LTC4007_15 Datasheet, PDF (11/24 Pages) Linear Technology – 4A, High Effi ciency, Standalone Li-Ion Battery Charger

LTC4007
OPERATION
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 cur-
rent. Error amp CA2 compares this current against the
desired current programmed by RPROG 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)
= ^{⎛⎝⎜}_RVPRREOFG
–
⎞
11.67μA_⎟ •
⎠
3.01kΩ
R_SENSE
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:
V_PROG
=
(^I_CHARGE
•
R_SENSE
+
11.67μA
•
3.01kΩ₎
•
R_PROG
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 voltage,
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
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 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 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
4007fc
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