Datasheet

LTC1730-4/LTC1730-4.2
9
sn1730 1730fs
End-of-Charge (C/10)
The LTC1730 includes a comparator to monitor the duty
cycle at the GATE pin to detect a near end-of-charge
condition. 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 end-of-charge comparator is disabled in
trickle charge mode.
Internal Pass Transistor
An N-channel MOSFET (0.35Ω) is included in the LTC1730
as the pass transistor. The gate of the MOSFET is con-
trolled 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
CC
pin; therefore, no
blocking diode is required in series with the battery or the
input supply. This will not only reduce the cost but also the
heat generated when in fast charge mode. An internal
thermal shutdown circuit turns off the pass transistor if
the die temperature exceeds approximately 140°C with
5°C of thermal hysteresis.
Gate Drive
The pass transistor gate drive consists of a regulated 10µA
current source charge pump. A series RC network is
required from the GATE pin to the V
CC
pin. When the pass
transistor is turned on, the voltage at the V
CC
pin starts
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
CC
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 40µA current
source to ground starts pulling the GATE voltage down.
Once the pass transistor is off, the voltage at the V
CC
pin
begins slewing up with the rate equal to 40µA/C. With this
external capacitor, the voltage at the V
CC
pin is ramping at
a controlled manner (Figure 2).
For higher current applications an external power N-MOSFET
can be connected in parallel with the internal pass transis-
tor. Because the charge pump output is clamped to 12V
above V
BAT
, the external N-MOSFET gate to source break-
down voltage should be rated at 20V or more.
Battery Temperature Detection
A negative temperature coefficient (NTC) thermistor lo-
cated close to the battery pack can be used to monitor
battery temperature and will not allow charging unless the
battery temperature is within an acceptable range. Connect
a 10kΩ thermistor between ground and the NTC/SHDN pin
and a 4.1k resistor from the NTC/SHDN pin to V
CC
. If the
temperature rises to 50°C, the resistance of the thermister
will be approximately 4.1kΩ (Dale NTHS-1206N02) and
the LTC1730 will go into a hold mode. For cold tempera-
tures, the threshold of the hold mode is at 0°C (R
NTC
≈
28kΩ). The pass transistor turns off and the timer is frozen
at hold mode while the output status at the CHRG pin
remains the same. The charge cycle begins or resumes
once the temperature is within the acceptable range.
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 device consists of two components:
1. Input supply current multiplied by the input voltage
2. The voltage drop across the switch (SENSE pin to BAT
pin) multiplied by the charge current
The LTC1730 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.
APPLICATIO S I FOR ATIO
WUU
U
GATE
VOLTAGE
V
CC
1730 TA02
10µA/C
10µA/C
40µA/C
40µA/C
40µA/C
10µA/C
Figure 2. Slew Rate at GATE and V
CC
Pins
with the RC Network from GATE to V
CC