a8504 Allegro MicroSystems, Inc., a8504 Datasheet - Page 11

• Manufacturer Part Number: a8504
• Description: Wled/rgb Backlight Driver For Medium Size Lcds
• Manufacturer: Allegro MicroSystems, Inc.

Design Example
This section provides a method for selecting component values
when designing an application using the A8504.
Assumptions
given as the application requirements:
• V
• V
• Quantity of LED channels: 6
• Quantity of LEDs per channel: 8
• LED current per channel, I
• V
• f
• T
Dimming
A8504 can work with wide range of PWM frequencies, taking
about 6 μs typical (10 μs maximum) to turn on. This delay may
have a noticeable effect at high PWM frequencies combined with
low duty cycles. For example, at 100 Hz and 10% duty cycle, the
PWM on-period is 1 ms. In that period, a delay of 6 μs causes
only a 0.6% error. If the PWM frequency is 1 kHz, this error
is 6%. However, error due to turn-on delay can be nullified by
increasing the applied PWM duty cycle.
Procedure
configuration and then the individual component values, in an
ordered sequence.
1. Identify the SELx pins to use. For 6 channels:
2. Connect LEDs to pins LED1 through LED6 (leave pins
3. Select resistor RISET (connected between pin ISET and
4. Select resistor RFSET (connected between pin FSET and
A8504
• connect pins SEL1 and SEL3 to V
• connect pin SEL2 to AGND
SW
A
LED7 and LED8 open).
AGND). Given I
then:
Select a common value: 14.3 kΩ, 1%.
AGND). Given:
for a 2 MHz switching frequency, select:
BAT
IN
f
(max): 65°C
at 40 mA: 3 to 3.4 V
: 2 MHz
: 5 V
: 8 to 21 V
Use a 100 Hz PWM signal on the PWM pin. The
The procedure consists of selecting the appropriate
For the purposes of this example, the following are
R
ISET
R
FSET
LED
= 1.23 / (40/460) = 14.2 kΩ .
R
FSET
= 40 mA and V
= 26.03 / 2 = 13 kΩ .
LED
= 26.03 /f
: 40 mA
IN
SW
WLED/RGB Backlight Driver for Medium Size LCDs
ISET
,
= 1.23 V typical,
Application Information
(1)
(2)
5. Select resistor ROVP (connect to the OVP pin to set the OVP
6. Select inductor L1. This should assume a maximum duty
level, V
With a 15% margin, to set the output OVP level, given an
I
Select a common value: 41.2 kΩ.
cycle, D(max), at V
Then calculate maximum switch on-time:
Maximum input current can be calculated as:
Set inductor ripple at 60% of I
Given, during switch on-time:
Select a common value, 6.8 μH.
It is recommended to select an inductor that can handle a DC
current level that is greater than 923 mA, at the peak current
level (saturation) of 923 mA + 554 / 2 = 1200 mA. This is
to ensure that the inductor does not saturate at any steady
state or transient condition, within specified temperature and
tolerance ranges. Inductor saturation level decreases with
increasing temperature. It is advisable to use a inductor with
a saturation level of 1.6 A, because the switch current limit
is 1.8 A typical. The inductor should have a minimum DC
resistance and core loss for better efficiency.
OVPH
I
BAT
of 49 μA typical, and V
OUT
(max) = [27.7 (40 × 6)] / (8 × 0.9) = 923 mA .
V
I
D(max) = 1– (8 × 0.9) / 27.7 = 74% .
BAT
(max)). Given V
OUT
R
OVP
= (V
(max) = 3.4 × 8 + 0.5 = 27.7 V .
∆I
V
t
D = 1– (V
on
= (32 – 30) / 49 = 40.8 kΩ .
BAT
L
(max) = D(max) / f
BAT
OUT
= 0.6 × 923 = 554 mA
8 = L × 0.554 × 2 / 0.74, and
L = 5.3 μH .
= L × ∆I
(min) and 90% efficiency.
× I
115 Northeast Cutoff
1.508.853.5000; www.allegromicro.com
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 U.S.A.
= 0.74 / 2 MHz = 370 ns .
BAT
OUT
f
(max) = 3.4 V, then:
OVP
BAT
) / (V
L
× η) / V
× f
(max):
= 30 V:
SW
BAT
SW
/ D
OUT
(min) × η)
(3)
(4)
(5)
(6)
(7)
(8)
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