ltc3455euf-trpbf Linear Technology Corporation, ltc3455euf-trpbf Datasheet - Page 21

• Manufacturer Part Number: ltc3455euf-trpbf
• Description: Dual Dc/dc Converter With Usb Power Manager And Li-ion Battery Charger
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
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):
If only USB power is used (no wall adapter), select the
R
charge current at 500mA. If a wall adapter is also used,
I
R
cally 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.
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 is
USB powered, as the battery charge current varies signifi-
cantly with normal load transients.
Programming the Battery Charger Timer
An external capacitor on the TIMER pin sets the total
charge time. When this timer elapses the charge cycle
CHARGE
PROG
PROG
R
I
BAT
PROG
value to be 2.49kΩ (or larger) to set the maximum
value), and the USB power manager will automati-
= 1000 • V
Figure 11. Monitoring Average Charge Current
can be programmed up to 1A (with a 1.24kΩ
= 1000 • 1.23V / I
LTC3455
GND
PROG
25
PROG
U
2
R
PROG
/ R
U
PROG
BAT
C
10k
FILTER
3455 F11
W
CHARGE
CURRENT
MONITOR
CIRCUITRY
U
terminates and the CHRG pin assumes a high impedance
state. The total charge time is programmed as:
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 4.2V float
voltage right away. This type of charging is undesirable for
some battery applications, and can be avoided by choos-
ing 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 battery charges to just above the 4.05V thresh-
old using this shorter timer method, more than 90%
charge capacity should easily be reached.
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 is
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.
T
TIMER
(hours) = C
TIMER
• (3 hours) / (0.1µF)
LTC3455
21
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