A6264KLPTR-T - Allegro MicroSystems Europe Ltd.

Description

The A6264 is a linear, programmable current regulator providing

up to 100 mA from each of four outputs to drive arrays of high

brightness LEDs. The LED current can be switched between

high current and low current for stop/tail applications. The

two LED current levels from each output, accurate to 5%,

are set by two reference resistors. Current matching in each

string is better than 10% without the use of ballast resistors.

Driving LEDs with constant current ensures safe operation

with maximum possible light output.

Optimum performance is achieved when driving 4 strings with

1 to 3 LEDs in each string, at a total current of up to 100 mA

in each string. Outputs can be connected in parallel or left

unused as required.

Short detection is provided to protect the LEDs and the A6264

during a short-to-ground at any LED output pin. An open LED

in any of the strings disables all outputs but can be overridden.

Shorted LED output pins or open LEDs are indicated by a

fault flag.

A temperature monitor is included to reduce the LED drive

current if the chip temperature exceeds a thermal threshold.

The device packages are a 10-pin MSOP (LY) and a 16-pin

TSSOP (LP), both with exposed pad for enhanced thermal

dissipation. They are lead (Pb) free, with 100% matte tin

leadframe plating.

A6264-DS, Rev. 5

Features and Benefits

 AEC Q-100 qualified

 Total LED drive current up to 400 mA

 Current shared equally up to 100 mA by up to 4 strings

 6 to 50 V supply

 Low dropout voltage

 LED output short-to-ground and thermal protection

 Enable input for PWM control

 Current slew rate limit during PWM

 Current set by reference resistor

 Automotive temperature range (–40°C to 150°C)

Applications:

Typical Application Diagram

A6264

Automotive Stop/Tail LED Array Driver

 Automotive tail, stop, and turn lights

A6264

–

LA1

VIN

Automotive

12 V power net

GND

LA2

LA3

LA4

FULL

Stop Switch

Tail Switch

IREFH

IREF

Packages

Not to scale

10-pin MSOP with

exposed thermal pad

(suffix LY)

16-pin TSSOP with

exposed thermal pad

(suffix LP)

Automotive Stop/Tail LED Array Driver

A6264

Allegro MicroSystems, LLC

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

Absolute Maximum Ratings1

Characteristic Symbol Notes Rating Unit

Load Supply Voltage VIN –0.3 to 50 V

Pin FULL –0.3 to 50 V

Pins LA[1:4] –0.3 to 50 V

Pin FF –0.3 to 50 V

Pins IREF, IREFH –0.3 to 6.5 V

Ambient Operating Temperature

Range2TAK temperature range –40 to 125 °C

Maximum Continuous Junction

Temperature TJ(max) 150 °C

Transient Junction Temperature TtJ

Over temperature event not exceeding 10 s, lifetime duration

not exceeding 10 h, guaranteed by design characterization 175 °C

Storage Temperature Range Tstg –55 to 150 °C

1With respect to GND.

2Limited by power dissipation.

Selection Guide

Part Number Ambient Operating

Temperature, TA (°C) Packing Package

A6264KLPTR-T –40 to 125 4000 pieces per 13-in. reel 16-pinTSSOP with exposed thermal pad,

4.4 × 5 mm case

A6264KLYTR-T –40 to 125 4000 pieces per 13-in. reel 10-pin MSOP with exposed thermal pad

3 × 3 mm case

Thermal Characteristics*may require derating at maximum conditions, see application information

Characteristic Symbol Test Conditions* Value Unit

Package Thermal Resistance

(Junction to Ambient) RθJA

LP package

On 4-layer PCB based on JEDEC standard 34 ºC/W

On 2-layer PCB with 3.8 in.2 of copper area each side 43 ºC/W

LY package

On 4-layer PCB based on JEDEC standard 48 ºC/W

On 2-layer PCB with 2.5 in.2 of copper area each side 48 ºC/W

Package Thermal Resistance

(Junction to Pad) RθJP 2 ºC/W

*To be verified by characterization. Additional thermal information available on the Allegro™ website.

