IRFR1018EPBF Datasheet, PDF (13/15 Pages) International Rectifier – HEXFET TM Power MOSFET

NCP5383
120
Calculate the current limit voltage:
The current limit function is based on the total sensed
100
current of two phases multiplied by a gain of 5.94. DCR
sensed inductor current is function of the winding
80
temperature. The best approach is to set the maximum
current limit based on the expected average maximum
60
temperature of the inductor windings.
40
DCRTmax + DCR25CĂ·Ă
(1 ) 0.00393Ă·ĂC-1Ă(TTmax-25Ă·ĂC))
(eq. 1)
20
0
100
200
300
400
FREQUENCY (KHz)
Figure 18. ROSC vs. Phase Frequency
ǒ ǒ ǓǓ VILIMIT ^ 5.94Ă·Ă
IMIN_OCPĂ·ĂDCRTmax ) ^{DC2RĂ·Ă5V0iCnĂĂ··ĂĂFVsoutĂ} ·Ă
Vin-Vout
L
*
(N-1)Ă·Ă
Vout
L
* 0.02
Solve for the individual resistors:
RLIM2
+
VILIMITĂ·ĂROSC
2Ă·ĂV
RLIM1 + ROSC-RLIM2
(eq. 2)
(eq. 3)
Final Equation for the Current Limit Threshold
ǒ Ǔ ILIMIT(Tinductor)
^
ǒ Ǔ 2Ă·ĂVĂ·ĂRLIM2
RLIM1)RLIM2
) 0.02
5.94Ă·Ă(DCR25CĂ·Ă(1 ) 0.00393Ă·ĂC-1(TInductor-25Ă·ĂC)))
*
2Ă·ĂVVoinuĂ·tĂFsĂ
·Ă
Vin-Vout
L
*
1Ă·Ă
Vout
L
Selecting the closest available values of 16.9ĂKW for
RLIM1 and 15.8 KW yield a nominal operating frequency
(eq. 4)
of 305 Khz and an approximate current limit of 180ĂA at
100C. The total sensed current can be observed at the
VDRP pin added to a positive, no-load offset of
approximately 0.8ĂV.
Inductor Selection:
When using the inductor current sensing it is
recommended that the inductor does not saturate by more
than 10% at the maximum load. The inductor also must not
go into hard saturation before current limit trips. Small
DCR values can be used, however current sharing accuracy
and droop accuracy decrease as DCR decreases.
Inductor Current Sense Compensation
The NCP5383 uses the inductor current sensing method.
This method uses an RC filter to cancel out the inductance
of the inductor and recover the voltage that is the result of
the current flowing through the inductor's DCR. This is
done by matching the RC time constant of the current sense
filter to the L/DCR time constant. The first cut approach is
to use a 0.47 mF capacitor for C and then solve for R.
Rsense(T) +
(eq. 5)
L
0.47Ă·ĂmFĂ·ĂDCR25CĂ·Ă(1 ) 0.00393Ă·ĂC-1Ă·Ă(T-25Ă·ĂC))
Figure 19.
The demoboard inductor measured 950 nH and 0.75 mW
at room temp. The actual value used for Rsense matches the
equation for Rsense at approximately 50C. Because the
inductor value is a function of load and inductor
temperature final selection of R is best done
experimentally on the bench by monitoring the Vdroop pin
and performing a step load test on the actual solution.
It is desirable to keep the Rsense resistor value below
1.0 k whenever possible by increasing the capacitor values
in the inductor compensation network. The bias current
flowing out of the current sense pins is approximately
100 nA. This current flows through the current sense
resistor and creates an offset at the capacitor which will
appear as a load current at the Vdroop pin. A 1.0 k resistor
will keep this offset at the droop pin below 2.5 mV.
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