IR3638SPBF データシートの表示（PDF） - International Rectifier

部品番号

コンポーネント説明

メーカー

IR3638SPBF

HIGH FREQUENCY SYNCHRONOUS PWM BUCK CONTROLLER FOR TRACKING APPLIACTIONS

International Rectifier 

Application Information

Design Example:

The following is a design of typical application for

IR3638. The application circuit is shown on page

14.

Vin = 5V

Vcc = Vc = 12V

Vp = 1V

Vo = 1.2V

Io = 6A

ΔVo ≤ 30mV

Output Voltage Programming

Output voltage is programmed by Vp voltage and

external voltage divider. The Fb pin is the

inverting input of the error amplifier, which is

externally referenced to Vp. The divider is set to

provide a voltage equal to Vp (in this case 1V)

when the output is at its desired value. The

output voltage is defined by using the following

equation:

Vo

= Vp

∗ ⎜⎜⎝⎛1 +

R1

R2

⎟⎟⎠⎞

--( 2 )

When an external resistor divider is connected to

the output as shown in figure 8.

IR3638

Fb

VOUT

R1

R2

Fig. 8: Typical application of the IR3638 for

programming the output voltage

Equation (2) can be rewritten as:

R2

=

R1

∗

⎜⎜⎝⎛

V

Vp

O−Vp

⎟⎟⎠⎞

--(3 )

Select R1 =1kOhm, this will result in R2=5kOhm.

Select the closest Standard value of 4.99kOhm.

IR3638SPbF

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 ≅ 22μA *Tstart --(1)

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

For a start-up time of 5ms, the soft-start

capacitor will be 0.1uF. Choose a ceramic

capacitor at 0.1uF.

Input Capacitor Selection

The input filter capacitor should be selected

based on how much ripple the supply can

tolerate on the DC input line. The ripple current

generated during the on time of upper MOSFET

should be provided by input capacitor. The RMS

value of this ripple is expressed by:

IRMS = Io ∗ D ∗(1 − D) --(4 )

Where:

D is the Duty Cycle

D = Vo

Vin

IRMS is the RMS value of the input capacitor

current.

Io is the output current.

For Io=6A and D=0.24, the IRMS=2.6A.

Ceramic capacitors are recommended due to

their peak current capabilities, they also feature

low ESR and ESL at higher frequency which

enhance better efficiency,

Use two 10uF, 16V ceramic capacitor from

Panasonic.

Inductor Selection

The inductor is selected based on output power,

operating frequency and efficiency requirements.

Low inductor value causes large ripple current,

resulting in the smaller size, faster response to a

load transient but poor efficiency and high output

noise. Generally, the selection of inductor value

can be reduced to desired maximum ripple

current in the inductor (Δi ) . The optimum point is

usually found between 20% and 50% ripple of

the output current.

3/19/07

9

Share Link: