A8513 ALLEGRO [Allegro MicroSystems], A8513 Datasheet - Page 17

• Manufacturer Part Number: A8513
• Description: Wide Input Voltage Range, High Efficiency Fault Tolerant LED Driver
• Manufacturer: ALLEGRO [Allegro MicroSystems]

A8513
Design Example
This section provides a method for selecting component values
when designing an application using the A8513. A typical circuit
using this design is shown in figure 21.
Assumptions: For the purposes of this example, the following are
given as the application requirements:
• V
• Quantity of series LEDs, #
• LED current, I
• V
• f
• T
• PWM dimming frequency: 200 Hz with a minimum duty cycle
Procedure: Select the appropriate configuration and the individual
component values in an ordered sequence.
Step 1: Connect the series LED string from VOUT to the LED
pin.
Step 2: Determine the value for the I
Choose an 8.66 kΩ resistor.
Step 3: Determine the values of the OVP resistors. The OVP
resistors are connected between the OVP pin and the output volt-
age (VOUT) and the OVP pin and ground.
Step 3a: The first step is to determine the maximum voltage
based on the LED V
voltage should be added, as well as another 2 V to account for
noise, output ripple, and resistor tolerances. The regulation volt-
age, V
of 1%.
SW
A
IN
f
(max): 85°C
at 120 mA: 3.2 V
: 2 MHz
: 5 to 16 V
R
LED
ISET
, of the A8513 is 880 mV. Then:
V
= V
= (1.003 × 1045) / 120 mA = 8.74 kΩ
OUT(OVP)
LED
ISET
: 120 mA
f
× A
requirements. To this value the regulation
= 6 × 3.2 V+ 0.880 V + 2 V
= 22.08 V
= #
ISET
SERIESLEDS
SERIESLEDS
/ I
LED
LED
× V
: 6
f
setting resistor, R
+ V
LED
+ 2 V
Wide Input Voltage Range, High Efficiency
ISET
:
(7)
(8)
To find the OVP resistor values, the user should choose a resis-
tor divider that has very low current (I
approximately 1 MΩ. A good starting point is 50 μA as I
(The I
current.) Then :
and:
Choose a value of resistor that is higher value than the calculated
R
of the minimum OVP trip level with the selected resistor:
STEP 3b: At this point a quick check should be done to determine
if the conversion ratio is acceptable for the selected frequency:
where the Minimum Switch Off-Time, t
Electrical Characteristics table.
The Theoretical Maximum V
where V
The Theoretical Maximum V
value V
sion of the A8513 should be chosen to meet the maximum duty
cycle requirements.
Step 4: Inductor selection. The inductor should be chosen such
that it can handle the necessary input current. In most applica-
tions, due to stringent EMI requirements, the system must operate
in continuous conduction mode throughout the whole input volt-
age range.
OVP
. In this case 422 kΩ was selected. Below is the actual value
V
OVP
R
R
V
D
OUT
OVP1
OVP2
OUT(OVP)
OUT(OVP)
maxofboost
d
is the diode forward voltage.
current is used later in calculating the total leakage
(max)
= (22.08 V – 1.218 V) / 50 μA = 417.2 kΩ
= 1.218 V / 50 μA = 24.36 kΩ
= (V
= V
Fault Tolerant LED Driver
OVPHI(th)
. If this is not the case, a lower frequency ver-
= R
= 422 kΩ × 50 μA + 1.218 V = 22.32 V
OUT(OVP)
= 1 – 85 ns × 2 MHz = 83%
= 1 – t
=
=
1 – D
OVP
1 – 0.83
V
5 V
SW(OFF)
IN
× I
maxofboost
/ I
(min)
– V
115 Northeast Cutoff
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Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 U.S.A.
OVP
OVP
OUT
OUT
OVPHI(th)
–
× V
× f
0.4 V 29.01 V
is then calculated as:
value must be greater than the
SW
OVPHI(th)
–
OVP
=
) / I
V
SW(OFF) ,
d
) and R
OVP
is found in the
OVP
should be
OVP
.
(10)
(11)
(12)
(13)
(9)
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