DW01A Battery protection Chip

The DW01A chip is a battery protection device for a single cell Lithium Ion battery protecting the cell from over and under charging (by controlling maximum and minimum voltage to the cell), reverse connection, and short circuit. You can typically find this chip on TP4056 breakout boards (Lithium ion charger chip).

DW01A purpose w.r.t TP4056

The real purpose of the DW01A is to protect an individual Lithium Ion battery, and as such it really should physically attach to the battery itself - and then you plug the battery-DW01A combo into a charger.

In the case of the TP4096, the chip is on the breakout board itself severely limiting the reason it is used because the TP4056 itself has fairly strict limits on output voltage i.e. the TP4056 won't allow the voltage to go too high or too low for charging anyway, making the DW01A chip mostly redundant!

The real reason the DW01A chip should be used (and not the way it is used on the breakout board), is if someone tried to charge a Lithium Ion battery with a non-compliant charger e.g. if someone used a standard plug block to charge it - in this case the DW01A is useful stopping a potentially dangerous action.

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.

This ensures that the Lithium ion battery kept in a safe operating area.

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

Load Protection

On the load side it protects from short circuits, high current, short circuit current and reverse charger connection. So if you short the battery output the ground line is disconnected.

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
DW01A Battery protector block diagram
        [Source: The DW01A 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.

DW01A Features

Charger input protection

The CS pin is connected to the negative terminal of the charger input (via a 1kâ� 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.

DW01A Datasheet

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 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 (see calculations below).
  • 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.

Dual MOSFET N Channel Datasheet

8025A MOSFET Datasheet

8205A MOSFET Current Limit

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).

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 Vgs values.

Since the battery voltage is close to 4.5V using that curve gives RDS(ON) as 20mâ� 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 25mâ� 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 DW01A

The circuit below shows how the DW01A chip is supposed to be used:

Typical circuit using the DW01A

DW01A typical lithium battery protection circuit
      [Source: The DW01A datasheet]

It is 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 following:
  • 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 values.
  • 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 (3uA datasheet).
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).

How the DW01A operates with the TP4056

DW01A and TP4056 breakout Board

On the breakout board, the chip is soldered to the TP4056 so this can never be connected the wrong way round at the "charger input". At the other side the DW01A does not protect from connecting the battery the wrong way round!

This chip will not activate for battery voltage level problems (unless the TP4056 fails) since the TP4056:
  • Stops discharging at voltages below 2.9V; Here trickle charge activates.
         The DW01A threshold is ~ 2.4V; So it will never activate.
  • Stops charging at voltages above 4.2V.
        The DW01A threshold is  ~ 4.3V; So it will never activate.

The only function that will operate is the over current protection and short circuit protection. These will activate at around 3A when using the 8205A dual Mosfet.

In the context of the TP4056, the DW01A:

  1. Cannot protect against battery reverse connection - solder it correctly!
  2. Cannot protect against over voltage - the TP4056 won't allow it.
  3. Cannot protect against over voltage - the TP4056 won't allow it.
  4. Can protect against short circuit output.

TP4056 incorrect use of DW01A

I am emphasizing the above incorrect usage of the chip on the TP4056 breakout board (except for short circuit protection).

There are 3 parts to this discussion

Part1: Introduction to the TP4056.
Part2: How to add a MOSFET for correct usage of the TP4056
Part3: Understanding the DW01A battery protector in the context of the TP4056 breakout board.

Written by John Main, who has a degree in Electronic Engineering.


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