ncp5381 ON Semiconductor, ncp5381 Datasheet - Page 30

• Manufacturer Part Number: ncp5381
• Description: Ac-dc Offline Switching Controllers/regulators
• 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
current of all phases multiplied by a gain of 5.84. 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 167 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.
V ILIMIT ^ I MIN_OCP · DCR Tmax · 5.84
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 ^ 5.84 · I MIN_OCP · DCR Tmax )
5.84 · (DCR 25C · (1 ) 0.00393 · C −1 (T Inductor −25 · C)))
RLIM1)RLIM2
2 · V · RLIM2
RLIM2 +
http://onsemi.com
NCP5381
Figure 26.
30
V ILIMIT · R OSC
DCR 50C · Vout
2 · Vin · F s
resistance and frequency is inversely proportional to the
resistance. The resistance may be estimated by equation 2.
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.
2 · V
When using inductor current sensing it is recommended
DCR Tmax + DCR 25C ·
(1 ) 0.00393 · C −1 (T Tmax −25 · C))
· Vin−Vout
ROSC ^ 9.98
33.3 · kW ^ 9.98
*
2 · Vin · F s
L
Vout
RLIM1 + R OSC −R LIM2
Frequency Phase
* (N−1) · Vout
· Vin−Vout
10 9 · Hz · W
300 · kHz
10 9 · Hz · W
L
L
* (N−1) · Vout
(eq. 4)
(eq. 5)
(eq. 6)
(eq. 2)
(eq. 3)
L