A4402_1 ALLEGRO [Allegro MicroSystems], A4402_1 Datasheet - Page 11

• Manufacturer Part Number: A4402_1
• Description: Constant On-Time Buck Converter with Integrated Linear Regulator
• Manufacturer: ALLEGRO [Allegro MicroSystems]

A4402
Switcher On-Time and Switching Frequency In order for
the switcher to maintain regulation, the energy that is transferred
to the inductor during the on-time must be transferred to the
capacitor during the off-time. Because of this relationship, the
load current, IR drops, as well as input and output voltages, affect
the on-time of the converter. The equation that governs switcher
on-time is:
The effects of the voltage drop on the inductor and trace resis-
tance affect the switching frequency. However, the frequency
variation due to these factors is small and is covered in the varia-
tion of the switcher period, T
Removing these current dependant terms simplifies the equation:
Be sure to use worst-case sense voltage and forward voltage of
the diode, including any effects due to temperature. For example,
given a 1 A converter with a supply voltage of 13.5 V, output
voltage is 5 V, V
quency is 2.0 MHz. We can solve for t
The formulas above describe how t
load conditions. Because load changes are minimal, and the out-
put voltage is fixed, the dominant factor that affects the on-time
is the input voltage. The converter is able to maintain a constant
period over a varying supply voltage because the on-time change
is based on the input voltage. The current into the TON terminal
is derived from a resistor tied to VIN1, which sets the on-time
proportional to the supply voltage. Selecting the resistor value,
based on the t
formula:
R
t
t
t
on
on
on
TON
=
=
=
=
T
V
V
13.5 + 0.5 + 0.15
IN1
SW
SW
5 + 0.5 + 0.15
V
on
IN1
+ V
+ V
× (V
3.12 × 10
calculated above, is done using the following
f
× (t
f
f
is 0.5 V, V
+ (V
+ (V
SW
on
+ R
+ 10 ns)
SENSE
SENSE
V
–12
IN1
LOAD
+ R
× I
× I
×
SENSE
× I
SW
DS(on)
PEAK
PEAK
2×10
PEAK
.
, which is ±25% of the target.
1
6
)
)
× I
is 0.15 V and the desired fre-
on
+ V
×
=
PEAK
changes based on input and
on
199 ns
f
f
SW
1
+ R
as follows:
+ V
SENSE
f
.
.
× I
Application Information
PEAK
)
.
(10)
(11)
(9)
Constant On-Time Buck Converter
After the resistor is selected and a suitable t
be demonstrated that t
exceed the minimum on-time or minimum off-time of the con-
verter. The minimum on-time occurs at maximum input voltage
and minimum load. The maximum off time is occurs at minimum
supply voltage and maximum load. For supply voltages below
9.5 V and above 7 V, refer to the Low Voltage Operation section.
Low Voltage Operation The converter can run at very low
input voltages. With a 5 V output, the minimum input supply can
be as low as 6 V. When operating at high frequencies, the on-time
of the converter must be very short because the available period
is short. At high input voltages the converter must maintain very
short on-times, while at low input voltages the converter must
maintain long off-times. Rather than limit the supply voltage
range, the converter solves this problem by automatically increas-
ing the period by a factor of 3.5. With the period extended, the
converter will not violate the minimum on-time or off-time. If the
input voltage is between 9.5 V and 17 V, the converter will main-
tain a constant period. When calculating worst-case on-times and
off-times, make sure to use the multiplier if the supply voltage is
between those values.
When operating at voltages below 8 V, additional care must be
taken when selecting the inductor and diode. At low voltages
the maximum current may be limited due to the IR drops in the
current path. When selecting external components for low voltage
operation, the IR drops must be considered when determining
on-time, so the complete formula should be used to make sure the
converter does not violate the timing specification.
Inductor Selection Choosing the right inductor is critical to
the correct operation of the switcher. The converter is capable of
running at frequencies above 2 MHz. This makes it possible to
use small inductor values, which reduces cost and board area.
The inductor value is what determines the ripple current. It is
important to size the inductor so that under worst-case conditions
I
margin. If the ripple current is too large, the converter will be
current limited. Typically peak-to-peak ripple current should be
limited to 20% to 25% of the maximum average load current.
VALLEY
with Integrated Linear Regulator
equals I
AV
minus half the ripple current plus reasonable
on
does not, under worst-case conditions,
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Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 U.S.A.
on
is found, it must
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