IR3621 datasheet(7/29 Pages) IRF

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IR3621&(PbF)

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FUNCTIONAL DESCRIPTION

Introduction

The IR3621 is a versatile device for high performance

Figure 4 - Loss-less inductive current sensing

and current sharing.

In the diagram, L1 and L2 are the output inductors. RL1

and RL2 are inherent inductor resistances. The resistor

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 RL1 and RL2. The time constant of the RC network

should be equal or at most three times larger than the

time constant L

1/RL1.

RL1

RL2

IR3621

Comp

0.8V

Fb1

VP2

FB2

Master E/A

Slave E/A

PWM Comp2

PWM Comp1

VOUT

RL1

×C1=(1~3)×

---(1)

Figure 5 - 30A Current Sharing using Inductor sensing

(5A/Div)

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).

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.

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