Automotive Stop/Tail LED Array Driver

A6264

Allegro MicroSystems, LLC

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

Terminal List Table

Number Name Function

LP LY

1,2,8,9,

15,16 – NC No connection

3 1 IREFH High current reference

4 2 IREF Base current reference

5 3 GND Ground reference

6 4 LA1 LED anode (+) connection 1

7 5 LA2 LED anode (+) connection 2

10 6 LA3 LED anode (+) connection 3

11 7 LA4 LED anode (+) connection 4

12 8 VIN Supply

13 9 FULL Full/reduced current select

14 10 FF Fault output

– – PAD Exposed thermal pad

LA1

VIN

VBAT

Current

Regulators

GND

PAD

Fault

Control

Tem p

Comp

Tem p

Monitor

Slew

Limit

Base Current

Reference

High Current

Reference

LA2

LA3

LA4

IREFH

IREF

RREF

RREFH

FULL

Functional Block Diagram

Pin-out Diagrams

LP Package

LY Package

IREFH

IREF

GND

LA1

LA2

FULL

VIN

LA4

LA3

PAD

IREFH

IREF

GND

LA1

LA2

FULL

VIN

LA4

LA3

PAD

Automotive Stop/Tail LED Array Driver

A6264

Allegro MicroSystems, LLC

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

Supply and Reference

VIN Functional Operating Range26 – 50 V

VIN Quiescent Current IINQ LA[1:4] connected to VIN – – 10 mA

Startup Time tON

VIN > 7 V to ILA1 < –5 mA, RREF = 125 Ω,

FULL = low 51530 μs

Current Regulation

Reference Voltage VIREFx 0.7 mA < IREFx < 8.8 mA 1.15 1.2 1.25 V

Reference Current Ratio GHILAx / IREFI , IREFI = IREF + IREFH – 12.5 – –

Current Accuracy3EILAx –10 mA > ILAx > –100 mA –5 ±4 5 %

Current Matching4EIMLAx

–20 mA > ILAx > –100 mA, VLAx match to

within 1 V – 5 10 %

Output Current, Low Level ILAx(L)

FULL = low – GH × IREF ––

IREF = 8 mA, FULL = low –105 –100 –95 mA

Output Current, High Level ILAx(H)

FULL = high – GH × (IREF

+ IREFH)

––

IREF = IREFH = 4 mA, FULL = high –105 –100 –95 mA

Maximum Output Current ILAxmax IREF = IREFH = 4.6 mA, FULL = high – – –110 mA

Minimum Drop-out Voltage VDO

VIN – VLAx , ILAx = –100 mA – – 800 mV

VIN – VLAx , ILAx = –40 mA – – 660 mV

Output Disable Threshold VODIS VIN – VLAx 65 – 160 mV

Current Slew Time Current rising or falling between 10% and 90% 50 80 110 μs

Logic FF and FULL Pins

Input Low Voltage VIL – – 0.8 V

Input High Voltage VIH 2–– V

Input Hysteresis (FULL pin) VIhys 150 350 – mV

Pull-Down Resistor (FULL pin) RPD –50– kΩ

FF Pin Output Low Voltage VOL IOL = 1 mA – – 0.4 V

Protection

Short Detect Voltage VSCD Measured at LAx 1.2 – 1.8 V

Short Circuit Source Current ISCS Short present LAx to GND –2 –0.8 –0.5 mA

Short Release Voltage VSCR Measured at LAx – – 1.9 V

Short Release Voltage Hysteresis VSChys VSCR – VSCD 200 – 500 mV

Open Load Detect Voltage VOCD VIN – VLAx 170 – 450 mV

Open Load Detect Delay tOCD –2–ms

Thermal Monitor Activation Temperature TJM TJ with ISEN = 90% 95 115 130 °C

Thermal Monitor Slope ATM ISEN = 50% –3.5 –2.5 –1.5 %/°C

Thermal Monitor Low Current

Temperature TJL TJ at ISEN = 25% 120 135 150 °C

Overtemperature Shutdown TJF Temperature increasing – 170 – °C

Overtemperature Hysteresis TJhys Recovery = TJF – TJhys –15– °C

1For input and output current specifications, negative current is defined as coming out of (sourcing) the specified device pin.

2Function is correct but parameters are not guaranteed outside the general limits (7 to 40 V).

3When FULL = low, EILAx = 100 × [( | ILAx | × RREF / 15 ) –1] ; when FULL = high, EILAx = 100 × { | ILAx | × [(RREF × RREFH ) / (15 × RREFI )] –1}

with ILAx in mA and RREF in kΩ.

