A4402
Constant On-Time Buck Converter
With Integrated Linear Regulator
Functional Description
? R1 + R2 ?
V SW = V FB1 ? ?
,
? (1)
V LIN = V FB2 ? ?
? R3 + R4 ?
.
?
R4
BasicOperation TheA4402containsafixedon-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 IN1 , is reached or the mini-
mum on-time at high V IN1 .
Switcher Overcurrent Protection The converter utilizes
pulse-by-pulse valley current limiting, which operates when the
current through the sense resistor rises to V ISEN . During an over-
load 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 
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 when the target for the V SW voltage is
between 3 and 5.5 V. For voltages outside of that range, the bias 
supply for the IC is taken from VIN1 directly and affects overall 
efficiency.
For applications where the switcher voltage is greater than 5 V, a 
second supply between 3 and 5.5 V can be used to supply VIN2 
bias current and the linear regulator. Note that the current into the
VIN2 supply must supply both the i dd bias current and any cur-
rent load on the linear 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:
R2 ?
? ?
? (2)
? ?
In order to maintain accuracy on the regulators the equivalent
impedance on the FB node (R1 parallel with R2) should be
approximately 10 k?.
Inductor current operating at maximum load
Current Limit level
Maximum load
Constant On-Time
Constant period
Time
Inductor current operating in a “soft” overload
Overload
Current Limit level
Constant On-Time
Extended period
Time
Figure 1. Current limiting during overload
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
8
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