NCP81174N Datasheet, PDF (14/18 Pages) ON Semiconductor – 4/3/2-Phase Synchronous Buck Controller

NCP81174N
DESIGN METHODOLOGY
Programming the Current Limit
The VREF pin provides a 2.0 V reference voltage which
is divided down with a resistor divider (RLIM1/RLIM2) and
fed into the current limit pin ILIM. 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. If not using thermal compensation, the best
approach is to set the maximum current limit based on
expected average maximum temperature of the inductor
windings,
Acssum
Adrp
RNOR
I1
I2
+
I3
I4
RISO1 RT2 RISO2
−
RSUM
+
+
−
Ilim
Figure 7. ACSSUM and ADRP
OCP
event
(eq. 5)
DCR_Tmax + DCR₂₅o @ (1 ) 0.00393 @ (T_max * 25))
For multiphase controller, the ripple current can be
calculated as,
I_PP
+
ǒ_Vin
* N @ V_out^Ǔ @
L @ F_SW @ V_in
V_out
(eq. 6)
Therefore calculate the current limit voltage as below,
V_LIMIT ^ A_CSSUM @ A_DRP @ DCR_TMAX
ǒ Ǔ _{@ IMIN_OCP @ ) 0.5 @ IPP}
(eq. 7)
V_LIMIT ^ A_CSSUM @ A_DRP @ DCR_TMAX
ǒ Ǔ @
I
MIN_OCP
@
)
0.5
@
ǒ_Vin
* N @ V_out^Ǔ @
L @ F_SW @ V_in
V_out
In Equation 7, A_CSSUM and A_DRP are the gain of current
summing amplifier and droop amplifier.
As introduced before, V_LIMIT comes from a resistor
divider connected to VREF, thus
V_LIMIT
+
2
V
@
R_LIM2
R_LIM1 ) R_LIM2
@
COEpsi
(eq. 8)
A_CSSUM X+ −3.93
R_NOR @ ^ǒR_ISO1 ) R_ISO2 ) R_T2^{Ǔ (eq. 9)}
A_DRP + − _{ǒRNOR ) RISO1 ) RISO2 ) RT2Ǔ @ RSUM}
RISO1 and RISO2 are in series with R_T2, the NTC
temperature sense resistor placed near inductor. RSUM is
the resistor connecting between pin VDFB and pin CSSUM.
In PS0 mode, the current limit follows the Equation 10; In
PS1 or PS2, the current limit calculation follows
Equation 11, COEpsi is a coefficient for the current limiting
related in power saving mode PS1, PS2. COEpsi value is one
over the original phase count N. Refer to the PSI and phase
shedding section for more details.
2 V@R_LIM2
ǒ Ǔ ^{ILIMIT(normal)}
^
3.93
@
R_LIM1)R_LIM2
R_NOR@ǒR_ISO1)R_ISO2)R_T2Ǔ
ǒR_NOR)R_ISO1)R_ISO2)R_T2Ǔ@R_SUM
@
DCR_25°
@
1 ) 0.00393 @ ^ǒT_inductor * 25^Ǔ
(eq. 10)
* 0.5 @ ^{ǒVin * N @ VoutǓ @ Vout}
L @ F_SW @ V_in
2 V@R_LIM2
R_LIM1)R_LIM2 ^{@ COEpsi}
ǒ Ǔ ^ILIMIT(PSI)
^
3.93
@
R_NOR@ǒR_ISO1)R_ISO2)R_T2Ǔ
ǒR_NOR)R_ISO1)R_ISO2)R_T2Ǔ@R_SUM
@
DCR_25°
@
1 ) 0.00393 @ ^ǒT_inductor * 25^Ǔ
ǒ_{Vin * Vout}Ǔ _{@ Vout}
* 0.5 @
(eq. 11)
L @ F_SW @ V_in
N is the number of phases involved in the circuit.
The first cut approach is to use a 0.1 mF capacitor for C and
Inductor Current Sensing Compensation
The NCP81174N uses the inductor current sensing
method. An RC filter is selected to cancel out the impedance
then solve for R.
R_sense(T)
+
0.1
@
mF
@
DCR_25C
@
L
(1 )
0.00393
@
(T
*
(eq.
25))
12)
from inductor and recover the current information through
Because the inductor value is a function of load and
the inductor’s DCR. This is done by matching the RC time inductor temperature final selection of R is best done
constant of the sensing filter to the L/DCR time constant. experimentally on the bench by monitoring the VDRP pin
and performing a step load test on the actual solution.
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