ICIN_RMS = IOUT x D(1 - D)

ICIN_RMS = IOUT

§

¨

©

D 1 - D +

'IL

2

12 x IOUT

2

FB

VIN

VIN

LM1770

VOUT

HG

LG

GND

COUT

Q1

CIN

Q2

RFB1

RFB2

RSNS

L1

LM1770

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SNVS403C – SEPTEMBER 2005 – REVISED APRIL 2013

This circuit uses an additional resistor in series with the inductor to add ripple at the output. It is placed in this

location and used in combination with the feed-forward capacitor (CFF) to provide ripple to the feedback pin,

without adding ripple or a DC offset to the output. The benefit of using a ceramic capacitor is still obtained with

this technique. Because the addition of the resistor results in power loss, this circuit implementation is only

recommended for low currents (2A and below). The power loss and rating of the resistor should be taken into

account when selecting this component.

This circuit implementation utilizing the feed-forward capacitor begins to experience limitations when the output

voltage is small. Previously the circuit relied on the CFF for all the ripple at the feedback node by assuming that

the resistor divider was negligible. As VOUT decreases this can not be assumed. The resistor divider contributes a

larger amount of ripple which is problematic as it is also out of phase. Therefore the resistor location should be

changed to be in series with the output capacitor. This can be viewed as adding an effective ESR to the output

capacitor.

FEED-FORWARD CAPACITOR

The feed-forward capacitor is used across the top feedback resistor to provide a lower impedance path for the

high frequency ripple without degrading the DC accuracy. Typically the value for this capacitor should be small

enough to prevent load transient errors because of the discharging time, but large enough to prevent attenuation

of the ripple voltage. In general a small ceramic capacitor in the range of 1nF to 10nF is sufficient.

If CFF is used then it can be assumed that the ripple voltage seen at the feedback pin is the same as the ripple

voltage at the output. The attenuation factor H no longer needs to be used. However, in these conditions, it is

recommended to have a minimum of 20mV ripple at the feedback pin. The use of a CFF capacitor is

recommended as it improves the regulation and stability of the design. However, its benefit is diminished as VOUT

starts approaching VREF , therefore it is not needed in this situation.

INPUT CAPACITOR

The dominating factor that usually sets an input capacitors’ size is the current handling ability. This is usually

determined by the package size and ESR of the capacitor. If these two criteria are met then there usually should

be enough capacitance to prevent impedance interactions with the source. In general it is recommended to use a

ceramic capacitor for the input as they provide a low impedance and small footprint. One important note is to use

a good dielectric for the ceramic capacitor such as X5R or X7R. These provide better over temperature

performance and also minimize the DC voltage derating that occurs on Y5V capacitors. To calculate the input

capacitor RMS current, the equation below can be used:

(15)

which can be approximated by,

(16)

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