A8521 datasheet(30/35 Pages) ALLEGRO

30 / 35 page

Wide Input Voltage Range, High Efficiency

Fault Tolerant LED Driver

A8521

Allegro MicroSystems, Inc.

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

Design Example for SEPIC Configuration

This section provides a method for selecting component values

when designing an application using the A8521 in SEPIC (Sin-

gle-Ended Primary-Inductor Converter) circuit. SEPIC topology

has the advantage that it can generate a positive output voltage

either higher or lower than the input voltage. The resulting design

is diagrammed in figure 34.

Assumptions: For the purposes of this example, the following are

given as the application requirements:

• VBAT: 6 to 14 V ( VIN(min): 5 V and VIN(max): 16 V )

• Quantity of LED channels, #CHANNELS: 4

• Quantity of series LEDs per channel, #SERIESLEDS: 4

• LED current per channel, ILED: 60 mA

• LED Vf at 60 mA: ≈ 3.3 V

• fSW: 2 MHz

• TA(max): 65°C

• PWM dimming frequency: 200 Hz, 1% duty cycle

Procedure: The procedure consists of selecting the appropriate

configuration and then the individual component values, in an

ordered sequence.

Step 1

Connecting LEDs to LEDx pins. If only some of the LED

channels are needed, the unused LEDx pins should be pulled to

ground using a 1.5 kΩ resistor.

Step 2

Determining the LED current setting resistor RISET:

RISET = (VISET × AISET) / ILED

(30)

= (1.003 (V) × 653) / 0.60 (A) = 10.92 kΩ

Choose an 11.00 kΩ 1% resistor.

Step 3

Determining the OVP resistor. The OVP resistor is

connected between the OVP pin and the output voltage of the

converter.

Step 3a

The first step is determining the maximum voltage based

on the LED requirements. The regulation voltage, VLED, of the

A8521 is 700 mV. A constant term, 2 V, is added to give margin

to the design due to noise and output voltage ripple.

VOUT(OVP) = #SERIESLEDS × Vf + VLED + 2 (V)

(31)

= 4 × 3.3 (V) + 0.7 (V) + 2 (V) = 15.9 V

Then the OVP resistor is:

ROVP = (VOUT(OVP) – VOVP(th) ) / IOVPH

(32)

= (15.9 (V) – 8.1 (V)) / 0.199 (mA) = 39.196 kΩ

where both IOVPH and VOVP(th) are taken from the Electrical

Characteristics table.

In this case a value of 39.2 kΩ was selected. Below is the actual

value of the minimum OVP trip level with the selected resistor:

VOUT(OVP) = 39.2 (kΩ) × 0.199 (mA) + 8.1 (V) = 15.9 V

Step 3b

At this point a quick check must be done to determine if

the conversion ratio is acceptable for the selected frequency.

Dmax = 1 – tSWOFFTIME × fSW

(33)

= 1 – 68 (ns) × 2 (MHz) = 86.4%

where the minimum off-time (tSWOFFTIME) is found in the Electri-

cal Characteristics table.

The Theoretical Maximum VOUT is then calculated as:

VOUT(max) =

–

1 – Dmax

Dmax

VIN(min)

0.4 (V)

30.3 V

–

1 – 0.86

0.86

5 (V)

(34)

where Vd is the diode forward voltage.

The Theoretical Maximum VOUT value must be greater than

the value VOUT(OVP). If this is not the case, it may be necessary

to reduce the frequency to allow the boost to convert the volt-

age ratios.

Step 4

Selecting the inductor. The inductor must be chosen such

that it can handle the necessary input current. In most applica-

A8521 Datasheet(PDF) 30 Page - Allegro MicroSystems

A8521 Datasheet(HTML) 30 Page - Allegro MicroSystems

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

Datasheet Download

Link URL