ML4861
INPUT CAPACITOR
LAYOUT
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.
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 pin
L
and from the output capacitor to the V
pin
OUT
REFERENCE CAPACITOR
• Use a single point ground for the ML4861 ground pins,
and the input and output capacitors
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
A sample PC board layout is shown in Figure 6.
(V ) with a small 10nF to 100nF ceramic capacitor. This
TOP LAYER
BOTTOM LAYER
REF
is particularly true if the ripple voltage at V is greater
IN
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 and R can be calculated using the following
A
B
equation:
(R +R )
A
B
V
= 0.2 ×
(5)
IN(MIN)
R
B
The value of R should be 100ký or less to minimize bias
B
current errors. R is then found by rearranging the
A
equation:
V
IN(MIN)
R
= R ×
−1
A
B
(6)
0.2
Figure 6. Sample PC Board Layout.
8
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
