SC2441AEVB datasheet(22/37 Pages) SEMTECH

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POWER MANAGEMENT

SC2441A

Assuming that the efficiency of the boost converter is

and the boost converter is running in CCM with duty ratio

D. The peak inductor current is

The saturation current rating of the selected inductor

should be at least 1.2 times of the calculated peak

current value.

Step-up Converter Capacitor Selection

Input capacitor::::: The input capacitance should be large

such that the input transients due to both the step-up

and the step-down converters do not trip the UVLO

threshold 1.71V. Since the SC2441A controls a 2-phase

low input voltage step-down converter, the input

capacitance is sized to handle the input ripple current of

the buck converter. This is usually sufficient for the

auxiliary boost converter because the input current in a

boost converter is continuous.

Output capacitor: Unlike buck converter, pulse current

is delivered to the output of a boost converter. To reduce

the output ripple voltage, low ESR capacitors should be

used. The output capacitor should also be able handle

the output ripple current. The SC2441A is designed to

use multi-layer ceramic capacitor as the sole output

capacitor.

Maximum Output Current of the Step-up Converter

Figure 3 shows that the boost switch current is sensed

with an internal sense resistor Rs and it is internally

limited at 0.6A. So the maximum output current can be

given as (

η is the efficiency of the step-up section):

max

o3,

0.6A

)

(

SW3

PGND2

SC2441A

PGND1

Small

Loop

Figure 9. Step-Up Section Layout Illustration

As shown in Figure 9, to minimize the switching noise

generated by the step-up converter, the loop formed by

1, C1, SW3 and PGND2 should be as small as possible.

And the PGND2 pin should be tied to PGND1 at one

spot close to the PGND1 pin.

Step-up Converter Inductor Selection

For a specified inductor current ripple ratio

3 (peak-to-

peak current ripple v.s. actual input current I

IN), the

inductor value is

)

(

Typically, select

3 <2 for a Continous Conduction Mode

(CCM) operation.

If V

in = 3.3V, Vo3 = 5V and Io3 = 100mA with δ

3 = 1.6 and

s3 = 1MHz, then, L1 = 4.7µH.

Applications Information

L1PEAK

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