I am stepping up ~4V to 12V to power an LED strip with a boost-regulator.
I don't want to run the LED strips (3 leds in series with a resistor) at full brightness so I need to PWM them.
Is it better to PWM the output of the boost circuit before the LED strip with some sort of FET, or to PWM the enable signal to the switching regulator itself?
Why use PWM? Since you require a boost converter anyway, the simplest option would be to use a variable output voltage type and set the voltage to the required level to give the brightness you require. e.g., Boost to 9 V for reduced brightness.
A range of 6 V to 12 V would be adequate for most applications and wouldn't require anything too fancy in hardware.
Is it better to PWM the output of the boost circuit before the LED strip with some sort of FET, or to PWM the enable signal to the switching regulator itself?
If your boost circuit does have an enable pin that's fast enough for your job (most have, some don't), use that. It saves you additional circuitry, at the very least, and you'll never run your boost circuit without load (which, again, most modern switchers could withstand, but not knowing yours, I can't tell).
Also, if you're concerned about efficiency, start by eliminating the series resistor on your LEDs! That's where energy is wasted. Since you already have a reliable power source, you should be able to run the LEDs directly if you just reduce the output voltage of your boost converter accordingly. In fact, for LED applications, there's even current-controlled boost converters, so that you build a constant current source instead of a constant voltage source – exactly what your LEDs need.
Other things being equal, they are more or less equivalent in efficiency. Obviously if you can modulate the switcher, you do not need the extra complexity of a PWM switch at its output.
You therefore have to see whether the hardware you have is actually capable of doing what you want.
Some switchers, when you toggle the enable, will start and stop quickly, some will start slowly. Does the one you have start and stop fast enough for practical flicker-free PWM rates?
