SC2441AEVB(2006) View Datasheet(PDF) - Semtech Corporation

Part Name

Description

MFG CO.

SC2441AEVB
(Rev.:2006)

1.8V to 20V Input 2-Phase Synchronous Step-down Controllers with Step-up Converter

Semtech Corporation 

SC2441A

POWER MANAGEMENT

Applications Information

VIN

L1

27

SW3

SC2441A

Small

Loop

D1

24

PGND1

PGND2

26

C1

VCC

21

C2

R1

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

D , C , SW3 and PGND2 should be as small as possible.

11

And the PGND2 pin should be tied to PGND1 at one

spot close to the PGND1 pin.

Step-up Converter Inductor Selection

Assuming that the efficiency of the boost converter is η

and the boost converter is running in CCM with duty ratio

D. The peak inductor current is

IL1PEAK

=

Vo3 × Io3

Vin × h

+

Vin

L1

×D×T× 1

2

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.

For a specified inductor current ripple ratio δ3 (peak-to-

peak current ripple v.s. actual input current I ), the

IN

inductor value is

L1

=

Vin

fs3d3Io3

(1-

Vin )

Vo3

Vin

Vo3

.

Typically, select δ3 <2 for a Continous Conduction Mode

(CCM) operation.

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):

Io3,max

=

(0.6A

-

d3

2

)×

Vin ×h

Vo3

If

V

in

=

3.3V,

V

o3

=

5V

and

I

o3

=

100mA

with

δ3

=

1.6

and

f

s3

=

1MHz,

then,

L

1

=

4.7µH.

 2006 Semtech Corp.

22

www.semtech.com

Share Link: