DW01A Battery protection Chip

The DW01A chip is a battery protection device for a single cell Lithium Ion battery. 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).

Battery Protection

On the battery side, it protects the battery from over-voltage (charging) or under-voltage (discharging). If the voltage goes too low you may 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 area.

This device is a back stop device with a high over-voltage and low under-voltage setting.

Load Protection

On the load side it protects from short circuits, high current, short circuit current and reverse charger connection.

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 over discharging. It also has low standby current 3uA when inactive, so it does not drain the battery significantly during storage.

DW01A Features

Charger input protection

· The CS pin is connected to the negative terminal of the charger input (via a 1k resistor) and performs the following functions:

· Short Circuit detector.

· Over current detector.

· Charger Detector.

· Reverse charger detection (overstress high current?).

· Battery monitoring

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.

Protection levels

· Overcharge protection voltage: 4.3V (typ) ± 50mV

· Overcharge release voltage: 4.1V (typ) ± 50mV

· Overdischarge protection voltage: 2.4V (typ) ± 100mV

· Overdischarge release voltage: 3.0V (typ) ± 100mV

· Overcurrent detection voltage: 0.15 (typ) ± 30mV

· Short circuit detection voltage: 1.35V (typ).

Note: The above over current and short circuit voltages are measured across the on-resistance of the MOSFET.

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 charging process.

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 .

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.

Using RDSON as the Current Limit

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). The below relation is also used to get the exact value ot on resistance with a desined value ot current limit.

You can find RDS(ON) values in the datasheet for the 8205, which is labelled "Rdson On-Resistance vs ID- Drain Current (A)" and shows the curves for various Vgs values.

Since the battery voltage is close to 4.5V using that curve gives RDS(ON) as 20mÃ¢‑¦ 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 25mÃ¢‑¦ 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