Datasheet

LTC4007
10
4007fc
OPERATION
keep a low forward voltage drop from drain to source. If the
voltage between DCIN and CLN drops to less than 25mV,
the input FET is turned off slowly. If the voltage between
DCIN and CLN is ever less than –25mV, then the input FET
is turned off in less than 10μs to prevent signifi cant reverse
current from fl owing in the input FET. In this condition, the
ACP pin is driven low and the charger is disabled.
Battery Charger Controller
The LTC4007 charger controller uses a constant off-time,
current mode step-down architecture. During normal
operation, the top MOSFET is turned on each cycle when
the oscillator sets the SR latch and turned off when the
main current comparator I
CMP
resets the SR latch. While
the top MOSFET is off, the bottom MOSFET is turned
on until either the inductor current trips the current
comparator I
REV
or the beginning of the next cycle. The
oscillator uses the equation:
t
VV
Vf
OFF
DCIN BAT
DCIN OSC
=
–
•
to set the bottom MOSFET on time. The result is a nearly
constant switching frequency over a wide input/output
voltage range. This activity is diagrammed in Figure 1.
Table 1. Truth Table For Indicator States
MODE DCIN SHDN ACP** LOBAT FLAG** FAULT** I
CL
TIMER
STATE CHG**
Shut down by low adapter voltage <BAT LOW LOW LOW HIGH HIGH LOW Reset HIGH
Charging a low bat >BAT LOW HIGH LOW HIGH* HIGH* HIGH* Running LOW
Normal charging >BAT LOW HIGH HIGH HIGH HIGH* HIGH* Running LOW
Input current limited charging >BAT LOW HIGH HIGH HIGH* HIGH* LOW Running LOW
Charger paused due to thermistor out of range >BAT LOW HIGH X X LOW
(from
NTC)
HIGH Paused LOW
Shut down by SHDN pin X HIGH X X HIGH HIGH LOW Reset HIGH
Terminated by low-battery fault (Note 1) >BAT LOW HIGH LOW HIGH* LOW LOW >T/4 HIGH
(Faulted)
Timer is reset when FLAG goes low, then
terminates after 1/4 T
>BAT LOW HIGH HIGH LOW HIGH LOW >T/4
after
FLAG =
LOW
HIGH
(Waiting for
Restart
Terminated by expired timer >BAT LOW HIGH HIGH HIGH HIGH LOW >T HIGH
(Waiting for
Restart
Charge termination defeated X X X X X X X X Forced LOW
Shut down by undervoltage lockout >BAT +
<UVL
LOW HIGH HIGH HIGH HIGH* LOW Reset HIGH*
*Most probable condition X = Don’t care, ** Open-drain output HIGH = OPEN with pull-up
Note 1: If a depleted battery is inserted while the charger is in this state,
the charger must be reset to initiate charging.
TGATE
OFF
ON
BGATE
INDUCTOR
CURRENT
t
OFF
TRIP POINT SET BY ITH VOLTAGE
ON
OFF
4006 F01
Figure 1