4EIMLA = 100 × [ max ( | ILAx– ILA(AV) | ) / ILA(AV) ] , where ILA(AV) is the average current of all active outputs.

ELECTRICAL CHARACTERISTICS1 Valid at TJ = –40°C to 150°C, VIN = 7 to 40 ; unless otherwise noted

Characteristics Symbol Test Conditions Min. Typ. Max. Unit

Automotive Stop/Tail LED Array Driver

A6264

Allegro MicroSystems, LLC

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

Functional Description

The A6264 is a linear current regulator that is designed to pro-

vide drive current and protection for parallel strings of series-

connected high brightness LEDs in automotive applications. It

provides up to four matched programmable current outputs, at

up to 100 mA, with low minimum dropout voltages below the

main supply voltage. For 12 V power net applications optimum

performance is achieved when driving 4 strings of 1 to 3 LEDs,

at currents up to 100 mA per string.

The A6264 is specifically designed for use in stop/tail applica-

tions where the LED current is switched between a high current

(indicating stop or brake) and a lower current (for normal tail

light operation).

Current regulation is maintained and the LEDs protected during a

short to ground at any point in the LED string. A short to ground

on any regulator output terminal will disable that output and set

the fault flag. An open load on any output will set the fault flag

and disable all outputs. Remaining outputs can be re-enabled

by pulling the fault flag output low. Individual outputs can be

disabled by connecting the output to VIN.

Integrated thermal management reduces the regulated current

level at high internal junction temperatures to limit power dis-

sipation.

Pin Functions

VIN Supply to the control circuit and current regulators. A small

value ceramic bypass capacitor, typically 100 nF, should be con-

nected from close to this pin to the GND pin.

GND Ground reference connection. Should be connected directly

to the negative supply.

FULL Logic input to enable high LED current output. Open or

low sets LED current to the base current level. High sets LED

current to the sum of the base current level, and the additional

high current (see Detailed Description of Regulator Operation

section)

. Typically connected through a resistor to the stop switch

input.

IREF 1.2 V base current reference. Used for base (low) level

current output, IREF

. Connect resistor, RREF

, to GND to set this

reference current.

IREFH 1.2 V additional high current reference. Summed with

IREF for full current output. Connect resistor, RREFH

, to GND to

set this reference current.

LA[1:4] Current source connected to the anode of the first LED in

each string. Connect directly to VIN to disable the respective out-

put. In this document “LAx” indicates any one of the four outputs.

FF Open drain fault flag, used with an external pull-up resistor,

to indicate open, short, or overtemperature conditions. FF is inac-

tive when a fault is present. During an open load condition, FF

can be pulled low to force the remaining outputs on.

LED Current Level

The LED current is controlled by four matching linear current

regulators between the VIN pin and each of the LAx outputs. The

basic equations that determine the nominal output current at each

LAx pin are:

Given FULL = low,

ILAx =RREF

and, given FULL = high,

ILAx =+

RREF

RREFH

(1)

where ILAx is in mA, and RREF and RREFH are in k.

In both cases, the output current may be reduced from the set

level by the thermal monitor circuit.

Conversely the reference resistors may be calculated from:

ILAx(LO)

RREF

and

ILAx(HI) – ILAx(LO)

RREFH

(2)

where ILAx(LO) is the required source current when FULL is low

and ILAx(HI) is the current when FULL is high. ILAx(x) are in mA,

and RREF and RREFH are in k.

Automotive Stop/Tail LED Array Driver

A6264

Allegro MicroSystems, LLC

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

For example, where the required high-level current (FULL =

high) is 90 mA and the required low-level current (FULL = low)

is 20 mA, the resistor values will be:

RREF 750 

and

(90 – 20)

RREFH 214 

These equations completely define the output currents with

respect to the setting resistors. However, for further reference, see

Detailed Description of Regulator Operation section.

It is important to note that because the A6264 is a linear regu-

lator, the maximum regulated current is limited by the power

dissipation and the thermal management in the application. All

current calculations assume adequate heatsinking for the dissi-

pated power. Thermal management is at least as important as the

electrical design in all applications. In high current high ambient

temperature applications the thermal management is the most

important aspect of the systems design. The application section

below provides further detail on thermal management and the

associated limitations.

