The purpose of the resistor between VDRV and CS is to apply a DC bias to the CS pin so that the voltage on the current resistor is reduced for a certain desired peak current limit. This allows the power dissipation of the current resistor to be reduced and improve efficiency.
Below is part of the MAX17599 schematic.
The resistor, R17 and capacitor, C11 are always selected at 220Ω and 1000pF respectively. VR18max can be chosen value between 290mV and 150mV. Below is the formula that calculates R16 and R18.
For example, you calculate the maximum primary peak current, Ipeak_max equal 5A and select VR18max=150mA. The R16 and R18 are calculated as table below.
|
R16(Ω) |
R18(Ω) |
VR18max(V) |
Vcs-peak(V) |
VDRV(V) |
R17(Ω) |
C119pF) |
Ipeakmax(A) |
|
7401.4 |
0.03 |
0.15 |
0.29 |
5 |
220 |
1000 |
5 |