LTC4110
37
4110fb
PROGRAMMING CHARGE VOLTAGE
Depending on the battery chemistry chosen by the TYPE
pin, a charge termination voltage or a fl oat voltage will be
required. The difference between the two is time. A fl oat
voltage is applied to a battery forever. The V
CHG
pin is used
to set any of these voltages and the equations remain the
same. For this document, we will use the term fl oat voltage
generically. If nickel chemistry is chosen, the V
CHG
pin is
disabled placing the charger in constant current mode. If
you are using a smart battery, wake-up charge is subject
to the V
CHG
pin setting when active.
Connecting the V
CHG
pin to GND will set the default per
cell fl oat voltage (4.2V for Li-ion, 2.35V for SLA). If a dif-
ferent fl oat voltage is needed, tie the V
CHG
pin to a voltage
between 0.25 V
BGR
and 0.75 V
BGR
using a resistor divider
on the V
REF
pin. Unlike V
REF
, V
BGR
is an internal reference
voltage of the same voltage as V
REF
but with a much tighter
(±0.5%) tolerance than V
REF
.
V
FLOAT
=
2 •
V
CHG
V
BGR
1
• 0.6
where
Δ
V
FLOAT
= Adjusted Float Voltage – Default Float Voltage
V
CHG
= V
CHG
Pin Voltage,
V
BGR
= 1.220V
The resistor divider connected to V
REF
pin will affect timer
(see the Programming Charge Time with TIMER and V
REF
Pins section for more details).
THERMISTOR FOR LEAD ACID BATTERIES
When the TYPE pin is programmed for Lead Acid, THA
pin will be force to V
BGR
, THB will be used to sense the
NTC resistance. The value of R1 is given by:
R1
=
R0 •
β −
2 • T0
β
+
2 • T0
where:
R0 = thermistor resistance (
Ω
) at T0
T0 = thermistor reference temperature (°K)
β
= exponential temperature coeffi cient of resistance
The LTC4110 is designed to work best with a 5% 10k NTC
thermistor with a
β
near 3750, such as the Siemens/EPCOS
B57620C103J062. In this case, R1 = 7256
Ω
.
APPLICATIONS INFORMATION
Figure 15. Lead Acid Thermistor
R
NTC
TH_HI
V
BGR
+
–
TH_LO
RES_OR
RES_HOT
HI_REF
REF LO_REF
+
–
THB
R1
THA
LOGIC
+
4110 F15
LEAD ACID BATTERY TEMPERATURE COMPENSATION
To program the temperature compensation for SLA charg-
ing, an external circuit is needed as shown in Figure 16.
The values are given by:
R1
=
R0 •
β −
2 • T0
β
+
2 • T0
k1
=
R0
R0
+
R1
TCk1
=
–
β
• R1• R0
(R1
+
R0)
2
• T0
2
k2
=
TCV
FLOAT
1.2 • TCk1
k3
=
0.5
+
Δ
V
FLOAT
/ 1.2
−
k1• k2
1
−
k2
where:
TCV
FLOAT
= temperature coeffi cient of the fl oat voltage
(Range: –2mV/°C – –6mV/°C)
Δ
V
FLOAT
= fl oat voltage at 25°C – default fl oat voltage
2.35V (Range: –0.15V – 0.15V)
For example, if a 10k NTC with
β
= 3750 is used, desired