Datasheet
12
LTC4006
4006fa
APPLICATIO S I FOR ATIO
WUUU
Charger Current Programming
The basic formula for charging current is:
I
mV
R
CHARGE MAX
SENSE
()
=
100
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 • 2
27
• 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.
Alternatively, a normally closed switch can be used to
detect when the battery is present (see Figure 8).
Figure 5. t
TIMER
vs R
RT
R
RT
(kΩ)
100
0
t
TIMER
(MINUTES)
20
60
80
100
200
140
200
300
350
4006 F05
40
160
180
120
150 250
400
450
500
33k
200k
4006 F06
V
DD
3.3V
µP
IN
OUT
LTC4006
CHG
2
Figure 6. Microprocessor Interface
DCIN
ACP/SHDN
470k
LTC4006
ADAPTER
POWER
SWITCH CLOSED IF
BATTERY CONNECTED
4006 F07
1
3
Figure 7
DCIN
ACP/SHDN
LTC4006
ADAPTER
POWER
SWITCH OPEN WHEN
BATTERY CONNECTED
4006 F08
1
3
Figure 8