ML4895
DESIGN CONSIDERATIONS (cont.)
INPUT CAPACITOR SELECTION
The choice of the input capacitor is based on its ripple
current and voltage ratings rather than its capacitance
value. The input capacitor should be a low ESR type and
located as close to the source of the P-MOS switch as
possible. The input capacitor’s ripple current is
determined by the load current and input voltage, with
the worst case condition occurring at V
IN
= 2 x V
OUT
:
I
RMS(C
IN
)
≈
(I
SENSE(MAX)
+
1
∆
I
L( MAX)
)
×
2
V
OUT
×
V
IN
−
V
OUT
V
IN
OUTPUT CAPACITOR SELECTION
The output capacitors determine the loop stability and the
output ripple voltage. Use only low ESR capacitors
intended for switching power supply applications, such as
AVX TPS, Sprague 593D, Sanyo OS-CON, or Nichicon PL
series. To ensure stability, the minimum capacitance value
is given by:
b
g
C
OUT
≥
T
ON( MAX)
4.3
×
V
OUT
R
SENSE
(14)
The maximum ESR value can be estimated using:
ESR
≤
∆
V
OUT
∆
I
L( MAX)
(15)
The capacitor’s voltage rating is based on the maximum
input voltage, V
IN(MAX)
. Capacitor manufacturers typically
recommend derating the capacitor voltage rating by 20%
to 50% for aluminum electrolytic types and 50% to 70%
for tantalum types.
In high current applications it may necessary to add a
small 0.1µF ceramic capacitor to bypass V
IN
(pin 8) of the
ML4895.
The selected capacitor must meet both the capacitance
and ESR requirements. As a final check, make sure the
output capacitor can handle the ripple current, I
RMS
:
I
RMS
≈
∆
I
L( MAX)
12
(16)
OUTPUT VOLTAGE
The output of the buck converter is adjustable and can be
set to any voltage between 2.5V and 4V by connecting a
resistor divider to the feedback pin as shown in Figure 1.
The resistor values R1 and R2 can be calculated using the
following equation:
V
OUT
=
250V
×
.
R1
+
R2
R2
(17)
The value of R2 should be 475kΩ or less to minimize bias
current errors.
It is important to note that the accuracy of these resistors
directly affects the accuracy of the output. Use precision
resistors and set the nominal voltage approximately 1% to
2% high in order to make up for the load regulation. This
offset results in the best overall output accuracy over line
and load.
8
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
