NCP81246
www.onsemi.com
21
During steady state operation, the duty cycle is centered
on the valley of the sawtooth ramp waveform. The steady
state duty cycle is still calculated by approximately
V
OUT
/V
IN
.
Two-Phase Rail Phase Detection Sequence
During start-up, the number of operational phases and
their phase relationship is determined by the internal
circuitry monitoring the CSP outputs. Normally, this rail
operates as a two-phase VCC_Rail2 PWM controller. If
CSP2_2ph is pulled high to VCC, the two-phase rail
operates as a single-phase rail.
Disable Single-Phase Rail
If the NCP81246 is to provide fewer than three rails, one
or both of the single-phase rails can be disabled by pulling
up their respective CSP pin. The main rail cannot be
disabled.
Single-Phase Rails
The architecture of the two single-phase rails makes use
of a digitally enhanced, high performance, current mode
RPM control method that provides excellent transient
response while minimizing transient aliasing. The average
operating frequency is digitally stabilized to remove
frequency drift under all continuous mode operating
conditions. At light load the single-phase rails automatically
transition into DCM operation to save power.
Single-Phase Rail Remote Sense Error Amplifier
A high performance, high input impedance, true
differential transconductance amplifier is provided to
accurately sense the regulator output voltage and provide
high bandwidth transient performance. The VSP and VSN
inputs should be connected to the regulator’s output voltage
sense points through filter networks describe in the Droop
Compensation and DAC Feedforward Compensation
sections. The remote sense error amplifier outputs a current
proportional to the difference between the output voltage
and the DAC voltage:
(eq. 13)
I
COMP
+ gm @
ǒ
V
DAC
*
ǒ
V
VSP
* V
VSN
Ǔ
Ǔ
This current is applied to a standard Type II compensation
network.
Single-Phase Rail Voltage Compensation
The Remote Sense Amplifier outputs a current that is
applied to a Type II compensation network formed by
external tuning components CLF, RZ and CHF.
Figure 15.
VSN
VSP
COMP
R
Z
CLF
CHF
VSN
VSP
+
−
+
DAC
gm
S
Single-Phase Rail – Differential Current Feedback
Amplifier
Each single-phase controller has a low offset, differential
amplifier to sense output inductor current. An external
lowpass filter can be used to superimpose a reconstruction
of the AC inductor current onto the DC current signal sensed
across the inductor. The lowpass 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
Phase
(eq. 14)
°
(eq. 15)
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 lowpass 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
