SC2441AEVB datasheet(21/37 Pages) SEMTECH

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 2006 Semtech Corp.

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POWER MANAGEMENT

SC2441A

switching frequency and very short duty cycle is not

practical. If the input voltage is 3.3V and the operating

frequency is 1MHz, the lowest output voltage will be 0.6V.

There will not be enough modulation headroom if the on

time is simply made equal to the minimum on time of the

SC2441A. For ease of control, we recommend that the

required pulse width be at least 1.5 times the minimum

on time.

Maximum Duty-cycle Consideration

The top MOSFET turns off for at least 200ns every cycle

regardless of the switching frequency. This places an

upper bound on the voltage conversion ratio at a given

switching frequency.

If the desired output voltage requires high operating duty-

cycle, then operating frequency will have to be lowered

to allow modulating headroom.

RC Filtering network for V

CC and VIN pins

A RC filtering network is recommended for the SC2441A

CC and VIN pin connections. As shown in Figure 1, R6 plus

8 and R2 plus C2 are the filtering networks for VCC pin

and V

IN pin respectively. The value of the R6 and R2 ranges

from 3.3

Ω to 5.11Ω. C

8 and C2 should be larger than

µF.

8 and C2 are the decoupling capacitors for the VCC pin

and V

IN pin. They should be placed as close as possible

to the pins of the SC2441A to achieve the best

decoupling performance. Due to the different

functionalities of the V

CC pin and VIN pin, C2 should be

placed between the V

IN pin and the signal ground of the

SC2441A. And C

8 should be placed between the VCC pin

and the power ground of the SC2441A. The

recommended connections for the V

CC pin and VIN pin are

illustrated in Figure 8.

GND

PGND1

SC2441A

Figure 8. RC Network Connections for V

IN and VCC pins

Step-Up Converter

The SC2441A features a step-up regulator and two step-

down controllers.

The boost section of the SC2441A comprises of pins 7,

8, 26 and 27. Pin 26 is the independent power ground

for the boost converter section, which should be

separated from the step-down section power ground pin

24 in layout to minimize the noise influence. The boost

section in SC2441A has an internal reference set at

1.25V. The output of the boost section can be

programmed with external resistors R

1 and R4 as shown

in Figure 1.

BOOST

1.25V

SC2441A utilizes a transconductance error amplifier for

the step-up controller and it can be compensated with

3, R5 and C5 as shown in Figure 1. The step-up controller

in the SC2441A employs cycle-by-cycle peak current limit

to protect the internal switching transistor. Current limit

threshold is typically 0.8A.

In the applications where only low input voltage is

available, the step-up converter in the SC2441A is very

useful for generating an auxiliary output to power the

gate drive of the step-down controllers.

Applications Information

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