A8513 ALLEGRO [Allegro MicroSystems], A8513 Datasheet - Page 19

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

A8513
For a stable system, the required minimum slope must be smaller
than the IC slope compensation.
Note: The slope compensation value is in A/μs; the 1×10
constant multiplier.
STEP 4e: Determine the inductor current rating :
Step 5: Choose the proper switching diode. The switching diode
should be chosen for three characteristics when it is used in LED
lighting circuitry. The first and most obvious are the current
rating of the diode and the reverse voltage rating. The reverse
voltage rating should be such that during operation condition the
voltage rating of the device is larger than the maximum output
voltage; in this case it is V
the diode is:
The third major component in deciding the switching diode is the
reverse current, I
is especially important when PWM dimming is implemented.
During PWM off-time the boost converter is not switching. This
results in a slow bleeding off of the output voltage, due to leakage
currents. I
tures. On the diode that was selected in this design, the current
varies between 1 and 100 μA.
Step 6: Choose the output capacitors. The output capacitors
should be chosen such that they provide filtering for both the
boost converter and for the PWM dimming function. The biggest
factors that contribute to the size of the output capacitor is PWM
dimming frequency and the PWM duty cycle. Another major
contributor is leakage current, I
of the OVP resistor divider, I
switching diode. In this design the PWM dimming frequency is
I
L
minimum rating = I
R
can be a large contributor, especially at high tempera-
R
, characteristic of the diode. This characteristic
I
dp
= 0.595 A + 0.20 A / 2 = 0.695 A
= 0.595 A + 0.20 A / 2 = 0.695 A
= I
IN
IN
OUT (OVP)
(max) +
(max) +
OVP
lkg
, and the reverse leakage of the
. This current is a combination
1
. The peak current through
/
1
2
/
2
ΔI
ΔI
Lused
Lused
Wide Input Voltage Range, High Efficiency
-6
is a
(22)
(23)
200 Hz and the minimum duty cycle is 1%. Typically the voltage
variation on the output during PWM dimming must be less than
250 mV (V
capacitance can be calculated as follows:
A capacitor larger than 2.38 μF capacitor should be selected due
to degradation of capacitance at high voltages on the capacitor.
One ceramic 4.7 μF, 50 V capacitor is a good choice to fulfill this
requirement. Corresponding capacitors include:
The rms current through the capacitor is given by:
The output capacitor should have a current rating of at least
230 mA. The current rating of the 4.7 μF, 50V capacitor is 1.5 A.
I
COUT
C
OUT
rms =
Vendor
Murata
Murata
COUT
=
=
=
Fault Tolerant LED Driver
I
120 μA
lkg
I
0.120 A
OUT
) so that no audible noise can be heard. The
4.7 μF 50 V
2.2 μF 50 V
f
Value
PWM(dimming)
D(max) +
0.78 +
200 Hz
115 Northeast Cutoff
1.508.853.5000; www.allegromicro.com
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 U.S.A.
1 – D
1 – D(max)
1 – 0.01
1 – 0.78
0.595 A
dimming
I
GRM32ER71H475KA88L
GRM31CR71H225KA88L
IN
0.250 V
0.20 A
(max)
V
COUT
(min)
Part number
∆I
L
12
=
12
2.38 μF
=
0.23 A
(24)
(25)
19