IR3621F International Rectifier, IR3621F Datasheet - Page 7

• 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

FUNCTIONAL DESCRIPTION
Introduction
The IR3621 is a versatile device for high performance
buck converters. It consists of two synchronous buck
controllers which can be operated either in two indepen-
dent mode or in current share mode.
The timing of the IC is provided by an internal oscillator
circuit which generates two out-of-phase clock that can
be programmed up to 500kHz per phase.
Supply Voltage
Vcc is the supply voltage for internal controller. The op-
erating range is from 5.5V to 14.5V. It also is fed to the
internal LDO. When Vcc is below under-voltage thresh-
old, all MOSFET drivers will be turned off.
Internal Regulator
The regulator powers directly from Vcc and generates a
regulated voltage (Typ. 6.2V@40mA). The output is pro-
tected for short circuit. This voltage can be used for charge
pump circuitry as shown in Figure12.
Input Supplies UnderVoltage LockOut
The IR3621 UVLO block monitors three input voltages
(Vcc, VcH1 and VcH2) to ensure reliable start up. The
MOSFET driver output turn off when any of the supply
voltages drops below set thresholds. Normal operation
resumes once the supply voltages rise above the set
values.
Mode Selection
The SS2 pin is used for mode selection. In current share
mode this pin should be floating and in dual output mode
a soft start capacitor must be connected from this pin to
ground to program the start time for the second output.
Independent Mode
In this mode the IR3621 provides control to two indepen-
dent output power supplies with either common or differ-
ent input voltages. The output voltage of each individual
channel is set and controlled by the output of the error
amplifier, which is the amplified error signal from the
sensed output voltage and the reference voltage. The
error amplifier output voltage is compared to the ramp
signal thus generating fixed frequency pulses of variable
duty-cycle, which are applied to the FET drivers, Fig-
ure19 shows a typical schematic for such application.
Currnt Share Mode
This feature allows to connect both outputs together to
increase current handling capability of the converter to
support a common load. The current sharing can be done
either using external resistors or sensing the DCR of
inductors (see Figure 4).
www.irf.com
In this mode, one control loop acts as a master and sets
the output voltage as a regular Voltage Mode Buck con-
troller and the other control loop acts as a slave and
monitors the current information for current sharing. The
voltage drops across the current sense resistors (or DCR
of inductors) are measured and their difference is ampli-
fied by the slave error amplifier and compared with the
ramp signal to generate the PWM pulses to match the
output current. In this mode the SS2 pin should be float-
ing.
In the diagram, L1 and L2 are the output inductors. R
and R
R1 and capacitor C1 are used to sense the average in-
ductor current. The voltage across the capacitors C1
and C2 represent the average current flowing into resis-
tance R
should be equal or at most three times larger than the
time constant L
Figure 5 - 30A Current Sharing using Inductor sensing
PWM Comp2
PWM Comp1
R1×C1=(1~3)×
Figure 4 - Loss-less inductive current sensing
L2
L1
are inherent inductor resistances. The resistor
and R
L2
1
Master E/A
/R
IR3621
. The time constant of the RC network
and current sharing.
Slave E/A
L1
.
R
L1
L1
(5A/Div)
0.8V
Comp
Fb1
FB2
VP2
IR3621 & (PbF)
---(1)
R1
R2
L1
L2
C2
C1
R
R
L1
L2
V
OUT
7
L1