR3621F

• Pwm Type: Voltage Mode
• Number Of Outputs: 2
• Frequency - Max: 345kHz
• Duty Cycle: 86.5%
• Voltage - Supply: 5.5 V ~ 14.5 V
• Buck: Yes
• Boost: No
• Flyback: No
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: No
• Operating Temperature: -40°C ~ 125°C
• Package / Case: 28-TSSOP
• Frequency-max: 345kHz
• Package: 28-Pin TSSOP
• Circuit: Dual Sync PWM Controller or 2Phase Single Output
• Vcc (min): 4.7
• Vcc (max): 16
• Vout (min): 0.8
• Vout (max): Vcc * 0.90
• Switch Freq (khz): programmable to 500kHz
• Pbf: PbF Option Available
• For Use With: IRDC3621 - BOARD EVAL DUAL SYNC BUCK CTRLR
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

IR3621 & (PbF)
Low Temperature Start-Up
The controller is capable of starting at -40 C ambient
temperature.
8
Dual Soft-Start
The IR3621 has programmable soft-start to control the
output voltage rise and limit the inrush current during
start-up. It provides a separate Soft-Start function for each
outputs. This will enable to sequence the outputs by
controlling the rise time of each output through selection
of different value soft-start capacitors. The soft-start pins
will be connected together for applications where, both
outputs are required to ramp-up at the same time.
To ensure correct start-up, the soft-start sequence ini-
tiates when the Vcc, VcH1 and VcH2 rise above their
threshold and generate the Power On Reset (POR) sig-
nal. Soft-start function operates by sourcing an internal
current to charge an external capacitor to about 3V. Ini-
tially, the soft-start function clamps the E/A’s output of
the PWM converter. During power up, the converter out-
put starts at zero and thus the voltage at Fb is about 0V.
A current (64µA) injects into the Fb pin and generates a
voltage about 1.6V (64µA×25K) across the negative
input of E/A and (see Figure6).
The magnitude of this current is inversely proportional to
the voltage at soft-start pin. The 28µA 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 E/A.
When the soft-start capacitor is around 1V, the current
flowing into the Fb pin is approximately 32µA. The volt-
age at the positive input of the E/A is approximately:
The E/A will start to operate and the output voltage starts
to increase. As the soft-start capacitor voltage contin-
ues to go up, the current flowing into the Fb pin will keep
decreasing. Because the voltage at pin of E/A is regu-
lated to reference voltage 0.8V, the voltage at the Fb is:
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 7 shows the theo-
retical operational waveforms during soft-start.
V
FB
= 0.8-(25K×Injected Current)
32µA×25K = 0.8V
www.irf.com
The output start-up time is the time period when soft-
start capacitor voltage increases from 1V to 1.8V. The
start-up time will be dependent on the size of the exter-
nal soft-start capacitor. The start-up time can be esti-
mated by:
Voltage at negative input
SS2 / SD
SS1 / SD
Comp2
Comp1
Figure 7 - Theoretical operational waveforms
Fb2
POR
V
Fb1
Voltage at Fb pin
P2
Current flowing
Figure 6 -Soft-start circuit for IR3621
Output of POR
of Error Amp
26
6
7
20
22
21
0.8V
8
into Fb pin
Soft-Start
28µA×T
Voltage
28uA
during soft-start.
64uA
1.6V
0V
0V
START
28uA
/C
SS
64uA
= 1.8V-1V
64uA
Max
Error Amp2
Error Amp1
0uA
0.8V
0.8V
1.8V
3V
1V