A4403 ALLEGRO [Allegro MicroSystems], A4403 Datasheet - Page 7

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

A4403
Then, the minimum on-time is:
The specified minimum on-time, T
there is reasonable margin in this case.
The specified minimum off-time, T
also has to be considered. The minimum off-time occurs at
minimum input voltage and maximum load. As was shown in the
minimum on-time calculation (equation 4), you have to exam-
ine the extreme operating conditions to ensure adequate margin
exists.
The switch on-time, T
TON pin. The current is determined by the input voltage, V
and the resistor R1. The on-time can be found as:
The switching frequency may be slightly modulated by load
changes. The on-time is always constant for a given input voltage
and across the load range. To compensate for any losses in the
circuitry (for example, in the series switch and inductor, or in the
voltage drop across the recirculation diode), the off-time, hence
the switching frequency, has to be adjusted. This effect is most
noticeable at low input voltages and high output currents.
To calculate the actual switching frequency, the T
tion 5 can be used in conjunction with the transfer function of the
converter:
An alternative approach to selecting the TON resistor (R1), to
accomplish an approximate switching frequency is found in the
following formula:
Figure 2 illustrates a range of switching frequencies that can be
achieved with various TON resistances and output voltages.
T
T
on
on
(min)
=
f
SW
R
⎛
⎜
⎜
⎝
1
V
=
=
=
IN
⎛
⎜
⎜
⎝
⎛
⎜
⎜
⎝
on
V
V
× 2.05 ×10
V
46 + 0.5
, is set by the current flowing into the
OUT
5 + 0.5
OUT
IN
R
+ V
1
+ V
× 2.05 × 10
f
f
f
SW
⎞
⎟
⎟
⎠
⎞
⎟
⎟
⎠
10
on(min)
×
×
off(min)
1× 10
T
⎞
⎟
⎟
⎠
1
on
+ 10 × 10
1
10
, is 60 ns maximum, so
, 350 ns maximum,
6
=
.
.
118 ns
–9
on
of equa-
Valley Current Mode Control Buck Converter
.
IN
,
(5)
(6)
(7)
Top-Off Charge Pump During light load operation, when
operating in PFM mode, the top-off charge pump provides
enough charge to drive the buck switch.
Light Load Operation To avoid the output voltage peak charg-
ing due to leakage effects from the buck switch and the charge
pump recirculation current, a minimum load of 1 mA must be
applied to the output.
The output feedback resistor network provides some loading.
Depending on the values selected, this network may provide all,
or at least some, of the minimum loading requirement.
Control Loop The process of closing the control loop for the
A4403 has been greatly simplified through the integration of
the compensation components into the device. The control loop
bandwidth has been optimized for operation across the full input
and output voltage range and for switching frequencies between
450 kHz and 2 MHz. Loop optimization is achieved with a 20 μF
ceramic capacitor placed across the output (VOUT to GND) and
a power inductor that achieves a peak to peak ripple current of
around 720 mA. For example, for a 3.3 V output operating at a
frequency of 1 MHz, the power inductor = 4.7 μH.
Larger output capacitors can be used; however, this tends to
decrease the bandwidth of the control loop. Note that the output
capacitance should not exceed 1000 μF or be less than 10 μF, as
this may cause a loop instability to occur.
Figure 2. Switching frequencies versus TON resistor values, at various
levels of V
2000
1800
1600
1400
1200
1000
800
600
400
OUT
10
0.8 V
1.5 V
3.3 V
115 Northeast Cutoff
1.508.853.5000; www.allegromicro.com
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 U.S.A.
Resistor R1 (kΩ)
5 V
100
12 V
V
OUT
1000
7