MC34673 Datasheet, PDF (15/22 Pages) Freescale Semiconductor, Inc – High Input Voltage 1.2A Charger for Single-cell Li-Ion Batteries

FUNCTIONAL DEVICE OPERATION
OPERATIONAL MODES
the ISET pin voltage to monitor the actual charge current as
given in equ. 1 during the whole charging cycle.
CHARGE CURRENT LIMITATION
The charge current is limited by multiple factors.
When the voltage difference between the input and the
battery (V_IN - V_BAT) is low, (V_IN - V_BAT) / R_DS(ON), where
R_DS(ON) is the on resistance of the power MOSFET, may be
less than the programmed CC-mode current I_CHG. The
charge current is limited by (V_IN - V_BAT) / R_DS(ON) in this case.
When the voltage difference between the input and the
battery is too high, the large power dissipation may lead to
the charge-current thermal-foldback operation due to the die
temperature regulation. The charge current is reduced to
prevent further temperature rise (See Charge Current
Thermal Foldback).
DC INPUT VOLTAGE
The MC34673 accepts up to 28V DC input. When all of the
following conditions are satisfied, the input is in a power-good
range for the charger to start charging. The conditions
include:
1. V_IN > V_POR
2. V_IN - V_BAT > V_OS
3. V_IN < V_OVP
where V_OS is the offset voltage for the comparator that
monitors the input and the battery voltages. The V_OS is for
preventing the reverse leakage current from the battery when
the power supply is off. V_OVP is the over-voltage protection
threshold. When the DC input voltage is above the over-
voltage protection threshold, the charger is disabled
internally. The 28V input voltage rating eliminates the need of
any additional input over-voltage protection circuitry.
CHARGE-ENABLE INPUT
The charge-enable input, EN, has a weak internal pull-
down current. Driving it to a low logic voltage, leaving it
floating, or shorting it to the ground will enable the charger, if
the input voltage is in the power-good range. Whenever the
EN pin is driven to a high logic voltage, the charger is
disabled.
INPUT POWER PRESENCE INDICATOR
When V_IN is applied and the voltage is above the power-on-
reset voltage threshold (V_POR), the PPR pin outputs a low
voltage to indicate the input power presence. The PPR out-
put is only controlled by the input voltage. All other functions,
such as the EN pin, the over-voltage protection, and the VIN-
BAT comparator, do not affect the PPR output. The PPR pin
is capable to sink at least 12.0mA current when outputting a
low voltage to drive an external LED.
CHARGE STATUS INDICATORS
The MC34673 has two charge status indicators, CHG and
FAST. CHG outputs a low voltage when the charger is
enabled and the charging is in progress. When the charge
cycle completes, CHG outputs high-impedance. If the
charger is disabled or the input voltage is out of the power-
good range, the CHG pin outputs high-impedance as well.
The CHG pin has at least 12.0mA current-sinking capability
to drive an external LED, same as the PPR pin.
FAST indicates whether the MC34673 is in the fast-charge
mode or not. When the charger is on and the battery voltage
is higher than the trickle-mode threshold, the charger enters
the fast-charge mode and FAST outputs a low voltage. The
open-drain FAST pin requires a pull-up resistor to output the
logic signal. If the charger is in the trickle-charge mode or is
disabled, or when the input voltage is out of the power-good
range, the FAST pin outputs high-impedance.
CHARGE CURRENT THERMAL FOLDBACK
An internal thermal feedback loop begins to reduce the
charge current when the die temperature reaches 110°C to
prevent further temperature rise. This feature protects the
MC34673 from over-temperature failures and allows the user
to push the limits of the power handling capability of a given
circuit board without the risk of damaging the MC34673. The
charge current can be programmed according to the typical
(not the worst-case) ambient temperature with the assurance
that the charger will automatically reduce the current in worst-
case conditions.
Analog Integrated Circuit Device Data
Freescale Semiconductor
34673
15