Datasheet

R
PL1
+ R
PL
ǒ
R
CS
R
DCS
Ǔ
R
PL2
+
R
PL1
ǒ
R
CS
R
DCS
Ǔ
* 1
R
CS
R
PL1
R
PL2
From power
MOSFET
To CS
R
DCS
R
PL
From power
MOSFET
To CS
(a) (b)
UCC28600
www.ti.com SLUS646J –NOVEMBER 2005– REVISED JULY 2011
PRACTICAL DESIGN NOTES
Non-Ideal Current Sense Value
Resistors R
CS
, R
PL
, R
OVP1
and R
OVP2
must be programmed as a set due to functional interactions in the
converter. Often, the ideal value for R
CS
is not available because the selection range of current sense resistors is
too coarse to meet the required power limit tolerances. This issue can be solved by using the next larger
available value of R
CS
and use a resistive divider with a Thevenin resistance that is equal to the ideal R
PL
value
in order to attenuate the CS signal to its ideal value, as shown in Figure 14. The equations for modifying the
circuit are:
(4)
• R
DCS
= ideal, but non-standard, value of current sense resistor.
• R
PL
= previously calculated value of the power limit resistor.
(5)
• R
CS
= available, standard value current sense resistor.
The board should be laid out to include R
PL2
in order to fascillitate final optimization of the design based upon
readily available components.
Figure 14. Modifications to Fit a Standard Current Sense Resistor Value
Copyright © 2005–2011, Texas Instruments Incorporated 21