LTC4001 Datasheet, PDF (11/20 Pages) Linear Technology – 2A Synchronous Buck Li-Ion Charger

LTC4001
APPLICATIO S I FOR ATIO
The equations for calculating R1 (used in single resistor
programming) differ from the equations for calculating
R_PROG and R_IDET (2-resistor programming) and reflect the
fact that the current from both the IDET and PROG pins
must flow through a single resistor R1 when a single
programming resistor is used.
CHRG Status Output Pin
When a charge cycle starts, the CHRG pin is pulled to
ground by an internal N-channel MOSFET which is capable
of driving an LED. When the charge current drops below
the end-of-charge (IDET) threshold for at least 4ms, and
the battery voltage is close to the float voltage, the N-channel
MOSFET turns off and a weak 30µA current source to
ground is connected to the CHRG pin. This weak pull-
down remains until the charge cycle ends. After charging
ends, the pin will become high impedance. By using two
different value resistors, a microprocessor can detect
three states from this pin (charging, end-of-charge and
charging stopped). See Figure 2.
To detect the charge mode, force the digital output pin,
OUT, high and measure the voltage on the CHRG pin. The
N-channel MOSFET will pull the pin low even with a 2k pull-
up resistor. Once the charge current drops below the end-
of-charge threshold, the N-channel MOSFET is turned off
and a 30µA current source is connected to the CHRG pin.
The IN pin will then be pulled high by the 2k resistor
connected to OUT. Now force the OUT pin into a high
impedance state, the current source will pull the pin low
through the 390k resistor. When charging stops, the
CHRG pin changes to a high impedance state and the 390k
resistor will then pull the pin high to indicate charging has
stopped.
Charge Termination
Battery charging may be terminated several different ways,
depending on the connections made to the TIMER pin. For
time-based termination, connect a capacitor between the
TIMER pin and GNDSENS (C_TIMER = Time(Hrs) 0.0733µF).
Charging may be terminated when charge current drops
below the IDET threshold by tying TIMER to GNDSENS.
Finally, charge termination may be defeated by tying
TIMER to IDET. In this case, an external device can
terminate charging by pulling the EN pin high.
Battery Temperature Detection
When battery temperature is out of range (either too hot or
too cold) charging is temporarily halted and the FAULT pin
is driven high. In addition, if the battery is still charging at
a high rate (greater than the IDET current) when a tem-
perature fault occurs, the CHRG pin NMOS turns on and off
at approximately 50kHz, alternating between a high and
low duty factor at an approximate rate of 1.5Hz (Figure 3).
This provides a low rate visual indication (1.5Hz) when
driving an LED from the CHRG pin while providing a fast
temperature fault indication (20useconds typical) to a
microprocessor by tying the CHRG pin to an interrupt line.
Serrations within this pulse are typically 500ns wide.
V_IN
V_DD
LTC4001
CHRG
R1
390k R2
2k
µPROCESSOR
OUT
IN
4001 F02
Figure 2. Microprocessor Interface
667ms
20µs
4001 F03
Figure 3. CHRG Temperature Fault Waveform
4001f
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