A6264
Automotive Stop/Tail LED Array Driver
Features and Benefits
Description
▪ 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 K-temperature range (–40°C to 150°C)
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.
Applications:
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.
▪ Automotive tail, stop, and turn lights
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.
Packages
10-pin MSOP with
exposed thermal pad
(suffix LY)
A temperature monitor is included to reduce the LED drive
current if the chip temperature exceeds a thermal threshold.
16-pin TSSOP with
exposed thermal pad
(suffix LP)
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.
Not to scale
Typical Application Diagram
Tail Switch
VIN
A6264
Stop Switch
+
Automotive
12 V power net
FULL
LA1
FF
LA2
IREFH
LA3
IREF
LA4
–
GND
A6264-DS, Rev. 4
A6264
Automotive Stop/Tail LED Array Driver
Selection Guide
Part Number
Ambient Operating
Temperature, TA (°C)
Packing
A6264KLPTR-T
–40 to 125
4000 pieces per 13-in. reel
A6264KLYTR-T
–40 to 125
4000 pieces per 13-in. reel
Package
16-pinTSSOP with exposed thermal pad,
4.4 × 5 mm case
10-pin MSOP with exposed thermal pad
3 × 3 mm case
Absolute Maximum Ratings1
Characteristic
Rating
Unit
–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
–40 to 125
°C
150
°C
175
°C
–55 to 150
°C
Load Supply Voltage
Symbol
Notes
VIN
Ambient Operating Temperature
Range2
TA
Maximum Continuous Junction
Temperature
TJ(max)
Transient Junction Temperature
TtJ
Storage Temperature Range
Tstg
K temperature range
Over temperature event not exceeding 10 s, lifetime duration
not exceeding 10 h, guaranteed by design characterization
1With
respect to GND.
2Limited by power dissipation.
Thermal Characteristics*may require derating at maximum conditions, see application information
Characteristic
Symbol
Test Conditions*
LP package
Package Thermal Resistance
(Junction to Ambient)
RθJA
LY package
Package Thermal Resistance
(Junction to Pad)
Value
Unit
34
ºC/W
43
ºC/W
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
2
ºC/W
On 4-layer PCB based on JEDEC standard
On 2-layer PCB with 3.8
RθJP
in.2
of copper area each side
*To be verified by characterization. Additional thermal information available on the Allegro® website.
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
2
A6264
Automotive Stop/Tail LED Array Driver
Functional Block Diagram
VBAT
VIN
FULL
Current
Regulators
IREFH
High Current
Reference
LA1
Temp
Comp
IREF
Temp
Monitor
Slew
Limit
LA2
LA3
Base Current
Reference
FF
LA4
Fault
Control
RREFH
RREF
PAD
GND
Pin-out Diagrams
Terminal List Table
NC 1
16 NC
NC 2
15 NC
IREFH 3
14 FF
Number
Name
Function
LP
LY
1,2,8,9,
15,16
–
NC
12 VIN
LA1 6
11 LA4
3
1
IREFH
High current reference
LA2 7
10 LA3
NC 8
9 NC
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
7
LA4
LED anode (+) connection 4
8
VIN
Supply
IREF 4
GND 5
PAD
13 FULL
LP Package
10 FF
IREFH 1
9 FULL
IREF 2
8 VIN
11
LA1 4
7 LA4
12
LA2 5
6 LA3
13
9
FULL
14
10
FF
–
–
PAD
GND 3
PAD
LY Package
No connection
Full/reduced current select
Fault output
Exposed thermal pad
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
3
A6264
Automotive Stop/Tail LED Array Driver
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
Supply and Reference
VIN Functional Operating Range2
6
–
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
5
15
30
μs
1.15
1.2
1.25
V
ILAx / IREFI , IREFI = IREF + IREFH
–
12.5
–
–
–10 mA > ILAx > –100 mA
–5
±4
5
%
–20 mA > ILAx > –100 mA, VLAx match to
within 1 V
–
5
10
%
FULL = low
–
GH × IREF
–
–
–105
–100
–95
mA
–
GH × (IREF
+ IREFH)
–
–
–105
–100
–95
mA
Current Regulation
Reference Voltage
Reference Current Ratio
VIREFx
GH
Current Accuracy3
EILAx
Current Matching4
EIMLAx
Output Current, Low Level
ILAx(L)
Output Current, High Level
ILAx(H)
0.7 mA < IREFx < 8.8 mA
IREF = 8 mA, FULL = low
FULL = high
IREF = IREFH = 4 mA, FULL = high
Maximum Output Current
ILAxmax
Minimum Drop-out Voltage
