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.)

sensed-current. In the SC2441 the compensation ramp For the rated output current Io, the first-order gain k is

is made duty-ratio dependent. The compensation ramp determined as

is approximately

Iramp = De1.1D * 4.8µA.

k

=

∆Io

∆Vc

.

The incremental slope of such current ramp is then

Se = (1+ 1.1D)e1.1Dfs * 4.8µA.

Furthermore the transfer function from the voltage error

amplifier output vc to the converter output vo can be

derived from Figure 15.

Once the inner current loop is stabilized, the output voltage

is then regulated with the outer voltage feedback loop. An

equivalent circuit of current-mode Buck converter is shown

in Figure 15.

Vo (s)

Vc (s)

:=

Gvc (s)

=

kRo

1+

1+

s

sz1

s

sp1

,

where the single dominant-pole is

sp1

=

(Ro

+

1

Roesr

)Co

and the zero associated with the output capacitor ESR is

s z1

=

1

R oesr C o

.

k

The dominant-pole changes with the converter output load.

The controller transfer function (from the converter output

vo to the voltage error amplifier output vc) is

C(s)

=

gmh

s(C2 + C3 )

1+

1+

s

sz2

s

sp2

,

Figure 15. A simple model of current-mode buck converter

The voltage transconductance error amplifier (in the

SC2441)

has

a

g

m

of

400µA/V.

C,

2

C

3

and

R

2

of

the

compensation network are to be determined for stable

operation with optimized load transient response.

The feedback gain h and the resistor values are determined

using the equations given in the “Setting the Output

Voltage” section with

h

=

0.5

Vo

.

where

and

sz2

=

1

R2C2

sp2

=

R2

1

C2C3

C2 + C3

.

The loop transfer function is then

T(s)=Gvc(s)C(s).

 2005 Semtech Corp.

23

www.semtech.com

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