NCP5392P
internally mirrored into a capacitor to create an oscillator.
The current limit function is based on the total sensed
current of all phases multiplied by a controlled gain
(Acssum*Adrp). DCR sensed inductor current is a function
of the winding temperature. The best approach is to set the
maximum current limit based on expected average
maximum temperature of the inductor windings,
The period is proportional to the resistance and frequency
is inversely proportional to the total resistance. The total
resistance may be estimated by Equation 1. This equation
is valid for the individual phase frequency in both three and
four phase mode.
Rosc ^ 20947 FSWā*1.1262
30.5ꢀkW ^ 20947 330*1.1262
(eq. 1)
(eq. 2)
DCRTmax + DCR25C(1 ) 0.00393 @ (Tmax * 25))
60
50
40
30
20
10
Calculation
Real
0
100
1000
Freq-kHz
Figure 11. ROSC vs. Frequency
For multiphase controller, the ripple current can be calculated as,
(Vin * N @ Vout) @ Vout
(eq. 3)
(eq. 4)
Ipp +
L @ FSW @ Vin
Therefore calculate the current limit voltage as below,
VLIMIT ^ ACSSUM @ ADRP @ DCRTmax @ (IMIN_OCP @ ) 0.5 @ Ipp)
(Vin * N @ Vout) @ Vout
@ ǒ
are the gain of current summing amplifier and droop amplifier.
Ǔ
VLIMIT ^ ACSSUM @ ADRP @ DCRTmax
IMIN_OCP @ ) 0.5 @
L @ FSW @ Vin
In Equation 4, A
and A
CSSUM
DRP
Acssum
Adrp
RNOR
R
temperature sense resistor placed near inductor. R
and R
are in series with R , the NTC
is
ISO1
ISO2 T2
SUM
RISO1
RSUM
the resistor connecting between pin VDFB and pin
CSSUM. If PSI = 1, PSI function is off, the current limit
follows the Equation 7; if PSI = 0, the power saving mode
will be enabled, COEpsi is a coefficient for the current
limiting related with power saving function (PSI), the
current limit can be calculated from Equation 8. COEpsi
value is one over the original phase count N. Refer to the
PSI and phase shedding section for more details.
RISO2
I1
RT2
-
+
I2
I3
I4
+
+
-
OCP
event
Ilim
Figure 12. ACSSUM and ADRP
As introduced before, V
comes from a resistor
LIMIT
divider connected to Rosc pin, thus,
RLIM2
(eq. 5)
(eq. 6)
VLIMIT + 2ꢀV @
RLIM1 ) RLIM2
@ COEpsi
ACSSUM + *4
RNOR @ (RISO1 ) RISO2 ) RT2
)
ADRP + *
(RNOR ) RISO1 ) RISO2 ) RT2) @ RSUM
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