LTC3455_15 Datasheet, PDF (21/28 Pages) Linear Technology – Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger

LTC3455/LTC3455-1
APPLICATIONS INFORMATION
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):
R_PROG = 1000 • 1.23V / I_BAT
If only USB power is used (no wall adapter), select the
R_PROG value to be 2.49kΩ (or larger) to set the maximum
charge current at 500mA. If a wall adapter is also used,
I_CHARGE can be programmed up to 1A (with a 1.24kΩ R_PROG
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.
I_BAT = 1000 • V_PROG / R_PROG
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.
LTC3455/
LTC3455-1
2
10k
PROG
GND
25
R_PROG C_FILTER
CHARGE
CURRENT
MONITOR
CIRCUITRY
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Figure 11. Monitoring Average Charge Current
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
and the CHRG pin assumes a high impedance state. The
total charge time is programmed as:
T_TIMER (hours) = C_TIMER • (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.
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