LTC4006_15 Datasheet, PDF (12/20 Pages) Linear Technology – 4A, High Efficiency, Standalone Li-Ion Battery Charger

LTC4006
APPLICATIO S I FOR ATIO
Charger Current Programming
The basic formula for charging current is:
I_CHARGE(MAX)
=
100mV
R_SENSE
Table 3. Recommended R_SENSE Resistor Values
I_MAX (A)
R_SENSE (Ω) 1%
R_SENSE (W)
1.0
0.100
0.25
2.0
0.050
0.25
3.0
0.033
0.5
4.0
0.025
0.5
Setting the Timer Resistor
The charger termination timer is designed for a range of 1
hour to 3 hours 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 (Refer to Figure 5)
(seconds)
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.
CHG Status Output Pin
When the charge cycle starts, the CHG pin is pulled down
to ground by an internal N-channel MOSFET that can drive
more than 100µA. When the charge current drops to 10%
of the full-scale current (C/10), the N-channel MOSFET is
turned off and a weak 25µA current source to ground is
connected to the CHG pin. After a time out occurs, the pin
will go into a high impedance state. By using two different
value pull-up resistors, a microprocessor can detect three
states from this pin (charging, C/10 and stop charging).
See Figure 6.
Battery Detection
It is generally not good practice to connect a battery while
the charger is running. The timer is in an unknown state
and the charger could provide a large surge current into
the battery for a brief time. The circuit shown in Figure 7
keeps the charger shut down and the timer reset while a
battery is not connected.
12
Alternatively, a normally closed switch can be used to
detect when the battery is present (see Figure 8).
200
180
160
140
120
100
80
60
40
20
0
100 150 200 250 300 350 400 450 500
R_RT (kΩ)
4006 F05
Figure 5. t_TIMER vs R_RT
3.3V
LTC4006
CHG 2
200k
33k
V_DD
µP
OUT
IN
4006 F06
Figure 6. Microprocessor Interface
ADAPTER
POWER
470k
SWITCH CLOSED IF
BATTERY CONNECTED
LTC4006
1 DCIN
3 ACP/SHDN
4006 F07
Figure 7
ADAPTER
POWER
SWITCH OPEN WHEN
BATTERY CONNECTED
LTC4006
1 DCIN
3 ACP/SHDN
4006 F08
Figure 8
4006fa