Datasheet

MAX16962
4A, 2.2MHz, Synchronous Step-Down
DC-DC Converter
10Maxim Integrated
Input Capacitor
The input filter capacitor reduces peak currents drawn
from the power source and reduces noise and voltage
ripple on the input caused by the circuit’s switching.
The input capacitor RMS current requirement (I
RMS
) is
defined by the following equation:
OUT PV1 OUT
RMS LOAD(MAX)
PV1
V(V V)
II
V
−
=
I
RMS
has a maximum value when the input voltage
equals twice the output voltage (V
PV1
= 2V
OUT
), so
I
RMS(MAX)
= I
LOAD(MAX)
/2.
Choose an input capacitor that exhibits less than +10NC
self-heating temperature rise at the RMS input current for
optimal long-term reliability.
The input-voltage ripple is composed of DV
Q
(caused
by the capacitor discharge) and DV
ESR
(caused by the
ESR of the capacitor). Use low-ESR ceramic capacitors
with high-ripple current capability at the input. Assume
the contribution from the ESR and capacitor discharge
equal to 50%. Calculate the input capacitance and ESR
required for a specified input voltage ripple using the fol-
lowing equations:
ESR
IN
L
OUT
V
ESR
I
I
2
∆
=
∆
+
where:
PV1 OUT OUT
L
PV1 SW
(V V ) V
I
VfL
−×
∆=
××
and:
OUT
IN
Q SW
I D(1 D)
C
Vf
×−
=
∆×
and
OUT
PV1
V
D
V
=
where I
OUT
is the maximum output current, and D is the
duty cycle.
It is strongly recommended that a 4.7FF small footprint
be placed close to PV1 and PV2 and a minimum of 100nF
small footprint be placed close to PV. Using a small
footprint such as 0805 or smaller helps to reduce the total
parasitic inductance.
Output Capacitor
The minimum capacitor required depends on output
voltage, maximum device current capability, and the
error amplifier voltage gain. Use the following formula to
determine the required output capacitor value:
×
×
= =
π× × π× ×
EAMP
OUT(MIN)
CO OUT OUT
4A G
4A 40
C
2 f V 2 250kHz V
where f
CO
, the target crossover frequency, is 250kHz,
and G
EAMP
, the error-amplifier voltage gain, is 40V/V.
Table 2 lists some of the inductor values for 4A out-
put current and several output voltages. For proper
functionality, ceramic capacitors must be used. Make
sure that the self-resonance of the ceramic capacitors at
the converter output is above 1MHz to avoid converter
instability.
Table 2. Output Capacitor Value vs. V
OUT_
Setting
VOUT (V)
3.3 2.5 1.5 0.8
COUT (µF),
I
MAX
= 4.0A
≥ 31 ≥ 41 ≥ 68 ≥ 127