The DW01A chip is a battery protection device for a single cell Lithium
Ion battery and it protects the cell from over and under charging,
reverse connection and short circuit. You can typically find this chip on TP4056 breakout boards (Lithium ion charger chip).
On the battery side, it protects the battery from over-voltage
(charging) or under-voltage (discharging). If the voltage goes too low
not be able to recover the battery. If the voltage goes too high there
is a risk of thermal runaway.
Keeping a Lithium ion battery from charging to too high a voltage or
too low a voltage ensures that the battery is kept in a safe operating
This device is a back stop device with a high over-voltage and low under-voltage setting.
On the load side it protects from short circuits, high current, short circuit current and reverse charger connection.
Tip: The correct usage of the DW01A is to include it as part of a
battery pack. It has reduced functionality when used on the TP4056 board.
DW01A Block Diagram
[Source: The DWO1A datasheet]
The DW01A requires an external dual FET to control the ground line to
the battery. By using the FET, the ground line is disconnected during
current error conditions (Short circuit, over discharge, over
charge). This isolates the Lithium battery until the load is removed (in
the case of a short circuit).
The DW01A also has extended voltage limits (above and below normal voltage charging values) to protect from
overcharging and overdischarging. It also
has low standby current 3uA when inactive, so it does not drain the
battery significantly during storage.
Charger input protection
The CS pin is connected to the negative terminal of the charger input (via a 1kΩ resistor) and performs the following functions:
Short Circuit detector.
Over current detector.
Reverse charger detection (overstress high current?).
VCC and GND are connected across the battery where two voltages are detected:
Overcharge Detector (battery voltage too high).
Overdischarge Detector (battery voltage too low).
Battery Over Current Protection
Protection is provided using the two control pins OD and OC (which
stand for over discharging and over charging respectively). These two
controls attach to the gates of two MOSFETS and stop current flow to the
battery if there is a problem.
Note: The above over current and short circuit voltages are measured across the on-resistance of the MOSFET.
Download the battery protection IC datasheet here.
How the DW01A works
The crucial part of the DW01A operation is the controlled dual MOSFET (N Channel); Specifically the RDS(ON) resistance of the N Channel MOSFET.
In the datasheet it states that " the threshold current for
overcurrent detection is determined by the turn-on resistance of the
charge and discharge control MOSFETs. "
There are problems in designing a current detection device this way, because as it also says in the datasheet:
" turn-on resistance of the MOSFET changes with temperature
variation due to heat dissipation, It changes with the voltage between
gate and source as well "
It is designed this way because it is a very cheap method.
However there are three points to this:
The threshold voltage levels fall outside the
normal charging voltages of a battery charger, and so do not interfere with the normal
The exact short circuit current value does not matter (as long as
it is reasonable i.e. not 100A! - it can be made to be 3A (see
The current limit reduces as the MOSFET's selected RDS(ON) gets worse - this is a good; If you use a high RDS(ON) value, the current needed to trigger the short circuit is smaller. Also increasing temperature increases RDS(ON).
This is a fail safe device, so as long as
the values chosen fall outside the normal operating state of the
charging battery, it will provide short circuit protection even if the exact
charging-cut off value changes with temperature and voltage.
You should simulate, analyse
and test the MOSFET operation to make sure it is acceptable for your application.
The current limiting voltage threshold is detected by a comparator, when
the voltage at the CS pin reaches 150mV. The comparator voltage will be reached when the voltage drop
across the resistance of the two (switched on) MOSFETs reaches 150mV -
this is caused by more current flowing through the two MOSFETS and is
therefore the voltage drop across 2 x RDS(ON).
You can find RDS(ON) values in Figure 6 of
the datasheet for the 8205, which is labelled "Rdson On-Resistance(mΩ)
vs ID- Drain Current (A)" and shows the curves for various
Since the battery voltage is close to 4.5V using that curve gives RDS(ON)
as 20mΩ which results in a short circuit current of 3.75A (0.15/(2*20e-3)).
When the battery discharges it will be closer to 2.5V giving RDS(ON)as 25mΩ resulting in a short circuit current of 3A (0.15/(2*25e-3)).
Once triggered, the DW01A the discharge MOSFET (OD) is turned off. It is only released when the load is removed.
Two Overcurrent Threshold levels
There are two over discharge values (the one above) 150mV and 1.35V.
The reason for the second one is that both are associated with
activation delays. For the 150mV one the delay is 10ms, while for the
second the delay is 5us.
So for an extremely large short circuit the activation delay is much faster.
When the short circuit current detector has been activated, you, must
remove the load, before the DW01A allows current to flow again (OC
MOSFET turned on).
DW01A Circuit Schematic
Correct use of the DWO1A
The circuit below shows how the DW01A chip is supposed to be used:
Typical circuit using the DW01A
[Source: The DWO1A datasheet]
in fact, supposed to be attached as part of battery pack making a
single unit with the battery and the DW01A chip.
This is true because of
The CS pin protects from reverse connecting of a charger.
Voltage limits for over and under voltage detection are outside
normal charging / discharged levels. They are specified to be extreme
Latchup protection when a charger is connected under over discharged condition.
Operating current is so low it won't discharge the battery if left in place attached to the battery.
When used as part of a battery pack it provides protection from the charger connected to BATT+
and BATT-, and in this configuration provides reverse polarity
protection from plugging in a charger the wrong way round and even
plugging in a totally inappropriate and dangerous charger e.g. a NiCad
or NiMh charger (do not try this).
TP4056 incorrect use of DW01A
I am emphasizing the above correct usage of the chip as it is not used in this way for the TP4056 breakout board
where it is used incorrectly (except for short circuit protection - but
even that assumes the TP4056 is attached permanently which is wrong
unless power sharing is implemented).