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A4407

2.2 MHz Constant On-Time Buck Regulator With Two External and Two Internal Linear Regulators

Allegro MicroSystems 

A4407

2.2 MHz Constant On-Time Buck Regulator

With Two External and Two Internal Linear Regulators

Functional Description

Overview

The A4407 contains a constant on-time (COT), buck switch-

ing pre-regulator with valley sensing current mode control, two

integrated 5 V linear regulators, and two N-channel FET drivers:

one for a 1.2 V, 1.5 V, or 1.8 V linear regulator, and the other for

a 3.3 V linear regulator. The 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 and

frequency. Valley mode current control allows the converter to

achieve very short on-times because current is measured during

the off-time, so there is no requirement for blanking.

With very low input voltages, the buck switch transitions to 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 the logic level input (ENB) or the high

voltage ignition input (ENBAT). When the device is enabled,

the converter starts up under the control of a 10 ms internal soft

start ramp. The two enable inputs are logically ORed together, so

either of the inputs can be used to enable the device. Both inputs

must be low to disable the device.

Under light load conditions, the switch enters pulse-skipping

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

wide input voltage range, the switcher period is extended when

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

supply is at either end of its range.

The A4407 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 between

the ISENx pins. Output current from the internal 5 V and 5 V

protected linear regulators is also monitored, and if shorted the

outputs would fold back. The external FET drivers have current

limit sensing that can be used with a sense resistor to trigger

fold back protection.

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 the

VREG pin then becomes simply the supply voltage minus the

drop across the buck switch and inductor. In this mode, 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 voltages

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 typical. The following conditions are

required to trigger a soft start:

• ENBAT or ENB transition high, and

• There is no thermal shutdown (TSD = 0), and

• 3V3 voltage is below its undervoltage lockout (UV)

threshold, and

• 1V2 voltage is below its UV threshold, and

• VREG voltage is below its UV 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 8.6 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 (100% duty cycle). Within

reasonable input voltage ranges, the period of the converter is

held constant. This results in a constant operating frequency

across the input supply range. More information on how to

choose RTON can be found in the Application Information section.

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

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

(1)

Buck 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,

Buck Current Sense (ISEN+, ISEN–)

The sense inputs are used to sense the current in the buck regula-

tor free-wheeling diode during the off-time. The value of the

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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