Operation with Fewer LED Strings or Higher Currents

The A6264 may be configured to use fewer than four LED

strings, either by connecting outputs together for higher cur-

rents, or by connecting the output directly to VIN to disable the

regulator for that output. When a regulator is disabled, it will

not indicate an open load and will not affect the fault flag or the

operation of the remaining regulator outputs.

Safety Features

The circuit includes several features to ensure safe operation and

to protect the LEDs and the A6264:

• The current regulators between VIN and each LAx output pro-

vide a natural current limit due to the regulation.

• Each LAx output includes a short-to-ground detector that will

disable the output to limit the dissipation.

• An open circuit on any output will disable all outputs.

• The thermal monitor reduces the regulated current as the tem-

perature rises.

• Thermal shutdown completely disables the outputs under ex-

treme overtemperature conditions.

Short Circuit Detection A short to ground on any LED

cathode (figure 1A) will not result in a short fault condition. The

current through the remaining LEDs will remain in regulation and

the LEDs will be protected. Due to the difference in the voltage

drop across the LEDs, as a result of the short, the current match-

ing in the A6264 may exceed the specified limits.

Any LAx output that is pulled below the short detect voltage

(figure 1B) will disable the regulator on that output and allow

the fault flag, FF, to go high. A small current will be sourced

from the disabled output to monitor the short and detect when

it is removed. When the voltage at LAx rises above the short

detect voltage the fault flag will be removed and the regulator

re-enabled.

A shorted LED (figure 1C) will not result in a short fault condi-

tion. The current through the remaining LEDs will remain in

regulation and the LEDs will be protected. Due to the difference

A. Any LED cathode short

to ground. Current remains

regulated in non-shorted LEDs.

Matching may be affected.

FF is low.

B. Any LAx output short to

ground. Shorted output is

disabled. Other outputs remain

active. FF is high.

C. Current remains regulated.

Matching may be affected. Only

the shorted LED is inactive.

FF is low.

D. Short between LEDs in

different strings. Current

remains regulated. Current

is summed and shared by

affected strings. Intensity match

dependent on voltage binning.

FF is low.

Figure 1. Short circuit conditions.

A6264

LA1

VIN

GND

LA2

LA3

LA4

A6264

LA1

VIN

GND

LA2

LA3

LA4

A6264

LA1

VIN

GND

LA2

LA3

LA4

A6264

LA1

VIN

GND

LA2

LA3

LA4

Automotive Stop/Tail LED Array Driver

A6264

Allegro MicroSystems, LLC

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

in the voltage drop across the LEDs, as a result of the short, the

current matching in the A6264 may exceed the specified limits.

A short between LEDs in different strings (figure 1D) will not

result in a short fault condition. The current through the remain-

ing LEDs will remain in regulation and the LEDs will be pro-

tected. The current will be summed and shared by the affected

strings. Current matching in the strings will then depend on the

LED forward voltage differences.

Open Load Detection An open load condition is detected

when the voltage across the regulator, VIN – VLAx , is less than

the open load detect voltage, VOCD , but greater than the output

disable threshold voltage, VODIS

. When this condition is present

for more than the open load detect time, tOCD , then all regulators

will be disabled and the fault flag allowed to go high.

The regulators will remain disabled until either the power is

cycled off and on, or the fault flag, FF, is pulled low. If the power

is cycled, the regulators will start in the enabled state, unless

disabled by tying the output to VIN, and the open load detection

timer will be reset. If the open load is still present the regulators

will again be disabled after the open load detect time.

Pulling the fault flag low will override the open load fault action

and all enabled regulators will be switched on. This state will

be maintained while the fault flag is held low. If the fault flag is

allowed to go high the A6264 will return to the open load fault

condition and will disable all regulators.

Each of the four regulators includes a limiter to ensure that

the output voltage will not rise higher than the output disable

threshold voltage below VIN when driven by the regulator. This

means that the voltage across the regulator will not be less than

the output disable voltage, unless it is forced by connecting the

LAx pin to VIN. However if a load becomes disconnected, the

regulator will pull the LAx pin up to the limit, which will ensure

that the voltage across the regulator, VIN – VLAx , is less than the

open load detect voltage, VOCD .

Note that an open load may also be detected if the sum of the for-

ward voltages of the LEDs in a string is close to or greater than

the supply voltage on VIN.

