LTC4012-3_15 Datasheet, PDF (16/28 Pages) Linear Technology – High Efficiency, Multi-Chemistry Battery Charger with PowerPath Control

LTC4012-3
Applications Information
Programming Charge Current
The formula for charge current is:
I_CHRG
=
R_IN
R_SENSE
•


1.2085V
R_PROG

– 11.67µA_
The LTC4012-3 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 discharged. 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
LTC4012-3 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.
For a truly continuous range of maximum charge current
control, pulse width modulation can be used as shown
in Figure 4.
LTC4012-3
PROG
13
BULK
CHARGE
PRECHARGE
R1
26.7k
Q1
R2
2N7002 53.6k
C_PROG
4.7nF
4012-3 F03
Figure 3. Programming 2-Level Charge Current
LTC4012-3
PROG
13
R_PROG
5V
C_PROG
R_MAX
511k
Q1
0V
2N7002
4012-3 F04
Figure 4. Programming PWM Current
40123fb
16