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Rev.1.1 Nov.05, 2019

AX3984

(1).Choose an appropriate inductor and output capacitance based on the allowed output voltage

ripple and load transient.

(2).Placing FC as high as possible can respond quickly to the load transient. Considering the output

capacitor’s tolerances and temperature effects, typically place FC approximately 1/10 of FS for the

multi-layer ceramic output capacitor (X5R, X7R). However, if the type of the output capacitor is the

aluminum electrolytic or that largely variable with the temperature, place FC approximately 1/20 of

FS .

(3).Set the compensation RC to zero to cancel the RLOAD COUT pole.

REF

CS

M

OUT

OUT

C

C

V

G

G

V

C

F

2

R















C

LOAD

OUT

C

R

R

C

C





GM: error amp transconductance

GCS: current sense transconductance

(4).Determine CP if required.

If ZESR (zero occurs by output capacitor ESR) is less than FC, it should be cancelled with a pole

set by capacitor CP connected between CC to GND.

C

ESR

OUT

P

R

R

C

C





Application Information

Input Capacitor Selection

It is necessary for the input capacitor to sustain the ripple current produced during the period of

“on” state of the upper MOSFET, so a low ESR is required to minimize the loss. The RMS value of

this ripple can be obtained by the following:

)

D

1

(

D

I

RMS

I

OUT

IN







Where D is the duty cycle, lINRMS is the input RMS current, and IOUT is the load current. The

equation reaches its maximum value with D = 0.5. The loss of the input capacitor can be

calculated by the following equation:

2

IN

CIN

CIN

RMS

I

ESR

P





Where PCIN is the power loss of the input capacitor and ESRCIN is the effective series resistance of