VDO
Output Disable Threshold
VODIS
Current Slew Time
IREF = IREFH = 4.6 mA, FULL = high
–
–
–110
mA
VIN – VLAx , ILAx = –100 mA
–
–
800
mV
VIN – VLAx , ILAx = –40 mA
–
–
660
mV
VIN – VLAx
65
–
160
mV
Current rising or falling between 10% and 90%
50
80
110
μs
–
–
0.8
V
Logic FF and FULL Pins
Input Low Voltage
Input High Voltage
VIL
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
–
–
0.4
V
IOL = 1 mA
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
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
Short Release Voltage
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, E
ILAx = 100 × [( | ILAx | × RREF / 15 ) –1] ; when FULL = high, EILAx = 100 × { | ILAx | × [(RREF × RREFH ) / (15 × RREFI )] –1}
with ILAx in mA and RREF in kΩ.
4E
IMLA = 100 × [ max ( | ILAx– ILA(AV) | ) / ILA(AV) ] , where ILA(AV) is the average current of all active outputs.
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
4
A6264
Automotive Stop/Tail LED Array Driver
Functional Description
The A6264 is a linear current regulator that is designed to provide drive current and protection for parallel strings of seriesconnected 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 applications 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 dissipation.
Pin Functions
VIN Supply to the control circuit and current regulators. A small
value ceramic bypass capacitor, typically 100 nF, should be connected 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 output. 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 inactive 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 =
15
RREF
and, given FULL = high,
ILAx =
15
+
15
(1)
RREF RREFH
where ILAx is in mA, and RREF and RREFH are in kΩ.
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:
RREF =
15
ILAx(LO)
RREFH =
15
ILAx(HI) – ILAx(LO)
and
(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 kΩ.
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
5
A6264
Automotive Stop/Tail LED Array Driver
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 = 15
20
and
RREFH =
= 750 Ω
15
= 214 Ω
(90 – 20)
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 regulator, 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 dissipated 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 currents, 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 provide 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 temperature rises.
• Thermal shutdown completely disables the outputs under extreme 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 matching 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 condition. The current through the remaining LEDs will remain in
regulation and the LEDs will be protected. Due to the difference
VIN
VIN
VIN
VIN
A6264
A6264
A6264
A6264
LA1
LA2
LA3
LA4
GND
A. Any LED cathode short
to ground. Current remains
regulated in non-shorted LEDs.
Matching may be affected.
FF is low.
Figure 1. Short circuit conditions.
LA1
LA2
LA3
LA4
GND
B. Any LAx output short to
ground. Shorted output is
disabled. Other outputs remain
active. FF is high.
LA1
LA2
LA3
LA4
LA1
LA2
LA3
LA4
GND
GND
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.
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
6
Automotive Stop/Tail LED Array Driver
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 remaining LEDs will remain in regulation and the LEDs will be protected. 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 forward 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% current level.
In extreme cases, if the chip temperature exceeds the overtemperature limit, TJF , all regulators will be disabled. The temperature 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 happen 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
Relative Sense Current (%)
A6264
90
80
60
TJM
40
25
20
TJF
TJL
0
70
130
90
110
Junction Temperature, TJ (°C)
150
170
Figure 2. Temperature monitor current reduction.
