TPS51640A, TPS59640, TPS59641
www.ti.com
SLUSAQ2 –JANUARY 2012
L
= C
´R
EQ
SENSE
R
DCR
where
•
REQ is the series/parallel combination of RSEQU, RNTC, RSERIES and RPAR
(9)
(10)
(11)
RP _N
REQ
=
R
SEQU + RP _N
R
´ R
+ R
+ R
(
+ R
)
PAR
NTC
SERIES
R
=
P _N
R
PAR
NTC
SERIES
CSENSE should be a capacitor type which is stable over temperature. Use X7R or better dielectric (C0G
preferred).
Since calculating these values by hand is difficult, TI has a spreadsheet using the Excel Solver function available
to calculate them. Contact a local TI representative to get a copy of the spreadsheet.
In this design, the following values are input to the spreadsheet:
•
•
•
•
•
L = 0.36 µH
RDCR = 0.825 mΩ
Load Line, RIMVP = -1.9 mΩ
Minimum overcurrent limit = 112 A
Thermistor R25 = 100 kΩ and "B" value = 4250 kΩ
The spreadsheet then calculates the OCP (overcurrent protection) setting and the values of RSEQU, RSERIES
,
RPAR, and CSENSE. In this case, the OCP setting is the resistor value selection of 56 kΩ from COCP-I to GND and
GOCP-I to GND. The nearest standard component values are:
•
•
•
•
RSEQU = 17.8 kΩ;
RSERIES = 28.7 kΩ;
RPAR = 162 kΩ
CSENSE =33 nF
Note the effective divider ratio for the inductor DCR. The effective current sense resistance (RCS(eff)) is shown in
Equation 12.
RP _N
RCS eff = RDCR
´
( )
R
SEQU + RP _N
where
•
RP_N is the series/parallel combination of RNTC, RSERIES and RPAR
.
(12)
(13)
RCS eff ´ ACS
( )
0.66mW ´12
RGDROOP
=
=
= 4.12kW
RLL ´ GM
3.9mW ´0.497mS
RCS(eff) is 0.66 mΩ.
Step Seven: Set current monitor (IMON) setting resistor.
After the OCP selection resistor is selected in Step 6, the IMON is set by the resistor from CIMON to COCP-I
(RCIMON) and GIMON to GOCP-I (RGIMON). Based on Equation 2 and Equation 3,
RCIMON = 71.5 kΩ and RGIMON = 309 kΩ
Step Eight: Set the load line.
The load-line for CPU channel is set by the resistor, RCDROOP from CCOMP to VREF. The load-line for GPU
channel is set by the resistor, RGDROOP from the GCOMP pin to VREF. Using the Equation 1, the droop setting
resistors are calculated in Equation 14 and Equation 15.
RCS eff ´ ACS
( )
0.66mW ´12
RCDROOP
=
=
= 8.45kW
RLL ´ GM
1.9mW ´0.497mS
(14)
41
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