6

LTC1734

4.2V (2.5V at amplifier A1’s input) the amplifier will divert

current away from the output driver thus limiting charge

current to that which will maintain 4.2V on the battery. This

is the constant voltage mode.

When in the constant voltage mode, the 1000:1 current

ratio is still valid and the voltage on the PROG pin will

indicate the charge current as a proportion of the maxi-

mum current set by the current programming resistor.

This feature allows a microcontroller with an ADC to easily

monitor charge current and if desired, manually shut down

the charger at the appropriate time.

An internal 3

µA current source pulls the PROG pin above

the 2.15V threshold of voltage comparator C1 initiating

shutdown.

For charging NiMH or NiCd batteries, the LTC1734 can

function as a constant current source by grounding the

BAT pin. This will prevent amplifier A1 from trying to limit

charging current and only A2 will control the current.

Fault conditions such as overheating of the die or exces-

sive DRIVE pin current are monitored and limited.

When input power is removed or manual shutdown is

entered, the charger will drain only tiny leakage currents

from the battery, thus maximizing battery standby time.

the battery by the charger. In manual shutdown the base

OPERATIO

The LTC1734 is a linear battery charger controller. Opera-

tion can best be understood by referring to the Block

external current programming resistor is connected be-

tween the PROG pin and ground. When charging, the

collector of the external PNP provides the charge current.

and through the internal 0.06

Ω current sense resistor.

This current is close in magnitude, but slightly more than

the collector current since it includes the base current.

Amplifier A3, along with the P-channel FET, will force the

same voltage that appears across the 0.06

Ω resistor to

appear across the internal 60

Ω resistor. The scale factor

of 1000:1 in resistor values will cause the FET’s drain

current to be 1/1000 of the charge current and it is this

current that flows through the PROG pin. In the constant

current mode, amplifier A2 is used to limit the charge

The PROG pin current, which is 1/1000 of the charge

current, develops a voltage across the program resistor.

When this voltage reaches 1.5V, amplifier A2 begins

diverting current away from the output driver, thus limit-

ing the charge current. This is the constant current mode.

As the battery accepts charge, its voltage rises. When it

reaches the preset float voltage of 4.2V (LTC1734-4.2

version), a precisely divided down version of this voltage

(2.5V) is compared to the 2.5V internal reference voltage

by amplifier A1. If the battery voltage attempts to exceed

APPLICATIO S I FOR ATIO

Charging Operation

Charging begins when an input voltage is present that

Li-Ion battery is connected to the charger output and a

program resistor is connected from the PROG pin to

ground. During the first portion of the charge cycle, when

the battery voltage is below the preset float voltage, the

charger is in the constant current mode. As the battery

voltage rises and reaches the preset float voltage, the

charge current begins to decrease and the constant

voltage portion of the charge cycle begins. The charge

current will continue to decrease exponentially as the

battery approaches a fully charged condition.

Should the battery be removed during charging, a fast

built-in protection circuit will prevent the BAT pin from ris-

ing above 5V, allowing the precision constant voltage

circuit time to respond.