NCP81255
www.onsemi.com
13
Differential Current Feedback Amplifier
The NCP81255 controller has a low offset, differential
amplifier to sense output inductor current. An external
low-pass filter can be used to superimpose a reconstruction
of the AC inductor current onto the DC current signal sensed
across the inductor. The low-pass filter time constant should
match the inductor L/DCR time constant by setting the filter
pole frequency equal to the zero of the output inductor. This
makes the filter AC output mimic the product of AC inductor
current and DCR, with the same gain as the filter DC output.
It is best to perform fine tuning of the filter pole during
transient testing.
F
Z
+
DCR @ 25 C
2 @ p @ L
(eq. 2)
°
(eq. 3)
F
P
+
1
2 @ p @
ǒ
R
PHSP
@
ǒ
R
TH
)R
CSSP
Ǔ
R
PHSP
)R
TH
)R
CSSP
Ǔ
@ C
CSSP
Forming the low-pass filter with an NTC thermistor (R
TH
)
placed near the output inductor, compensates both the DC
gain and the filter time constant for the inductor DCR change
with temperature. The values of R
PHSP
and R
CSSP
are set
based on the effect of temperature on both the thermistor and
inductor. The CSP and CSN pins are high impedance inputs,
but it is recommended that the low-pass filter resistance not
exceed 10 kW in order to avoid offset due to leakage current.
It is also recommended that the voltage sense element
(inductor DCR) be no less than 0.5 mW for sufficient current
accuracy. Recommended values for the external filter
components are:
R
PHSP
= 7.68 kW
R
CSSP
= 14.3 kW
R
TH
= 100 kW, Beta = 4300
(eq. 4)
C
CSSP
+
L
PHASE
R
PHSP
@
ǒ
R
TH
)R
CSSP
Ǔ
R
PHSP
)R
TH
)R
CSSP
@ DCR
Using 2 parallel capacitors in the low-pass filter allows
fine tuning of the pole frequency using commonly available
capacitor values.
The DC gain equation for the current sense amplifier
output is:
(eq. 5)
V
CURR
+
R
TH
) R
CSSP
R
PHSP
) R
TH
) R
CSSP
@ I
OUT
@ DCR
Figure 6.
R
CSSP
CSP
A
V
= 1
−
+
R
TH
R
PHSP
C
CSSP
CSN
To Inductor
Current
Sense AMP
CURR
COMP
RAMP PWM
PWM
Generator
t
The amplifier output signal is combined with the COMP
and RAMP signals at the PWM comparator inputs to
produce the Ramp Pulse Modulation (RPM) PWM signal.
PSYS
The PSYS pin is an analog input to the NCP81255. It is
a system input power monitor that facilitates the monitoring
of the total platform system power. For more details about
PSYS please contact Intel, Inc.
