9

LTC1734

Monitoring Charge Current

The voltage on the PROG pin indicates the charge current

as a proportion of the maximum current set by the

program resistor. The charge current is equal to 1000 •

ler with an ADC to easily monitor charge current and if

desired, manually shut down the charger at the appropri-

ate time. See Figure 1 for an example. The minimum PROG

pin current is about 3

Errors in the charge current monitor voltage on the PROG

pin are inversely proportional to battery current and can be

statistically approximated as follows:

One Sigma Error(%)

Dynamic loads on the battery will cause transients to

appear on the PROG pin. Should they cause excessive

errors in charge current monitoring, a simple RC filter as

shown in Figure 2 can be used to filter the transients. The

filter will also quiet the PROG pin to help prevent inadvert-

ent momentary entry into the manual shutdown mode.

Because the PROG pin is in a closed-loop signal path the

pole frequency must be kept high enough to maintain

adequate AC stability. This means that the maximum

resistance and capacitance presented to the PROG pin

must be limited. See the Stability section for more details.

Constant Current Source

The LTC1734 can be used as a constant current source by

disabling the voltage control loop as shown in Figure 3.

This is done by pulling the BAT pin below the preset float

voltages of 4.1V or 4.2V by grounding the BAT pin. The

program resistor will determine the output current. The

output current range can be between approximately 50mA

and 700mA, depending on the maximum power rating of

the external PNP pass transistor.

External PNP Transistor

The external PNP pass transistor must have adequate

beta, low saturation voltage and sufficient power dissipa-

tion capability (including any heat sinking, if required).

To provide 700mA of charge current with the minimum

available base drive of approximately 30mA requires a

PNP beta greater than 23. If lower beta PNP transistors are

used, more base current is required from the LTC1734.

This can result in the output drive current limit being

reached, or thermal shutdown due to excessive power

dissipation. Excessive beta can affect AC stability (see

Stability section)

With low supply voltages, the PNP saturation voltage

than the minimum supply voltage minus the maximum

voltage drop across the internal sense resistor and bond

wires (0.1

Ω) and battery float voltage. If the PNP transis-

tor can not achieve the low saturation voltage required,

base current will dramatically increase. This is to be

avoided for a number of reasons: output drive may reach

current limit resulting in the charger’s characteristics to

go out of specifications, excessive power dissipation may

force the IC into thermal shutdown, or the battery could

become discharged because some of the current from the

DRIVE pin could be pulled from the battery through the

forward biased collector base junction.

For example, to program a charge current of 500mA with

a minimum supply voltage of 4.75V, the minimum operat-

than the expected charge current because the charger

senses the emitter current and the battery charge current

will be reduced by the base current. In terms of

If

Another important factor to consider when choosing the

PNP pass transistor is the power handling capability. The

transistor’s data sheet will usually give the maximum rated

power dissipation at a given ambient temperature with a

power derating for elevated temperature operation. The

maximum power dissipation of the PNP when charging is:

the minimum battery voltage when discharged.