High-Efficiency, 2A, Current-Mode

Synchronous, Step-Down Switching Regulator

14 _____________________________________________________________________________________

capacitor’s ESL. Estimate the output-voltage ripple due

to the output capacitance, ESR, and ESL as follows:

OUT

OUT

OUT

ESR_COUT

SW

IN

SW

OUT

V

V

1

V

1

R

f

L

V

8 f

C









∆

=

× −

×

+









×

×

×



 



For ceramic capacitors, ESR contribution is negligible:

ESR_OUT

SW

OUT

1

R

8 f

C

<<

×

×

For tantalum or electrolytic capacitors, ESR contribution

is dominant:

ESR_OUT

SW

OUT

1

R

8 f

C

>>

×

×

Use these equations for initial output-capacitor selec-

tion. Determine final values by testing a prototype or an

evaluation circuit. A smaller ripple current results in less

output-voltage ripple. Since the inductor ripple current is

a factor of the inductor value, the output-voltage ripple

decreases with larger inductance. Use ceramic capaci-

tors for low ESR and low ESL at the switching frequency

of the converter. The ripple voltage due to ESL is negli-

gible when using ceramic capacitors.

Load-transient response also depends on the selected

output capacitance. During a load transient, the output

can respond, the output deviates further, depending on

the inductor and output capacitor values. After a short

time, the controller responds by regulating the output

voltage back to the predetermined value.

light load operation or transition between heavy load and

light load, triggering skip mode, causing output under-

shooting or overshooting. When applying the load, limit

following formula:

LOAD

OUT

CO

OUT

I

C

3f

x V

∆

≅

∆

tor unity-gain bandwidth (or zero crossover frequency),

removing the load and entering skip mode, the device

cannot control output overshooting, since it has no sink

current capability; see the Skip Mode Frequency and

Skip Mode Frequency and Output Ripple

culated as follows:

reached is:

IN

OUT

SKIP LIMIT

ON

V

V

I

t

L

−

−

=

×

Figure 2. Skip Mode Waveform