LTC4007_1 Datasheet, PDF (11/24 Pages) Linear Technology – 4A, High Efficiency, Standalone Li-Ion Battery Charger

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OPERATIO
The peak inductor current, at which I_CMP resets the SR
latch, is controlled by the voltage on ITH. ITH is in turn
controlled by several loops, depending upon the situation
at hand. The average current control loop converts the
voltage between CSP and BAT to a representative current.
Error amp CA2 compares this current against the desired
current programmed by R_PROG at the PROG pin and
adjusts ITH until:
V_{REF =} V_CSP – V_BAT + 11.67µA • 3.01kΩ
R_PROG
3.01kΩ
therefore,
I_CHARGE(MAX)
=
⎛
⎝⎜
V_REF
R_PROG
–
11.67µA^{⎞⎠⎟}
•
3.01kΩ
R_SENSE
The voltage at BAT is divided down by an internal resistor
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:
( ) V_PROG =
I_CHARGE • R_SENSE + 11.67µA • 3.01kΩ
• ^RPROG
3.01kΩ
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
1.2
1.19V
1.0
0.8
0.6
0.4
0.309V
0.2
0
0
20
40
60
80 100
I_CHARGE (% OF MAXIMUM CURRENT)
4007 F02
Figure 2. V_PROG vs I_CHARGE
LTC4007
voltage, thereby reducing charging current. The I_CL indica-
tor 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 lim-
iting, 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 tran-
sient 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 protec-
tion 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 func-
tion. An internal clock, whose frequency is determined by
4007fa
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