Temperature Monitor A temperature monitor function,

included in the A6264, reduces the LED current as the silicon

junction temperature of the A6264 increases (see figure 2). By

mounting the A6264 on the same thermal substrate as the LEDs,

this feature can also be used to limit the dissipation of the LEDs.

As the junction temperature of the A6264 increases, the regulated

current level is reduced, reducing the dissipated power in the

A6264 and in the LEDs. The current is reduced from the 100%

level at typically 4% per degree Celsius until the point at which

the current drops to 25% of the full value, defined at TJL

. Above

this temperature the current will continue to reduce at a lower

rate until the temperature reaches the overtemperature shutdown

threshold temperature, TJF. The temperature at which this effect

begins is defined as the thermal monitor activation temperature,

TJM, and is specified, in the characteristics table, at the 90% cur-

rent level.

In extreme cases, if the chip temperature exceeds the overtem-

perature limit, TJF

, all regulators will be disabled. The tempera-

ture will continue to be monitored and the regulators re-activated

when the temperature drops below the threshold provided by the

specified hysteresis.

Note that it is possible for the A6264 to transition rapidly

between thermal shutdown and normal operation. This can hap-

pen if the thermal mass attached to the exposed thermal pad is

small and TJM is increased to close to the shutdown temperature.

The period of oscillation will depend on TJM

, the dissipated

power, the thermal mass of any heatsink present, and the ambient

temperature.

100

TJM

TJL

TJF

70 90 110

Junction Temperature, TJ (°C)

Relative Sense Current (%)

130 150 170

Figure 2. Temperature monitor current reduction.

Automotive Stop/Tail LED Array Driver

A6264

Allegro MicroSystems, LLC

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

Detailed Description of Regulator Operation

The current sourced from each LAx output is determined by the

internal reference current as:

ILAx = GH × IREFI (3)

where ILAx is the current sourced from each LAx pin, GH is the

current gain, typically 12.5, and IREFI is the internal current refer-

ence.

The internal current reference, IREFI , has two possible values

depending on the state of the FULL input:

• When FULL is low, IREFI is defined by IREF , the current drawn

from the IREF pin.

• When FULL is high, IREFI is defined by the sum of IREF and

IREFH, the current drawn from the IREFH pin.

Two external resistors determine IREF and IREFH:

• Resistor RREF

, from IREF to GND, such that

IREF = 1200 / RREF (4)

• Resistor RREFH, from IREFH to GND, such that

IREFH = 1200 / RREFH (5)

where IREFx are in mA and RREFX are in .

The voltage at the IREF and IREFH pins is a fixed, 1.2 V refer-

ence.

Automotive Stop/Tail LED Array Driver

A6264

Allegro MicroSystems, LLC

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

Application Information

Power Dissipation

The most critical design considerations when using a linear regu-

lator such as the A6264 are the power produced internally as heat

and the rate at which that heat can be dissipated.

There are three sources of power dissipation in the A6264:

• The quiescent power to run the control circuits

• The power in the reference circuit

• The power due to the regulator voltage drop

The elements relating to these dissipation sources are illustrated

in figure 3.

Quiescent Power The quiescent power is the product of the

quiescent current, IINQ

, and the supply voltage, VIN , and is not

related to the regulated current. The quiescent power, PQ, is there-

fore defined as:

PQ = VIN × IINQ (6)

Reference Power The reference circuit draws the reference

current from the supply and passes it through the reference resis-

tor to ground. The reference current is 8% of the output current

on any one active output. When FULL is high, the reference

circuit power is the product of the reference current and the dif-

ference between the supply voltage and the reference voltage,

typically 1.2 V. The reference power, PREF , is therefore defined

as:

PREF =RREFH

(VIN – VREF) × VREF

RREF

VREF +













(7)

Regulator Power In most application circuits the largest dis-

sipation will be produced by the output current regulators. The

power dissipated in each current regulator is simply the product

of the output current and the voltage drop across the regulator.

The total current regulator dissipation is the sum of the dissipa-

tion in each output regulator. The regulator power for each output

is defined as:

PREGx =(VIN – VLEDx ) × ILEDx

(8)

where x is 1, 2, 3, or 4.

Note that the voltage drop across the regulator, VREG , is always

greater than the specified minimum drop-out voltage, VDO

. The

output current is regulated by making this voltage large enough

to provide the voltage drop from the supply voltage to the total

forward voltage of all LEDs in series, VLED .

