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A4402

Constant On-Time Buck Converter With Integrated Linear Regulator

Allegro MicroSystems 

A4402

Constant On-Time Buck Converter

With Integrated Linear Regulator

Functional Description

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 VIN1 , is reached or the mini-

mum on-time at high VIN1 .

Switcher Overcurrent Protection The converter utilizes

pulse-by-pulse valley current limiting, which operates when the

current through the sense resistor rises to VISEN . 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 VSW 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 idd 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:

VSW =

VFB1







R1 + R2

R2







,

(1)

VLIN =

VFB2







R3 + R4

R4







.

(2)

In order to maintain accuracy on the regulators the equivalent

impedance on the FB node (R1 parallel with R2) should be

approximately 10 kΩ.

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

Constant On-Time

Current Limit level

Extended period

Time

Figure 1. Current limiting during overload

Allegro MicroSystems, Inc.

8

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

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