LTC4012 datasheet(16/28 Pages) LINER

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LTC4012/

LTC4012-1/LTC4012-2

4012f

APPLICATIONS INFORMATION

Programming Charge Current

The formula for charge current is:

μA

CHRG

SENSE

PROG

⎛

⎝⎜

⎞

⎠⎟

•

–.

12085

11 67

The LTC4012 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 RIN.

RSENSE should be chosen according to the following

equation:

SENSE

MAX

100

where IMAX is the desired maximum charge current ICHRG.

The 100mV target can be adjusted to some degree to obtain

standard RSENSE values and/or a desired RPROG 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 RSENSE while solving the ﬁrst equation given above for

charge current with ICHRG = IMAX:

VμA R

PROG MIN

()

.•

•

12085

0 1

11 67

If RIN is chosen to be 3.01k with a sense voltage of 100mV,

this equation indicates a minimum value for RPROG 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 cur-

rent for initial conditioning when they are heav-

ily 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 IMAX/10 for battery conditioning.

When Q1 is on, the LTC4012 is programmed to allow

full IMAX current for bulk charge. This technique can be

expanded through the use of additional digital control

inputs for an arbitrary number of pre-programmed cur-

rent values.

For a truly continuous range of maximum charge current

control, pulse width modulation can be used as shown

in Figure 4.

Figure 3. Programming 2-Level Charge Current

2N7002

4012 F03

53.6k

PROG

LTC4012

26.7k

CPROG

4.7nF

BULK

CHARGE

PRECHARGE

Figure 4. Programming PWM Current

2N7002

4012 F04

PROG

LTC4012

RPROG

RMAX

511k

CPROG

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