A4447 Просмотр технического описания (PDF) - Allegro MicroSystems

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A4447

High Voltage Step Down Regulator

Allegro MicroSystems 

A4447

High Voltage Step Down Regulator

Component Selection

L1. The inductor must be rated to handle the total load current.

The value should be chosen to keep the ripple current to a reason-

able value. The ripple current, IRIPPLE, can be calculated by:

IRIPPLE = VL(OFF) × tOFF / L

(3)

VL(OFF) = VOUT + Vf + IL(AV) × RL

(4)

Example:

Given VOUT = 5 V, Vf = 0.55 V, VIN = 42 V, ILOAD = 0.5 A, power

inductor with L = 180 μH and RL = 0.5 Ω Rdc at 55°C, tOFF =

7 μs, and RDS(on) = 1 Ω.

Substituting into equation 4:

VL(OFF) = 5 V + 0.55 V+ 0.5 A × 0.5 Ω = 5.8 V

Substituting into equation 3:

IRIPPLE = 5.8 V × 7 μs / 180 μH = 225 mA

The switching frequency, fSW, can then be estimated by:

fSW = 1 / ( tON + tOFF )

(5)

tON = IRIPPLE × L / VL(ON)

(6)

VL(ON) = VIN – IL(AV) × RDS(on) – IL(AV) × RL– VOUT

(7)

Substituting into equation 7:

VL(ON) = 42 V – 0.5 A × 1 Ω – 0.5 A × 0.5 Ω – 5 V = 36 V

Substituting into equation 6:

tON = 225 mA × 180 μH / 36 V = 1.12 μs

Substituting into equation 7:

fSW = 1 / (7 μs +1.12 μs) = 123 kHz

Higher inductor values can be chosen to lower the ripple cur-

rent. This may be an option if it is required to increase the total

maximum current available above that drawn from the switching

regulator. The maximum total current available, ILOAD(MAX) , is:

ILOAD(MAX) = ICL(min) – IRIPPLE / 2

(8)

where ICL(min) is 2.2 A, from the Electrical Characteristics table.

D1. The Schottky catch diode should be rated to handle 1.2 times

the maximum load current. The voltage rating should be higher

than the maximum input voltage expected during all operating

conditions. The duty cycle for high input voltages can be very

close to 100%.

COUT. The main consideration in selecting an output capacitor

is voltage ripple on the output. For electrolytic output capacitors,

a low-ESR type is recommended.

The peak-to-peak output voltage ripple is simply IRIPPLE ×

ESR. Note that increasing the inductor value can decrease the

ripple current. The minimum voltage rating of the capacitor is

10 V. However, because ESR decreases with voltage, the most

cost-effective choice may be rated higher in voltage. It is recom-

mended that the ESR be less than 100 mΩ.

Allegro MicroSystems, Inc.

6

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

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