LTC4070

9

4070f

General Charging Considerations

The LTC4070 uses a different charging methodology from

previous chargers. Most Li-Ion chargers terminate the

charging after a period of time. The LTC4070 does not have

a discrete charge termination. Extensive measurements

on Li-Ion cells show that the cell charge current drops to

nanoamps with the shunt charge control circuit effectively

terminating the charge. For long cell life, operate the charger

at 100mV lower charge voltage normally used.

The simplest application of the LTC4070 is shown in

Figure 1. This application requires only an external resis-

tor to program the charge/shunt current. Assume the wall

rent is calculated as:

(

)

=

12V

− 3V

(

)

162

Ω

= 55.5mA

Care must be taken in selecting the input resistor. Power

following equation:

(

=

12V

− 3V

(

162

Ω

= 0.5W

The charge current decreases as the battery voltage

increases. If the rising battery voltage is 40mV less than

APPLICATIONS INFORMATION

the programmed float voltage, the LTC4070 consumes

only 450nA of current, and all of the input current flows

into the battery. As the battery voltage reaches the float

voltage, the LTC4070 shunts current from the wall adapter

shunt current the LTC4070 sinks, the less charge current

the battery gets. Eventually, the LTC4070 shunts all the

current from the battery; up to the maximum shunt cur-

rent. The maximum shunt current in this case, with no

NTC adjustment, is determined by the input resistor and

is calculated as:

(

)

=

12V

− 4.1V

(

)

162

Ω

= 49mA

At this point the power dissipated in the input resistor is

388mW.

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

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.

Figure 1. Single-Cell Battery Charger

Figure 2. Single-Cell Charger with Boosted Drive

4070 F01

LTC4070

ADJ

162Ω

0.5W

GND

12V WALL

ADAPTER

Li-Ion

BATTERY

NTCBIAS

FLOAT

IF NOT NEEDED

FLOAT

NTC

+

4070 F02

LTC4070

ADJ

Q1: FDN352AP

110Ω

4W

GND

24V WALL

ADAPTER

Li-Ion

BATTERY

NTCBIAS

DRV

Q1

NTC

+

FLOAT

IF NOT NEEDED