The total power dissipated in the A6264 is the sum of the qui-

escent power, the reference power, and the power in each of the

four regulators:

PDIS =PQ + PREF

+ PREGA + PREGB + PREGC + PREGD

(9)

The power that is dissipated in each string of LEDs is:

PLEDx =VLEDx × ILEDx

(10)

where x is A, B, C, or D, and VLEDx is the voltage across all

LEDs in the string.

A6264

LAx

ILAx

IINQ

IREF

VIN

GND

IREF

RREF

VREF

VLED

VREG

VIN

Figure 3. Internal power dissipation sources.

Automotive Stop/Tail LED Array Driver

A6264

Allegro MicroSystems, LLC

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

From these equations (and as illustrated in figure 4) it can be seen

that, if the power in the A6264 is not limited, then it will increase

as the supply voltage increases but the power in the LEDs will

remain constant.

Dissipation Limits

There are two features limiting the power that can be dissipated

by the A6264: thermal shutdown and thermal foldback.

Thermal Shutdown If the thermal resistance from the A6264

to the ambient temperature is high, then the silicon temperature

will rise to the thermal shutdown threshold and the current will

be disabled. After the current is disabled the power dissipated will

drop and the temperature will fall. When the temperature falls by

the hysteresis of the thermal shutdown circuit, then the current

will be re-enabled and the temperature will start to rise again.

This cycle will repeat continuously until the ambient temperature

drops or the A6264 is switched off. The period of this thermal

shutdown cycle will depend on several electrical, mechanical,

and thermal parameters, and could be from a few milliseconds to

a few seconds.

Thermal Foldback If there is a good thermal connection to the

A6264, then the thermal foldback feature will have time to act.

This will limit the silicon temperature by reducing the regulated

current and therefore the dissipation.

The thermal monitor will reduce the LED current as the tempera-

ture of the A6264 increases above the thermal monitor activation

temperature, TJM , as shown in figure 5. The figure shows the

operation of the A6264 with 4 strings of 3 red LEDs, each string

running at 50 mA. The forward voltage of each LED is 2.3 V and

the graph shows the current as the supply voltage increases from

14 to 17 V. As the supply voltage increases, without the thermal

foldback feature, the current would remain at 50 mA, as shown by

the dashed line. The solid line shows the resulting current decrease

as the thermal foldback feature acts.

If the thermal foldback feature did not affect LED current, the

current would increase the power dissipation and therefore the

silicon temperature. The thermal foldback feature reduces power

in the A6264 in order to limit the temperature increase, as shown

in figure 6. The figure shows the operation of the A6264 under

the same conditions as figure 5. That is, 4 strings of 3 red LEDs,

each string running at 50 mA with each LED forward voltage at

2.3 V. The graph shows the temperature as the supply voltage

Figure 4. Power Dissipation versus Supply Voltage.

3.0

2.5

2.0

1.5

1.0

0.5

70 80 10090

Supply Voltage, VIN (V)

Power Dissipation, PD (W)

120110 150140130

LED Power

A6262 Power 4 Strings

VLED = 6.9 V

ILED = 50 mA

Figure 5. LED current versus Supply Voltage.

Figure 6. Junction Temperature versus Supply Voltage.

Without thermal monitor

With thermal monitor

14.0 14.5 15.0 16.0 17.015.5

Supply Voltage, VIN (V)

ILED (mA)

16.5

4 Strings

VLED = 6.9 V

ILED = 50 mA

TA = 50°C

130

125

120

115

110

105

100

Without thermal monitor

With thermal monitor

14.0 14.5 15.0 16.0 17.015.5

Supply Voltage, VIN (V)

TJ (°C)

16.5

4 Strings

VLED = 6.9 V

ILED = 50 mA

TA = 50°C

Automotive Stop/Tail LED Array Driver

A6264

Allegro MicroSystems, LLC

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

increases from 14 to 17 V. Without the thermal foldback feature

the temperature would continue to increase up to the thermal

shutdown temperature as shown by the dashed line. The solid line

shows the effect of the thermal foldback function in limiting the

temperature rise.

Figures 5 and 6 show the thermal effects where the thermal

resistance from the silicon to the ambient temperature is 40°C/W.

