ncp5389 ON Semiconductor, ncp5389 Datasheet - Page 22

• Manufacturer Part Number: ncp5389
• Description: 2/3 Phase Buck Controller For 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.
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
100
100
90
90
80
80
70
70
60
60
50
50
40
40
30
30
20
20
10
10
0
0
0
0
F
OSC
V ILIMIT ^ 5.94 · I MIN_OCP · DCR Tmax )
Figure 14. ROSC vs. Phase Frequency
200
200
F
F
(2, Calculated)
OSC
OSC
(2, Measured)
(2, Measured)
5.94 · (DCR 25C · (1 ) 0.00393 · C - 1 (T Inductor - 25 · C)))
400
400
RLIM2 +
F
OSC
(kHz)
600
600
V ILIMIT · R OSC
RLIM1)RLIM2
2 · V
2 · V · RLIM2
800
800
http://onsemi.com
1000
1000
) 0.02
NCP5389
DCR 50C · Vout
2 · Vin · F s
22
This equation is valid for the individual phase frequency in
both three and four phase mode.
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.
100
When using inductor current sensing it is recommended
90
80
70
60
50
40
30
20
10
RLIM1 + R OSC - R LIM2
0
0
F
· Vin- Vout
OSC
DCR Tmax + DCR 25C ·
(1 ) 0.00393 · C - 1 (T Tmax - 25 · C))
32.36 kW ^ 10.14
ROSC + 10.14
(3, Measured)
Figure 15. R
ROSC + 9.711
*
200
F
L
OSC
2 · Vin · F s
(3, Calculated)
Vout
* (N- 1) · Vout
2 Phase Mode
3 Phase Mode
400
Frequency
Frequency
OSC
F
300 · k
OSC
· Vin- Vout
vs. 3-Phase Mode
(kHz)
10 9
10 9
L
10 9
600
L
* 1440
* 1111
* 1440
* 0.02
* (N- 1) · Vout
800
1000
(eq. 5)
(eq. 6)
(eq. 7)
(eq. 2)
(eq. 3)
(eq. 4)
L