ncp5381a ON Semiconductor, ncp5381a Datasheet - Page 32

• Manufacturer Part Number: ncp5381a
• Description: 2/3/4 Phase Buck Controller For Vr10 And Vr11 Pentium Iv Processor Applications
• Manufacturer: ON Semiconductor

100 W for maximum phase boost. The value of RF is
typically set to 4.0 kW.
Droop Injection and Thermal Compensation
output currents for each phase and applying a gain of
approximately six. VDRP is externally summed into the
feedback network by the resistor RDRP. This induces an
offset which is proportional to the output current thereby
forcing the controlled resistive output impedance.
effect out to a great degree by adding an NTC (negative
temperature coefficient resistor) in parallel with RFB to
reduce the droop gain as the temperature increases. The
NTC device is nonlinear. Putting a resistor in series with the
equation 13.
Beta of 4300. Figure 21 shows the uncompensated and
compensated output impedance versus temperature.
help with the selection of the NTC. The actual selection of
the NTC will be effected by the location of the output
inductor with respect to the NTC and airflow, and should
be verified with an actual system thermal solution.
Figure 21. Uncompensated and Compensated Output
RFB is always set to 1.0 kW and RFB1 is usually set to
The VDRP signal is generated by summing the sensed
The system can be thermally compensated to cancel this
The output impedance varies with inductor temperature by the equation:
By including the NTC RT2 and the series isolation resistors the new equation becomes:
The typical equation of a NTC is based on a curve fit
The demo board is populated with a 10 kW NTC with a
ON Semiconductor provides an excel spreadsheet to
RT2(T) + RT2 25C · e b
Impedance vs. Temperature
Zout(T) +
Zout(T) +
RFB · (RISO1)RT2(T))RISO2)
RFB)RISO1)RT2(T))RISO2
273 ) T
DCR Tmax + DCR 25C · (1 ) 0.00393 · C −1 (T Tmax −25 · C))
1
RFB · DCR 25C · (1 ) 0.00393 · C −1 (T max −25C)) · 5.94
*
298
1
(eq. 14)
· DCR 25C · (1 ) 0.00393 · C −1 (T max −25C)) · 5.94
http://onsemi.com
32
Rdroop
Rdroop
basic equation:
according to the following equation 10:
NTC helps make the device appear more linear with
temperature. The series resistor is split and inserted on both
sides of the NTC to reduce noise injection into the feedback
loop. The recommended value for RISO1 and RISO2 is
approximately 1.0 kW.
VRHOT and VRFAN
comparators for thermal monitoring. The circuit consists of
two comparators that compare the voltage on the NTC pin
to an internal resistor divider connected to VREF. By
powering the external temperature sense divider with
VREF the tolerance of the VREF voltage is canceled out.
The data sheet specifications for the thresholds are shown
as ratios with respect to VREF.
VR_FAN Upper Threshold Ratio = 0.3625
VR_FAN Lower Threshold Ratio = 0.3025
VR_HOT Upper Threshold Ratio = 0.2815
VR_HOT Lower Threshold Ratio = 0.2190
temperature trip points.
Beta of 4750. RNTC1 is populated with 15 kW and RNTC2
is populated with a zero ohm resistor. Figure 22 is a plot of
equation 15. The horizontal trip thresholds intersect the
Ratio
RRDP determines the target output impedance by the
The value of the inductor’s DCR varies with temperature
Thermal monitoring provides two threshold sensitive
The following equations can be used to find the
The demo board contains a 68 K NTC for RT1 with a
RT1(T) + RT1 25C · e b
Ratio NTC (T) :
NTC
curve.
Vout
RDRP + RFB · DCR · 5.94
Iout
+ Zout + RFB · DCR · 5.94
RNTC1 ) RNTC2 ) RT1(T)
RNTC2 ) RT1(T)
Zout
273 ) T
1
RDRP
*
298
1
(eq. 12)
(eq. 13)
(eq. 11)
(eq. 15)
(eq. 16)
(eq.
10)