LTC4078/LTC4078X
10
4078xfb
OPERATION
programmed termination current. The 1.6ms fi lter time
(t
TERMINATE
) on the termination comparator ensures that
transient loads of this nature do not result in premature
charge cycle termination. Once the average charge current
drops below the programmed termination threshold, the
LTC4078/LTC4078X terminate the charge cycle and stops
providing current out of the BAT pin. In this state, any load
on the BAT pin must be supplied by the battery.
Low-Battery Charge Conditioning (Trickle Charge)
This feature ensures that near-dead batteries are gradually
charged before applying full charge current. If the BAT pin
voltage is below 2.9V, the LTC4078 supply 1/10th of the full
charge current to the battery until the BAT pin rises above
2.9V. For example, if the charger is programmed to charge
at 800mA from the wall adapter input and 500mA from
the USB input, the charge current during trickle charge
mode would be 80mA and 50mA, respectively.
The LTC4078X does not include the trickle charge feature;
it outputs full charge current to the battery when the
BAT pin voltage is below 2.9V. The LTC4078X are useful
in applications where the trickle charge current may be
insuffi cient to supply a load during low-battery voltage
conditions.
Automatic Recharge
In standby mode, the charger sits idle and monitors the
battery voltage using a comparator with a 4.1ms fi lter time
(t
RECHRG
). A charge cycle automatically restarts when the
battery voltage falls below 4.075V (which corresponds to
approximately 80% to 90% battery capacity). This ensures
that the battery is kept at, or near, a fully charged condi-
tion and eliminates the need for periodic charge cycle
initiations.
Manual Shutdown
The ENABLE pin has a 2MΩ pulldown resistor to GND. The
defi nition of this pin depends on which source is supplying
power. When the wall adapter input is supplying power,
logic low enables the charger and logic high disables it (the
pulldown defaults the charger to the charging state). The
opposite is true when the USB input is supplying power;
logic low disables the charger and logic high enables it
(the default is the shutdown state).
The DCIN input draws 40μA when the charger is in shutdown
mode. The USBIN input draws 40μA during shutdown if
no voltage is applied to DCIN, but draws only 23μA when
V
DCIN
provides valid voltage (see Table 1).
Status Indicators
The charge status open-drain output (CHRG) has two states:
pulldown and high impedance. The pulldown state indicates
that the LTC4078/LTC4078X are in a charge cycle. Once the
charge cycle has terminated or the LTC4078/LTC4078X are
disabled, the pin state becomes high impedance.
The power supply status open-drain output (PWR) has two
states: pulldown and high impedance. The pulldown state
indicates that power is present at either DCIN or USBIN.
Table 1. Power Source Selection (V
BATDET
< 1.75V)
V
USBIN
< 3.95V or
V
USBIN
< BAT + 50mV
6V > V
USBIN
> 3.95V and
V
USBIN
> BAT + 50mV 22V > V
USBIN
> 6V
ENABLE HIGH LOW or No Connect HIGH LOW or No Connect HIGH LOW or No Connect
V
DCIN
< 4.15V or
V
DCIN
< BAT + 50mV
No Charging.
PWR: Hi-Z
CHRG: Hi-Z
No Charging.
PWR: Hi-Z
CHRG: Hi-Z
Charging from
USBIN source.
PWR: LOW
CHRG: LOW
No Charging.
PWR: LOW
CHRG: Hi-Z
No Charging.
PWR: Hi-Z
CHRG: Hi-Z
No Charging.
PWR: Hi-Z
CHRG: Hi-Z
6V > V
DCIN
> 4.15V
and V
DCIN
> BAT +
50mV
No Charging.
PWR: LOW
CHRG: Hi-Z
Charging from DCIN
source.
PWR: LOW
CHRG: LOW
No Charging.
PWR: LOW
CHRG: Hi-Z
Charging from DCIN
source.
PWR: LOW
CHRG: LOW
No Charging.
PWR: LOW
CHRG: Hi-Z
Charging from DCIN
source.
PWR: LOW
CHRG: LOW
22V > V
DCIN
> 6V No Charging.
PWR: Hi-Z
CHRG: Hi-Z
No Charging.
PWR: Hi-Z
CHRG: Hi-Z
No Charging.
PWR: LOW
CHRG: Hi-Z
No Charging.
PWR: LOW
CHRG: Hi-Z
No Charging.
PWR: Hi-Z
CHRG: Hi-Z
No Charging.
PWR: Hi-Z
CHRG: Hi-Z
