SC2441ITSTRT View Datasheet(PDF) - Semtech Corporation

Part Name

Description

MFG CO.

SC2441ITSTRT

Very Low Input Voltage 2-Phase Synchronous Step-down Controllers with Step-up Converter

Semtech Corporation 

SC2441

POWER MANAGEMENT

Applications Information (Cont.)

form of foldback current limiting. The following

calculations estimate the average inductor current when

the converter output is shorted to the ground.

a) The time taken to discharge the capacitor from 3.25V

to 0.47V is

t ssf

=

C32

(3.25 − 0.47)V

1.4µA

.

resistance may not be available as a standard value

resistor. As a result, there will be a set error in the

converter output voltage. This non-random error is

caused by the feedback voltage divider ratio. It cannot

be corrected by the feedback loop.

The following table lists a few standard resistor

combinations for realizing some commonly used output

voltages.

If C = 0.1µF, t will then be 200ms.

32

ssf

b) The soft-start time from 0.47V to 3.25V is

t ssr

= C 32

(3.25 − 0.47

2.3µA

)V

.

If C32 = 0.1µF, tssr will then be 121ms. Note that during

soft-start, the converter only starts switching when the

voltage at SS/EN exceeds 1.3V.

c) The effective start-up time is

Vo (V)

0.6 0.9 1.2 1.5 1.8 2.5 3.3

(1-h)/h

0.2 0.8 1.4 2 2.6 4

5.6

Ro1 (Ohm) 200 806 1.4K 2K 2.61K 4.02K 5.62K

Ro2 (Ohm) 1K 1K 1K 1K 1K 1K 1K

Only the voltages in boldface can be precisely set with

standard 1% resistors.

t sso

=

C32

(3.25 − 1.3)V

2.3µA

.

The average inductor current is then

ILeff

= ILMcp

t sso

tssf + tssr

.

The input bias current of the error amplifier also causes

an error in the output voltage. The inverting input bias

currents of error amplifiers 1 and 2 are –160nA and –

260nA respectively. Since the non-inverting input is biased

to 0.5V, the percentage error in the second output voltage

will be –100% ·

keep this error

(0.26µA) · Ro1Ro2

below 0.2%, Ro2 <

/[0.5

4kΩ.

·

(Ro1+Ro2)].

To

Loop Compensation

I

Leff

=

0.27I

LMcp

and

is

independent

of

the

soft

start

capacitance. The converter will not overheat in hiccup.

Setting the Output Voltage

The non-inverting inputs of the error amplifiers are

internally biased to 0.5V voltage reference. A simple

voltage divider (Ro1 at top and Ro2 at bottom) sets the

converter output voltage. R can be expressed as a

o2

function of the voltage feedback gain h=0.5/V and R

o

o1

Ro2

=

1

h

−

h

R

o1.

The SC2441 uses current-mode control for both step-down

channels. Current-mode control is a dual-loop control

system in which the inductor peak current is loosely

controlled by the inner current-loop. The higher gain outer

loop regulates the output voltage. Since the current loop

makes the inductor appear as a current source, the complex

high-Q poles of the output LC networks is split into a

dominant pole determined by the output capacitor and

the load resistance and a high frequency pole. This pole-

splitting property of current-mode control greatly simplifies

loop compensation.

Once either R or R is chosen, the other can be calculated

o1

o2

for the desired output voltage V . Since the number of

o

standard resistance values is limited, the calculated

The inner current-loop is unstable (sub-harmonic oscillation)

unless the inductor current up-slope is steeper than the

inductor current down-slope. For stable operation above

50% duty-cycle, a compensation ramp is added to the

 2005 Semtech Corp.

22

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