SC2620 datasheet(18/26 Pages) SEMTECH

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SC2620

POWER MANAGEMENT

Applications Information

If C

1 is ceramic, then its ESR zero can be neglected as it

situates well beyond half the switching frequency. The low

frequency gain of the control-to-output transfer function is

simply the product of power stage transconductance and

the equivalent load resistance (Figure 12).

The transfer functions of the feedback network and the

error amplifier are:

() 



















OUT

(9)

and

()

() (

)

COMP

⋅

≈

(10)

provided that

and

In Equation (10), C

5 forms a low frequency pole p1 with the

output resistance R

O of the error amplifier and C6 forms a

high frequency pole p

3 with R5:

Ω

−

280

tan

Transcondu

Gain

Loop

Open

Amplifier

−

In addition C

5 and R5 form a zero with angular frequency:

−

The

output-to-control

transfer

function

OUT

COMP

OUT

COMP

⋅

is also shown in Figure 12. Its mid-

band gain (between z

1 and p3) is









. The

overall loop gain T(s) is the product of the control-to-output

and the output-to-control transfer functions. To simplify

)

(

ω Bode plot, the feedback network is assumed to be

resistive. If the overall loop gain is to cross 0dB at one

tenth of the switching frequency (

) at –20dB/

decade, then its mid-band gain (between z

1 and p2) will

OUT

POWER

STAGE

GMP = 8

Ω

REFERENCE

VOLTAGE

C11

ESR

ROUT

VOUT

VIN

I OUT

-1

GMA =

280

µΩ

-1

COMP

1.6M

Ω

Figure 11. Simplified Control Loop Equivalent Circuit

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