Datasheet

ripple-current requirement (I
RMS
) imposed by the
switching currents as defined by the following equations:
for (N x D) ≤ 1:
for (N x D) > 1.
where N is the number of phases, D is the duty cycle,
and I
OUT_MAX
is the maximum output current.
Use the minimum input voltage for calculating the duty
cycle to obtain the worst-case input-capacitor RMS rip-
ple current. Low-ESR aluminum electrolytic, polymer, or
ceramic capacitors should be used to avoid large volt-
age transients at the input during a large step load
change at the output. The ripple-current specifications
provided by the manufacturer should be carefully
reviewed for temperature derating. Additional small-
value, low-ESL ceramic capacitors (1μF to 10μF with
proper voltage rating) can be used in parallel to reduce
any high-frequency ringing.
Output Capacitor
The minimum output capacitance, C
OUT(MIN)
, is
required to meet load-dump requirements. The worst-
case load dump is a sudden transition from full load
current (I
2
OUT_MAX
) to minimum load current
(I
2
OUT_MIN
). C
OUT(MIN)
is estimated based on energy
balance from:
where I
2
OUT_MAX
and I
2
OUT_MIN
are the initial and final
values of the load current during the worst-case load
dump, V
INIT
2
is the voltage prior to the load dump, V
FIN
is the steady-state voltage after the load dump, and
V
OV
is the allowed voltage overshoot above V
FIN
. The
term (V
FIN
+ V
OV
) represents the maximum transient
output voltage reached during the load dump. The
above equation is an approximation, and the output
capacitance value obtained serves as a good starting
point. The final value should be obtained from actual
measurements. For ceramic output capacitors, the out-
put capacitor requirement is determined mostly by load
dump requirements due to their low ESR and ESL. See
Figures 7 and 8.
Compensation Design
The MAX8686 uses an internal transconductance error
amplifier whose output compensates the control loop.
The external inductor, output capacitor, compensation
C
L
N
II
VV
OUT MIN
OUT MAX OUT MIN
FIN OV
()
__
≥
×−
()
+
(
22
))
−
2
2
V
INIT
IDI
ND
ND
RMS OUT MAX
=× ×
×
−
×
−
_
()
32
1
2
IDI
ND
RMS OUT MAX
=× ×
×
−
_
1
1
MAX8686
Single/Multiphase, Step-Down,
DC-DC Converter Delivers Up to 25A Per Phase
______________________________________________________________________________________ 19
MAX8686
R
C
C
C
COMP
C
F
Figure 6. Compensation Components
GAIN
(dB)
FREQUENCY (Hz)
fp
MOD
fz
MOD
f
C
CLOSED LOOP
ERROR
AMPLIFIER
0
VOLTAGE-
DIVIDER
POWER
MODULATOR
Figure 7. Simplified Gain Plot for the f
zMOD
> f
C
Case
GAIN
(dB)
FREQUENCY (Hz)
fp
MOD
fz
MOD
f
C
0
POWER
MODULATOR
CLOSED LOOP
ERROR
AMPLIFIER
VOLTAGE-
DIVIDER
Figure 8. Simplified Gain Plot for the f
zMOD
< f
C
Case