a4403geu-t Allegro MicroSystems, Inc., a4403geu-t Datasheet - Page 10

• Manufacturer Part Number: a4403geu-t
• Description: Valley Current Mode Control Buck Converter
• Manufacturer: Allegro MicroSystems, Inc.

A4403
The amount of capacitance required for a given ripple voltage can
be found:
As mentioned in the previous section, E-field biasing effects
can reduce the actual capacitance and this should be taken into
account when making the selection.
Again, there is generally no need to consider the heating effects
of the rms current flowing through the esr of a ceramic capaci-
tor. If an electrolytic device is used, then the ripple current rating
should be considered. Note that most manufacturers only con-
sider the rms current rating at 100 kHz.
Recirculation Diode This diode (D1) conducts during the
switch off-time. A Schottky diode is recommended to minimize
both the forward drop and switching losses. The worst-case
dissipation occurs at maximum V
minimum.
The average current through the diode can be found:
The forward voltage drop, V
characteristics by using the actual load current (not the average
current).
The static power dissipation can be found:
It is also important to take into account the thermal rating of
the package, R
enough heatsinking is provided to maintain the diode junction
temperature within the safe operating area for the device.
To minimize the heating effects from the A4403 on the diode and
vice-versa, it is recommended that the diode be mounted on the
reverse side of the printed circuit board.
Sense Resistor The sense resistor should be a surface mount
package, with low inductance. On no account should a wire-
wound or through hole package be used. To prevent potential
mistriggering problems from occurring in noisy systems, it is
recommended that an R-C filter be applied across the sense resis-
tor, as shown in figure 3.
I
θJA
DIODE(av)
, and the ambient temperature, to ensure that
C
P
IN
STAT
=
= I
= I
I
LOAD
V
rms
f
, can be found from the diode
DIODE(av)
RIPPLE
× T
× (1 – D (min)) .
IN
on
, when the duty cycle is at a
× V
.
f
.
Valley Current Mode Control Buck Converter
(14)
(15)
(16)
The sense resistor value is selected depending on the maximum
output load current. The typical sense voltage that causes a cur-
rent limit is 180 mV. So, for example, a 50 mΩ value would be
appropriate for a maximum load of 3 A, as it allows for margin
between maximum load and the current limit. A tolerance of up to
±5% is acceptable.
The power rating of the resistor has to be considered. The current
flowing in the resistor is essentially the same as the current flow-
ing through the recirculation diode, although the power dissipa-
tion is worked out using the rms current.
To a first approximation, the sense resistor dissipation can be
worked out as:
For a converter working with a load of 3 A, a very narrow duty
cycle, and a sense resistor of 50 mΩ, the power dissipation would
be 450 mW.
The optimal solution from a cost perspective is to use two
100 mΩ, 1206-style resistors connected in parallel. Each resistor
is generally rated at 250 mW at 70°C ambient. Check the vendor
datasheet to verify the maximum ambient at full power.
When laying out the PCB, it is essential that the sense resistor
connections, carrying the power current (see figure 3), are as
short and wide as possible to minimize the effects of leakage
inductance noise. In addition, the Kelvin sense circuit connec-
tions should be as close to the sense resistor pads as possible.
Figure 3. R-C filter added to the current sense circuit
P
A4403
SENSE
SGND
ISEN
= I
LOAD
C
Kelvin
connection
1 nF
Kelvin
connection
2
FILTER
R
47 Ω
115 Northeast Cutoff
1.508.853.5000; www.allegromicro.com
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 U.S.A.
× (1 – D (min)) × R
FILTER
R
SENSE
Power
current
SENSE
.
(17)
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