Datasheet

MAX8660/MAX8660A/MAX8660B/MAX8661
High-Efficiency, Low-I
Q
, PMICs with Dynamic
Voltage Management for Mobile Applications
39
inductance values than L
IDEAL
can be used to obtain
higher output current, but typically require physically
larger inductor size. Refer to the MAX8660 EV kit data
sheet for specific inductor recommendations.
Input Capacitor Selection
The input capacitor in a step-down DC-DC converter
reduces current peaks drawn from the battery or other
input power source and reduces switching noise in the
controller. The impedance of the input capacitor at the
switching frequency should be less than that of the
input source so that high-frequency switching currents
do not pass through the input source.
The input capacitor must meet the input-ripple-current
requirement imposed by the step-down converter.
Ceramic capacitors are preferred due to their resilience
to power-up surge currents. Choose the input capacitor
so that the temperature rise due to input ripple current
does not exceed approximately 10°C. For a step-down
DC-DC converter, the maximum input ripple current is
1/2 of the output. This maximum input ripple current
occurs when the step-down converter operates at 50%
duty factor (V
IN
= 2 x V
OUT
).
Refer to the MAX8660 EV kit data sheet for specific
input capacitor recommendations.
Output Capacitor Selection
The step-down DC-DC converter output capacitor
keeps output ripple small and ensures control-loop sta-
bility. The output capacitor must also have low imped-
ance at the switching frequency. Ceramic, polymer,
and tantalum capacitors are suitable, with ceramic
exhibiting the lowest ESR and lowest high-frequency
impedance.
Output ripple due to capacitance (neglecting ESR) is
approximately:
Additional ripple due to capacitor ESR is:
V
RIPPLE(ESR)
= I
L(PEAK)
x ESR
Refer to the MAX8660 EV kit data sheet for specific out-
put capacitor recommendations.
Step-Down Converter Output Current
The maximum output current for each step-down con-
verter is listed in the Electrical Characteristics table.
This current is guaranteed by correlation to the p-chan-
nel current-limit threshold, p-channel on-resistance, n-
channel on-resistance, oscillator frequency, input
voltage range, and output voltage range. The maximum
output current in the Electrical Characteristics table is
for the components shown in Figure 3 over then entire
specified range of input and output voltage. For differ-
ent components or voltage ranges, the maximum out-
put current changes. Typically, inductors with a higher
inductance increase the maximum output current, but
they are physically larger and decrease the output volt-
age response time due to a load transient.
Calculate the maximum output current for a particular
application using following the two-step process (see
Figure 12). Use the maximum expected value for input
voltage (V
IN
). Using the minimum expected values for
the p-channel current-limit (I
LIM
), oscillator frequency
(f), and inductance (L) provides the absolute worst-
case maximum output current (i.e., the lowest value).
V
I
xf xC
RIPPLE
L PEAK
OSC OUT
=
()
2π
TO FIND THE ABSOLUTE WORST CASE MAXIMUM OUTPU-TT CURRENT FOR REG WITH V V TO V
V
IN
OUT
33242
12
=
=
..,
.VVL H ANDR m
D
VI
L
OUT OUTTAR
,. %,=± =
=
+
1 2 30 50µΩ
(()
()
..(.
RR
VI RR
VA
NL
IN OUTTAR N P
+
+−
=
++12 16 008Ω
0005
42 16 008 012
034
.)
..(. .)
.
Ω
ΩΩVA
I
I
OUTMAX
+−
=
=
LLIM
OUT
NL
VD
fL
RR
D
fL
A
−
−
××
++
−
××
=
−
()
()
.
1
2
1
1
2
185
112 1 034
2 1910 1210 07
1
6
6
.( .)
(. ) (. .)
V
Hz H
−
×× ×× ×
−
+++
−
×× ××
−
(. . )
.
(. ) (.
008 005
1034
2 1910 1210
6
ΩΩ
Hz
66
07
156
H
A
×
=
.)
.
Figure 12. Step-Down Converter Maximum Output Current Example