XRP7725 Datasheet, PDF (16/34 Pages) Exar Corporation – Power Management System

XRP7725
Intel Node Manager Compatible Programmable
Power Management System
voltage ripple is well controlled and is much
lower than in other architectures which use a
“burst” methodology.
If the output voltage goes outside the
high/low windows, PFM mode is exited and the
PWM loop is reactivated.
Although the PFM mode is effective at
improving efficiency at light load, at very light
loads the dead zone time can increase to the
point where the switching frequency can enter
the audio hearing range. When this happens
some components, like the output inductor
and ceramic capacitors, can emit audible
noise. The amplitude of the noise depends
mainly on the board design and on the
manufacturer and construction details of the
components. Proper selection of components
can reduce the sound to very low levels. In
general Ultrasonic Mode is not used unless
required as it reduces light load efficiency.
Ultrasonic Mode
Ultrasonic mode is an extension of PFM to
ensure that the switching frequency never
enters the audible range. When this mode is
entered, the switching frequency is set to
30kHz and the duty cycle of the upper and
lower FETs, which are fixed in PFM mode, are
decreased as required to keep the output
voltage in regulation while maintaining the
30kHz switching frequency.
Under extremely light or zero load currents,
the GH on time pulse width can decrease to its
minimum width. When this happens, the lower
FET on time is increased slightly to allow a
small amount of reverse inductor current to
flow back into V_IN to keep the output voltage
in regulation while maintaining the switching
frequency above the audio range.
Oversampling OVS Mode
Oversampling (OVS) mode is a feature added
to the XRP7725 to improve transient
responses. This mode can only be enabled
when the channel switching frequency is
operating in 1x frequency mode. In OVS mode
the output voltage is sampled four times per
switching cycle and is monitored by the AFE
window comparators. If the voltage goes
outside the set high or low limits, the OVS
control electronics can immediately modify the
pulse width of the GH or GL drivers to respond
accordingly, without having to wait for the
next cycle to start. OVS has two types of
response depending on whether the high limit
is exceeded during an unloading transient
(Over Voltage), or the low limit is exceeded
during a loading transient (Under Voltage).
Under Voltage OVS: If there is an increasing
current load step, the output voltage will drop
until the regulator loop adapts to the new
conditions to return the voltage to the correct
level. Depending on where in the switching
cycle the load step happens there can be a
delay of up to one switching cycle before the
control loop can respond. With OVS enabled if
the output voltage drops below the lower
level, an immediate GH pulse will be
generated and sent to the driver to increase
the output inductor current toward the new
load level without having to wait for the next
cycle to begin. If the output voltage is still
below the lower limit at the beginning of the
next cycle, OVS will work in conjunction with
the PID to insert additional GH pulses to
quickly return the output voltage back within
its regulation band. The result of this system
is transient response capabilities on par or
exceeding those of a constant on-time control
loop.
Over Voltage OVS: When there is a step load
current decrease, the output voltage will
increase (bump up) as the excess inductor
current that is no longer used by the load
flows into the output capacitors causing the
output voltage to rise. The voltage will
continue to rise until the inductor current
decreases to the new load current. With OVS
enabled, if the output voltage exceeds the
high limit of the window comparator, a
blanking pulse is generated to truncate the GH
signal. This causes inductor current to
immediately begin decreasing to the new load
level. The GH signal will continue to be
blanked until the output voltage falls below
the high limit. Again, since the output voltage
is sampled at four times the switching
frequency, over shoot will be decreased and
the time required to get back into the
regulation band is also decreased.
© 2014 Exar Corporation
16/34
Rev. 1.0.0