A8513 データシートの表示（PDF） - Allegro MicroSystems

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A8513

Wide Input Voltage Range, High Efficiency Fault Tolerant LED Driver

Allegro MicroSystems 

A8513

Wide Input Voltage Range, High Efficiency

Fault Tolerant LED Driver

ISET pin. The limitation of this form of dimming is that the inter-

nal ISET error amplifier is designed to work from 40 to 160 μA,

thus limiting the dimming ratios that can be achieved.

Figure 7, top panel is a typical application using a DAC to control

the LED current using a single resistor connected to the ISET pin.

The ISET current is controlled by the following formula:

ISET

=

VISET – VDAC

RISET

(3)

Where VISET is the ISET pin voltage and VDAC is the DAC out-

put voltage.

When the DAC voltage is equal to VISET , the internal reference,

there is no current through RISET . When the DAC voltage starts

to decrease, the ISET current starts to increase, thus increasing

the LED current. When the DAC voltage is 0 V, the LED current

will be at its maximum.

• For a dual-resistor configuration (lower panel of figure 7), the

ISET current is controlled by the following formula:

ISET

=

VISET

RISET

–

VDAC – VISET

R1

(4)

The advantage of this circuit is that the DAC voltage can be

higher or lower, thus adjusting the LED current to a higher or

lower value of the preset LED current set by the RISET resistor:

▫ VDAC = 1.003 V; the output is strictly controlled by RISET

▫ VDAC > 1.003 V; the LED current is reduced

▫ VDAC < 1.003 V; the LED current is increased

Output Overvoltage Protection (OVP) and Output

Undervoltage Protection (UVP)

The A8513 has output overvoltage protection (OVP), output

undervoltage protection and secondary overvoltage protection

(open diode).

Overvoltage Protection The OVP pin has a threshold, VOVPHI(th) ,

of 1.218 V. A resistive divider can be used to set the VOUT over-

voltage protection threshold up to 45 V (see figure 8). There is

no restriction on the value of the resistor chosen, but it is recom-

mended that the divider current be kept between 10 and 60 μA.

This will minimize the effect of sense current on the accuracy of

OVP, and minimize output voltage bleed-off during PWM dim-

ming.

Formulas for calculating the OVP resistor voltage divider are

shown below.

ROVP1 = (VOVP – 1.218 V) / ISET

(5)

ROVP2 = 1.218 V / ISET

(6)

The OVP function is not inherently a latched fault. If the OVP

condition occurs during a load dump, the IC will stop switching

but not shut down.

DAC

VDAC

GND

RISET

A8513

ISET

GND

DAC

VDAC

GND

R1

RISET

A8513

ISET

GND

VOUT

A8513

OVP

ROVP1

1.218 V

ROVP2

100 mV

Figure 7. Simplified diagram of voltage LED current control (upper) single

resistor, and (lower) dual resistors.

Figure 8. Simplified diagram of the OVP pin functions.

Allegro MicroSystems, Inc.

11

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

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