Datasheet

LTC4070
9
4070fc
applicaTions inForMaTion
Figure 3. 2-Cell Battery Charger
Figure 4. 2-Cell Battery Charger with Boosted Drive
LTC4070
ADJ
R
IN
GND
WALL
ADAPTER
FLOAT
IF NOT NEEDED
FLOAT
FLOAT
IF NOT NEEDED
FLOAT
Li-Ion
BATTERY
NTCBIAS
NTC
V
CC
V
CC1
+
4070 F03
LTC4070
ADJ
GND
Li-Ion
BATTERY
NTCBIAS
NTC
V
CC
V
CC2
+
LTC4070
ADJ
R
IN
GND
WALL
ADAPTER
FLOAT
IF NOT NEEDED
FLOAT
FLOAT
IF NOT NEEDED
FLOAT
Li-Ion
BATTERY
NTCBIAS DRV
Q1
NTC
V
CC
V
CC1
V
CC2
+
4070 F04
LTC4070
ADJ
GND
Li-Ion
BATTERY
NTCBIAS
Q1, Q2: Si3469DV
DRV
Q2
NTC
V
CC
+
Figure 2. Single-Cell Charger with Boosted Drive
4070 F02
LTC4070
ADJ
Q1: FDN352AP
R
IN
110Ω
4W
GND
24V WALL
ADAPTER
Li-Ion
BATTERY
NTCBIAS DRV
Q1
NTC
V
CC
+
FLOAT
IF NOT NEEDED
Figure 2 shows a charge circuit that can boost the charge
current as well as the shunt current with an external
P-channel MOSFET, Q1. In this case, if the wall adapter
voltage (V
WALL
) is 24V and the minimum battery voltage
(V
BAT
) is 3V, then the initial charge current is set to 191mA
by selecting R
IN
= 110Ω. Note that this resistor dissipates
over 4W of power, so select the resistor taking power rating
into account. When the battery voltage reaches the float
voltage, the LTC4070 and the external P-channel MOSFET
begin to shunt current from the wall adapter. Eventually,
the LTC4070 and the external P-channel MOSFET shunts
all available current (182mA) and no current flows to the
battery. Take the full shunt current and power into account
when selecting the external MOSFET.
The LTC4070 can also be used to regulate series-connected
battery stacks as illustrated in Figures 3 and 4. Here two
LTC4070 devices are used to charge two batteries in series;
with or without boosted drive. A single resistor sets the
maximum charge/shunt current. The GND pin of the top
device is simply connected to the V
CC
pin of the bottom
device. Care must be taken in observing the status output
pins of the top device as these signals are not ground ref-
erenced. Also, the wall adapter must have a high enough
voltage rating to charge both cells.
NTC Protection
The LTC4070 measures battery temperature with a negative
temperature coefficient thermistor thermally coupled to the
battery. NTC thermistors have temperature characteristics
which are specified in resistance-temperature conversion
tables. Internal NTC circuitry protects the battery from
excessive heat by reducing the float voltage for each
10°C rise in temperature above 40°C (assuming a Vishay
thermistor with a B
25/85
value of 3490).
The LTC4070 uses a ratio of resistor values to measure
battery temperature. The LTC4070 contains an internal fixed
resistor voltage divider from NTCBIAS to GND with four tap
points; NTC
TH1
-NTC
TH4
. The voltages at these tap points
are periodically compared against the voltage at the NTC
pin to measure battery temperature. To conserve power,
the battery temperature is measured periodically by biasing
the NTCBIAS pin to V
CC
about once every 1.5 seconds.