A4402 ALLEGRO [Allegro MicroSystems], A4402 Datasheet - Page 9

• Manufacturer Part Number: A4402
• Description: Constant On-Time Buck Converter with Integrated Linear Regulator
• Manufacturer: ALLEGRO [Allegro MicroSystems]

TSET The TSET pin serves a dual function by controlling the
timing for both the soft start ramp and the WDI input. The current
sourced from the TSET pin is dependant on the state of NPOR.
There are two formulas for calculating the time constants. C
must be selected so that both the WDI frequency and soft start
requirements are met. The formulas for calculating WDI and soft
start timing are:
where C
t
Watchdog The WDI input is used to monitor the state of a DSP
or microcontroller. A constant current is driven into the capacitor
on TSET, causing the voltage on the TSET pin to ramp upward
until, at each rising edge on the WDI input, the ramp is pulled
down to V
ramp reaches V
The watchdog timer is not activated until the WDI input sees
one rising edge. The WDI pin should be pulled to ground with a
4.7 kΩ resistor.
Soft Start During soft start, an internal ramp generator and the
external capacitor on TSET are used to ramp the output voltage
in a controlled fashion. This reduces the demand on the exter-
nal power supply by limiting the current that charges the output
capacitor and any DC load at startup. Either of the following
conditions are required to trigger a soft start:
• ENB pin input rising edge
• Reset of a TSD event
When a soft start event occurs, VO2 is held in the off state until
the soft start ramp timer expires. Then the regulator will power
up normally. Refer to timing diagrams for details.
BOOT A bootstrap capacitor is used to provide adequate charge
to the NMOS switch. The boot capacitor is referenced to LX
and supplies the gate drive with a voltage larger than the supply
voltage. The size of the capacitor must be 0.01 μF, X7R type, and
rated for at least 25 V.
A4402
x
, are in s.
TSET
RESET
is the value of the capacitor and the results,
t
WDI
TRIP
t
. If no edge is seen on the WDI pin before the
SS
=
=
, the NPOR pin is pulled low.
7.2 × 9.6 × 10
6.0 × 6.0 × 10
4
4
× C
× C
TSET
TSET
, and
,
TSET
(2)
(3)
Constant On-Time Buck Converter
TON A resistor from the TON input to VIN1 sets the on-time of
the converter for a given input voltage. The formula to calculate
the on-time is:
When the supply voltage is between 9 and 17.5 V, the switcher
period remains constant, at a level based on the selected value
of R
period is reduced by a factor of 3.5.
If a constant period is desired over varying input voltages, it is
important to select an on-time that under worst case conditions
will not exceed the minimum off-time or minimum on-time of the
converter. For reasonable input voltage ranges, the period of the
converter can be held constant, resulting in a constant operating
frequency over the input supply range.
More information on how to choose R
Application Information section.
ISEN The sense input is used to sense the current in the diode
during the off-time cycle. The value for R
formula:
where I
tor during the off-time cycle.
It is recommended that the current sense resistor be sized so that,
at peak output current, the voltage on ISEN does not exceed
–0.5 V. Because the diode current is measured when the inductor
current is at the valley, the average output current is greater than
the I
where:
with Integrated Linear Regulator
I
I
K is a guardband margin. The peak current in the switch is then:
OUT(av)
RIPPLE
ton
VALLEY
. At voltages lower than 9 V and higher than 17.5 V, the
VALLEY
is the inductor ripple current, and
is the average of both output currents,
value. The value for I
t
on
I
VALLEY
is the lowest current measured through the induc-
=
V
I
R
PEAK
IN1
R
ton
SENSE
= I
OUT(av)
= I
3.12
115 Northeast Cutoff
1.508.853.5000; www.allegromicro.com
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 U.S.A.
VALLEY
= 0.2 / I
–12
– 0.5 I
VALLEY
+ 30 × 10
+ I
VALLEY
ton
RIPPLE
RIPPLE
can be found in the
should be:
SENSE
–9
,
+ K ,
.
.
is obtained by the
(4)
(5)
(6)
(7)
9