LTC4007_15 Datasheet, PDF (14/24 Pages) Linear Technology – 4A, High Effi ciency, Standalone Li-Ion Battery Charger

LTC4007
APPLICATIONS INFORMATION
amount as the full-scale voltage is reduced then, R4 =
R5 = 2.49k and R_PROG = 26.7k, the maximum charging
current is still 2.5A but the FLAG trip point is maintained
at 10% of full scale.
There are other effects to consider. The voltage across the
current comparator is scaled to obtain the same values as
the 100mV sense voltage target, but the input referred sense
voltage is reduced, causing some careful consideration of
the ripple current. Input referred maximum comparator
threshold is 117mV, which is the same ratio of 1.4x the
DC target. Input referred I_REV threshold is scaled back to
–24mV. The current at which the switcher starts will be
reduced as well so there is some risk of boost activity.
These concerns can be addressed by using a slightly larger
inductor to compensate for the reduction of tolerance to
ripple current.
Charger Voltage Programming
Pins CHEM and C3C4 are used to program the charger
final output voltage. The CHEM pin programs Li-Ion bat-
tery chemistry for 4.1V/cell (low) or 4.2V/cell (high). The
C3C4 pin selects either 3 series cells (low) or 4 series
cells (high). It is recommended that these pins be shorted
to ground (logic low) or left open (logic high) to effect
the desired logic level. Use open-collector or open-drain
outputs when interfacing to the CHEM and 3C4C pins from
a logic control circuit.
Table 3. Charger Voltage Programming
V_FINAL (V)
12.3
3C4C
LOW
12.6
LOW
16.4
HIGH
16.8
HIGH
CHEM
LOW
HIGH
LOW
HIGH
Setting the Timer Resistor
The charger termination timer is designed for a range
of 1hour to 3 hour with a ±15% uncertainty. The timer
is programmed by the resistor R_RT using the following
equation:
t_TIMER = 10 • 2²⁷ • R_RT • 17.5pF (seconds)
200
180
160
140
120
100
80
60
40
20
0
100 150 200 250 300 350 400 450 500
R_RT (kΩ)
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Figure 7. t_TIMER vs R_RT
It is important to keep the parasitic capacitance on the R_T
pin to a minimum. The trace connecting R_T to R_RT should
be as short as possible.
Soft-Start
The LTC4007 is soft started by the 0.12μF capacitor on
the ITH pin. On start-up, ITH pin voltage will rise quickly
to 0.5V, then ramp up at a rate set by the internal 40μA
pull-up current and the external capacitor. Battery charging
current starts ramping up when ITH voltage reaches 0.8V
and full current is achieved with ITH at 2V. With a 0.12μF
capacitor, time to reach full charge current is about 2ms and
it is assumed that input voltage to the charger will reach
full value in less than 2ms. The capacitor can be increased
up to 1μF if longer input start-up times are needed.
Input and Output Capacitors
The input capacitor (C2) is assumed to absorb all input
switching ripple current in the converter, so it must have
adequate ripple current rating. Worst-case RMS ripple
current will be equal to one half of output charging current.
Actual capacitance value is not critical. Solid tantalum
low ESR capacitors have high ripple current rating in a
relatively small surface mount package, but caution must
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