A8514KLPTR-T Allegro Microsystems Inc, A8514KLPTR-T Datasheet - Page 32

• Manufacturer Part Number: A8514KLPTR-T
• Description: IC LED DVR WHT 4X80MA 16-ETSSOP
• Manufacturer: Allegro Microsystems Inc

Specifications of A8514KLPTR-T

• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

A8514
The reverse breakdown voltage rating for the output diode in a
SEPIC circuit should be:
because the maximum output voltage in this case is V
The peak current through the diode is calculated as:
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. It is often advantageous to pick a
diode with a much higher breakdown voltage, just to reduce the
reverse current. Therefore for this example, pick a diode rated for
a V
Step 7
must be chosen such that they can 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 are:
PWM dimming frequency and PWM duty cycle. Another major
contributor is leakage current, I
tion of the OVP leakage current as well as the reverse current of
the switching diode. In this design the PWM dimming frequency
is 200 Hz and the minimum duty cycle is 1%. Typically, the volt-
age variation on the output, V
be less than 250 mV, so that no audible hum can be heard. The
capacitance can be calculated as follows:
BD
C
of 60 V, instead of just 40 V.
OUT
Choosing the output capacitors. The output capacitors
V
R
BD
=
=
can be a large contributor, especially at high tempera-
I
200 (μA)
> 15.9 (V) + 16 (V) = 31.9 V
I
> V
dp
LK
= 0.848 (A) + 0.096 (A) = 0.944 A
= I
R
OUT(OVP)
, characteristic of the diode. This characteristic
IN
f
(max) +
PWM(dimming)
200 (Hz)
(max) + V
1 – D(min)
COUT
1
/
LK
2
1 – 0.01
ΔI
. This current is the combina-
, during PWM dimming must
Lused
IN
V
0.250 (V)
(max)
COUT
=
Wide Input Voltage Range, High Efficiency
3.96 μF
OUT(OVP)
(44)
(45)
(46)
.
A capacitor larger than 3.96 μF should be selected due to degra-
dation of capacitance at high voltages on the capacitor. Select a
4.7 μF capacitor for this application.
The rms current through the capacitor is given by:
The output capacitor must have a ripple current rating of at least
500 mA. The capacitor selected for this design is a 4.7 μF 50 V
capacitor with a 1.5 A current rating.
Step 8
selected such that it provides a good filtering of the input voltage
waveform. A estimation rule is to set the input voltage ripple,
ΔV
input capacitor requirements are as follows:
The rms current through the capacitor is given by:
A good ceramic input capacitor with a rating of 2.2 μF 25 V will
suffice for this application.
IN
C
, to be 1% of the minimum input voltage. The minimum
I
IN
COUT
Selecting input capacitor. The input capacitor must be
C
rms =
IN
rms =
=
=
=
Fault Tolerant LED Driver
8
8
∆I
0.191 (A)
=
Lused
12
I
2 (MHz) 0.05 (V)
0.240 (A)
f
OUT
12
SW
∆I
0.191 (A)
Lused
∆V
1 – D(max)
115 Northeast Cutoff
1.508.853.5000; www.allegromicro.com
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 U.S.A.
=
D(max)
IN
0.055 A
1 – 0.765
0.765
=
=
0.24 μF
0.433 A
(47)
(48)
(49)
32