A8697 datasheet(8/12 Pages) ALLEGRO

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Wide Input Voltage 4.0 A Step Down Regulator

A8697

Allegro MicroSystems, Inc.

115 Northeast Cutoff, Box 15036

Worcester, Massachusetts 01615-0036 (508) 853-5000

www.allegromicro.com

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

(5)

VL(OFF) = VOUT + Vf + IL(AV) × RL

(6)

Example:

Given VOUT = 1.06 V, Vf = 0.3 V, VIN = 12 V, ILOAD = 4.0 A,

power inductor with L = 3.8 μH and RL = 0.05 Ω Rdc at 55°C,

tOFF = 5.0 μs, and RDS(on) = 0.2 Ω.

Substituting into equation 6:

VL(OFF) = 1.06 V + 0.3 V+ 4 A × 0.05 Ω = 1.56 V

Substituting into equation 5:

IRIPPLE = 1.56 V × 5 μs / 3.8 μH = 2.05 A

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

fSW = 1 / ( tON + tOFF )

(7)

tON = IRIPPLE × L / VL(ON)

(8)

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

(9)

Substituting into equation 9:

VL(ON) = 12 V – 4 A × 0.2 Ω – 4 A × 0.05 Ω – 1.06 V = 9.94 V

Substituting into equation 8:

tON = 2.05 A × 3.8 μH / 9.94 V = 0.785 μs

Substituting into equation 7:

fSW = 1 / (5 μs + 0.785 μs) = 173 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. Please refer to the Maximum Load Current graph for

the maximum load recommended.

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 ESR should be in the range from 50 to 500 mΩ.

The switching frequency is calculated as follows:

fSW

tON + tOFF

(4)

Shorted Load. If the voltage on the FB pin falls below 0.4 V, the

regulator will invoke a 1.5 A typical overcurrent limit to handle

the shorted load condition at the regulator output. For low output

voltages at power up and in the case of a shorted output, the off-

time is extended to prevent loss of control of the current limit due

to the minimum on-time of the switcher.

The extension of the off-time is based on the value of the TSET

multiplier and the FB voltage, as shown in the following table:

VFB (V)

TSET Multiplier

< 0.16

8 × tOFF

< 0.32

4 × tOFF

< 0.5

2 × tOFF

> 0.5

tOFF

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