MAX15053_V01 datasheet(14/21 Pages) MAXIM

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ESL. Estimate the output-voltage ripple due to the output

capacitance, ESR, and ESL as follows:

OUT

ESR_COUT

OUT



∆

×−



××

 

Forceramiccapacitors,ESRcontributionisnegligible:

ESR_OUT

OUT

8 f

××

Fortantalumorelectrolyticcapacitors,ESRcontribution

is dominant:

ESR_OUT

OUT

8 f

××

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 capacitors

for low ESR and low ESL at the switching frequency of

the converter. The ripple voltage due to ESL is negligible

when using ceramic capacitors.

Load-transient response also depends on the selected

output capacitance. During a load transient, the output

instantly changes by ESR x DILOAD. Before the control-

ler 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 volt-

age back to the predetermined value.

Use higher COUT values for applications that require light

load operation or transition between heavy load and light

load, triggering skip mode, causing output undershooting

or overshooting. When applying the load, limit the output

undershoot by sizing COUT according to the following

formula:

LOAD

OUT

x V

∆

≅

∆

where DILOAD is the total load change, fCO is the regula-

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

and DVOUT is the desired output undershooting. When

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

Output Ripple

section to properly size COUT.

Skip Mode Frequency and Output Ripple

In skip mode, the switching frequency (fSKIP) and output

ripple voltage (VOUT-RIPPLE)showninFigure2arecal-

culated as follows:

tON is a fixed time (300ns, typ); the peak inductor current

reached is:

OUT

SKIP LIMIT

−

Figure 2. Skip Mode Waveform

VOUT

ISKIP-LIMIT

tON

ILOAD

VOUT-RIPPLE

tOFF1

tOFF2 = n × tCK

MAX15053

High-Efficiency, 2A, Current-Mode Synchronous,

Step-Down Switching Regulator

www.maximintegrated.com

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