Datasheet

15
4009fd
LTC4009
LTC4009-1/LTC4009-2
applicaTions inForMaTion
Programming Charge Current
The formula for charge current is:
I
R
R
V
R
µA
CHRG
IN
SENSE PROG
=






•
.
– .
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11 67
The LTC4009 operates best with 3.01k input resistors,
although other resistors near this value can be used to
accommodate standard sense resistor values. Refer to
the subsequent discussion on inductor selection for other
considerations that come into play when selecting input
resistors R
IN
.
R
SENSE
should be chosen according to the following
equation:
R
mV
I
SENSE
MAX
=
100
where I
MAX
is the desired maximum charge current I
CHRG
.
The 100mV target can be adjusted to some degree to obtain
standard R
SENSE
values and/or a desired R
PROG
value, but
target voltages lower than 100mV will cause a proportional
reduction in current regulation accuracy.
The required minimum resistance between PROG and GND
can be determined by applying the suggested expression
for R
SENSE
while solving the first equation given above for
charge current with I
CHRG
= I
MAX
:
R
V R
V µA R
PROG MIN
IN
IN
( )
. •
. . •
=
+
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0 1 11 67
If R
IN
is chosen to be 3.01k with a sense voltage of 100mV,
this equation indicates a minimum value for R
PROG
of
26.9k. Table 6 gives some examples of recommended
charge current programming component values based
on these equations.
The resistance between PROG and GND can simply be
set with a single a resistor, if only maximum charge cur-
rent needs to be controlled during the desired charging
algorithm. However, some batteries require a low charge
current for initial conditioning when they are heavily dis-
charged. The charge current can then be safely switched
to a higher level after conditioning is complete. Figure 3
illustrates one method of doing this with 2-level control
of the PROG pin resistance. Turning Q1 off reduces the
charge current to I
MAX
/10 for battery conditioning. When
Q1 is on, the LTC4009 is programmed to allow full I
MAX
current for bulk charge. This technique can be expanded
through the use of additional digital control inputs for an
arbitrary number of pre-programmed current values.
Figure 3. Programming 2-Level Charge Current
13
Q1
2N7002
4009 F03
R2
53.6k
PROG
LTC4009
R1
26.7k
C
PROG
4.7nF
BULK
CHARGE
PRECHARGE
For a truly continuous range of maximum charge current
control, pulse width modulation can be used as shown in
Figure 4. The value of R
PROG
controls the maximum value
of charge current which can be programmed (Q1 continu-
ously on). PWM of the Q1 gate voltage changes the value
of R
PROG
to produce lower currents. The frequency of this
modulation should be higher than a few kHz, and C
PROG
must be increased to reduce the ripple caused by switch-
ing Q1. In addition, it may be necessary to increase loop