A4403 ALLEGRO [Allegro MicroSystems], A4403 Datasheet - Page 8

• Manufacturer Part Number: A4403
• Description: Valley Current Mode Control Buck Converter
• Manufacturer: ALLEGRO [Allegro MicroSystems]

A4403
When the output voltage is set for 0.8 V, the typical bandwidth
is 90 kHz with a phase margin of 45° at full load. As the load is
reduced, the bandwidth remains largely constant; however, the
phase margin tends to reduce slightly because the output power
pole is shifted down in frequency, introducing the phase lag
sooner. At light loads, before pulse frequency modulation occurs,
the phase margin reduces to approximately 40°, which is reason-
able given that it is the worst-case condition. Note that when
pulse frequency modulation occurs, the system no longer operates
as a linear system, therefore, the control laws do not apply.
When the output voltage is set for higher voltages, the DC gain
is reduced by the resistor feedback network from the output. This
effectively reduces the bandwidth of the control loop. An optional
speed-up capacitor (C6) can be used in parallel with the feedback
resistor (R5) to compensate for this effect. The addition of this
capacitor introduces an additional zero which increases the gain
and extends the bandwidth to maintain it in the region of 90 kHz.
The position of the zero depends on the values of R5 and C6.
The following time constants should be used for various output
voltages:
For example, assume a target output voltage of 5 V, and an R5
of 3.92 kΩ to achieve that voltage. Then C6 = 9.18 × 10
nearest commonly available value is 10 nF.
For applications that require output voltages (V
what is defined above, the following formula should be used to
calculate the time constant:
Inductor The main factor in selecting the inductance value is
the ripple current. The ripple current affects the output voltage
ripple and also has an effect on the current limit. Because slope
compensation is not used, the ripple current is not constrained by
this factor.
Output Voltage
(V)
3.3
2.5
1.5
0.8
τ = V
5
OUT
× 7.2 × 10
–6
Time Constant
Not required
,
3.6 × 10
2.4 × 10
1.8 × 10
1.1 × 10
OUT
(
τ)
) other than
–5
–5
–5
–5
Valley Current Mode Control Buck Converter
–9
. The
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A good starting point in selecting the inductance for a given
application is to specify a maximum peak-to-peak ripple current
of about 25% of the maximum load. The equates to a ripple cur-
rent of approximately 750 mA for a maximum load of 3 A. This
often gives a good compromise between size, cost, and perfor-
mance.
The maximum peak to peak ripple current, I
maximum input voltage. Therefore the duty cycle, D, should be
found under these conditions:
where V
and the sense resistor.
The required inductance can be found:
Note that the manufacturers inductance tolerance should also be
taken into account. This value may be as high as ±20%.
In addition, because the control is dependant on the valley signal,
it is important to consider the minimum peak to peak valley
voltage that is developed across the sense resistor. The minimum
peak to peak ripple current occurs at minimum input voltage. The
peak to peak voltage is simply the peak to peak current multiplied
by the sense resistor value. It is recommended that the peak to
peak sense voltage should be greater than 25 mV.
It is recommended that gapped ferrite solutions be used as
opposed to powdered iron solutions. The latter exhibit relatively
high core losses that can have a large impact on long term reli-
ability.
Inductors are typically specified at two current levels:
• RMS current. It is important to understand how the RMS cur-
rent level is specified, in terms of ambient temperature. Some
manufacturers quote an ambient only, whilst others quote a tem-
perature that includes a self-temperature rise. For example, if an
inductor is rated for 85°C and includes a self-temperature rise of
25°C at maximum load, then the inductor cannot be safely oper-
ated beyond an ambient temperature of 60°C at full load. The
RMS current can be assumed to be simply the maximum load
L (min)
f
is the forward voltage drop of the recirculation diode
=
V
D (min) =
IN
I
RIPP
– V
OUT
115 Northeast Cutoff
1.508.853.5000; www.allegromicro.com
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 U.S.A.
V
×
IN
V
OUT
(max)+V
D (min)
+V
f
f
×
RIPP
f
,
SW
, occurs at the
1
(min)
.
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8