LTC4008 Linear Technology, LTC4008 Datasheet - Page 8

• Manufacturer Part Number: LTC4008
• Description: Multi-Chemistry Battery Charger
• Manufacturer: Linear Technology

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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
and a sense resistor (R
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
sampling interval for the thermistor.
LTC4008
TEST CIRCUIT
8
Table 1. Truth Table For Indicator States
MODE
Shutdown by low adapter voltage (Disabled on LTC4008-1)
Normal charging
Input current limited charging
Charger shut down due to thermistor out of range
Shut down by ACP/SHDN pin (USER)
Shut down by undervoltage lockout
*Most probable condition, **Open-drain output, HIGH = Open with pull-up, X = Don’t care
U
PROG
SENSE
) from the PROG pin to ground
) between the CSP and BAT
7
V
FB
13
90.325k
9.675k
BATMON
T
pin sets the
+
–
11
CSP
12
BAT
V
>BAT + <UVL
REF
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.
<BAT
>BAT
>BAT
>BAT
DCIN
X
–
+
EA
LT1055
Forced LOW
ACP/SHDN
HIGH
HIGH
HIGH
HIGH
LOW
+
–
LTC4008
9
4008 TC
I
TH
FLAG**
HIGH*
HIGH
HIGH
HIGH
HIGH
X
0.6V
FAULT**
HIGH*
HIGH*
HIGH*
HIGH
HIGH
LOW
HIGH*
HIGH
LOW
LOW
LOW
LOW
I
CL
4008fa