ALT80600 datasheet(21/33 Pages) ALLEGRO

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LED Driver with Pre-Emptive Boost

for Ultra-High Dimming Ratio and Low Output Ripple

ALT80600

Allegro MicroSystems, LLC

955 Perimeter Road

Manchester, NH 03103-3353 U.S.A.

www.allegromicro.com

FAULT DETECTION AND PROTECTION

LED String Partial-Short Detect

All LED current sink pins (LED1 to LED4) are designed to with-

stand the maximum output voltage, as specified in the AbsMax

section. This prevents the IC from being damaged if VOUT is

directly applied to an LED pin due to an output connector short.

In case of direct-short or partial-shorted fault in any LED string dur-

ing operation, the LED pin with voltage exceeding VLEDSC will be

removed from regulation. This prevents the IC from dissipating too

much power due to large voltage drop across the LED current sink.

Figure 35: Normal startup sequence showing voltage at LED2 and

LED3 pins. VIN = 6 V, output = 6 × WLED in series, current = 4 × 100 mA

Figure 36: Startup sequence when LED string#2 has a partial-short

fault (4 × WLED instead of 6). As soon as LED2 pin rises above

VLEDSC (~4.6 V), the channel is disabled. Output is now 300 mA.

While the IC is being PWM dimmed, the IC will recheck the

disabled LED every time the PWM signal goes high. This allows

for some self-correction in case an intermittent LED pin shorted

to VOUT fault is present.

At least one LED pin must be at regulation voltage (below

~1.2 V) for the LED string partial-short detection to activate.

In case all of the LED pins are above regulation voltage (this

could happen when the input voltage rises too high for the LED

strings), they will continue to operate normally.

Overvoltage Protection

The ALT80600 offers a programmable output overvoltage

protection (OVP), plus a fixed secondary overvoltage protection

(OVP2).

The OVP pin has a threshold level of 2.5 V typical. Overvoltage

protection is tripped when current into this pin exceeds ~150 µA.

A resistor can be used to set the OVP threshold up to 40 V approxi-

mately. This is sufficient for driving 11 white LEDs in series.

The formula for calculating the OVP resistor is shown below:

Equation 6:

ROVP = (VOVP – VOVP(th)) / iOVP(th)

where VOVP is the desired OVP threshold, VOVP(th) = 2.5 V

typical, iOVP(th) = 150 µA typical.

To determine the desired OVP threshold, take the maximum LED

string voltage at cold and add ~10% margin on top of it.

The OVP event is not a latched fault and, by itself, does not pull

the FAULT pin to low. If the OVP condition occurs during a load

dump, for example, the IC will stop switching but not shut down.

There are several possibilities of why an OVP condition is

encountered during operation. The two most common being an

open LED string and a disconnected output connector.

The waveform below shows a typical OVP condition. When one

LED string becomes open, current through its LED driver drops

to zero. The ALT80600 responses by boosting the output voltage

higher. When output reaches OVP threshold, the LED string with-

out current is removed from regulation. The rest of LED strings

continue to draw current and drain down VOUT. Once VOUT falls

below ~94% OVP, boost will resume switching to power the

remaining LED strings.

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