"Soft start" power input control for PCBs?

P-FET with an R-C filter (differentiator) on the gate to the supply?

O--+-----------+^++--------------------O | ||| | === | || | +----||-----+ || | | .-. | | | | '-' | --- GND

Can you put a TVS across the ceramic cap?

John

Why not a 5v1 zener across the 5v supply? Will the transient overshoot kill it? Will shunting the overshoot kill the 5V source?

Possibly, but that needs tweaked for every new design with component selection to match the switcher controller used.

I'm looking for a universal solution I can use with every PCB (more or less) knowing it'll stop this problem.

Nial.

A zener was my first reaction, but looking at the data sheets of a few likely candidates the max and min zener voltage for each was either below 5V or above 5.5V.

Any this needs component selection per design.

Nial.

--
Version 4
SHEET 1 880 680
WIRE 208 16 96 16
WIRE 368 16 208 16
WIRE 528 16 464 16
WIRE 576 16 528 16
WIRE 208 80 208 16
WIRE 528 144 528 16
WIRE 208 192 208 144
WIRE 256 192 208 192
WIRE 384 192 384 64
WIRE 384 192 336 192
WIRE 96 224 96 16
WIRE 208 256 208 192
WIRE 96 368 96 304
WIRE 208 368 208 336
WIRE 208 368 96 368
WIRE 528 368 528 224
WIRE 528 368 208 368
WIRE 96 416 96 368
FLAG 96 416 0
SYMBOL voltage 96 208 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 5
SYMBOL res 192 240 R0
SYMATTR InstName R1
SYMATTR Value 10k
SYMBOL cap 192 80 R0
SYMATTR InstName C1
SYMATTR Value 1e-6
SYMBOL res 512 128 R0
SYMATTR InstName R3
SYMATTR Value 1
SYMBOL pmos 464 64 M270
SYMATTR InstName M1
SYMATTR Value IRF7210
SYMBOL res 352 176 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R2
SYMATTR Value 100k
TEXT 120 392 Left 0 !.tran .01 startup

y

ooting

er

hen

with

Gonna be real hard to beat the series resistor approach- even at 90% efficiency the input current is ( 1.1 x 3 /0.9)/5=3D0.73A for a dissipation of 0.04 x (0.73)^2=3D20mW in the 40 milliohm, somewhere around 0.5% of output power delivered.

Unfortunately

overshooting

bigger

when

instantly with

My revised circuit / sim:

Version 4 SHEET 1 1180 680 WIRE 208 16 96 16 WIRE 368 16 208 16 WIRE 528 16 464 16 WIRE 576 16 528 16 WIRE 208 80 208 16 WIRE 528 144 528 16 WIRE 208 192 208 144 WIRE 256 192 208 192 WIRE 384 192 384 64 WIRE 384 192 336 192 WIRE 96 224 96 16 WIRE 208 240 208 192 WIRE 320 240 208 240 WIRE 208 256 208 240 WIRE 320 256 320 240 WIRE 96 368 96 304 WIRE 208 368 208 336 WIRE 208 368 96 368 WIRE 320 368 320 320 WIRE 320 368 208 368 WIRE 528 368 528 224 WIRE 528 368 320 368 WIRE 96 416 96 368 =46LAG 96 416 0 SYMBOL voltage 96 208 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 WINDOW 3 39 216 Left 0 SYMATTR InstName V1 SYMATTR Value PWL(0 0 1m 6.3 5m 5 10m 4.7 50m 5.1 53m 2.25 55m 2.05 57m

5.3 60m 5.2 95m 4.9) SYMBOL res 192 240 R0 SYMATTR InstName R1 SYMATTR Value 10k SYMBOL cap 192 80 R0 SYMATTR InstName C1 SYMATTR Value 1e-6 SYMBOL res 512 128 R0 SYMATTR InstName R3 SYMATTR Value 1 SYMBOL pmos 464 64 M270 SYMATTR InstName M1 SYMATTR Value IRF7210 SYMBOL res 352 176 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R2 SYMATTR Value 100k SYMBOL schottky 304 320 M180 WINDOW 0 24 72 Left 0 WINDOW 3 24 0 Left 0 SYMATTR InstName D1 SYMATTR Value 1N5817 SYMATTR Description Diode SYMATTR Type diode TEXT 120 392 Left 0 !.tran .1 startup