Hi,
In order to make a lowcost circuit, i used Application report SBEA001, in partilcular the laserdiode driver - 1.
but instead of using vcc of OPA350 at the same voltage of transistor, i tried a variant schematic, with 2.7V for OPA in order to get the same current if VBAT goes to 3V, but my pcba failed, led always off.
Could you help me please?
Regards
Fruh,
In order for MJD44H11 N-channel pass transistor to turn-on, the voltage on its gate must be at least 1V above its source, Vgs>1V. Since your LED forward voltage is Vf=2.9V, for LED to turn-on, the minimum gate voltage must be at least >3.9V (Vf+Vgs). However, since the output voltage of OPA350 cannot get higher than its supply voltage of Vs=2.7V, the pass device will always be in off state and that is the problem you see. To fix the issue, connect OPA350 (V+) supply to a higher voltage like your 4.2V external supply as shown on the schematic below.