Thermal performance can be enhanced further by using a signifi-

cant amount of thermal vias as described below.

Thermal Dissipation

The amount of heat that can pass from the silicon of the A6264

to the surrounding ambient environment depends on the thermal

resistance of the structures connected to the A6264. The thermal

resistance, RJA

, is a measure of the temperature rise created by

power dissipation and is usually measured in degrees Celsius per

watt (°C/W).

The temperature rise, T, is calculated from the power dissipated,

, and the thermal resistance, RJA

, as:

T = PD × RJA (11)

A thermal resistance from silicon to ambient, RJA

, of approxi-

mately 30°C/W (LP package) or 34°C/W (LY package) can be

achieved by mounting the A6264 on a standard FR4 double-sided

printed circuit board (PCB) with a copper area of a few square

inches on each side of the board under the A6264. Multiple

thermal vias, as shown in figure 7, help to conduct the heat from

the exposed pad of the A6264 to the copper on each side of the

board. The thermal resistance can be reduced by using a metal

substrate or by adding a heatsink.

Supply Voltage Limits

In many applications, especially in automotive systems, the avail-

able supply voltage can vary over a two-to-one range, or greater

when double battery or load dump conditions are taken into con-

sideration. In such systems is it necessary to design the applica-

tion circuit such that the system meets the required performance

targets over a specified voltage range.

To determine this range when using the A6264 there are two

limiting conditions:

• For maximum supply voltage the limiting factor is the power

that can be dissipated from the regulator without exceeding the

temperature at which the thermal foldback starts to reduce the

output current below an acceptable level.

• For minimum supply voltage the limiting factor is the maximum

drop-out voltage of the regulator, where the difference between

the load voltage and the supply is insufficient for the regulator

to maintain control over the output current.

Minimum Supply Limit: Regulator Saturation Voltage

The supply voltage, VIN

, is always the sum of the voltage drop

across the high-side regulator, VREG , and the forward voltage of

the LEDs in the string, VLED, as shown in figure 3.

VLED is constant for a given current and does not vary with

supply voltage. Therefore VREG provides the variable difference

between VLED and VIN . VREG has a minimum value below which

the regulator can no longer be guaranteed to maintain the output

current within the specified accuracy. This level is defined as the

regulator drop-out voltage, VDO.

The minimum supply voltage, below which the LED current does

not meet the specified accuracy, is therefore determined by the

sum of the minimum drop-out voltage, VDO , and the forward

voltage of the LEDs in the string, VLED . The supply voltage must

always be greater than this value and the minimum specified sup-

Figure 7. Board via layout for thermal dissipation: (top) LP package

(bottom) LY package.

Automotive Stop/Tail LED Array Driver

A6264

Allegro MicroSystems, LLC

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

ply voltage, that is:

VIN > VDO + VLED, and

VIN > VIN

(min) (12)

As an example, consider the configuration used in figures 5 and

6 above, namely 4 strings of 3 red LEDs, each string running at

50 mA, with each LED forward voltage at 2.3 V. The minimum

supply voltage will be approximately:

VIN(min) = 0.55 + (3 × 2.3) = 7.45 V

Maximum Supply Limit: Thermal Limitation As described

above, when the thermal monitor reaches the activation tempera-

ture, TJM (due to increased power dissipation as the supply volt-

age rises), the thermal foldback feature causes the output current

to decrease. The maximum supply voltage is therefore defined as

the voltage above which the LED current drops below the accept-

able minimum.

This can be estimated by determining the maximum power that

can be dissipated before the internal (junction) temperature of the

A6264 reaches TJM.

The maximum power dissipation is therefore defined as:

PD(max) =RJA

T(max)

(13)

where T(max) is difference between the thermal monitor activa-

tion temperature, TJM

, of the A6264 and the maximum ambient

temperature, TA(max), and RJA is the thermal resistance from the

internal junctions in the silicon to the ambient environment.

If minimum LED current is not a critical factor, then the maxi-

mum voltage is simply the absolute maximum specified in the

parameter tables above.

Application Examples

Figure 8 shows a typical configuration for driving tail and stop

light LEDs. Although the functional features of the A6264 are

specifically designed for use with automotive tail and stop lights,

the IC can be used in many other general lighting applications.

• Figure 9 shows the A6264 driving LEDs in a low voltage incan-

descent lamp replacement. In such replacement applications the

supply may be provided by a PWM-driven, high-side switch.

