MPM3695-25 Remote sense

DC-DC Power Converters
1

Hi,

We want to use the VOSNS+/- pins for remote sensing using dedicated sense pins in the load ASIC.
The problem is that those sense pins can source and sink only very little current which will not be enough to create the required voltage drop on the resistor divider of R2 and R1 on VOSNS pins.

Currently, the resistor divider is based on the recommended value of R2=1K.
Is it possible to use larger resistor values for the resistor divider, e.g. R2=100K?

Thanks,

2

Hi rbenbassat,

Yes, you can use larger resistor values, just be aware that R1 and R2 are used to set the feedback gain. The equation can be found under “Output Voltage Setting” on page 23 of the datasheet.

Thanks,
Cindy

3

Thanks Cindy.
Will this have an effect on the feedback loop response time or bandwidth?
Do I need to change the value of Cff also?

4

It shouldn’t have any effect if you compensate with the feed forward capacitor correctly. But you will need to figure the optimal capacitance since the recommended values are for the smaller resistors.

Cindy

5

Can you give any equation or guide lines for choosing the correct Cff capacitance?

  • Rafi
6

Sure, so Cff is used to improve the phase margin and bandwidth by adding a zero and a pole at the crossover frequency. The value chosen will depend on your circuity and crossover frequency. You’ll need to measure this using either transient analysis or a network analyzer. The following equations can be used to calculated the zero(fz) and pole(fp) frequency based off Cff, R1 and R2.
fz = 1/(2piR1Cff)
fp = 1/(2piCff)(I/R2 +1/R1)