LTC4052-4.2_15 Datasheet, PDF (8/12 Pages) Linear Technology – Lithium-Ion Battery Pulse Charger with Overcurrent Protection

LTC4052-4.2
APPLICATIO S I FOR ATIO
C/10 Detection (Near End-of-Charge)
The LTC4052 includes a comparator to monitor the duty
cycle at the GATE pin to detect when the battery is nearing
full charge. When the duty cycle falls below 10%, the
comparator trips and turns off the N-MOSFET at the CHRG
pin and switches in a weak (40µA) current source to
ground. The 40µA turns off when the timer terminates the
charge cycle. C/10 detection is disabled in trickle charge
mode.
V_IN
GATE
VOLTAGE
10µA/C
50µA/C
10µA/C
10µA/C
50µA/C
50µA/C
4052 TA02
Figure 2. Slew Rate at GATE and V_IN Pins
with the RC Network from GATE to V_IN
Internal Pass Transistor
An N-channel MOSFET (0.35Ω) pass transistor is included
in the LTC4052. The gate of the MOSFET is controlled by
an internal charge pump. The body is connected to ground
instead of source terminal. There is no body diode from the
BAT pin back to the V_IN pin; therefore, no blocking diode
is required in series with the battery or the input supply.
This will not only reduce cost but also reduce the heat
generated while in full charge mode. An internal thermal
shutdown circuit turns the pass transistor off if the die
temperature exceeds approximately 140°C with 5°C of
thermal hysteresis.
Gate Drive
The MOSFET gate drive consists of a regulated 10µA
current source charge pump. A series RC network is
required from the GATE pin to the V_IN pin. When the
MOSFET is turned on, the voltage at the V_IN pin will begin
slewing down to a voltage equal to V_BAT plus the voltage
drop across the pass transistor and R_SENSE. The slew rate
is equal to 10µA/C. By ramping the V_IN pin down slowly,
the inrush current is reduced. The resistor in series with
the capacitor is required to limit the transient current when
the input supply is first applied.
When the charge pump is turned off, a 50µA current
source to ground will start pulling the GATE voltage down.
Once the pass transistor is off, the voltage at the V_IN pin
will begin slewing up with the rate equal to 50µA/C. With
this external capacitor, the voltage at the V_IN pin is ramping
in a controlled manner (Figure 2).
For higher current applications an external power
N-channel MOSFET can be connected in parallel with the
internal pass transistor. Because the charge pump output is
clamped to 12V above GND, the external MOSFET gate to
source voltage rating should be 16V or more.
Thermal Considerations
The power handling capability is limited by the maximum
rated junction temperature (125°C) and the amount of PC
board copper used as a heat sink. The power dissipated by
the IC consists of two components:
1. Input supply quiescent current multiplied by the input
voltage
2. The voltage drop across the switch (SENSE pin to BAT
pin) multiplied by the charge current
The LTC4052 has internal thermal shutdown designed to
protect the IC from overtemperature conditions. For con-
tinuous charging in the fast charge mode, the maximum
junction temperature must not be exceeded. It is important
to give careful consideration to all sources of thermal
resistance from junction to ambient. Additional heat sources
mounted nearby must also be considered.
Surface mount packages rely primarily on the PC board
copper to dissipate the heat generated by the package to the
surrounding air. Since the PC board copper is the heat sink,
generous amounts of copper surrounding the package are
recommended to increase the effectiveness of the heat sink.
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