The A6264 can be used in this application by applying the

PWM supply directly to VIN. When power is applied there will

be a short startup delay, tON , before the current starts to rise.

The current rise time will be limited by the internal current slew

rate control. In this example the A6264 is operating with FULL

high and with a fault output.

• Figure 10 shows a typical configuration for a higher voltage

supply.

• If neither fault action nor fault reporting is required, then FF

may be tied to ground as in figure 11. This shows two A6264

ICs driving a single string of two HB LEDs.

• Figure 12 shows on combination of outputs tied together.

Automotive Stop/Tail LED Array Driver

A6264

Allegro MicroSystems, LLC

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

A6264

LA1

VIN

GND

LA2

LA3

LA4

FULL

IREFH

IREF

A6264

LA1

VIN

GND

LA2

LA3

LA4

FULL

IREFH

IREF

A6264

VIN

GND

FULL

IREFH

IREF

LA1

LA2

LA3

LA4

A6264

VIN

GND

FULL

IREFH

IREF

LA1

LA2

LA3

LA4

A6264

VIN

Fused or PWM

12 V supply

Fused or PWM

12 V supply

GND

FULL

IREFH

IREF

A6264

VIN

GND

IREFH

IREF

+Automotive

12 V power net

Automotive

12 V power net

Stop Switch

Tail Switch

Stop Switch

Tail Switch

+Automotive

24 V power net

High-side

PWM source

FULL LA1

LA2

LA3

LA4

LA1

LA2

LA3

LA4

Figure 8. Common tail / stop lamp configuration. Figure 9. Incandescent lamp replacement.

Figure 12. LED outputs options.

Figure 10. Higher voltage supply application. Figure 11. Disabling FF, driving high brightness (HB) LEDs with two A6264s.

Automotive Stop/Tail LED Array Driver

A6264

Allegro MicroSystems, LLC

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

Package LP, 16-Pin TSSOP with Exposed Thermal Pad

1.20 MAX

0.15

0.00

0.30

0.19

0.20

0.09

8º

0º

0.60 ±0.15

1.00 REF

SEATING

PLANE

C0.10

16X

0.65 BSC

0.25 BSC

5.00±0.10

4.40±0.10 6.40±0.20

GAUGE PLANE

SEATING PLANE

ATerminal #1 mark area

For Reference Only; not for tooling use (reference MO-153 ABT)

Dimensions in millimeters

Dimensions exclusive of mold flash, gate burrs, and dambar protrusions

Exact case and lead configuration at supplier discretion within limits shown

Exposed thermal pad (bottom surface); dimensions may vary with device

6.10

0.65

0.45

1.70

3.00

Reference land pattern layout (reference IPC7351

SOP65P640X110-17M);

All pads a minimum of 0.20 mm from all adjacent pads; adjust as

necessary to meet application process requirements and PCB layout

tolerances; when mounting on a multilayer PCB, thermal vias at the

exposed thermal pad land can improve thermal dissipation (reference

EIA/JEDEC Standard JESD51-5)

PCB Layout Reference View

Branded Face

3±0.05

Automotive Stop/Tail LED Array Driver

A6264

Allegro MicroSystems, LLC

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

Package LY, 10-Pin MSOP with Exposed Thermal Pad

Terminal #1 mark area

Gauge Plane

Seating Plane

0.86 ±0.05

SEATING

PLANE

0.50

REF

0.25

0.53 ±0.10

0.15 ±0.05

0.05

0.15

0° to 6°

3.00 ±0.10

3.00 ±0.10 4.88 ±0.20

1.98

1.73

0.27

0.18

For Reference Only; not for tooling use (reference JEDEC MO-187)

Dimensions in millimeters

Dimensions exclusive of mold flash, gate burrs, and dambar protrusions

Exact case and lead configuration at supplier discretion within limits shown

Exposed thermal pad (bottom surface)

Automotive Stop/Tail LED Array Driver

A6264

Allegro MicroSystems, LLC

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

For the latest version of this document, visit our website:

www.allegromicro.com

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the information being relied upon is current.

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failure of that life support device or system, or to affect the safety or effectiveness of that device or system.

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Revision History

Revision Revision Date Description of Revision

Rev. 5 January 15, 2013 Update Features List