A6265KLPTR-T Datasheet A6265KLPTR-T | P10-P12

Automotive High Current LED Controller

A6265

Allegro MicroSystems, LLC

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

Diagnostics

The circuit includes several diagnostic and safety functions to

assist in ensuring safe operation of the LEDs, the A6265, and the

external components. When any fault is detected, one or both of

the fault flag outputs, FF1 and FF2, will be inactive (high imped-

ance, open drain) until the fault is removed. The action taken by

the A6265 when a fault occurs is defined in table 1. To be able to

monitor the state of FF1 and FF2, add a suitable external pull-up

resistor.

The A6265 will continue to drive the LEDs under most fault con-

ditions and will only disable the drive to the LEDs when a high

voltage hazard is present or the external components are likely to

be over-stressed. For output short circuits or open LED condi-

tions, the fault status is latched until EN is taken low or a power

cycle occurs. For output short circuits or a shorted LED string,

the fault status is latched until either EN is taken low for a period

greater than the disable time, or a power cycle occurs.

At start-up, a Fault Blank period, t

FB ,

occurs before the fault

detection circuitry becomes active. This period allows steady

state conditions to be established before fault monitoring takes

place.

Note that no fault blanking is applied to open LED faults. This

is generally not an issue because the charging of the output filter

capacitor provides a degree of filtering. In addition, extremely

high voltages are prevented from causing potential device break-

down, for example in the external switching MOSFET.

VREG Undervoltage If the voltage at VREG, V

REG

, drops

below the specified turnoff voltage, V

REGUV

, the gate drive

output, SG, will be driven low and both fault flags, FF1 and

FF2, will be high impedance. V

REG

must rise above the turn-on

threshold, V

REGUV

+ V

REGUV

, before the output circuits are

activated. This ensures that the external FET is operating in its

fully enhanced state and avoids permanent damage to the FET,

caused by overheating.

LED Undercurrent Under some circuit conditions, particularly

during a low input voltage condition, it is possible that there

could be insufficient drive to maintain the current to the LEDs

at the required level. If the voltage across the LED current sense

resistor, R

, falls below the target sense voltage, V

IDL

, by an

amount that is more than the LED undercurrent voltage differ-

ence, V

UCL

, the A6265 will indicate an LED undercurrent condi-

tion by setting FF2 to high impedance. However, the A6265 will

continue to drive the output. When the output again reaches the

required current level, FF2 will go low.

Overtemperature Warning If the chip temperature exceeds

the overtemperature threshold, T

, fault flag FF2 will be high

impedance. No action will be taken by the A6265 to limit the

chip temperature. An external control circuit must take action

to avoid permanent damage to the A6265 and/or the LEDs. The

temperature will continue to be monitored and the fault flags will

be deactivated when the temperature drops below the recovery

threshold provided by the hysteresis, T

Jhys

LED Diagnostics The status of the LEDs in the load can be

determined by monitoring the voltage with respect to ground at

the three pins LP, LF, and LA, namely V

, V

, and V

. These

voltages provide two differential voltage measurements:

• the voltage across a single reference LED:

LED

= V

– V

(5)

• the ratio of the voltage across all LEDs in a single string:

STR

= V

– V

(6)

The voltage, V

STR

, is derived from the voltage across all LEDs in

the string, by an external resistor divider with a ratio equal to the

quantity of LEDs in the string. To minimize the effects of the bias

currents introducing an offset voltage, it is recommended that the

resistor between LP and LA should be approximately 560 .

So for example, if eight LEDs were used, the ratio required

would be an eighth, therefore the resistor connected between LA

and the anode end of the LED string would be 3.9 k;

560 / [560 + 3900] = 1/8 .

