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Date: 5/25/04

SP6650 High Efficiency 600mA Synchronous Buck Regulator

© Copyright 2004 Sipex Corporation

The additional Rf/Cf network used in Figure 16

generates an artificial ramp from the LX pin

voltage and superimposes it to the feedback pin.

As a result, the internal loop comparator doesn’t

have to rely on output ripple to run PFM. Now

low ESR output capacitors, such as ceramic

capacitors, can be used, and the output ripple can

be reduced by two to three times. For the best

result, size the Cf and Rf values so the network

would introduce 10 to 30mV ripples to the FB

RB

100k

SP6650

U1

10

9

8

7

6

1

2

3

4

5

L1 22µH

47µF

R2

160k

R3

100k

Cf

33nF

BLON

SHDN

LX

PGND

GND

FB

®

®

Rf

4.99k

3.3V/600mA

Vin 2.7-6.5 VDC

1.0µF

10.0

47µF

ADDITIONAL APPLICATION CIRCUITS

Figure 16. Additional Application Circuit with Low Output Ripple

pin. Oversized ripple would compromise the

load regulation and also cause oscillation during

load transient. Load transient response and out-

put ripples from Figure 16 circuit are shown in

Figure 17 and Figure 18, respectively. The added

ripple voltage can be calculated from

∆V =

RfCf