NCP5314 ON Semiconductor, NCP5314 Datasheet - Page 16

• Manufacturer Part Number: NCP5314
• Description: Two/Three/Four-Phase Buck CPU Controller
• Manufacturer: ON Semiconductor

current sense signal and the actual inductor current during a
positive step in load current with values of L = 500 nH,
R
ideal current signal compensation would require R
31 k . Due to the faster than ideal RC time constant, there is
an overshoot of 50% and the overshoot decays with a 200 ms
time constant. With this compensation, the I
must be set more than 50% above the full load current to avoid
triggering current limit during a large output load step.
Transient Response and Adaptive Voltage Positioning
filter is frequently sized larger than ripple currents require in
order to reduce voltage excursions during load transients.
Adaptive voltage positioning can reduce peak−peak output
voltage deviations during load transients and allow for a
smaller output filter. The output voltage can be set higher
than nominal at light loads to reduce output voltage sag
when the load current is applied. Similarly, the output
voltage can be set lower than nominal during heavy loads to
reduce overshoot when the load current is removed. For low
current applications, a droop resistor can provide fast,
accurate adaptive positioning. However, at high currents,
the loss in a droop resistor becomes excessive. For example,
a 50 A converter with a 1 m
50 mV change in output voltage between no load and full
load and would dissipate 2.5 W.
alternative to using a droop resistor, but it must respond to
changes in load current. Figure 21 shows how AVP works.
The waveform labeled “normal” shows a converter without
AVP. On the left, the output voltage sags when the output
current is stepped up and later overshoots when current is
stepped back down. With fast (ideal) AVP, the peak−to−peak
excursions are cut in half. In the slow AVP waveform, the
L
The waveforms in Figure 20 show a simulation of the
For applications with fast transient currents, the output
Lossless adaptive voltage positioning (AVP) is an
Load Step with Fast RC Time Constant (50 s/div)
Figure 20. Inductive Sensing Waveform During a
= 1.6 m , R
CSx
= 20 k and C
resistor would provide a
CSx
= .01 mF. In this case,
LIM
pin threshold
CSx
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to be
NCP5314
16
output voltage is not repositioned quickly enough after
current is stepped up and the upper limit is exceeded.
voltage based on the output current of the converter. (Refer to
the application diagram in Figure 1). The no−load positioning
is now set internally to VID − 20 mV, reducing the potential
error due to resistor and bias current mismatches.
network is connected between V
During no−load conditions, the V
voltage as the V
V
V
a loadline set by the resistor divider network.
transient is controlled primarily by power stage output
impedance, and by the ESR and ESL of the output filter. The
transition between fast and slow positioning is controlled by
the total ramp size and the error amp compensation. If the
ramp size is too large or the error amp too slow, there will be
a long transition to the final voltage after a transient. This
will be most apparent with low capacitance output filters.
Overvoltage Protection
the normal operation of the Enhanced V
with synchronous rectifiers. The control loop responds to an
overvoltage condition within 40 ns, causing the GATEx
output to shut off. The (external) MOSFET driver should
react normally to turn off the top MOSFET and turn on the
bottom MOSFET. This results in a “crowbar” action to
clamp the output voltage and prevent damage to the load.
The regulator will remain in this state until the fault latch is
reset by cycling power at the V
Power Good
(PWRGD) signal must be asserted when the output voltage
is within a window defined by the VID code, as shown in
Figure 22.
comparators to accurately sense the output voltage. The
effect of the PWRGD lower threshold can be modified using
a resistor divider from the output to PWRLS to ground, as
shown in Figure 23.
DRP
FB
The controller can be configured to adjust the output
In order to realize the AVP function, a resistor divider
The response during the first few microseconds of a load
Overvoltage protection (OVP) is provided as a result of
According to the latest specifications, the Power Good
The PWRLS pin is provided to allow the PWRGD
voltage higher, causing V
pin voltage increases proportionally. This drives the
Figure 21. Adaptive Voltage Positioning
Normal
Slow
Limits
Fast
FB
Adaptive Positioning
Adaptive Positioning
pin. As the output current increases, the
OUT
CC
pin.
DRP
to “droop” according to
FB
, V
pin is at the same
2
DRP
control topology
and V
OUT
.