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- 15th September 2009, 14:35 #1

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## voltage drop formula

what factors effect the voltage drop on PWM dc-dc converter?

- 15th September 2009, 14:35

- 15th September 2009, 20:20 #2

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## why current drop in pwm

The voltage drop input-output? That's the step-down

ratio which becomes roughly the PWM duty cycle,

VOUT/VIN. The drop is (duh) VIN-VOUT.

The voltage droop, or load regulation, is pretty much

just the error amplifier (and powertrain) composite

gain as a chopped transconductor. If you are current

mode controlled then you have a volts-in, current-out

relation somewhere in there (say, 1A/V) and a volts/volts

error amp gain (say, 60dB=10,000). So at the error amp

input you would see for a 1A load step, a 1/10,000 (or 0.1

mV) input deflection. Now scale that up by the feedback

network ratio (say, 5:1 for a 28V to 5V buck) and you get

a 0.5mV output deflection. All happy, right?

Well, probably not. A major, major term in load regulation

is the current induced offsets in the ground and the feedback

lines, which develop current-dependent error voltages. If

the load current return and the PWM ground are offset,

this may go right into the error amp (prime reference is

chip ground, != package, !=board ground). You would like

a Kelvin ground and load sense, but this is not always a

practical thing.

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