I'm looking for a way to drive a half-bridge with a 100% duty cycle on the high side... i.e. the default state of the bridge will be to have a motor brake on by activating the high side gate, while the other state is of course to have the low side fet active (duty cycle will be anywhere from around 15% to 100% depending on a supplied PWM signal). I'm open to either discrete inputs (in which case dead time on the driver is optional as it can be software controlled), or a single PWM input (in which case dead time on the driver is required).
In the past I've used a 5v microcontroller to directly drive the low side n-fet, and drive a p-fet high side through a smaller sot-23 n-fet. I am moving to a 3.3v microcontroller and this will cause issues driving the low side fet directly.
So far, I have found that there is a number of triple half-bridge drivers with a bootstrap trickle charger that would support this scenario (presumably for BLDC ESC's). For a single half-bridge, the only one I have come across is the Allegro A3946 - which is both expensive, and rare.
Some options I would like to consider are:
a) Does anyone know a fairly accessible smd chip that will do this?
b) Can I connect the switching node of a general non-100% duty cycle driver chip to a signal generator - such as a 555 timer instead of the motor to keep the bootstrap cap charged?
c) Is it just easier to use a logic level shifter to drive the low side gate and just continue to drive the fets the same way?
Most half-bridge drivers specifically do not allow this - simple bootstrapping requires the output to be switching regularly to charge up the high-side capacitor. Since the output voltage is completely independent of the driver supply you either have to switch the output or have a separate charge pump circuit of some kind that can handle the output voltage (remember the high side driver swings with the output voltage which can be much higher than 12V).
If you find a good half-H-bridge with charge pump do let us know. The HIP4081A full-H-bridge chip is the obvious example of a charge-pump driver. You are allowed to use only half the chip if you want, of course.
MarkT:
If you find a good half-H-bridge with charge pump do let us know.
I was afraid you'd say that. But based on this, I might go and hunt around for a small QFN BLDC chip with a charge pump - if one exists. Otherwise I'll try and run with the chip you mentioned and use DFN 3x3 fets to make up the space.