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A4406

Constant On-Time Buck Regulator With One External and One Internal Linear Regulator

Allegro MicroSystems 

A4406

Constant On-Time Buck Regulator

With One External and One Internal Linear Regulator

Functional Description

Basic Operation

The A4406 contains a fixed on-time, buck switching pre-reg-

ulator with valley sensing current mode control, an integrated

5 V linear regulator, and an N-channel FET driver for a 3.3 V

linear regulator. The constant on-time (COT) 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, which maintains a relatively constant

period. Valley mode current control allows the converter to

achieve very short on-times because current is measured during

the off-time.

With very low input voltages the buck switch maintains a 100%

duty cycle. This turns the buck switch on 100% of the time (no

switching) and allows the regulator to operate in dropout mode.

The device is enabled via logic level ENB or high voltage igni-

tion ENBAT input. When the device is enabled the converter

starts up under the control of an internal soft start routine. The

two enable inputs are logically ORed together internally so either

of the inputs can be used to enable the device.

Under light load conditions the switch enters pulse-skipping

mode to ensure regulation is maintained. In order to maintain a

wide input voltage range the switcher period is extended when

the minimum on- or off-time is reached, or when the input supply

is at either end of its range.

Overcurrent Protection

The A4406 features overcurrent protection on all regulators

including the VREG pre-regulator. The buck switch current limit

is determined by the selection of the sense resistor at the ISENx

pins. Output current is also monitored on the 5VP and V33 linear

regulators, and if shorted the outputs fold back. The external FET

driver has a current limit tap that can be used with a sense resistor

to trigger a current limit based on an external resistor and trip

voltage.

Dropout Mode

The topology of a COT timer is ideal for systems that have high

input voltages. Because current is measured during the off-time,

very short on-times can be achieved. With low input voltages

the switcher must maintain very short off-times. To prevent

the switcher from reaching its minimum off-time, the switcher

is designed to enter a 100% duty cycle mode. This causes the

switcher to stop acting as a buck switch. The voltage at VREG

then becomes the simply the supply voltage minus the drop

across the buck switch and inductor. In this mode the maximum

available current may be lower, depending on ambient tempera-

ture and supply voltage, while in dropout mode.

Soft Start

An internal ramp generator and counter allow the output voltage

to ramp-up. This limits the maximum demand on the external

power supply by controlling the inrush current required to charge

the external capacitor and any DC load at startup. Internally, the

ramp is set to 10 ms nominal.

The following conditions are required to trigger a soft start:

• ENBAT or ENB transition to high, and

• There is no thermal shutdown, and

• V33 voltage is below its UVLO threshold, and

• VREG voltage is below its UVLO threshold.

Buck Pulse Width ( tON )

A resistor from the TON input to VIN sets the on-time of the

converter for a given input voltage. When the supply voltage is

between 8.3 and 31 V, the switcher period remains constant based

on the selected value of RTON . At voltages lower than 6.5 V the

switch is in dropout mode. For reasonable input voltage ranges

the period of the converter is held constant resulting in a constant

operating frequency over the input supply range. More informa-

tion on how to choose RTON can be found in the Application

Information section.

The formula to calculate the value for the on-time resistor is:

ton = ( RTON / VIN ) × 6.36 × 10 –12 + 5 × 10 –9 (ns). (1)

ISEN+ and ISEN–

The sense inputs are used to sense the current in the buck, free-

wheeling diode during the off-time cycle. The value for RSENSE is

obtained by the formula:

RSENSE = 220 (mV) / IVALLEY ,

(2)

where IVALLEY is the lowest current measured through the induc-

tor during the off-time cycle. It is recommended that the current

sense resistor be sized so that, at peak output current, the voltage

Allegro MicroSystems, Inc.

11

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

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