> Why not just PWM all the way to the LEDs? Then you don't need a regulator=
..

Pure PWM which applies Vin to the LEDs may cause I_LED to be 3+ times
more than rated value when on.
Some will die very soon, others just have lifetimes reduced or much reduced=
..

Say I =3D 20 mA at 12V with V_LED =3D 3.2V.
R series =3D (12- 3 x 3.2)/0.02  =3D 120 Ohms

Now apply 18V which is required top end of Vin range.
Assume V_LED rises to 3.5V.

I =3D (18- 3 x 3.5) /120 =3D 62.5 mA

Adding a series R and a cap on the LED side will allow the LED Voltage
rise to be controlled, at the expense of capacitor ripple current.

    Russell McMahon
--=20
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