A4402 ALLEGRO [Allegro MicroSystems], A4402 Datasheet - Page 8

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

A4402
Basic Operation The A4402 contains a fixed on-time, adjust-
able voltage buck switching regulator with valley sensing current
mode control, and an adjustable linear regulator designed to run
off the buck regulator output. The constant on-time converter
maintains a constant output frequency because the on-time is
inversely proportional to the supply voltage. As the input voltage
decreases, the on-time is increased, maintaining a relatively con-
stant period. Valley mode current control allows the converter to
achieve very short on-times because current is measured during
the off-time.
The device is enabled via the ENB input. When the ENB pin
is pulled high, the converter starts-up under the control of an
adjustable soft start routine whose ramp time is controlled by an
external capacitor.
Under light load conditions, the switch enters pulse-skipping
mode to ensure regulation is maintained. This effectively changes
the switcher frequency. The frequency also is affected when the
switcher is operating in discontinuous mode. In order to maintain
a wide input voltage range, the switcher period is extended when
either the minimum off-time at low V
mum on-time at high V
Switcher Overcurrent Protection The converter utilizes
pulse-by-pulse valley current limiting, which operates when the
current through the sense resistor creates a voltage on the sense
pin (ISEN) that equals –0.2 V. During an overload condition, the
switch is turned on for a period determined by the constant on-
time circuitry. The switch off-time is extended until the current
decays to the current limit value set by the selection of the sense
resistor, at which point the switch turns on again. Because no
slope compensation is required in this control scheme, the current
limit is maintained at a reasonably constant level across the input
voltage range.
Figure 1 illustrates how the current is limited during an overload
condition. The current decay (period with switch off) is propor-
tional to the output voltage. As the overload is increased, the out-
put voltage tends to decrease and the switching period increases.
VIN1 and VIN2 VIN1 is a high voltage input, designed to with-
stand 50 V. Bulk capacitance of at least 10 μF should be used to
IN
.
IN
, is reached or the mini-
Functional Description
Constant On-Time Buck Converter
decouple input supply VIN1. The VIN2 input is used to supply
the linear regulator and should be connected directly to the output
of the switching regulator.
Output Voltage Selection The output voltage on each of the
two regulators is set by a voltage divider off the regulator output,
as follows:
In order to maintain accuracy on the regulators the equivalent
impedance on the FB node (R1 parallel with R2) should be
approximately 10 kΩ.
Figure 1. Current limiting during overload
with Integrated Linear Regulator
Inductor current operating at maximum load
Inductor current operating in a “soft” overload
V
V
LIN
Constant period
SW
Constant On-Time
Constant On-Time
=
=
V
V
FB1
FB2
Extended period
115 Northeast Cutoff
1.508.853.5000; www.allegromicro.com
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 U.S.A.
⎛
⎜
⎜
⎝
⎛
⎜
⎜
⎝
Time
Time
R1 + R2
R1 + R2
Current Limit level
R2
R2
Current Limit level
⎞
⎟
⎟
⎠
⎞
⎟
⎟
⎠
Overload
,
.
Maximum load
(1)
8