ncp5381a ON Semiconductor, ncp5381a Datasheet - Page 29

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

Programming the Current Limit and the Oscillator Frequency
approximately 300 kHz. The OSC pin provides a 2.0 V
reference voltage which is divided down with a resistor
divider and fed into the current limit pin ILIM. Calculate
the total series resistance to set the frequency and then
calculate the individual values for current limit divider.
ground. This current is internally mirrored into a capacitor
to create an oscillator. The period is proportional to the
resistance and frequency is inversely proportional to the
resistance. The resistance may be estimated by equation 2
or 3 depending on the phase count.
current of all phases multiplied by a gain of 5.94. DCR
sensed inductor current is function of the winding
temperature. The best approach is to set the maximum
Calculate the current limit voltage:
Solve for the individual resistors:
Final Equation for the Current Limit Threshold
0.75 mW. Selecting the closest available values of 16.9 kW
for RLIM1 and 15.8 kW for RLIM2 yield a nominal
operating frequency of 305 kHz and an approximate
current limit of 180 A at 100°C. The total sensed current
can be observed as a scaled voltage at the VDRP pin added
to a positive, no−load offset of approximately 1.3 V.
I LIMIT (T inductor ) ^
The demo board is set for an operating frequency of
The series resistors RLIM1 and RLIM2 sink current to
The current limit function is based on the total sensed
The inductors on the demo board have a DCR at 25°C of
V ILIMIT ^ 5.94 · I MIN_OCP · DCR Tmax )
5.94 · (DCR 25C · (1 ) 0.00393 · C −1 (T Inductor −25 · C)))
RLIM2 +
100
90
80
70
60
50
40
30
20
10
0
V ILIMIT · R OSC
Figure 17. ROSC vs. Phase Frequency
RLIM1)RLIM2
2 · V
2 · V · RLIM2
http://onsemi.com
) 0.02
Frequency (kHz)
DCR 50C · Vout
2 · Vin · F s
29
current limit based on the expected average maximum
temperature of the inductor windings.
Inductor Selection
that the inductor does not saturate by more than 10% at
maximum load. The inductor also must not go into hard
saturation before current limit trips. The demo board includes
a four phase output filter using the T50−8 core from
Micrometals with 4turns and a DCR target of 0.75 mW @
25°C. Smaller DCR values can be used, however, current
sharing accuracy and droop accuracy decrease as DCR
decreases. Use the excel spreadsheet for regulation accuracy
calculations for a specific value of DCR.
When using inductor current sensing it is recommended
RLIM1 + R OSC −R LIM2
· Vin−Vout
4 Phase Mode
3 Phase Mode
DCR Tmax + DCR 25C ·
(1 ) 0.00393 · C −1 (T Tmax −25 · C))
32.36 kW ^ 10.14
ROSC + 10.14
ROSC + 9.711
*
L
2 · Vin · F s
Vout
* (N−1) · Vout
4 Phase Mode
3 Phase Mode
Frequency
Frequency
300 · k
· Vin−Vout
10 9
10 9
L
10 9
L
* 1440
* 1111
* 1440
* 0.02
* (N−1) · Vout
(eq. 5)
(eq. 6)
(eq. 7)
(eq. 2)
(eq. 3)
(eq. 4)
L