LTC4001 Datasheet, PDF (10/20 Pages) Linear Technology – 2A Synchronous Buck Li-Ion Charger

LTC4001
APPLICATIO S I FOR ATIO
Soft-Start and Compensation Capacitor Selection
The LTC4001 has a low current trickle charger and a
PWM-based high current charger. Soft-start is used when-
ever the high rate charger is initially turned on, preventing
high start-up current. Soft-start ramp rate is set by the
internal 12.8µA pull-up current and an external capacitor.
The control range on the SS pin is approximately 0.3V to
1.6V. With a 0.1µF capacitor, the time to ramp up to
maximum duty cycle is approximately 10ms.
The external capacitor on the SS pin also sets the compen-
sation for the current control loop and the float voltage
control loop. A minimum capacitance of 10nF is required.
Charge Current and IDET Programming
The LTC4001 has two different charge modes. If the
battery is severely depleted (battery voltage less than
2.9V) a 50mA trickle current is initially used. If the battery
voltage is greater than the trickle charge threshold, high
rate charging is used.
This higher charge current is programmable and is ap-
proximately 915 times the current delivered by the PROG
pin. This current is usually set with an external resistor
from PROG to GNDSENS, but it may also be set with a
current output DAC connected to the PROG pin. The
voltage on the PROG pin is nominally 1.213V.
For 2A charge current:
R_PROG
=
915
• 1.213V
2A
≅
554.9Ω
The IDET threshold (a charge current threshold used to
determine when the battery is nearly fully charged) is
programmed in much the same way as the PROG pin,
except that the IDET threshold is 91.5 times the current
delivered by the IDET pin. This current is usually set with
an external resistor from IDET to ground, but it may also
be set with a current output DAC. The voltage on the PROG
pin is nominally 1.213V.
For 200mA IDET current (corresponding to C/10 for a
2AHr battery):
R_IDET
=
91.5 •1.213V
0.2A
≅
554.9Ω
1.10kΩ programs approximately 100mA and 274Ω ap-
proximately 400mA.
For applications where IDET is set to one tenth of the high
rate charge current, and slightly poorer charger current
and IDET threshold accuracy is acceptable, the PROG and
IDET pins may be tied together and a single resistor, R1,
can program both (Figure 1).
R1=
457.5 •1.213
I_CHARGE
and
IDET = ^ICHARGE
10
LTC4001
PROG IDET
R1
274Ω FOR 2A
GNDSENS
4001 F01
Figure 1. Programming Charge Current and IDET Threshold
with a Single Resistor
4001f
10