LTC4001-1_15 Datasheet, PDF (11/20 Pages) Linear Technology – 2A Synchronous Buck Li-Ion Charger

LTC4001-1
APPLICATIONS INFORMATION
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 charg-
ing 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 charg-
ing at a high rate (greater than the IDET current) when a
temperature 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 (20μs typical) to a mi-
croprocessor by tying the CHRG pin to an interrupt line.
Serrations within this pulse are typically 500ns wide.
V_IN
LTC4001-1
CHRG
V_DD
R1
390k R2
2k
μPROCESSOR
OUT
IN
40011 F02
Figure 2. Microprocessor Interface
20μs
667ms
40011 F03
Figure 3. CHRG Temperature Fault Waveform
40011fa
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