LTC4008 Datasheet, PDF (8/24 Pages) Linear Technology – 4A, High Efficiency, Multi-Chemistry Battery Charger

LTC4008
TEST CIRCUIT
V_FB
7
LTC4008
BATMON
13
90.325k
9.675k
V_REF
+
EA
–
CSP
BAT
11
12
LT1055
I_TH
9
0.6V
4008 TC
U
OPERATIO
OVERVIEW
The LTC4008 is a synchronous current mode PWM step
down (buck) switcher battery charger controller. The
charge current is programmed by the combination of a
program resistor (R_PROG) from the PROG pin to ground
and a sense resistor (R_SENSE) between the CSP and BAT
pins. The final float voltage is programmed with an exter-
nal resistor divider and the internal 1.19V reference volt-
age. Charging begins when the potential at the DCIN pin
rises above the voltage at BAT (and the UVLO voltage) and
the ACP/SHDN pin is high. An external thermistor network
is sampled at regular intervals. If the thermistor value
exceeds design limits, charging is suspended and the
FAULT pin is set low. If the thermistor value returns to an
acceptable value, charging resumes and the FAULT pin is
set high. An external resistor on the R_T pin sets the
sampling interval for the thermistor.
As the battery approaches the final float voltage, the
charge current will begin to decrease. When the current
drops to 10% of the full-scale charge current, an internal
C/10 comparator will indicate this condition by latching
the FLAG pin low. If this condition is caused by an input
current limit condition, described below, then the FLAG
indicator will be inhibited. When the input voltage is not
present, the charger goes into a sleep mode, dropping
battery current drain to 15µA. This greatly reduces the
current drain on the battery and increases the standby
time. The charger can be inhibited at any time by forcing
the ACP/SHDN pin to a low voltage. Forcing ACP/SHDN
low, or removing the voltage from DCIN, will also clear the
FLAG pin if it is low.
Table 1. Truth Table For Indicator States
MODE
DCIN
ACP/SHDN FLAG**
Shutdown by low adapter voltage (Disabled on LTC4008-1)
<BAT
LOW
HIGH
Normal charging
>BAT
HIGH
HIGH
Input current limited charging
>BAT
HIGH
HIGH*
Charger shut down due to thermistor out of range
>BAT
HIGH
X
Shut down by ACP/SHDN pin (USER)
X
Forced LOW HIGH
Shut down by undervoltage lockout
>BAT + <UVL HIGH
HIGH
*Most probable condition, **Open-drain output, HIGH = Open with pull-up, X = Don’t care
FAULT**
HIGH
HIGH*
HIGH*
LOW
HIGH
HIGH*
I_CL
LOW
HIGH*
LOW
HIGH
LOW
LOW
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