A6263 Datasheet, PDF (8/11 Pages) Allegro MicroSystems – Protected LED Array Driver

A6263
Protected LED Array Driver
Application Information
Power Dissipation
The most critical design considerations when using a linear regu-
lator such as the A6263 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 A6263:
• 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 4.
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, V_LED .
The total power dissipated in the A6263 is the sum of the qui-
escent power, the reference power, and the power in each of the
four regulators:
P_DIS = P_Q + P_REF
+ P_REGA + P_REGB + P_REGC + P_REGD
(7)
The power that is dissipated in each string of LEDs is:
Quiescent Power The quiescent power is the product of the
quiescent current, I_INQ , and the supply voltage, V_IN , and is not
related to the regulated current. The quiescent power, P_Q, is there-
fore defined as:
P_Q = V_IN × I_INQ
(4)
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. The reference circuit power is the prod-
uct of the reference current and the difference between the supply
voltage and the reference voltage, typically 1.2 V. The reference
power, P_REF , is therefore defined as:
P_REF = ^{(VIN – VREF) × VREF}
(5)
R_REF
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.
P_LEDx = V_LEDx × I_LEDx
(8)
where x is A, B, C, or D, and V_LEDx is the voltage across all
LEDs in the string.
From these equations it can be seen that, if the power in the
A6263 is not limited, then it will increase as the supply voltage
increases but the power in the LEDs will remain constant.
A6263
VIN
I_LAx
LAx
V_REG
V_IN
I_REF
I_INQ
The total current regulator dissipation is the sum of the dissipa-
IREF
V_LED
tion in each output regulator. The regulator power for each output
is defined as:
P_REGx = (V_IN – V_LEDx ) × I_LEDx
V_REF R_REF
(6)
GND
where x is 1, 2, 3, or 4.
Note that the voltage drop across the regulator, V_REG , is always
greater than the specified minimum drop-out voltage, V_DO . The
Figure 4. Internal power dissipation sources.
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