LTC4009-2_15 Datasheet, PDF (15/28 Pages) Linear Technology – High Efficiency, Multi-Chemistry Battery Charger

LTC4009
LTC4009-1/LTC4009-2
Applications Information
Programming Charge Current
The formula for charge current is:
I_CHRG
=
R_IN
R_SENSE
•


1.2085V
R_PROG

– 11.67µA_
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_SENSE
=
100mV
I_MAX
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_PROG(MIN)
=
1.2085V • R_IN
0.1V + 11.67µA • R_IN
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.
LTC4009
PROG
13
BULK
CHARGE
PRECHARGE
R1
26.7k
Q1
R2
2N7002 53.6k
C_PROG
4.7nF
4009 F03
Figure 3. Programming 2-Level Charge Current
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
4009fd
15