LTC4067
12
4067f
OPERATIO
U
current availability at OUT. This mode is typically used
when there is power available from a wall adapter.
Suspend Mode
When I
LIM1
is high and I
LIM0
is low, the LTC4067 enters
suspend mode to comply with the USB specifi cation. In
this mode, the power path between IN and OUT is put in
a high impedance state to reduce the IN input current to
50μA. If no other power source is available to drive OUT,
the system load connected to OUT is supplied through
the ideal diode connected to BAT.
Ideal Diode From BAT to OUT
The LTC4067 has an internal ideal diode as well as a
controller for an optional external ideal diode. Both the
internal and external ideal diodes will respond quickly
whenever OUT drops below BAT.
If the load increases beyond the input current limit, ad-
ditional current will be pulled from the battery via the ideal
diodes. Furthermore, if power to IN (USB) is removed, then
all of the application’s power will be provided by the battery
via the ideal diodes. The ideal diodes are fast enough to
keep OUT from dropping with just the recommended output
capacitor. The ideal diode consists of a precision amplifi er
that enables an on-chip P-channel MOSFET whenever the
voltage at OUT is approximately 30mV (V
FWD
) below the
voltage at BAT. The resistance of the internal ideal diode is
approximately 200mΩ. If this is suffi cient for the applica-
tion, then no external components are necessary. However,
if more conductance is needed, an external P-channel
MOSFET can be added from BAT to OUT.
The GATE pin of the LTC4067 drives the gate of the external
P-channel MOSFET for automatic ideal diode control. The
source of the MOSFET should be connected to OUT and
the drain should be connected to BAT. Capable of driving
a 1nF load, the GATE pin can control an external P-channel
MOSFET having extremely low on-resistance.
If the BAT voltage is below the V
BUVLO
threshold the ideal
diodes are disabled.
IN Undervoltage Lockout (UVLO)
An internal undervoltage lockout circuit monitors IN and
keeps the input current limit circuitry off until IN rises above
the rising UVLO threshold (3.8V) and at least 50mV above
OUT. Hysteresis on the UVLO turns off the input current
limit if IN drops below 3.675V or 50mV below OUT. When
this happens, system power at OUT will be drawn from the
battery via the ideal diode. To minimize the possibility of
oscillation in and out of UVLO when using resistive input
supplies, the input current limit is reduced when IN falls
to within a few hundred millivolts of the UVLO threshold.
To ensure that the full input current limit is available and
a complete battery charge cycle can be achieved, apply
at least 4.25V to IN.
Battery Charger
The LTC4067 includes a constant-current/constant-volt-
age battery charger with automatic recharge, automatic
termination by safety timer, low voltage trickle charging,
bad cell detection and thermistor sensor input for out of
temperature charge pausing.
When a battery charge cycle begins, the battery charger
fi rst determines if the battery is deeply discharged. If
the battery voltage is below 2.8V, an automatic trickle
charge feature sets the battery charge current to 10%
of the programmed value. If the low voltage persists for
more than 1/2 hour, the battery charger automatically
terminates and indicates via the
⎯
C
⎯
H
⎯
R
⎯
G pin that the battery
was unresponsive.
Once the battery voltage is above 2.8V, the battery charger
begins charging in full power constant-current mode. The
current delivered to the battery will try to reach 1000V/
R
PROG
. Depending on available input power and external
load conditions, the battery charger may or may not be
able to charge at the full programmed rate. The external
load will always be prioritized over the battery charge cur-
rent. The USB current limit programming will always be
observed and only additional current will be available to
charge the battery. When system loads are light, battery
charge current will be maximized.
Charge Termination
The battery charger has a built-in safety timer. When the
battery voltage approaches the 4.2V required to maintain
a full charge, otherwise known as the fl oat voltage, the
