IP1837 datasheet(21/40 Pages) IRF

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iP1837

Highly Integrated 35A Single‐input Voltage,

Synchronous Buck Regulator

March 5, 2012 | V1.26

97600

For our design, Rbot is selected to be 604 ohm.

This selection is based on a trade‐off between two

considerations:

1) The resistive divider should be as low impedance as

possible in order to have minimal impact on the

impedance seen at the Vosp and Vosm pins.

2) The resistive divider should have high enough

impedance so as to minimize the bleed current from

the output.

Hence, from Equation (9), Rtop = 1.21K.

In order to ensure that the Vosp and Vosm see balanced

impedances, it is advisable to use Rcomp such that:

(10)

Ω

402



bot

top

comp

SOFT‐START PROGRAMMING

The soft‐start timing can be programmed by selecting the

soft‐start capacitance value. From (1), for a desired start‐

up time of the converter, the soft start capacitor can be

calculated by using:

(11)

0.033

)

(





start



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

For a start‐up time of 3ms, the soft‐start capacitor will be

0.099μF. Choose a 0.1μF ceramic capacitor.

INPUT CAPACITOR SELECTION

The ripple current generated during the on time of the

upper MOSFET should be provided by the input capacitor.

The RMS value of this ripple is expressed by:

(12)

)

(

RMS







(13)



Where:

D is the Duty Cycle

IRMS is the RMS value of the input capacitor current.

Io is the output current.

For Io=35A and D = 0.15, the IRMS = 12.5A.

Ceramic capacitors are recommended due to their peak

current capabilities. They also feature low ESR and ESL at

higher frequency which enables better efficiency. For this

application, it is advisable to have 7x22uF 16V ceramic

capacitors ECJ‐3YX1C106K from Panasonic. In addition to

these, although not mandatory, a 1X330uF, 25V SMD

capacitor EEV‐FK1E331P may also be used as a bulk

capacitor and is recommended if the input power supply is

not located close to the converter.

INDUCTOR SELECTION

The inductor is selected based on output power, operating

frequency and efficiency requirements. A low inductor

value causes large ripple current, resulting in the smaller

size, and a faster response to a load transient but poor

efficiency and high output noise. Generally, the selection

of the inductor value can be reduced to the desired

maximum ripple current in the inductor. The optimum

point is usually found between 20% and 50% ripple of the

output current.

For the buck converter, the inductor value for the desired

operating ripple current can be determined using the

following relation:



(14)

;





















Where:

Vin = Maximum input voltage

V0 = Output Voltage

Δi = Inductor Ripple Current

Fs = Switching Frequency

Δt – Turn on time

D – Duty Cycle

If Δi ≈ 35%(Io), then the output inductor is calculated to be

0.21μH. Select L = 0.215 μH.

The PCDC1008‐R215EMO from Cyntec provides a compact

inductor suitable for this application.

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