LM25085AMYSLASHNOPB datasheet(20/31 Pages) TI1

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R4 =

VRIP(min)

IOR(min)

LM25085A

PGATE

GND

Cff

COUT

RFB2

RFB1

VOUT

GND

Cff =

3 x tON(max)

(RFB1//RFB2)

R4 =

25 mV

IOR(min)

LM25085A

SNVS601B – JANUARY 2009 – REVISED MARCH 2013

www.ti.com

Alternate Output Ripple Configurations

The minimum ripple configuration, using C1, C2 and R3, used in the example circuit Figure 28, results in a low

ripple amplitude at VOUT determined mainly by the characteristics of the output capacitor and the ripple current in

L1. This configuration allows multiple ceramic capacitors to be used for VOUT if the output voltage is provided to

several places on the PC board. However, if a slightly higher level of ripple at VOUT is acceptable in the

application, and distributed capacitance is not used, the ripple required for the FB comparator pin can be

generated with fewer external components using the circuits shown below.

Reduced ripple configuration: In Figure 33, R3, C1 and C2 are removed (compared to Figure 28). A low value

resistor (R4) is added in series with COUT, and a capacitor (Cff) is added across RFB2. Ripple is generated at

VOUT by the inductor’s ripple current flowing through R4, and that ripple voltage is passed to the FB pin via Cff.

The ripple at VOUT can be set as low as 25 mVp-p since it is not attenuated by RFB2 and RFB1. The minimum

value for R4 is calculated from:

where IOR(min) is the minimum ripple current, which occurs at minimum input voltage. The minimum value for Cff

is determined from:

where tON(max) is the maximum on-time, which occurs at minimum VIN. The next larger standard value capacitor

should be used for Cff.

Figure 33. Reduced Ripple Configuration

b) Lowest cost configuration: This configuration, shown in Figure 34, is the same as Figure 33 except Cff is

removed. Since the ripple voltage at VOUT is attenuated by RFB2 and RFB1, the minimum ripple required at VOUT is

equal to:

VRIP(min) = 25 mV x (RFB2 + RFB1)/RFB1

The minimum value for R4 is calculated from:

where IOR(min) is the minimum ripple current, which occurs at minimum input voltage.

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