LTC4065L/
LTC4065LX/LTC4065L-4.1
OPERATION
Charge Current Soft-Start and Soft-Stop
loop, whichever is trying to reduce the charge current
the most. The output of the other amplifier saturates low
which effectively removes its loop from the system. When
in constant-current mode, CA servos the voltage at the
PROG pin to be precisely 1V. VA servos its inverting input
to an internal reference voltage when in constant-voltage
mode and the internal resistor divider, made up of R1 and
R2, ensures that the battery voltage is maintained at 4.2V
for LTC4065L/LTC4065LX or 4.1V for LTC4065L-4.1. The
PROGpinvoltagegivesanindicationofthechargecurrent
during constant-voltage mode as discussed in “Program-
ming Charge Current”.
The LTC4065L includes a soft-start circuit to minimize
the inrush current at the start of a charge cycle. When a
charge cycle is initiated, the charge current ramps from
zerotothefull-scalecurrentoveraperiodofapproximately
170μs.Likewise,internalcircuitryslowlyrampsthecharge
current from full-scale to zero when the charger is shut off
or self terminates. This has the effect of minimizing the
transient current load on the power supply during start-up
and charge termination.
Constant-Current/Constant-Voltage/
Constant-Temperature
Thetransconductanceamplifier,TA,limitsthedietempera-
turetoapproximately115°Cwheninconstant-temperature
mode. DiodeD3ensuresthatTAdoesnotaffectthecharge
current when the die temperature is below approximately
115°C. The PROG pin voltage continues to give an indica-
tion of the charge current.
The LTC4065L use a unique architecture to charge a bat-
tery in a constant-current, constant-voltage and constant-
temperature fashion. Figure 1 shows a simplified block
diagram of the LTC4065L. Three of the amplifier feedback
loops shown control the constant-current, CA, constant-
voltage,VA,andconstant-temperature,TAmodes.Afourth
amplifierfeedbackloop,MA,isusedtoincreasetheoutput
impedanceofthecurrentsourcepair;M1andM2(notethat
M1 is the internal P-channel power MOSFET). It ensures
that the drain current of M1 is exactly 205 times greater
than the drain current of M2.
In typical operation, the charge cycle begins in constant-
currentmodewiththecurrentdeliveredtothebatteryequal
. If the power dissipation of the LTC4065L
PROG
to 205V/R
results in the junction temperature approaching 115°C,
the amplifier (TA) will begin decreasing the charge current
to limit the die temperature to approximately 115°C. As
the battery voltage rises, the LTC4065L either returns to
constant-current mode or enters constant-voltage mode
straight from constant-temperature mode. Regardless of
mode, the voltage at the PROG pin is proportional to the
current delivered to the battery.
Amplifiers CA and VA are used in separate feedback loops
to force the charger into constant-current or constant-
voltage mode, respectively. Diodes D1 and D2 provide
priority to either the constant-current or constant-voltage
4065lfb
11
Linear Systems [ Linear Systems ]
