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LTC3455/LTC3455-1

3455fc

Programming the Battery Charger Timer

An external capacitor on the TIMER pin sets the total charge

time. When this timer elapses the charge cycle terminates

Programming Charge Current

The maximum charge current is programmed using one

external resistor connected between the PROG pin and

GND (use the closest 1% resistor value):

RPROG = 1000 • 1.23V / IBAT

If only USB power is used (no wall adapter), select the

RPROG value to be 2.49kΩ (or larger) to set the maximum

charge current at 500mA. If a wall adapter is also used,

ICHARGEcanbeprogrammedupto1A(witha1.24kΩRPROG

value), and the USB power manager will automatically

throttle back the charge current to below 500mA when

under USB power.

Monitoring Charge Current

The voltage on the PROG pin is an accurate indication of the

battery charge current under all charging conditions.

IBAT = 1000 • VPROG / RPROG

Capacitance on the PROG pin should be minimized to

ensure loop stability when in constant-current mode. Do

not place a capacitor directly from the PROG pin to ground.

Adding an external R-C network (see Figure 11) allows the

monitoring of average, rather than instantaneous, battery

charge current. Average charge current is typically of more

interest to the user, especially when the LTC3455/LTC3455-1

are USB powered, as the battery charge current varies

signiﬁcantly with normal load transients.

and the CHRG pin assumes a high impedance state. The

total charge time is programmed as:

TTIMER (hours) = CTIMER • (3 hours) / (0.1μF)

For most applications, a two to three hour timer will provide

sufﬁcient time to completely recharge the battery. But for

some applications with larger capacity batteries, four to

ﬁve hours of charging may be needed. A potential problem

arises with setting such long timer periods (longer than 3

hours): If the battery is just below the recharge threshold

(meaning it is almost fully charged) it will still be charged

for the total timer period when external power is applied.

This means that the battery will be continually charged

at a very, very low charge current for the full four to ﬁve

hours, even if the battery reaches the ﬂoat voltage right

away. This type of charging is undesirable for some battery

applications, and can be avoided by choosing a shorter

timer period (but not less than 1 hour). At the end of a

charge cycle, the LTC3455 will measure the battery voltage

to see if it is above the 4.05V recharge threshold. If it is

not above 4.05V, a new charge cycle will begin, repeating

until the battery voltage is above 4.05V. Even if the bat-

tery charges to just above the 4.05V threshold using this

shorter timer method, more than 90% charge capacity

should easily be reached (Note: The LTC3455-1 recharge

threshold is 3.95V instead of 4.05V).

Trickle Charge and Defective Battery Detection

If the battery voltage is below 2.85V at the beginning of the

charge cycle, the charger goes into trickle charge mode,

reducing the charge current to 10% of its programmed

full-scale value. If the low battery voltage remains for one

quarter of the programmed total charge time, the battery is

assumed to be defective, the charge cycle is terminated, and

the CHRG pin goes to a high impedance state. This fault is

cleared if any of the following occurs: The battery voltage

rises above 2.85V, external power is removed and reapplied,

the PROG pin is ﬂoated temporarily, or the SUSPEND pin

is temporarily pulled high (if the LTC3455/LTC3455-1 are

under USB power). The device will still operate normally

from USB or wall power even if the charger has turned

off due to a trickle-charge timeout.

Figure 11. Monitoring Average Charge Current

APPLICATIONS INFORMATION

RPROG

CFILTER

CHARGE

CURRENT

MONITOR

CIRCUITRY

10k

3455 F11

LTC3455/

LTC3455-1

GND

PROG

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