ML4861
INPUT CAPACITOR
Unless the input source is a very low impedance battery, it
will be necessary to decouple the input with a capacitor
with a value of between 47µF and 100µF. This provides
the benefits of preventing input ripple from affecting the
ML4861 control circuitry, and it also improves efficiency
by reducing I-squared R losses during the charge and
discharge cycles of the inductor. Again, a low ESR
capacitor (such as tantalum) is recommended.
REFERENCE CAPACITOR
Under some circumstances input ripple cannot be
reduced effectively. This occurs primarily in applications
where inductor currents are high, causing excess output
ripple due to “pulse grouping”, where the charge-
discharge pulses are not evenly spaced in time. In such
cases it may be necessary to decouple the reference pin
(V
REF
) with a small 10nF to 100nF ceramic capacitor. This
is particularly true if the ripple voltage at V
IN
is greater
than 100mV.
SETTING THE
RESET
THRESHOLD
To use the
RESET
comparator as an input voltage monitor,
it is necessary to use an external resistor divider tied to the
DETECT pin as shown in the block diagram. The resistor
values R
A
and R
B
can be calculated using the following
equation:
V
IN(MIN)
=
0.2
×
(R
A
+
R
B
)
R
B
LAYOUT
Good PC board layout practices will ensure the proper
operation of the ML4861. Important layout considerations
include:
• Use adequate ground and power traces or planes
• Keep components as close as possible to the ML4861
• Use short trace lengths from the inductor to the V
L
pin
and from the output capacitor to the V
OUT
pin
• Use a single point ground for the ML4861 ground pins,
and the input and output capacitors
A sample PC board layout is shown in Figure 6.
TOP LAYER
BOTTOM LAYER
(5)
The value of R
B
should be 100k�½ or less to minimize bias
current errors. R
A
is then found by rearranging the
equation:
V
R
A
=
R
B
×
IN(MIN)
−
1
0.2
(6)
Figure 6. Sample PC Board Layout.
8
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
