NCP5392Q
http://onsemi.com
25
Actual DCR increases by temperature, the system can be
thermally compensated to cancel this effect to a great
degree by adding an NTC in parallel with R
NOR
to reduce
the droop gain as the temperature increases. The NTC
device is nonlinear. Putting a resistor in series with the NTC
helps make the device appear more linear with
temperature. The series resistor is split and inserted on both
sides of the NTC to reduce noise inj ection into the feedback
loop. The rec ommended tot al value for R
ISO1
plus R
ISO2
is
approximately 1.0 kΩ.
The output impedance varies with inductor temperature
by the e quation:
Z
out
(T) =
R
FB
⋅ DCR
25C
⋅ (1 + 0.00393 ⋅ (T − 25)) ⋅ A
CSSUM
⋅ A
DRP
R
DRP
(eq. 15)
By incl uding the NTC R
T2
and the series isolation resistors the new equation becomes:
Z
out
(T) =
R
FB
⋅ DCR
25C
⋅ (1 + 0.00393 ⋅ (T − 25)) ⋅ A
CSSUM
⋅
R
NOR
⋅(R
ISO1
+R
ISO2
+R
T2
)
(R
NOR
+R
ISO1
+R
ISO2
+R
T2
)⋅R
SUM
R
DRP
(eq. 16)
The typica l equation of an NTC is based on a curve fit
Equation 17
RT2(T) = RT2
25C
⋅ e
β
1
273+T
−
1
298
(eq. 17)
The demo board use a 10 kΩ NTC with a β value of 3740.
Figure 16 shows the comparison of the compensated output
impedance and uncompensated output impedance varying
with temperature.
0.0006
0.0007
0.0008
0.0009
0.001
0.0011
0.0012
0.0013
25 45 65 85 105
Celsius
Ohm
Zout
Zout(uncomp)
Figure 16. Z
out
vs. Temperature
IMON for Current Monitor
Since VDRP signal reflects the current information of all
phases. It can be fed into the IMON amplifier for current
monitoring as shown in Figure 17. IMON amplifier has a
fixed gain of 2 with an offset when VDRP is equal to 1.3 V,
the internal floating reference voltage. The IMON
amplifier wil l be saturated at an maximum output of 1.09 V
theref ore the total gai n of current should be carefully
considered to make the maximum load c urrent i ndicated by
the IMON output. Figure 18 shows a typical of the relation
betwee n IMON output and the load current.
+
I1
I2
I3
I4
Ilim
Acssum Adrp
OCP
event
+
--
+
--
RISO1
RISO2
RT2
RSUM
RNOR
+
--
Imon
Figure 17. IMON Circuit
Gain = 2
Vimon vs. Iout
0
0.21
0.42
0.63
0.84
1.05
0 102030405060708090100
Iout--A
Vimon--V
Figure 18. IMON Output vs. Output Current
Power Saving Indicator (PSI) and Phase Shedding
VR11.1 requires the processor to provide an output
signal to the VR controller to indicate when the processor
is in a low power state. NCP5392Q use the status of PSI pin
to decide if there is a need to change its operating state to
maximize efficiency at light loads. When PSI = 0, the PSI
function will be enabled, and VR system will be running at
a single phase power saving mode.
The PSI signal will de--assert 1 ms prior to moving to a
normal power state.
