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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

1.2 V/1.5 V/1.8 V Linear Regulator (1V2)

A 1.2 V, 1.5 V, or 1.8 V linear regulator can be implemented using

an external MOSFET. In the event this regulator output is shorted

to ground, the A4407 protects the external MOSFET by folding

back when the programmed current limit, I1V2LIM , is exceeded.

The current limit is determined by the voltage developed across

the external sense resistor, RCL2 , shown in the Functional Block

diagram. The 1.2 V/1.5 V/1.8 V regulator current limit can be

calculated using the following formula:

I1V2LIM = VCL1V2 / RCL2

(7)

where VCL1V2 is documented in the Electrical Characteristic

table, 218 mV typical. Usually RCL2 has a fairly low value so it

will not dissipate significant power (1/4W should be adequate) but

the tolerance should be 1% or less. When I1V2LIM is exceeded, the

maximum load current through the external MOSFET is folded

back to 27% typical of I1V2LIM , as shown in figure 9.

This regulator is designed to provide 1.2 V, but by using an exter-

nal resistive divider between VOUT1V2 and the 1V2 pin, other

voltages can be achieved, such as1.5 V or 1.8 V.

Charge Pump

The charge pump is used to generate a supply above VIN . A

0.22 μF monolithic ceramic capacitor should be connected

between VCP and VIN to act as a reservoir to run the internal

DMOS and the external MOSFETs. The VCP voltage is internally

monitored to ensure that the switching regulator would be dis-

abled in the case of a fault condition. A 0.22 μF ceramic mono-

lithic capacitor should be connected between CP1 and CP2.

ENBAT

ENBAT is a level-triggered enable input, used to enable the

device based on a high voltage ignition or battery switch (via a

1 kΩ resistor). The ENBAT comparator thresholds are VIGN(L) =

2.2 V minimum and VIGN(H) = 4.0 V maximum. ENBAT is used

only as a momentary switch to enable, or wake up, the A4407.

The ENB and ENBAT signals are logically ORed together inter-

nally, so individually either can wake up the A4407, that is, only

one of these two inputs must be pulled high in order to enable the

A4407. However, when ENBAT is removed, ENB must be high

to keep the A4407 enabled. If there is no need for the ignition

switch, ENBAT can be pulled low, making ENB a single reset

control. Power-up and power-down scenarios using these inputs

are shown in figures 10 and 11.

When an external resistor and capacitor are used to form a

low-pass filter to the ENBAT pin, then a 100 Ω resistor must be

used to prevent the external capacitor from discharging into and

damaging the ENBAT pin. See the Functional Block diagram for

connection of these three components.

ENBATS

When a logic high is sensed on the ENBAT input, the ENBATS

open drain output goes high, signaling to the user that the ignition

input is high. When a logic low is sensed on the ENBAT input,

then ENBATS transitions low. The ENBATS input logic levels

are identical to the ENBAT input logic levels.

ENB

This pin can be used as an enable input from either a DSP or a

microcontroller. This input has an internal pull down resistor so it

can be left unconnected if not needed.

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0

50 10 20 30 40 50 60 70 80 90 100 110 120

Percentage of Normal Current Se ng (%)

Figure 9. Fold back current limiting of the 1V2 regulator

Allegro MicroSystems, Inc.

16

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

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