NCP802
http://onsemi.com
18
Undervoltage Detection
The undervoltage detector (VD2) monitors the VCELL
pin voltage. When the VCELL voltage crosses the
undervoltage threshold (VDET2) from a high value to a
value lower than VDET2, VD2 senses an undervoltage
condition, and an external, discharge control, N−channel
MOSFET turns off by driving the DO pin to its low level.
The low level of DO is set to GND and the high level to
VCELL.
To reset the DO pin to its high level, one must connect a
charger to the battery pack. While the VCELL voltage
remains under VDET2, charge−current can flow through the
parasitic diode of the external discharge control MOSFET.
Once the VCELL voltage rises above VDET2, the NCP802
drives DO high. Connecting a charger to the battery pack
drives the DO level high instantaneously when the VCELL
voltage is higher than VDET2. VD2 has no hysteresis.
After VD2 detects an undervoltage condition, the
NCP802 enters a low supply current, standby mode.
Maximum standby current equals 0.1 mA at VCELL equal
to 2.0 V. An internal pull−up disables all the device functions
and thus drastically lowers quiescent current. When the
charger connects to the battery, it pulls small levels of
current from the P− pin. This overcomes the internal pull−up
and allows the NCP802 to reset.
There are internal, fixed delay times for both the detection
and release from an undervoltage condition. If the fault or
reset conditions are shorter than their respective delay times,
the NCP802 ignores that condition and stays in its previous
state.
Excess Discharge−Current/Short Circuit Detection
The excess discharge−current detector (VD3) and the
short circuit detector can function when the control
MOSFET’s are on. When the P− pin voltage is below the
short circuit detection voltage (VSHORT) and above the
excess discharge−current threshold (VDET3), VD3
operates. When the P− pin voltage rises higher than
VSHORT, the NCP802 enables the short circuit detector.
When either detector activates, the NCP802 turns off an
external, discharge control, N−channel, MOSFET by
driving the DO pin to its low level.
The output delay time for the excess discharge−current
detector is internally fixed. If the P− pin, voltage level
recovers from a level between VSHORT and VDET3 within
the delay time, the discharge MOSFET stays in its high state.
Output delay time for release from excess discharge−current
detection is typically 1.2 ms. When the short circuit detector
activates, DO transitions to its low state after a delay time of
approximately 400 ms.
There is an integrated pull−down resistor (RSHORT)
connected between the P− and GND pins. After VD3 or the
short circuit detector has activated; removing the cause of
that activation turns the discharge MOSFET back on. This
occurs because RSHORT pulls the P− pin, voltage level
down to the GND pin, voltage level. The NCP802 internally
disconnects RSHORT during a normal, fault−free, state. The
NCP802 only connects RSHORT if it has detected an excess
discharge−current or short circuit fault. In other words, VD3
is automatically released from excess discharge−current and
short circuit faults when the user removes the load.
The output delay time of excess discharge−current
detection is set shorter than the delay time for undervoltage
detection. Therefore, if VCELL voltage drops below
VDET2 during an excess discharge−current or short circuit
fault, the NCP802 detects the current fault first. This
prevents large discharge current faults from activating the
undervoltage detector and putting the NCP802 into standby
mode. Standby mode requires the charger to reset the
NCP802, while excess discharge−current and short circuit
faults only require that the fault be removed.
Excess Charge−Current Detection
When the battery pack is chargeable and discharge is also
possible, VD4 senses the P− pin voltage. For example, if the
user connects the battery to an inappropriate charger, excess
current can flow. Then, the P− voltage drops below the
excess charge−current threshold (VDET4). Next, the output
of CO becomes low. This prevents excess current flow into
the circuit by turning off the external MOSFET.
The output delay of the excess charge−current detector is
internally fixed. If the fault condition is within the delay time
window, the detector will not sense it and the MOSFET will
not change state. VD4 can be released by disconnecting a
charger and applying a load.
Delay Shortening Function
The output delay time of over−charge, over−discharge,
excess discharge−current, excess charge−current, and the
release from those detecting modes can be made shorter than
the pre−set value by forcing the VCELL voltage to the DS
pin. When one forces the specified middle range voltage to
the DS pin, the output delay circuit becomes disabled.
Therefore, under this condition, when over−charge or excess
charge current is detected, output level can be checked
without waiting for the delay.
A 1.3 MW pull−down resistor is connected between DS
pin and GND internally. For normal operation, the DS pin
should be at no connection state.
Zero Battery Voltage Charging
If the charger voltage is equal or higher than the zero−volt
charge, minimum voltage (VST), the NCP802 drives the CO
pin high. Therefore, it allows charging for batteries as low
as zero volts.
