ML4871
DESIGN CONSIDERATIONS
(Continued)
INPUT CAPACITOR
Due to the high input current drawn at startup and
possibly during operation, it is recommended to decouple
the input with a capacitor with a value of 47µF to 100µF.
This filtering prevents the input ripple from affecting the
ML4871 control circuitry, and also improves the efficiency
by reducing the I squared R losses during the charge cycle
of the inductor. Again, a low ESR capacitor (such as
tantalum) is recommended.
It is also recommended that low source impedance
batteries be used. Otherwise, the voltage drop across the
source impedance during high input current situations will
cause the ML4871 to fail to start-up or to operate
unreliably. In general, for two cell applications the source
impedance should be less than 200mW, which means that
small alkaline cells should be avoided.
LAYOUT
Good layout practices will ensure the proper operation of
the ML4871. Some layout guidelines follow:
• Use adequate ground and power traces or planes
• Keep components as close as possible to the ML4871
• Use short trace lengths from the inductor to the V
L1
and
V
L2
pins and from the output capacitor to the V
OUT
pin
• Use a single point ground for the ML4871 ground pin,
and the input and output capacitors
• Separate the ground for the converter circuitry from
the ground of the load circuitry and connect at a single
point
A sample layout is shown in Figure 8.
BATTERY MONITORING
The condition of the batteries can be monitored using the
DETECT pin. For primary batteries, the comparator can be
used to signal that the batteries will soon need to be
replaced. For rechargeable batteries, the comparator can
be used to signal the start of a charging cycle.
For input voltages greater than the minimum operating
voltage, the RESET pin can be set to go low at a specified
battery voltage by connecting a resistor divider across the
battery stack and to the DETECT pin of the ML4871 as
shown in Figure 7. The low battery trip voltage is
determined by first choosing a minimum battery voltage,
V
IN(MIN)
, and then calculating the values of R
A
and R
B
:
V
IN( MIN)
=
125
.
VIN
2
RA
DETECT
4
RB
VREF
+
–
COMP
7
RESET
1
R
A
+
R
B
6
FROM
START-UP
CIRCUITRY
R
B
(3)
Figure 7. Battery Monitoring Circuit
The values of R
A
and R
B
should be sufficiently large to
minimize the power dissipation in the divider. Also, use
care when selecting the low battery trip point. Too high a
trip voltage can lead to memory effects in the battery,
while too low a trip point can lead to reduced service life
or polarity reversal. Refer to the manufacturer’s data sheets
for more information on selecting and designing battery
systems.
Figure 8. Sample PC Board Layout
7
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
