High-side 75V n-channel MOSFET driver for BMS
Note: the bq78PL114 is not available anymore and is replaced by the BQ34Z100.
With the Li-Ion battery controller bq78PL114 from Texas Instruments, an advanced battery management system (BMS) can be built. The bq78PL114 is designed for driving high-side p-channel MOSFETs. Preferred however are n-channel MOSFETs, because of the lower on-resistance. N-channel MOSFETs on the high-side however require a gate voltage above the positive rail. Here we need so called high-side n-channel MOSFET driver, which contains a charge pump. There are several of these ICs available, but none of them are suitable for a maximum voltage of 75V, which was necessary for a particular application.
The LM5050-1 is a high side OR-ing FET controller for voltages up to 75V. I have developed a high-side n-channel MOSFET driver for battery management systems with the LM5050-1. Here, this IC has to be used in another way than it is designed for. The circuit is simulated with Multisim but it turned out that the LM5050-1 spice file and the datasheet is not completely correct. So I have further developed the circuit on a breadboard.
Download here the LM5050-1 datasheet.
The LM5050-1 circuit
- The bq78PL114 digital output signals (charge and discharge) levels are 0 .. 2.5V.
- To achieve a zero quiescent current (0μA), which is required for a BMS, Q3 and Q4 turns off U1 and U2 completely.
- The two comparators inside the LM5050-1 are not used and disabled by the open OUT pin.
- The LM5050-1 OFF pin must be > 0.25V. To start U2, Rboot pulls the OFF pin above 0.25V momentarily.
- Q2 has to be turned off very fast in case of a short circuit. This is done by the U2 OFF pin. The off-time is 6μs, see the scope image.
- When the U2 is turned off with Q3 and the OFF pin is high (2.5V), the OFF pin input current is 240uA. The quiescent current should be zero however. Therefore C3 generates just a short pulse on the OFF pin during shutdown; it prevents DC current flow into the LM5050-1 OFF pin.
- If Q2 fails then Rboot may become very hot, so take the value not too small.