A8450KLBTR-T【PDF 9/17 页】IC REG QD BUCK/LINEAR 24-SOIC

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Automotive Multioutput Voltage Regulator

A8450

Allegro MicroSystems, LLC

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

to 0 V at 50 mA, as shown in figure 4a. The voltage recovers to its

regulated output when the short is removed.

The V5A and V5D regulators track each other during power-on, and

when the device is enabled and ramped up out of disabled mode,

the regulators will start to track when V

REG

reaches approximately

1.8 V. These regulators are guaranteed to track to within 0.5% of

each other under normal operating conditions.

3.3 V and Adjustable Linear Regulators. Two additional

linear regulators, one that outputs at 3.3 V, and another that has a

1.2 V to 3.3 V adjustable output, can be implemented using external

npn pass transistors. The output voltage of the adjustable regulator,

OUTVADJ

(V), is set by the values of the output resistors, R1 and

R2 (?. It can be calculated as

OUTVADJ

= V

(1+R1 D R2)

where V

(V) is the voltage on the feedback pin, FB.

Additional pins, CL33 and CLADJ, are provided for setting cur-

rent limits. These are used to protect the external pass transis-

tors from a short-to-ground condition. The current limit setting,

(mA), is calculated using the formula

= V

D R

RCL

where RCL (? is the current-limiting resistor corresponding to

that regulator (R3 for the 3.3 V regulator, and R4 for the adjustable

regulator). When I

is exceeded, the maximum load current through

that regulator is folded back to 40% of I

?0%, as shown in figure

4b. If current limiting is not needed, the CL33 and CLADJ pins

should be shorted to the VREG pin.

Disabled Mode. When the two input signal pins, ENBAT and

ENB, are pulled low, the A8450 enters disabled mode. This is a

sleep mode, in which all internal circuitry is disabled in order to

draw a minimal current from VBB. When either of these pins is

pulled high, the device is enabled. When emerging from disabled

mode, the buck converter switching regulator does not operate

until the charge pump has stabilized (H 300 約).

Enabled Mode. When one or both of the signal input pins,

ENBAT and ENB, are in the high state, the A8450 is enabled.

ENBAT is an edge-triggered enable (logic 1 e 2.7 V), which is

used to enable the A8450 in response to a high-voltage signal,

such as from an automobile ignition or battery switch. In this

capacity, ENBAT is used only as a momentary switch to wake up

the device. If there is no need for a high-voltage signal, ENBAT

can be pulled low continuously.

ENB is used to initiate the reset of the device. If ENBAT is pulled

100

150

200

250

300

1600

5V Regulators Overcurrent Foldback

OUT

(mA)

OUTV5DLIM

and I

OUTV5ALIM

Figure 4a. Linear foldback to 50 mA. Foldback occurs at the typical

current limit for the 5 V regulator.

Figure 4b. Linear foldback to a percentage of I

. Foldback occurs at

the current limit setting for the regulator.

1600

3.3 V and Adjustable Regulators Overcurrent Foldback

OUT

(mA)

0.4

?0%

OUTVADJ(min)

OUTV33

and V

OUTVADJ(max)

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