I
= I
× (1 – DC(min)) ,
(22)
V SW +V f (23)
DC (min) =
P D(diode) = I LOAD(av) × DC(min) × V f
,
A4402
drop and switching losses. In order to size the diode correctly, it
is necessary to find the average diode conduction current using
the formula below:
D(av) LOAD
where DC (min) is defined as:
,
V IN1 +V f
where V IN1  is the maximum input voltage and V f is the maximum
forward voltage of the diode.
Average power dissipation in the diode is:
(24)
The power dissipation in the sense resistor must also be consid-
ered using I 2 R and the minimum duty cycle.
PCB Layout The board layout has a large impact on the per-
formance of the device. It is important to isolate high current
ground returns, to minimize ground bounce that could produce
reference errors in the device. The method used to isolate power
ground from noise sensitive circuitry is to use a star ground. This
approach makes sure the high current components such as the
input capacitor, output capacitor, and diode have very low imped-
Constant On-Time Buck Converter
With Integrated Linear Regulator
ance paths to each other. Figure 3 illustrates the technique.
The ground from each of the components should be very close to
each other and be connected on the same surface as the com-
ponents. Internal ground planes should not be used for the star
ground connection, as vias add impedance to the current path.
In order to further reduce noise effects on the PCB, noise sensi-
tive traces should not be connected to internal ground planes.
The feedback network from the switcher output should have an
independent ground trace that goes directly to the exposed pad
underneath the device. The exposed pad should be connected to
internal ground planes and to any exposed copper used for heat
dissipation. If the grounds from the device are also connected
directly to the exposed pad the ground reference from the feed-
back network will be less susceptible to noise injection or ground
bounce.
To reduce radiated emissions from the high frequency switching
nodes it is important to have an internal ground plane directly
under the LX node. The plane should not be broken directly
under the node as the lowest impedance path back to the star
ground would be directly under the signal trace. If another trace
does break the return path, the energy will have to find another
path, which is through radiated emissions.
LX
VIN1
Current path (on-cycle)
L1
A4402
R SENSE
Current path
(off-cycle)
R LOAD
Star Ground
Figure 3. Star Ground Connection
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
13
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