IR3621F International Rectifier, IR3621F Datasheet - Page 9

• 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

If using one soft start capacitor in dual configuration for a
precise power up the OCP needs to be set to latch mode.
Out-of-Phase Operation
The IR3621 drives its two output stages 180 out-of-phase.
In 2-phase configuration, the two inductor ripple currents
cancel each other and result in a reduction of the output
current ripple and yield a smaller output capacitor for the
same ripple voltage requirement.
In single input voltage applications, the input ripple current
reduces. This results in much smaller input capacitor's
RMS current and reduces the input capacitor quantity.
Over-Current Protection
The IR3621 can provide two different schemes for Over-
Current Protection (OCP). When the Hiccup pin is pulled
high, the OCP will operate in hiccup mode. In this mode,
during overload or short circuit, the outputs enter hiccup
mode and stay in that mode until the overload or short
circuit is removed. The converter will automatically re-
cover.
When the Hiccup pin is pulled low, the OCP scheme
will be changed to the latch up type, in this mode the
converter will be turned off during Overcurrent or short
circuit. The power needs to be recycled for normal
operation.
Each phase has its own independent OCP circuitry.
The OCP is performed by sensing current through the
R
external resistor (R
and the drain of low side MOSFET (Q2) which sets the
current limit set point.
For a given start up time, the soft-start capacitor can be
calculated by:
The soft-start is part of the Over Current Protection
scheme, during the overload or short circuit condition
the external soft start capacitors will be charged and
discharged in certain slope rate to achieve the hiccup
mode function.
DS(ON)
Figure 8 - 3uA current source for discharging soft
of low side MOSFET. As shown in Figure 9, an
start-capacitor during Hiccup mode
SS1 / SD
C
SET
SS
20
) is connected between OCSet pin
28µA×T
28uA
3uA
START
Hiccup
/0.8V
www.irf.com
The internal current source develops a voltage across
R
tor current flows through the Q2 and results a voltage
which is given by:
The critical inductor current can be calculated by set-
ting:
The value of R
circuit to ensure that the Over Current Protection
circuit activates as expected. The IR3621 current limit
is designed primarily as disaster preventing, "no blow
up" circuit, and is not useful as a precision current
regulator.
In two independent mode, the output of each channel
is protected independently which means if one output
is under overload or short circuit condition, the other
output will remain functional. The OCP set limit can be
programmed to different levels by using the external
resistors. This is valid for both hiccup mode and latch
up mode.
In 2-phase configuration, the OCP's output depends on
any one channel, which means as soon as one
channel goes to overload or short circuit condition the
output will enter either hiccup or latch-up, dependes on
status of Hiccup pin.
Pre-bias Startup
The IR3621 allows pre-bias startup without discharging
the output capacitors. The output starts in asynchro-
nous fashion and keeps the synchronous MOSFET off
until the first gate signal for control MOSFET is gener-
ated.
SET
Hiccup
Control
Figure 9 - Diagram of the over current sensing.
. When the low side switch is turned on, the induc-
V
V
I
SET
OCSET
OCSET
IR3621
= I
L(CRITICAL)
= I
= I
OCSET
OCSET
SET
I
OCSET
should be checked in an actual
×R
×R
=
SET
SET
R
R
OCSet
SET
DS(ON)
-R
- R
IR3621 & (PbF)
×I
DS(ON)
DS(ON)
OCSET
R
SET
×i
×I
L
L
= 0
Q1
Q2
---(2)
L1
---(3)
V
OUT
9