A4407 데이터 시트보기 (PDF) - Allegro MicroSystems

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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

Figures 6 and 7 show the A4407 operation when the V5P pin is

shorted to ground and VIN (battery). In both cases, the V5P output

is disabled and/or disconnected while the other outputs (VREG,

V5, V33, and 1V5) remain active.

Tracking Control

The TRACK input sets the master reference for the V5P track-

ing regulator. TRACK is meant to be either connected to ground

or left unconnected by the PCB routing. When TRACK is left

unconnected, it is pulled high by an internal current source and

V5P tracks the 3V3 regulator. When TRACK is connected to

ground, then V5P tracks the V5 regulator.

3V3 Linear Regulator (3V3)

A 3.3 V linear regulator can be implemented using an external

MOSFET. In the event the 3.3 V regulator output is shorted to

ground, the A4407 protects the external MOSFET by folding

back when the programmed current limit, I3V3LIM , is exceeded.

The current limit is determined by the voltage developed across

the external sense resistor, RCL1 , shown in the Functional Block

diagram. The 3.3 V regulator current limit can be calculated using

the following formula:

I3V3LIM =VCL3V3 / RCL1

(6)

where VCL3V3 is documented in the Electrical Characteristics

table, 235 mV typical. Usually, RCL1 has a fairly low value so it

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

but the tolerance should be 1% or less. When I3V3LIM is exceeded,

the maximum load current through the external MOSFET is

folded back to 27% typical of I3V3LIM as shown in figure 8.

VREG

30 V

V3V3

VIN pin

C1

VV5

25 V

Ringing due to parasitics from a long wire

V5P is clamped to a safe level above VIN

VREG

by D2 (see application schematic)

C2

V1V5

V3V3

C3

5V

C4

VV5P

VV5P

C5

All

t

t

Figure 6. V5P shorted to ground in 5 μs (DV5P is populated); shows VREG

(ch1, 2 V/div.), V3V3 (ch2, 1 V/div.), VV5 (ch3, 2 V/div.), V1V5 (ch4, 1 V/div.),

VV5P (ch5, 2 V/div.), t = 10 μs/div.

3.5

Figure 7. V5P is shorted to a 25 V battery; shows VVREG (ch1, 2 V/div.),

V3V3 (ch2, 2 V/div.), VIN pin (ch3, 5 V/div.), VV5P (ch4, 5 V /div.),

t = 10 μs/div.

3.0

2.5

2.0

1.5

1.0

05

0

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

Percentage of Normal Current Se ng (%)

Figure 8. Fold back current limiting of the 3V3 regulator

Allegro MicroSystems, Inc.

15

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

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