Picor Corporation • www.picorpower.com QPO-2L Data Sheet Rev. 1.2 Page 3 of 12
Product Highlights
Picor’s QPO-2, System-in-a-Package (SiP) output
ripple attenuator, is easy to apply and provides the
user with features that can be tailored to optimize
the product’s performance to meet their system
needs. It uses active filtering to achieve greater than
30 dB of attenuation of Periodic And Random
Deviation (PARD) over the frequency range of 1kHz
to 500kHz. For converters running off-line with
greater low frequency output ripple, the attenuation
can be extended to be greater than 20dB down at
50Hz by connecting a 25uF capacitor between the
VREF and REFGRD pins.
The QPO-2 operates over an output voltage range of
0.3 to 5.5Vdc and requires an external input bias
voltage of 7 volts above the QPO output for proper
operation. It is compatible with most switching
power supplies and converters and regulates the
output load by using either a converter’s remote
sensing feature or the SC/trim function of the QPO-2
with the converter. The SC/Trim feature will correct
the converter’s output voltage to compensate for the
headroom voltage drop of the filter if remote
sensing is not available or not preferred. The QPO-2
SC function works with converters that feature a
positive reference trim adjustment by sourcing
correction current into the trim reference pins
commonly found on many power supplies. The QPO-
2 can also be used when remote sense or SC/trim is
not possible. In this mode of operation the QPO-2
will still provide greater than 30dB of ripple and
noise attenuation but DC errors will not be corrected
for once the converter and headroom voltages are
set, resulting in reduced load and transient
performance.
The QPO-2’s closed loop architecture greatly
improves load transient response of the converter
while ensuring steady-state precise point of load
voltage regulation. The headroom setting of the
filter dramatically reduces the capacitance needed at
the converter output to provide the equivalent
transient performance and ripple reduction. Figure 2
demonstrates how the product can be an ideal
solution for noise sensitive applications providing
ripple and noise reduction and improved output
regulation with high current transient load demands.
Functional Description
The QPO-2 is an active power filter that provides
conducted differential attenuation of power supply
output PARD. It is design to be inserted between the
output of the supply and the load, providing closed
loop regulation through remote sensing or by means
of the SC/Trim feature of supplies having a positive
referenced based trim capability and is set by RSC.
The core of the design is a high bandwidth closed
loop function that forces the QPOOUT pins to be
equal to the VREF pin. The VREF pin is a filtered ratio
metric representation of the input voltage that is
determined by the RHR value selection. The voltage
difference between the input to the QPO-2 and VREF
pin is defined as the headroom voltage VHR. The
filter time constant of the VREF pin determines the
low frequency attenuation response of the QPO-2.
The high frequency attenuation response is
determined by the roll-off characteristics of the
active loop. To speed up the charging of the Vref pin
the RCP resistor can be used to clamp the pin just
below the steady state regulation point avoiding
excessive delay and headroom during start-up.
The QPO-2 has a current sensing function that
creates a voltage at the Slope Adjust pin that is
proportional with the load current. This feature can
be used to improve the efficiency of the filter when
supply ripple amplitude reduces with increasing load
as with Vicor products. By selecting the appropriate
RSA resistor value the slope of the headroom
reduction versus load can be set. The effect of this
function is to reduce the headroom voltage by the
amount determined by the RSA value resulting in
reduced power dissipation and increased efficiency
as compared to a fixed headroom setting.
There is also a Peak Detection function that adds the
input peak of the ripple voltage to the headroom
voltage. The QPO-2 will track the input ripple
adjusting the headroom within the dynamic range of
the filter as the peak of the ripple changes. The peak
of the ripple will automatically be summed with the
DC setting of the headroom voltage. This feature in
combination with the slope adjust feature allows the
user to optimize the initial headroom voltage and
overall efficiency required for their specific
application.