IR3629 データシートの表示（PDF） - International Rectifier

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IR3629

HIGH FREQUENCY SYNCHRONOUS PWM BUCK CONTROLLER WITH POWER GOOD OUTPUT

International Rectifier 

Application Information

Design Example:

The following example is a typical application for

IR3629. The application circuit is shown on page

18.

Vin=12V,(13.2V,max )

Vo=1.5V

Io=12 A

ΔVo≤30mV(output voltage ripple)

Fs=600 kHz

Output Voltage Programming

Output voltage is programmed by reference

voltage and external voltage divider. The Fb pin

is the inverting input of the error amplifier, which

is internally referenced to 0.6V. The divider is

ratioed to provide 0.6V at the Fb pin when the

output is at its desired value. The output voltage

is defined by using the following equation:

Vo

= Vref

∗ ⎜⎜⎝⎛1 +

R8

R9

⎟⎟⎠⎞

--( 4 )

When an external resistor divider is connected to

the output as shown in figure 10.

IRIR33662249

Fb

VOUT

R8

R9

Fig. 10: Typical application of the IR3629 for

programming the output voltage

Equation (4) can be rewritten as:

R9

=

R8

∗

⎜⎜⎝⎛

V

Vref

−V O ref

⎟⎟⎠⎞

--(5 )

For the calculated values of R8 and R9 see

feedback compensation section.

11/26/2007

IR3629/IR3629A MPbF

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:

CSS ≅ 20μA *Tstart --(1)

Where Tstart is the desired start-up time (ms).

For a start-up time of 10ms, the soft-start

capacitor will be 0.2uF. Choose a ceramic

capacitor at 0.22uF.

Vc supply for single input voltage

To drive the high side switch, it is necessary to

supply a gate voltage at least 4V greater than the

bus voltage. This is achieved by using a charge

pump configuration as shown in figure 11. This

method is simple and inexpensive. The operation

of the circuit is as follows: when the lower

MOSFET is turned on, the capacitor (C1) is

pulled down to ground and charges, up to VBUS

value, through the diode (D1). The bus voltage

will be added to this voltage when the upper

MOSFET turns on in the next cycle, and

providing supply voltage (Vc) through diode (D2).

Vc is approximately:

( ) VC ≅ 2 ∗Vbus − VD1 +VD2 --(6 )

A capacitors in the range of 0.1uF is generally

adequate for most applications. Fast recovery

diodes must be used to minimize the amount of

charge fed back from the charge pump capacitor

into VBUS. 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 applications, schottky diodes can be

used to minimize forward drop.

VBUS

IRIR3632692/249A

D1

C3

D2

Vc

C2

VBUS

C1

Q1

L

HDrv

Q2

Fig. 11: Charge pump circuit to generate

Vc voltage

11

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