A6267
Automotive High Current LED Controller
Two pull-up resistors are used to limit the voltage across the gate-
source junction during high input voltages or load dump condi-
tions. If the battery voltage is restricted, one resistor across the
gate-source junction can be used. The FF1 provides a sink current
of up to 1.3 mA.
exceeds the input fuse rating, the fuse will blow, creating an
open circuit).
• In buck-boost mode:
▫ If the cathode end of the LED string is shorted to ground, the
LED sense resistor effectively appears between VBAT and
ground. Depending on the current limit of the source supply or
the input fuse rating, the fault current may damage the resistor.
▫ If the LF node is shorted to ground, the reference LED and
the LED sense resistor effectively appear between VBAT and
ground. The reference LED will be reversed biased and will
probably be damaged.
▫ If the cathode end of the LED string (LP) is shorted to VBAT,
the impedance of the short appears in parallel with the series
combination of the series protection MOSFET and the LED
sense resistor. This will tend to reduce the effective impedance
of the LED sense resistor and correspondingly increase the
LED current.
This circuit can be used to avoid most hazardous conditions and
protect the circuit components from over-stress. Note that under
extreme cases, the circuit cannot protect against certain fault con-
ditions. For example, when any of the following occurs:
• In boost mode:
▫ If the cathode end of the LED string is shorted to VBAT, the
LED sense resistor effectively appears between VBAT and
ground. Depending on the current limit of the source supply or
the input fuse rating, the fault current may damage the resistor.
▫ If the LF node is shorted to VBAT, the reference LED and
the LED sense resistor effectively appear between VBAT and
ground. Depending on the current limit of the source supply or
the input fuse rating, the fault current may damage the resistor
and/or the reference LED.
▫ If the cathode end of the LED string is shorted to ground. A
fault current determined by the impedance of the shorting link
(now effectively the LED sense resistor) flows through the
power circuit. The fault current will either: be limited by the
maximum switch current sense, VIDS, or if the source supply
cannot maintain this current, be limited by the source supply.
(The source supply will either: fold back, or if the current
To ensure the A6267 inputs (LP and LN) are not damaged dur-
ing any of the above faults, it is necessary to add a differential
resistor in series with the LN connection (between the sense
resistor and the LN pin). This resistor value should be approxi-
mately 150 Ω.
If an output short is detected but it is necessary to keep the output
active, the FF1 output can be pulled low. This will override the
output disable but will not clear the fault.
Allegro MicroSystems, Inc.
115 Northeast Cutoff
12
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
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