I'm using a 1A constant current buck LED driver (the TPS92518 from TI), in order to implement "shunt FET dimming", where the load (1A LEDs) is short-circuited using a MOSFET in order shut-off a LED string very fast (but without ripple, hence the need for the output capacitor).
However, since an output capacitor will be problematic when shorting the LED string (because it will discharge through the Q2 switch when it's closed, and when Q2 will open it will sink current for awhile before any current goes to the LEDs again), I want to add a second switch, Q1, in order to disconnect the capacitor when the LEDs are shorted (Q1 OFF when Q2 is ON and vice-versa).
Ideally, Q2 needs to close a few nanoseconds after Q1 opens so that the capacitor doesn't begin to discharge through Q2.
However, I'm not sure what kind of switching topology to use. Q2 seems relatively easy, only a NMOS would suffice. However, for Q1 I'm lost. If I only use an NMOS for Q1 I won't be able to switch it because the body diode will conduct afaik.
I was wondering if anybody has a good suggestion for a fast circuit to implement Q1 (and make Q2 slower a bit). Basically, Q1 needs to be a few nanoseconds faster than Q2. Q2 needs to be able to switch for approx. 1us at worse.
There is in my circuit a voltage that is much higher than the voltage at the buck output, so this could be used to switch Q1, but since Q1 et Q2 must be triggered by an FPGA, there needs to be some kind of transistor arrangement. However I'm not too sure what to do to optimize the speed. Perhaps somebody has some clever transistor tricks?
EDIT: For those asking why shunt LED driving would even be a thing, see answer here.