LT3651-8.2/LT3651-8.4
11
36518284fa
For more information www.linear.com/LT3651-8.2
OPERATION
The I
TH
error voltage corresponds linearly to average cur-
rent sensed across the inductor current sense resistor.
Maximum
charge current is controlled by clamping the
maximum voltage of I
TH
to 1V. This limits the maximum
current sense voltage (voltage across R
SENSE
) to 95mV
setting the maximum charge current. Manipulation of
maximum charge current is possible through the RNG/SS
and I
LIM
pins (see the RNG/SS: Dynamic Charge Current
Adjust, RNG/SS: Soft-Start and I
LIM
Control sections).
If the voltage on the BAT pin (V
BAT
) is below V
BAT(PRE)
, A7
initiates the precondition mode. During the precondition
interval, the charger continues to operate in constant cur
-
rent mode, but the I
TH
clamp is reduced to 0.15V reducing
charge current to 15% of the maximum programmed value.
As V
BAT
approaches the float voltage (V
FLOAT
) the voltage
error amp V-EA takes control of I
TH
and the charger transi-
tions into constant voltage (CV) mode. As this occurs, the
I
TH
voltage falls from the limit clamp and charge current is
reduced from the maximum value. When the I
TH
voltage
falls below 0.1V, A8 signals C/10. If the charger is config-
ured for
C/10 termination the charge cycle is terminated.
Once
the charge cycle is terminated, the CHRG status
pin becomes high impedance and the charger enters low
current standby mode.
The LT3651-8.2/LT3651-8.4 contain an internal charge
cycle timer that terminates a successful charge cycle af
-
ter a programmed amount of time. This timer is typically
programmed to achieve end-of-cycle in three hours, but
can be configured for any amount of time by setting an
appropriate timing capacitor value (C
TIMER
). When timer
termination is used, the charge cycle does not terminate
after C/10 is achieved. Because the CHRG status pin re
-
sponds to the C/10 current level, the IC will indicate a fully
charged battery status, but the charger will continue to
source low currents. At the programmed end of the cycle
time the charge cycle stops and the part enters standby
mode. If the battery did not achieve at least 97.5% of the
full float voltage at the end-of-cycle, charging is deemed
unsuccessful and another full-timer cycle is initiated.
Use of the timer function also enables bad battery detec
-
tion. This
fault condition is achieved if the battery does
not
respond to preconditioning and the charger remains
in (or enters) precondition mode after one-eighth of the
programmed
charge cycle time. A bad battery fault halts
the charging cycle, the CHRG status pin goes high imped
-
ance and the FAULT pin is pulled low.
When the LT3651-8.2/LT3651-8.4 terminate a charging
cycle, whether through C/10 detection or by reaching
timer end-of-cycle, the average current mode analog loop
remains active but the internal float voltage reference is
reduced by 2.5%. Because the voltage on a successfully
charged battery is at the full float voltage, the voltage er
-
ror amp detects an overvoltage condition and rails low.
When
the voltage error amp output drops below 0.3V,
the IC enters standby mode, where most of the internal
circuitry is disabled and the V
IN
bias current is reduced
to <100µA. When the voltage on the BAT pin drops below
the reduced float reference level, the output of the voltage
error amp will climb, at which point the IC comes out of
standby mode and a new charging cycle is initiated.
The system current limit allows charge current to be
reduced in order to maintain a constant input current.
Input current is measured via a resistor (R
CL
) that is
placed between
the CLP and CLN pins. Power is applied
through this resistor and is used to supply both V
IN
of the
chip and other system loads. An offset produced on the
inputs of A12 sets the threshold. When that threshold is
achieved, I
TH
is reduced, lowering the charge current thus
maintaining the maximum input current.
50µA of current is sourced from I
LIM
to a resistor (R
ILIM
)
that is placed from that pin to ground. The voltage on I
LIM
determines the regulating voltage across R
CL
. 1V on I
LIM
corresponds to 95mV across R
CL
. The I
LIM
pin clamps
internally to 1V maximum.
If the junction temperature of the die becomes excessive,
A10 activates decreasing I
TH
and reduces charge current.
This reduces on-chip power dissipation to safe levels but
continues charging.
