LTC4070
11
4070fc
Typical applicaTions
The LTC4070 can be used to charge a battery to a 4.2V
float voltage from an AC line with a bridge rectifier as
shown in the simple schematic in Figure 5. In this example,
the four input 249k resistors are sized for acceptable UL
leakage in the event that one of the resistors short. Here,
the LTC4070 will fully charge the battery from the AC line
while meeting the UL specification with only 104µA of
available charge current.
A photovoltaic (PV) application for the LTC4070 is illus-
trated in Figure 6. In this application, transistor Q1 has
been added to further reduce the already low quiescent
current of the LTC4070 to achieve extremely low battery
discharge when the PV cells are not charging the battery.
In long battery life applications, Q1 isolates the battery
from the LTC4070 when Q1’s base voltage falls. Under
normal operation, the PV cells provide current through the
V
BE
and V
BC
diodes of Q1. While the battery is charging,
the majority of PV current flows to the battery. When V
CC
reaches the programmed float voltage, in this case 4.1V
with ADJ floating, then the LTC4070 shunts base-collector
junction current from Q1, effectively reducing the battery
charging current to zero and saturating Q1. In the event
that the thermistor temperature rises and the float voltage
drops, the LTC4070 shunts more current, and Q1 is forced
to operate in reverse active mode until the battery voltage
falls. Once equilibrium is achieved, the difference between
V
BAT
and V
CC
should be less than a few mV, depending on
the magnitude of the shunt current.
Add a series input resistor, R
IN
, to limit the current from
high current solar cells. Solar cells are limited in current
normally, so for small cells no resistor is needed. With
high current PV cells, select R
IN
taking into account the
PV cell’s open-circuit voltage and short-circuit current,
the temperature coefficient of the V
BC
and V
BE
diodes and
the maximum collector current and operating junction
temperature of Q1. Using an isolating transistor reduces
discharge current to a few nanoamps, and may be extended
to other applications as well.
The PV application schematic in Figure 6 also illustrates
using the LTC4070 with a 10k, 5% curve 2 type NTC
thermistor, NTHS0402N02N1002F. Here R
NOM
is 10k,
and the rising temperature trip points are 40°C, 50°C,
60°C and 70°C.
Figure 6. Photovoltaic Charger with Extremely
Low Leakage When Not Charging
Figure 5. 4.2V AC Line Charging, UL Leakage Okay
FLOAT
IF NOT
NEEDED
4070 F05
LTC4070
AC 110
DANGER! HIGH VOLTAGE!
GND
NTCBIAS
MB4S
NTC
Li-Ion
V
CC
R3
249k
R1
249k
ADJ
+
R4
249k
R2
249k
– +
DANGEROUS AND LETHAL POTENTIALS ARE PRESENT IN AC
LINE-CONNECTED CIRCUITS! BEFORE PROCEEDING ANY FURTHER,
THE READER IS WARNED THAT CAUTION MUST BE USED IN THE
CONSTRUCTION, TESTING AND USE OF AC LINE-CONNECTED
CIRCUITS. EXTREME CAUTION MUST BE USED IN WORKING WITH
AND MAKING CONNECTIONS TO THESE CIRCUITS. ALL TESTING
PERFORMED ON AN AC LINE-CONNECTED CIRCUIT MUST BE DONE
WITH AN ISOLATION TRANSFORMER CONNECTED BETWEEN THE
AC LINE AND THE CIRCUIT. USERS AND CONSTRUCTORS OF AC
LINE-CONNECTED CIRCUITS MUST OBSERVE THIS PRECAUTION
WHEN CONNECTING TEST EQUIPMENT TO THE CIRCUIT TO AVOID
ELECTRIC SHOCK.
4070 F06
LTC4070
ADJ
R
NTC
: NTHS0402N02N1002F 10k
GND
FLOAT
Q1
MP5650
T
R
NOM
10k
Li-Ion
NTCBIAS
NTC
V
CC
V
BAT
OR 2N3904
C
IN
0.1µF
+
+
–
+
–
