SC2441ITSTRT datasheet(25/35 Pages) SEMTECH

25 / 35 page

 2005 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC2441

Given the input voltage V

in, the output voltage Vo3 and

the load current I

o3, the following circuit parameters are

to be determined.

Inductor Selection

For a specified inductor current ripple (peak-to-peak)

the inductor value is

)

(

−

If V

in = 3.3V, Vo3 = 5V and Io3 = 1A withδ

3 = 0.3 and fs3 =

1MHz, then, L

3 = 2.5µH.

For operation above 1MHz, Ferrite core material should

be used for lower core losses and better circuit efficiency.

The peak inductor current is

(1+0.5*

3)Io3.

The saturation current rating of the selected inductor

should be at least 1.2~1.5 times of the calculated peak

current value. In the example above, the peak inductor

current is 1.74A.

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 1.59V. Since the SC2441 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. A small (< 1uF) ceramic bypass

capacitor can be soldered close to the PVIN pin for high

frequency filtering.

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 SC2441 is designed to use multi-layer

ceramic capacitor as the sole output capacitor.

Maximum Output Current

Figure 4 shows that the boost switch current is sensed

with an internal sense resistor Rs. With a current limit

threshold voltage of 14mV, the maximum output current

)

(

)

(

max

−

The boost switch current limit is internally set at 1.7A.

Voltage Feedback

With the internal reference voltage V

ref3=1.25V, the voltage

feedback gain is

ref

The values of the feedback resistors can be determined

from the equation in “Setting the Output Voltage” section.

Loop Compensation

A simple small signal model for current-mode boost

converter in continuous-conduction mode is shown in

Figure 17.

Figure 17. Small signal model of Boost converter.

Applications Information (Cont.)









−

)

(

)

(

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