LTC4008_15 Datasheet, PDF (9/24 Pages) Linear Technology – 4A, High Effi ciency, Multi-Chemistry Battery Charger

TEST CIRCUIT
V_FB
7
LTC4008
BATMON
13
90.325k
9.675k
V_REF
+
EA
–
CSP
BAT
11
12
LT1055
9 ^ITH
0.6V
4008 TC
LTC4008
OPERATION
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 external
resistor divider and the internal 1.19V reference voltage.
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
Shutdown by low adapter voltage (Disabled on LTC4008-1)
<BAT
LOW
Normal charging
>BAT
HIGH
Input current limited charging
>BAT
HIGH
Charger shut down due to thermistor out of range
Shut down by ACP/SHDN pin (USER)
>BAT
HIGH
X
Forced LOW
Shut down by undervoltage lockout
>BAT + <UVL HIGH
*Most probable condition, **Open-drain output, HIGH = Open with pull-up, X = Don’t care
FLAG**
HIGH
HIGH
HIGH*
X
HIGH
HIGH
FAULT**
HIGH
HIGH*
HIGH*
LOW
HIGH
HIGH*
I_CL
LOW
HIGH*
LOW
HIGH
LOW
LOW
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