Rainbow Electronics MAX15058 User Manual
Page 14
High-Efficiency, 3A, Current-Mode
Synchronous, Step-Down Switching Regulator
MAX15058
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
∆
=
Ч −
Ч
+
Ч
Ч
Ч
For ceramic capacitors, ESR contribution is negligible:
ESR_OUT
SW
OUT
1
R
8 f
C
<<
Ч
Ч
For tantalum or electrolytic capacitors, ESR contribution
is dominant:
ESR_OUT
SW
OUT
1
R
8 f
C
>>
Ч
Ч
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
instantly changes by ESR x DI
LOAD
. Before the controller
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.
Use higher C
OUT
values for applications that require
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
the output undershoot by sizing C
OUT
according to the
following formula:
LOAD
OUT
CO
OUT
I
C
3f
x V
∆
≅
∆
where DI
LOAD
is the total load change, f
CO
is the regula-
tor unity-gain bandwidth (or zero crossover frequency),
and DV
OUT
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 C
OUT
.
Skip Mode Frequency and Output Ripple
In skip mode, the switching frequency (f
SKIP
) and output
ripple voltage (V
OUT-RIPPLE
) shown in Figure 2 are cal-
culated as follows:
t
ON
is a fixed time (300ns, typ); the peak inductor current
reached is:
IN
OUT
SKIP LIMIT
ON
V
V
I
t
L
−
−
=
×
Figure 2. Skip Mode Waveform
I
L
V
OUT
I
SKIP-LIMIT
t
ON
I
LOAD
V
OUT-RIPPLE
t
OFF1
t
OFF2
= n × t
CK