IR3621 datasheet(11/29 Pages) IRF

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IR3621&(PbF)

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APPLICATION INFORMATION

Design Example:

The following example is a typical application for the

IR3621, the schematic is Figure19 on page18.

Output Voltage Programming

Output voltage is programmed by the reference voltage

and an external voltage divider. The Fb1 pin is the invert-

ing input of the error amplifier, which is referenced to the

voltage on the non-inverting pin of error amplifier. For this

application, this pin (VP2) is connected to the reference

voltage (VREF). The output voltage is defined by using the

following equation:

When an external resistor divider is connected to the

output as shown in Figure 11.

Figure 11 - Typical application of the IR3621 for pro-

gramming the output voltage.

Equation (4) can be rewritten as:

Will result to:

VOUT(2.5V) = 2.5V

VREF = 0.8V

R9= 2.15K, R5= 1K

If the high value feedback resistors are used, the input

bias current of the Fb pin could cause a slight increase

in output voltage. The output voltage can be set more

accurately by using low value, precision resistors.

For a start-up time of 4ms for both output, the soft-start

capacitor will be 0.1

µF. Connect two 0.1

µFceramic

capacitors from SS1 pin and SS2 pin to GND.

Supply VcH1 and VcH2

To drive the high side MOSFET, it is necessary to sup-

ply a gate voltage at least 4V greater than the bus volt-

age. This is achieved by using a charge pump configu-

ration as shown in Figure 12. This method is simple

and inexpensive. The operation of the circuit is as fol-

lows: when the lower MOSFET is turned on, the ca-

pacitor (C1) charges up to VOUT3, through the diode

(D1). The bus voltage will be added to this voltage when

upper MOSFET turns on in next cycle, and providing

supply voltage (VcH1) through diode (D2). VcH1 is ap-

proximately:

Capacitor in the range of 0.1

µF is generally adequate

for most applications. The diode must be a fast recov-

ery device to minimize the amount of charge fed back

from the charge pump capacitor into VOUT3. The diodes

need to be able to block the full power rail voltage, which

is seen when the high side MOSFET is switched on.

For low voltage application, Schottky diodes can be

used to minimize forward drop across the diodes at

start up.

Figure 12 - Charge pump circuit.

R6 = R5

- 1

VOUT

(

)

IR3621

VOUT

VREF

VP2

VCH1

≅ VOUT3 + VBUS - (VD1 + VD2)

VIN = 12V

VOUT(2.5V) = 2.5V @ 10A

VOUT(1.8V) = 1.8V @ 10A

∆VOUT = Output voltage ripple

≅ 3% of VOUT

FS = 400kHz

IR3621

VcH1

HDrv

Regulator

VOUT3

VBUS

Css

≅ 28×tSTART (µF)

---(5)

Where

tSTART is the desired start-up time (ms)

VOUT = VP2

× 1 +

---(4)

VP2 = VREF = 0.8V

(

)

VOUT(1.8V) = 1.8V

VREF = 0.8

R7= 1.24K, R8 = 1K

Soft-Start Programming

The soft-start timing can be programmed by selecting

the soft-start capacitance value. The start-up time of

the converter can be calculated by using:

IR3621 Datasheet(PDF) 11 Page - International Rectifier

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