XR76116 Datasheet, PDF (15/19 Pages) Exar Corporation – PowerBloxTM 15A and 20A Synchronous

XR76116/20
Applications Information (Continued)
Feed-Forward Capacitor (C_FF)
The feed-forward capacitor C_FF is used to set the necessary
phase margin when using ceramic output capacitors.
Calculate C_FF from the following equation:
CFF
=
2
x
S
x
1
R1
x
5
x
fLC
Where f_LC, the output filter double-pole frequency is
calculated from:
fLC =
1
2 x S x √ L x COUT
You must use manufacturer’s DC derating curves to
determine the effective capacitance corresponding to V_OUT.
A load step test (and/or a loop transient response test)
should be performed and if necessary C_FF can be adjusted
in order to get a critically damped transient load response.
In applications where output voltage ripple is less than
about 3mV, such as when a large number of ceramic
C_OUT are paralleled, it is necessary to use ripple injection
from across the inductor. The circuit and corresponding
calculations are explained in the Exar design note.
Feed-Forward Resistor (R_FF)
R_FF is required when C_FF is used. R_FF, in conjunction with
C_FF, functions similar to a high frequency pole and adds
gain margin to the frequency response. Calculate R_FF from:
Thermal Design
Proper thermal design is critical in controlling device
temperatures and in achieving robust designs. There are
a number of factors that affect the thermal performance.
One key factor is the temperature rise of the devices in
the package, which is a function of the thermal resistances
of the devices inside the package and the power
being dissipated.
The thermal resistance of the XR76116/20 is specified in
the Operating Ratings section of this datasheet. The _JA
thermal resistance specification is based on the XR76116/20
evaluation board operating without forced airflow. Since the
actual board design in the final application will be different,
the thermal resistances in the final design may be different
from those specified.
The package thermal derating curves for the XR76120 are
shown in Figures 7 and 8. These correspond to input voltage
of 12V and 5V, respectively. The package thermal derating
curves for the XR76116 are shown in Figures 9 and 10.
RFF
=
2
xS
1
xf
x
CFF
Where f is the switching frequency.
If R_FF is greater than 0.1xR1, then instead of C_FF/R_FF, use
ripple injection circuit as described in Exar design note.
REV1A
15/19