LP3958 Datasheet, PDF (15/36 Pages) National Semiconductor (TI) – Lighting Management Unit with High Voltage Boost Converter

LP3958
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SNVS423C – JANUARY 2006 – REVISED MARCH 2013
Switch Mode / Constant Current Mode
Each keypad LED output can be set to act as a switch or a constant current sink. Selection of mode is done with
the CC_SW bit in the Control Register. If bit is set high, then the switch mode is selected. Default is switch mode.
1. SWITCH MODE
In switch mode, the keypad LED outputs are low ohmic switches to ground. Resistance is typically 3.5Ω.
External ballast resistors must be used to limit the current through the LED.
2. CONSTANT CURRENT MODE
In constant current mode, the maximum output current is defined with a single external resistor (R_KEY) and the
maximum current control register (address 02H).
Name
IK1[1:0]
IK2[1:0]
IK3[1:0]
KEYPAD MAX CURRENT REGISTER (02H)
Bit
Description
5:4
KEY1 maximum current
3:2
KEY2 maximum current
1:0
KEY3 maximum current
Maximum current for each LED output is adjusted with the Keypad max current register in following way:
IK1[1:0], IK2[1:0], IK3[1:0]
00
01
10
11
Maximum current / output
0.25 x I_MAX
0.50 x I_MAX
0.75 x I_MAX
1.00 x I_MAX
External ballast resistors are not needed in this mode. The maximum current for all keypad LED drivers is set
with R_KEY. The equation for calculating the maximum current is:
I_MAX = 100 × 1.23V / (R_KEY + 50 Ω)
where
• I_MAX = maximum KEY current in any KEY output (during constant current mode)
• 1.23V = reference voltage
• 100 = internal current mirror multiplier
• R_KEY = resistor value in Ohms
• 50 Ω = Internal resistor in the I_KEY input
(3)
Table with example resistance values and corresponding output currents:
KEY resistor R_KEY (kΩ)
8.2
9.1
10
12
15
18
24
Maximum current / output I_MAX
(mA)
14.9
13.4
12.2
10.2
8.2
6.8
5.1
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