lm2756tmx National Semiconductor Corporation, lm2756tmx Datasheet - Page 11

• Manufacturer Part Number: lm2756tmx
• Description: Multi-display Inductorless Led Driver With 32 Exponential Dimming Steps In ?smd
• Manufacturer: National Semiconductor Corporation

Note: DxA4-DxA0, D53, D62: Sets Brightness for DxA pins (GroupA).
Brightness Level Control Table
(GroupA)
GroupB and GroupC Brightness Levels (% of Full-Scale) =
10%, 20%, 30%, 40%, 50%, 60%, 70%, 100%
Internal Hex Address: 0xA0 (GroupA), 0xB0 (GroupB),
Brightness Code (hex)
11111=Fullscale
DxB2-DxB0: Sets Brightness for DxB pins (GroupB). 111=Fullscale
DxC2-DxC0: Sets Brightness for D1C pin. 111 = Fullscale
Full-Scale Current set externally by the following equation:
I
Dxx
= 189 × 1.25V / R
0A
0B
0C
0D
0E
0F
1A
1B
1C
1D
1E
1F
00
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
17
18
19
0xC0 (GroupC)
SET
Perceived Brightness
Level (%)
16.875
0.125
0.313
0.625
1.125
1.313
1.688
2.063
2.438
2.813
3.125
4.375
18.75
26.25
31.25
43.75
61.25
3.75
5.25
6.25
8.75
12.5
22.5
37.5
52.5
87.5
100
7.5
10
15
70
1
11
Note: RS1-RS0: Sets Brightness Ramp Step Time. The Brightness ramp
Note: VF1-VF0: Sets the Gain Transition Delay Time. The VF Monitor Delay
Application Information
LED CONFIGURATIONS
The LM2756 has a total of 8 current sinks capable of sinking
180mA of total diode current. These 8 current sinks are con-
figured to operate in three independently controlled lighting
regions. GroupA has four dedicated current sinks, while
GroupB and GroupC each have one. To add greater lighting
flexibility, the LM2756 has two additional drivers (D53 and
D62) that can be assigned to either GroupA or GroupB
through a setting in the general purpose register.
At start-up, the default condition is four LEDs in GroupA, three
LEDs in GroupB and a single LED in GroupC (NOTE: GroupC
only consists of a single current sink (D1C) under any con-
figuration). Bits 53A and 62A in the general purpose register
control where current sinks D53 and D62 are assigned. By
writing a '1' to the 53A or 62A bits, D53 and D62 become as-
signed to the GroupA lighting region. Writing a '0' to these bits
assigns D53 and D62 to the GroupB lighting region. With this
added flexibility, the LM2756 is capable of supporting appli-
cations requiring 4, 5, or 6 LEDs for main display lighting,
while still providing additional current sinks that can be used
for a wide variety of lighting functions.
SETTING LED CURRENT
The current through the LEDs connected to DxA and DxB can
be set to a desired level simply by connecting an appropriately
sized resistor (R
GND. The DxA, DxB and D1C LED currents are proportional
to the current that flows out of the I
189 times greater than the I
of the internal amplifiers set the voltage of the I
(typ.). The statements above are simplified in the equations
below:
Once the desired R
has the ability to internally dim the LEDs using analog current
scaling. The analog current level is set through the I
patible interface. LEDs connected to GroupA can be dimmed
FIGURE 8. VF Monitor Delay Register Description
FIGURE 7. Ramp Step Time Register Description
settings only affect GroupA current sinks. ('00' = 100µs, '01' = 25ms,
'10' = 50ms, '11' = 100ms).
can be set to four different delay times. ('00' (Default) = 3-6msec., '01'
= 1.5-3msec., '10' = 0.4-0.8msec., '11' = 60-90µsec.).
R
I
DxA/B/C
SET
Internal Hex Address: 20h
Internal Hex Address: 60h
SET
(Ω)= 189 × (1.25V / I
) between the I
SET
(A)= 189 × (V
value has been chosen, the LM2756
SET
current. The feedback loops
SET
SET
ISET
pin of the LM2756 and
pin and are a factor of
/ R
DxA/B/C
SET
SET
)
)
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pin to 1.25V
2
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30009739
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