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NCP5385MNR2G

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27P28-P30P31-P33P34-P34
NCP5385
http://onsemi.com
28
Figure 16. VR10 Legacy Startup
Programming the Current Limit and the Oscillator Frequency
The demo board is set for an operating frequency of
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.
The series resistors RLIM1 and RLIM2 sink current to
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
or 3 depending on the phase count.
ROSC +
10.14 10
9
Frequency
* 1440
(eq. 2)
32.36 k ^
10.14 10
9
300 · k
* 1440
4 Phase Mode
ROSC +
9.711 10
9
Frequency
* 1111
(eq. 3)
3 Phase Mode
Figure 17. ROSC vs. Phase Frequency
0
10
20
30
40
50
60
70
80
90
100
100
200
300
400
500
600
700
800
900
1000
Frequency (kHz)
ROSC (kOhms)
4 Phase Mode
3 Phase Mode
The current limit function is based on the total sensed
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
current limit based on the expected average maximum
temperature of the inductor windings.
DCR
Tmax
+ DCR
25C
·
(eq. 4)
(1 ) 0.00393 · C
−1
(T
Tmax
−25 · C))
Calculate the current limit voltage:
V
ILIMIT
^ 5.94 ·
ǒ
I
MIN_OCP
· DCR
Tmax
)
DCR
50C
· Vout
2·Vin·F
s
·
ǒ
Vin−Vout
L
* (N−1) ·
Vout
L
Ǔ
Ǔ
* 0.02
(eq. 5)
Solve for the individual resistors:
(eq. 6)
RLIM2 +
V
ILIMIT
·R
OSC
2·V
RLIM1 + R
OSC
−R
LIM2
NCP5385
http://onsemi.com
29
Final Equation for the Current Limit Threshold
I
LIMIT
(T
inductor
) ^
ǒ
2·V·RLIM2
RLIM1)RLIM2
Ǔ
) 0.02
5.94 · (DCR
25C
·(1) 0.00393 · C
−1
(T
Inductor
−25 · C)))
*
Vout
2·Vin·F
s
·
ǒ
Vin−Vout
L
* (N−1) ·
Vout
L
Ǔ
(eq. 7)
The inductors on the demo board have a DCR at 25°C of
0.75 m. Selecting the closest available values of 16.9 k
for RLIM1 and 15.8 k 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.
Inductor Selection
When using inductor current sensing it is recommended
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 m @
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.
NCP5385
http://onsemi.com
30
Inductor Current Sense Compensation
The NCP5385 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 F capacitor for C and then solve for R.
Rsense(T) +
L
0.47 · F · DCR
25C
·(1) 0.00393 · C
−1
·(T−25 · C))
(eq. 8)
Figure 18.
The demoboard inductor measured 350 nH and 0.75 m
at room temp. The actual value used for Rsense was 953
which 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.
Simple Average PSPICE Model
A simple state average model shown in Figure 19 can be
used to determine a stable solution and provide insight into
the control system.
Figure 19.
+
−
+
−
−
+
VRamp_min
1.3 V
+
−
E
E1
GAIN = 6
0
Vin
12
0
12
0
L
12
(250e−9/4)
DCR
(0.85e−3/4)
Voff
RDRP
5.11 k
CH
22 p
RF
4.3 k
CF
1.5 n
+
−
1E3
R6
1 k
Unity
Gain
BW = 15 MHz
C3
10.6 n
0
Voffset
−
+
−
+
0
−
+
0
+
−
VDAC
1.25 V
CFB1
RFB1
680 p
100
RFB
1 k
RBRD
0.75 m
LBRD
12
100 p
CBulk
(560e−6*10)
ESRBulk
(7e−3/10)
ESLBulk
(3.5e−9/10)
2
1
+
−
I2
TD = 10u
TF = 50n
PW = 40u
PER = 80u
I1 = 10
I2 = 110
TR = 50n
+
−
1Aac
0Adc
ESRCer
(1.5e−3/18)
ESLCer
(1.5e−9/18)
1
CCer
(22e−6*18)
2
0
4
Voff
1.3
Vout
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NCP5385MNR2G

Mfr. #:
Buy NCP5385MNR2G
Manufacturer:
ON Semiconductor
Description:
IC REG CTRLR INTEL 6OUT 40QFN
Lifecycle:
New from this manufacturer.
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