A8434
Setting LED Current
6-Channel High Efficiency Charge Pump
White LED Driver
Application Information
Transitions Between 1.5× and 2× Modes
The transition from 1.5× to 2× mode depends upon dropout
conditions.
When operating in 2× mode, the IC switches back to 1.5× mode
every 130 ms, and then reenters 2× mode again if necessary.
Furthermore, when in 2× mode, whenever a channel disabling
is sensed (that is, whenever ENM is held low for > 0.5 ms with
ENF high, or vice versa), the IC automatically reverts to 1.5×
mode, and then reenters 2× mode again if necessary.
Dimming
Main Display LED1 through LED4 Dimming.
The main
display WLEDs (LED1 through LED4) brightness and on/off can
be controlled using digital input at the ENM pin. The ENM pin
accepts one-wire serial pulse input to enable the A8434 and to set
up to 11 dimming levels, from 100% down to 5%.
When ENM is initially pulled up from shutdown, after a soft-
start, the current for the WLEDs is programmed to 100% of the
setting current, which is determined by the current through the
ISET pin. Each subsequent pulse reduces the backlight LEDs cur-
rent by 10%, and the 10
th
pulse reduces the current by 5%. The
next pulse restores 100% (full) brightness. Figure 5 shows the
timing diagram for ENM control.
Use the following formula to set the display backlight LED full
current (100%) using RSET on LED1 through LED6. The maxi-
mum current through one LED should not exceed 30 mA:
R
SET
= 0.6 V × 220 /
I
LEDx
where R
SET
is in
Ω
and I
LEDx
in amperes.
Transitions Between 1× and 1.5× or 2× Modes
The A8434 adaptively selects operating mode. When V
IN
is suf-
ficiently high to maintain V
LEDx
> 150 mV, the A8434 operates
in 1 × mode unless, as V
IN
drops, the LEDx nodes fall below
the 150 mV threshold. When V
LEDx
falls below 150 mV, the IC
enters 1.5× mode.
When the A8434 switches from 1× to 2× mode, it first switches
to 1.5× mode for a typical duration of 1 ms before entering
2× mode.
When operating in 1.5× mode, if V
OUT
< V
IN
, then the IC
switches back to 1× mode every 130 ms, and then reenters 1.5×
mode again if necessary.
95.0
90.0
85.0
75.0
70.0
65.0
60.0
55.0
50.0
3.3
3.4
3.5
3.6
3.7
3.8
V
IN
Rising
3.9
80.0
V
IN
Falling
Eff (%)
4.0
4.1
4.2
V
IN
(V)
Figure 4. Mode change transition
t
HI(Init)
1
2
3
4
5
6
7
8
9
10
11
0
ENM or
ENS
t
SS
100%
IILED
x
SHDN
90%
80%
70%
t
LO
t
HI
100% 90%
t
SHDN
60%
50%
40%
30% 20%
10%
5%
SHDN or
Dimming
Reset
Figure 5. Single-Wire Serial Dimming Control; at pins ENM and ENS.
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
8
ALLEGRO [ ALLEGRO MICROSYSTEMS ]
