a8481 Allegro MicroSystems, Inc., a8481 Datasheet - Page 9

• Manufacturer Part Number: a8481
• Description: Dual Output Boost Regulator Using Single Inductor
• Manufacturer: Allegro MicroSystems, Inc.

A8481
Dimming with serial programming is disabled during startup, for
the initialization period, t
longer than t
When changing from one specific dimming level to another,
the user may not want to have to store the latest dimming level.
A simpler method is to program a memory reset and apply the
required number of pulses, from 100% to the target dimming
level. The total “LED off” time during shutdown, reenable, and
dimming level programming can be kept sufficiently short, such
that no delay is discernable to the human eye.
• PWM Control through the ON2 pin
The average current for the LEDs can be determined by control-
ling the duty cycle of the LED current using external PWM on
ON2 pin. When the ON2 pin is high, current at the 100% level
flows through the LEDs. When ON2 is low, the LED current is
less than 1 μA.
PWM can be applied only after the t
and frequency range will be limited due to t
PWM control through the range of 0 to 80% can be achieved at
100 Hz PWM frequency.
• PWM Control through the ISET pin
The average current for the LEDs can be controlled with external
PWM on the ISET pin, as shown in figure 4a.
PWM dimming accuracy is shown in figure 3. The PWM
source used had a high level of 3 V and low level of 0 V. Also,
R
SET
V
=30 kΩ and R
I
OUT2
ON2
SHDN
SHDN
Figure 2. ON2 Clock Timing Definition
0
t
SS
, the counter resets.
P
Initial t
=100 Ω.
100%
HII
HII
. When ON2 is pulled low for time
1
87%
Dual Output Boost Regulator Using Single Inductor
HII
2
period. The duty cycle
74%
SHDN
3
61%
. Typically,
4
47%
5
t
LO
34%
Figure 3. Accuracy of External PWM on ISET
6
21%
25
20
15
10
t
5
0
HI
0
7
8%
LED Supply Current versus Duty Cycle
0
100%
20
1
PWM control via ISET pin
87%
2
115 Northeast Cutoff
1.508.853.5000; www.allegromicro.com
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 U.S.A.
40
74%
DC (%)
60
t
SHDN
Memory
Reset
80
f
PWM
(Hz)
1000
200
400
100
9