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
7
A6264
Automotive Stop/Tail LED Array Driver
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 reference.
The internal current reference, IREFI , has two possible values
depending on the state of the FULL input:
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)
• When FULL is low, IREFI is defined by IREF , the current drawn
from the IREF pin.
where IREFx are in mA and RREFX are in Ω.
• When FULL is high, IREFI is defined by the sum of IREF and
IREFH, the current drawn from the IREFH pin.
The voltage at the IREF and IREFH pins is a fixed, 1.2 V reference.
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
8
A6264
Automotive Stop/Tail LED Array Driver
Application Information
Power Dissipation
The most critical design considerations when using a linear regulator 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:
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 quiescent power, the reference power, and the power in each of the
four regulators:
• The quiescent power to run the control circuits
• The power in the reference circuit
• The power due to the regulator voltage drop
PDIS = PQ + PREF
+ PREGA + PREGB + PREGC + PREGD
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 therefore defined as:
(6)
PQ = VIN × IINQ
Reference Power The reference circuit draws the reference
current from the supply and passes it through the reference resistor 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 difference between the supply voltage and the reference voltage,
typically 1.2 V. The reference power, PREF , is therefore defined
as:
V
V
(7)
PREF = (VIN – VREF) × REF + REF
RREF RREFH
Regulator Power In most application circuits the largest dissipation 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 dissipation in each output regulator. The regulator power for each output
is defined as:
(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.
VIN
A6264
VREG
ILAx
LAx
VIN
IREF
IINQ
VLED
IREF
VREF
RREF
GND
PREGx = (VIN – VLEDx ) × ILEDx
where x is 1, 2, 3, or 4.
(8)
Figure 3. Internal power dissipation sources.
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
9
A6264
Automotive Stop/Tail LED Array Driver
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.
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
54
52
Without thermal monitor
50
ILED (mA)
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.
48
46
44
The thermal monitor will reduce the LED current as the temperature 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
With thermal monitor
4 Strings
VLED = 6.9 V
ILED = 50 mA
TA = 50°C
42
40
14.0
14.5
15.0
15.5
16.0
Supply Voltage, VIN (V)
16.5
17.0
Figure 5. LED current versus Supply Voltage.
3.0
130
125
2.0
120
TJ (°C)
Power Dissipation, PD (W)
2.5
1.5
LED Power
1.0
A6264 Power
0.5
0
70
80
90
Figure 4. Power Dissipation versus Supply Voltage.
130
140
115
Without thermal monitor
With thermal monitor
110
4 Strings
VLED = 6.9 V
ILED = 50 mA
120
100
110
Supply Voltage, VIN (V)
4 Strings
VLED = 6.9 V
ILED = 50 mA
TA = 50°C
105
150
100
14.0
14.5
15.0
15.5
16.0
Supply Voltage, VIN (V)
16.5
17.0
Figure 6. Junction Temperature versus Supply Voltage.
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10
A6264
Automotive Stop/Tail LED Array Driver
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 significant 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, RθJA , 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, ΔT, is calculated from the power dissipated,
PD , and the thermal resistance, RθJA , as:
ΔT = PD × RθJA
• 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-
(11)
A thermal resistance from silicon to ambient, RθJA , of approximately 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 available supply voltage can vary over a two-to-one range, or greater
when double battery or load dump conditions are taken into consideration. In such systems is it necessary to design the application 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.
Figure 7. Board via layout for thermal dissipation: (top) LP package
(bottom) LY package.
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11
A6264
Automotive Stop/Tail LED Array Driver
ply voltage, that is:
Application Examples
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 temperature, TJM (due to increased power dissipation as the supply voltage 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 acceptable minimum.
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 incandescent 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.
This can be estimated by determining the maximum power that
can be dissipated before the internal (junction) temperature of the
A6264 reaches TJM.
• 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.
The maximum power dissipation is therefore defined as:
• Figure 12 shows on combination of outputs tied together.
