A8514 Ver la hoja de datos (PDF) - Allegro MicroSystems

Número de pieza

componentes Descripción

Lista de partido

A8514

Wide Input Voltage Range, High Efficiency Fault Tolerant LED Driver

Allegro MicroSystems 

A8514

Wide Input Voltage Range, High Efficiency

Fault Tolerant LED Driver

Functional Description

The A8514 incorporates a current-mode boost controller with

internal DMOS switch, and four LED current sinks. It can be

used to drive four LED strings of up to 12 white LEDs in series,

with current up to 80 mA per string. For optimal efficiency,

the output of the boost stage is adaptively adjusted to the mini-

mum voltage required to power all of the LED strings. This is

expressed by the following equation:

Powering up: LED pin short-to-ground check

The VIN pin has a UVLO function that prevents the A8514

from powering-up until the UVLO threshold is reached. After

the VIN pin goes above UVLO, and a high signal is present on

the PWM/EN pin, the IC proceeds to power-up. As shown in

figure 3, at this point the A8514 enables the disconnect switch

and checks if any LEDx pins are shorted to ground and/or are not

used.

VOUT = max ( VLED1 ,..., VLED4 ) + VREG

(1)

where

VLEDx is the voltage drop across LED strings 1 through 4, and

VREG is the regulation voltage of the LED current sinks (typi-

cally 0.7 V at the maximum LED current).

The LED detect phase starts when the GATE voltage of the

disconnect switch is equal to VIN – 4.5 V. After the voltage

threshold on the LEDx pins exceeds 120 mV, a delay of between

3000 and 4000 clock cycles is used to determine the status of the

pins. Thus, the LED detection duration varies with the switching

frequency, as shown in the following table:

Enabling the IC

The IC turns on when a logic high signal is applied on the

PWM/EN pin with a minimum duration of tPWMH for the first

clock cycle, and the input voltage present on the VIN pin is

greater than the 4.35 V necessary to clear the UVLO (VUVLOrise )

threshold. The power-up sequence is shown in figure 2. Before

the LEDs are enabled, the A8514 driver goes through a system

check to determine if there are any possible fault conditions that

might prevent the system from functioning correctly. Also, if the

FSET/SYNC pin is pulled low, the IC will not power-up. More

information on the FSET/SYNC pin can be found in the Sync

section of this datasheet.

Switching Frequency

(MHz)

2

1

0.800

0.600

Detection Time

(ms)

1.5 to 2

3 to 4

3.75 to 5

5 to 6.7

The LED pin detection voltage thresholds are as follows:

LED Pin Voltage LED Pin Status

<70 mV

Short-to-ground

150 mV

Not used

325 mV

LED pin in use

Action

Power-up is halted

LED removed from operation

None

VDD

C1

FSET/SYNC

C2

ISET

C3

PWM/EN

C4

GATE

GATE = VIN – 4.5 V

C1

LEDx

C2

LED detection period

ISET

C3

C4

PWM/EN

t

t

Figure 2. Power-up diagram; shows VDD (ch1, 2 V/div.), FSET/SYNC (ch2,

1 V/div.), ISET (ch3, 1 V/div.), and PWM/EN (ch4, 2 V/div.) pins,

time = 200 μs/div.

Figure 3. Power-up diagram; shows the relationship of an LEDx pin with

respect to the gate voltage of the disconnect switch (if used) during the

LED detect phase, as well as the duration of the LED detect phase for a

switching frequency of 2 MHz; shows GATE (ch1, 5 V/div.), LED (ch2,

500 mV/div.), ISET (ch3, 1 V/div.), and PWM/EN (ch4, 5 V/div.) pins,

time = 500 μs/div.

Allegro MicroSystems, Inc.

11

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

Share Link: