SP6127 datasheet(8/12 Pages) EXAR

8 / 12 page

Jan28-08 RevE

SP6127 TSOT-6 PFET Buck Controller

 2007 Exar Corporation

Page 8

______________________________________________________ GENERAL OVERVIEW

Output Capacitor Selection

Select the output capacitor for voltage rating,

capacitance and Equivalent Series Resistance

(ESR). Nominally the voltage rating is selected to

be twice as large as the output voltage. Select

the capacitance to satisfy the specification for

output voltage overshoot/undershoot caused by

current step load. A steady-state output current

IOUT corresponds to inductor stored energy of ½ L

IOUT

2. A sudden decrease in IOUT forces the

energy surplus in L to be absorbed by

COUT. This

causes an overshoot in output voltage that is

corrected by the reduced duty cycle of the power

switch. Use the following equation to calculate

COUT:













−

Vout

Vos

Cout

Where:

L is the output inductance

is the step load high current

1 is the step load low current

Vos is output voltage including overshoot

VOUT is steady state output voltage

Output voltage undershoot calculation is more

complicated.

Test

results

for

SP6127

buck

circuits show that undershoot is approximately

equal to overshoot. Therefore the above equation

provides a satisfactory method for calculating

COUT.

Select ESR such that output voltage ripple (VRIP)

specification is met. There are two components to

VRIP: The first component arises from charge

transferred to and from COUT during each cycle.

The second component of VRIP is due to inductor

ripple

current

flowing

through

the

output

capacitor’s ESR. It can be calculated from:













Cout

ESR

Irip

Vrip

Where:

IRIP

is inductor ripple current

fs is switching frequency

COUT is output capacitor calculated above

Note that a smaller inductor results in a higher

IRIP, therefore requiring a larger COUT and/or

lower ESR in order to meet

VRIP.

Input Capacitor Selection

Select

the

input

capacitor

for

Voltage,

Capacitance, ripple current, ESR and ESL.

Voltage rating is nominally selected to be twice

the input voltage. The RMS value of the input

capacitor current, assuming a low inductor ripple

current (IRIP), can be calculated from:

(

)

Iout

Icin

−

In general, total input voltage ripple should be

kept below 1.5% of VIN (not to exceed 180mV).

Input voltage ripple has three components: ESR

and ESL cause a step voltage drop upon turn on

of the MOSFET. During on time, the capacitor

discharges linearly as it supplies IOUT-IIN. The

contribution to Input voltage ripple by each term

can be calculated from:

(

)

Vin

Cin

Vout

Vin

Vout

Iout

Cin

−

∆

(

)

Irip

Iout

ESR

−

∆

(

)

Trise

Irip

Iout

ESL

−

∆

Where TRISE is the rise time of current through

capacitor

Total input voltage ripple is sum of the above:

ESL

ESR

Cin

Tot

∆

In circuits where converter input voltage is

applied via a mechanical switch, excessive

ringing may be present at turn-on that may

interfere with smooth startup of the SP6127. The

addition of an inexpensive 100µF Aluminum

Electrolytic capacitor at the input will help reduce

ringing and restore a smooth startup.

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