Picor Corporation • www.picorpower.com QPO-2 Data Sheet Rev. 1.6 Page 5 of 11
Application of the QPO-2
This product can be used over a 0.3 Vdc to 5.5 Vdc output
voltage range using either the remote sense or the
voltage trim feature of the selected converter. These
circuit configurations are shown in Figures 3 and 4.
In either configuration, the source output voltage will
increase to accommodate the headroom voltage of the
QPO-2 filter in order to maintain the load voltage at the
required level. In the case where remote sense or SC/Trim
use is not possible the QPO-2 can still be used to provide
PARD attenuation with the DC loss of the headroom
voltage at the load. If the supply output can be trimmed
up, the headroom voltage drop of the QPO-2 can be
compensated for at a given load. Further DC correction
for load variation at the QPO-2 output will occur only
within the supply’s control loop. The QPO-2’s output will
be controlled to the voltage present at the VREF pin in
this open loop filter configuration.
The user must decide on the control mode to be used and
to select the appropriate circuit configuration for that
mode. They must take into consideration the effects of
the headroom setting and power dissipation versus PARD
attenuation. The majority of the power dissipation of the
QPO-2 is the product of the headroom voltage times the
load current and must always be less than 4 watts. The
dynamic headroom range of the QPO-2 is 75 mv to 425 mv
as long as the maximum power is not exceeded. It is
important that the user understands the range of
expected ripple and transient performance of their power
source to properly bias and utilize the QPO features. The
objective is to maximize attenuation and minimize
dissipation while staying within the QPO-2 dynamic
operating range. Knowing the worse case maximum
steady state ripple, output impedance and transient
response time of the power source will determine the
minimum required headroom of the QPO-2, which is set
by the value of R
HR. See figure 5 below for the safe
operating power curve.
If the peak detector option is enabled the headroom will
automatically increase by the peak of the ripple amplitude
from the setting determined by R
HR. This makes the initial
headroom setting less critical because the headroom
and dynamic range will track the peak of the ripple,
maintaining the required QPO-2 biasing to actively
attenuate. Caution must be taken such that the added
peak detection headroom does not cause power
dissipation in excess of 4 watts. The time constant of this
feature is roughly 30 ms in response to ripple amplitude
changes. This feature can be enabled by connecting the
PEAKIN pin to the QPOIN pins and disabled by putting a
resistor between QPOIN and the PEAKIN pin as shown in
Figure 6.
Conversely the optional slope adjust feature will reduce
the headroom proportional to load current depending on
the R
SA value selected. This will reduce the maximum
ripple range so this feature is most useful when the
converter ripple amplitude decreases with increased load
current. The feature can be enabled by selecting the
proper R
SA value as described in the headroom slope
adjust section of the datasheet and effectively disabled by
using R
SA = 100 Kohms.
Figure 7 shows the relationship of the headroom voltage
versus attenuation of the QPO-2 for a 3.3 volt output with
a 15 amp load. This relationship is relatively constant over
the full output voltage rating of the product so this graph
can be used for the 0.3 V to 5.5 V range when selecting
the headroom voltage. The value of headroom resistor
will be dependent on desired output and headroom
voltages. The selection of the final headroom voltage
should be based on the maximum expected ripple, desired
attenuation, based on the curves in Figure 7, and the
transient response time of the converter. Formulas for SC
current setting resistor, R
SC and the RCP clamp setting
resistor, are provided in their respective sections. The
headroom range indicated in Figure 7 shows that
increasing the headroom voltage will increase the
attenuation, up to a point of diminishing returns, over the
range of 10 kHz to over 1 Mhz. With an external 25 uF