Table 1. Fault Table

Fault

Action Latched

FF1 FF2

No Fault L L No Action –

VREG Undervoltage Z Z Disable* No

Output Short Z L Disable* Yes

LED Undercurrent L Z No Action No

Overtemperature L Z No Action No

Open LED L Z Disable* Yes

Shorted LED L Z No Action No

Shorted LED String Z L Disable* Yes

* SG low, MOSFET off

L = active pull-down, Z = inactive, open drain

Automotive High Current LED Controller

A6265

Allegro MicroSystems, LLC

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

These measurements are used to determine if there is an open

circuit, if one or more LEDs are shorted, if the output is shorted,

or if there is a short across the LED string. Each condition is

described in turn in the following sections.

Open LED–An open circuit is evaluated when:

STR

> V

OCL

(7)

where V

OCL

is the LED open circuit voltage defined in the Elec-

trical Characteristics table.

Because the output is current-controlled it is possible for an open

circuit on the output to cause extremely high voltages to be pres-

ent. Therefore, to prevent any hazardous voltages or damage to

the circuits, the gate drive output, SG, is immediately driven low

when an open circuit is detected. After an open circuit fault has

been detected, FF2 will become high impedance, and the open

circuit fault state will remain until the open fault time-out period

toto, expires. When the gate drive output is re-enabled at the end

of the open fault time-out period, the output is again monitored

for an open circuit. If the open circuit is still present, then the

fault will again be flagged and the switch drive disabled. This

cycle will continue, as long as the open circuit condition is pres-

ent.

Note that the Fault Blank timer is not used when an open LED

fault occurs. This is to avoid potentially damaging voltages

appearing in the power circuitry.

Shorted LED – A short circuit on one or more LEDs is detected

when:

• for the first (reference) LED:

STR

> V

LED

+ V

SCOR

(8)

• for other than the first (reference) LED:

LED

> V

STR

+ V

SCO

(9)

where V

STR

and V

LED

are as defined above, V

SCO

is the nonrefer-

ence LED short offset voltage, and V

SCOR

is the reference LED

short offset voltage. V

SCO

and V

SCOR

are defined in the Electrical

Characteristics table.

When a short is present, the fault flag FF2 is high impedance, but

the regulator continues to operate and drives the remaining LEDs

with the correct regulated current. FF2 will remain high imped-

ance while the short circuit condition is present.

A short circuit on one or more LEDs will not cause a hazard

because the output is current-controlled. If one LED fails and

becomes a short circuit, then the remaining LEDs will continue

to be lit with the same current through, and voltage across,

each LED.

Note—Accuracy: The output status monitor relies on all the

LEDs in the load having a similar forward voltage drop. Where

possible all the LEDs forming the load for a single controller

should be taken from the same voltage bin. With only two or

three LEDs a wider variation in forward voltage is acceptable,

but the selection of LEDs from the same bin is more critical when

higher numbers of LEDs are used in a single string.

Shorted LED String or Output Short – A short circuit across

the LED string, is detected when:

STR

< V

SCL

(10)

An output short can consist of the LP, LN, or LF terminals of

the LED string being shorted, either to the battery terminal or

to ground. Either a shorted LED string or output short will be

latched and will only be cleared by pulling EN low for a period

greater than the disable time, or by cycling the power.

If either a shorted LED string, or an output short is detected, the

A6265 will stop the switching action by pulling SG low. The

fault flag FF1 will go high impedance and should be pulled up to

the supply with suitable external pull-up resistors to indicate the

fault. Either a shorted LED string or output short will be latched

and will only be cleared by pulling EN low for a period greater

than the disable time, or by cycling the power.

The FF1 output can also be used with pull-up resistors and a

P-channel MOSFET in the supply, to isolate the switching ele-

ments and the load from the supply. This MOSFET should be

connected, as shown in figure 3, with the source connected to the

supply and the drain connected to the inductor of the converter.

To FF1

VBAT

To VIN

Figure 3. Example of a supply isolation MOSFET

Automotive High Current LED Controller

A6265

Allegro MicroSystems, LLC

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

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.

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, V

IDS

, 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

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.

To ensure the A6265 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 .

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.

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A6265KLPTR-T

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