LTC3553 Linear Technology Corporation, LTC3553 Datasheet - Page 16

• Manufacturer Part Number: LTC3553
• Description: Micropower USB Power Manager
• Manufacturer: Linear Technology Corporation

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LTC3553
OPERATION
MOSFET whenever the voltage at V
15mV (V
the internal ideal diode is approximately 240mΩ.
Suspend Mode
When the SUSP pin is pulled high the LTC3553 enters
suspend mode to comply with the USB specifi cation. In
this mode, the power path between V
in a high impedance state to reduce the V
to 15μA. The system load connected to V
through the ideal diode connected to BAT.
V
Current Limit (UVCL)
An internal undervoltage lockout circuit monitors V
and keeps the input current limit circuitry off until V
rises above the rising UVLO threshold (3.8V) and at least
200mV above V
input current limit circuitry if V
within 50mV of V
V
minimize the possibility of oscillation in and out of UVLO
when using resistive input supplies, the input current limit
is reduced as V
Battery Charger
The LTC3553 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 V
typically 2.9V, 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. Once the battery voltage
is above 2.9V, the battery charger begins charging in full
power constant current mode. The current delivered to
the battery will try to reach 750V/R
available input power and external load conditions, the
battery charger may or may not be able to charge at the
full programmed current. The external load will always be
16
OUT
BUS
will be drawn from the battery via the ideal diode. To
Undervoltage Lockout (UVLO) and Undervoltage
FWD
) below the voltage at BAT. The resistance of
BUS
BAT
BAT
. Hysteresis on the UVLO turns off the
. When this happens, system power at
falls below 4.45V typical.
BUS
OUT
drops below 3.6V or
PROG
BUS
is approximately
BUS
and V
. Depending on
OUT
input current
is supplied
OUT
is put
TRKL
BUS
BUS
,
prioritized over the battery charge current. The USB cur-
rent 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 fl oat voltage, the charge
current begins to decrease as the LTC3553 enters con-
stant-voltage mode. Once the battery charger detects that
it has entered constant-voltage mode, the four hour safety
timer is started. After the safety timer expires, charging
of the battery will terminate and no more current will be
delivered to the battery.
Automatic Recharge
After the battery charger terminates, it will remain off
drawing only microamperes of current from the battery.
If the portable product remains in this state long enough,
the battery will eventually self discharge. To ensure that
the battery is always topped off, a charge cycle will au-
tomatically begin when the battery voltage falls below
V
is running when the battery voltage falls below V
the timer will reset back to zero. To prevent brief excur-
sions below V
battery voltage must be below V
2ms. The charge cycle and safety timer will also restart
if the V
removed and then replaced).
Charge Current
The charge current is programmed using a single resis-
tor from PROG to ground. 1/750th of the battery charge
current is delivered to PROG which will attempt to servo
to 1.000V. Thus, the battery charge current will try to
reach 750 times the current in the PROG pin. The program
resistor and the charge current are calculated using the
following equations:
RECHRG
R
PROG
BUS
(typically 4.1V). In the event that the safety timer
=
UVLO cycles low and then high (e.g., V
750V
I
CHG
RECHRG
,I
CHG
from resetting the safety timer, the
=
R
750V
PROG
RECHRG
for approximately
RECHRG
BUS
3553f
, is
,