PD(max) =
∆T(max)
RθJA
(13)
where ΔT(max) is difference between the thermal monitor activation temperature, TJM , of the A6264 and the maximum ambient
temperature, TA(max), and RθJA 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 maximum voltage is simply the absolute maximum specified in the
parameter tables above.
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115 Northeast Cutoff
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12
A6264
Automotive Stop/Tail LED Array Driver
+
Fused or PWM
12 V supply
Tail Switch
VIN
Stop Switch
Stop Switch
VIN
A6264
A6264
FULL
+
Automotive
12 V power net
FF
IREFH
LA1
FULL
LA1
LA2
FF
LA2
LA3
IREFH
LA3
LA4
IREF
LA4
IREF
GND
GND
Figure 8. Common tail / stop lamp configuration.
Figure 9. Incandescent lamp replacement.
+
Fused or PWM
12 V supply
Tail Switch
VIN
Stop Switch
A6264
FULL
+
Automotive
24 V power net
LA1
LA2
LA3
LA4
FF
IREFH
VIN
VIN
A6264
A6264
FULL
LA1
LA1
FF
LA2
LA2
FF
IREFH
LA3
LA3
IREFH
IREF
LA4
LA4
IREF
FULL
IREF
GND
GND
Figure 10. Higher voltage supply application.
GND
Figure 11. Disabling FF, driving high brightness (HB) LEDs with two A6264s.
+
Automotive
12 V power net
VIN
A6264
High-side
PWM source
FULL
LA1
LA2
FF
LA3
IREFH
LA4
IREF
GND
Figure 12. LED outputs options.
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115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
13
A6264
Automotive Stop/Tail LED Array Driver
Package LP, 16-Pin TSSOP with Exposed Thermal Pad
0.45
5.00±0.10
16
0.65
16
8º
0º
0.20
0.09
1.70
B
3±0.05
4.40±0.10
3.00
6.40±0.20
6.10
0.60 ±0.15
A
1
1.00 REF
2
3±0.05
0.25 BSC
Branded Face
16X
SEATING
PLANE
0.10 C
0.30
0.19
C
3.00
C
PCB Layout Reference View
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
1.20 MAX
0.65 BSC
1 2
SEATING PLANE
GAUGE PLANE
0.15
0.00
A Terminal #1 mark area
B
Exposed thermal pad (bottom surface); dimensions may vary with device
C 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)
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
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14
A6264
Automotive Stop/Tail LED Array Driver
Package LY, 10-Pin MSOP with Exposed Thermal Pad
3.00 ±0.10
0° to 6°
10
0.15 ±0.05
3.00 ±0.10
4.88 ±0.20
A
0.53 ±0.10
1
2
0.25
1.98
1
Seating Plane
Gauge Plane
2
B
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
1.73
A Terminal #1 mark area
B Exposed thermal pad (bottom surface)
10
0.86 ±0.05
SEATING
PLANE
0.27
0.18
0.50
REF
0.05
0.15
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115 Northeast Cutoff
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15
A6264
Automotive Stop/Tail LED Array Driver
Revision History
Revision
Revision Date
Rev. 4
January 13, 2012
Description of Revision
Update RθJA
Copyright ©2009-2012, Allegro MicroSystems, Inc.
Allegro MicroSystems, Inc. reserves the right to make, from time to time, such departures from the detail specifications as may be required to permit improvements in the performance, reliability, or manufacturability of its products. Before placing an order, the user is cautioned to verify that the
information being relied upon is current.
Allegro’s products are not to be used in life support devices or systems, if a failure of an Allegro product can reasonably be expected to cause the
failure of that life support device or system, or to affect the safety or effectiveness of that device or system.
The information included herein is believed to be accurate and reliable. However, Allegro MicroSystems, Inc. assumes no responsibility for its use;
nor for any infringement of patents or other rights of third parties which may result from its use.
For the latest version of this document, visit our website:
www.allegromicro.com
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115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
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16