Automotive LED Array Driver
A6269
9
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 5) it can be seen
that, if the power in the A6269 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 A6269: thermal shutdown and thermal foldback.
Thermal Shutdown If the thermal foldback feature is disabled
by connecting the THTH pin to GND, or if the thermal resistance
from the A6269 to the ambient environment 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 A6269 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
A6269, 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 A6269 increases above the thermal monitor activation
temperature, T
JM
, as shown in figure 6. The figure shows the
operation of the A6269 with 2 strings of 3 red LEDs, each string
running at 100 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 100 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 A6269 in order to limit the temperature increase, as shown
in figure 7. The figure shows the operation of the A6269 under
the same conditions as figure 6. That is, 2 strings of 3 red LEDs,
each string running at 100 mA with each LED forward voltage
Figure 5. Power Dissipation versus Supply Voltage
3.0
2.5
2.0
1.5
1.0
0.5
0
A6269 Power
Supply Voltage, V
IN
(V)
Power Dissipation, P
D
(W)
LED Power
2 Strings
V
LED
= 6.9 V
I
LED
= 100 mA
89 1110 1312 161514
Figure 6. LED current versus Supply Voltage
Figure 7. Junction Temperature versus Supply Voltage
54
52
50
48
46
44
42
40
Without thermal monitor
With thermal monitor
14.0 14.5 15.0 16.0 17.015.5
Supply Voltage, V
IN
(V)
I
LED
(mA)
16.5
2 Strings
V
LED
= 6.9 V
I
LED
= 100 mA
T
A
= 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, V
IN
(V)
T
J
(°C)
16.5
2 Strings
V
LED
= 6.9 V
I
LED
= 100 mA
T
A
= 50°C
