LTC4007-1
12
40071fa
For more information www.linear.com/LTC4007-1
operaTion
desired current programmed by RPROG at the PROG pin
and adjusts ITH until:
R
k
REF
CSP BAT
=
+μ Ω
Ω
.
301
therefore,
I
V
R
A
k
R
CHARGE MAX
REF
()
–. •
.
⎛
⎝
⎜
⎞
⎠
⎟
Ω
11 67
301
The voltage at BAT is divided down by an internal resis-
tor divider and is used by error amp EA to decrease ITH
if the divider voltage is above the 1.19V
reference. When
the charging current begins to decrease, the voltage at
PROG will decrease in direct proportion. The voltage at
PROG is then given by:
VI RAk
PROG CHARGE SENSE
PROG
=+μΩ
•.•. •
11 67 301
V
PROG
is plotted in Figure 2.
The amplifier CL1 monitors and limits the input current,
normally from the AC adapter to a preset level (100mV/
R
CL
). At input current limit, CL1 will decrease the ITH volt-
age, thereby reducing charging current. The I
CL
indicator
output will go low when this condition is detected and the
FLAG indicator will be inhibited if it is not already LOW.
If the charging current decreases below 10% to 15%
of programmed current while engaged in input current
limiting, BGATE will be forced low to prevent the charger
from discharging the battery. Audible noise can occur in
this mode of operation.
An overvoltage comparator guards against voltage transient
overshoots (>7% of programmed value). In this case, both
MOSFETs are turned off until the overvoltage condition
is cleared. This feature is useful for batteries which “load
dump” themselves by opening their protection switch
to perform functions such as calibration or pulse mode
charging.
PWM Watchdog Timer
There is a watchdog timer that observes the activity on
the BGATE and TGATE pins. If TGATE stops switching for
more than 40µs, the watchdog activates and turns off the
top MOSFET for about 400ns. The watchdog engages to
prevent very low frequency operation in dropout—a po
-
tential source of audible noise when using ceramic input
and output capacitors.
Charger Start-Up
When the charger is enabled, it will not begin switching
until the ITH voltage exceeds a threshold that assures initial
current will be positive. This threshold is 5% to 15% of the
maximum programmed current. After the charger begins
switching, the various loops will control the current at a
level that
is higher or lower than the initial current. The
duration of this transient condition depends upon the loop
compensation, but is typically less than 100µs.
Thermistor Detection
The thermistor detection circuit is shown in Figure 3. It
requires an external resistor and capacitor in order to
function properly.
The thermistor detector performs a sample-and-hold
function. An internal clock, whose frequency is determined
I
CHARGE
(% OF MAXIMUM CURRENT)
0
0
PROG
0.2
0.4
0.6
0.8
20 40
60 80
100
40071 F02
1.0
0.309V
Figure 2. V
PROG
vs I
CHARGE
