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參數(shù)資料
| 型號： | LP3958TLX |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 顯示驅(qū)動器 |
| 英文描述： | Lighting Management Unit with High Voltage Boost Converter |
| 中文描述： | LED DISPLAY DRIVER, PBGA25 |
| 封裝： | 2.54 X 2.54 MM, 0.60 MM HEIGHT, 0.50 MM PITCH, LEAD FREE, MICRO SMD PACKAGE-25 |
| 文件頁數(shù)： | 13/28頁 |
| 文件大?。?/td> | 1091K |
| 代理商： | LP3958TLX |
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Functionality of Keypad LED Outputs (KEY1, KEY2, KEY3)
(Continued)
20175588
Non-overlapping Mode
The non-overlapping mode has 8-programmed balance ra-
tios. Since the KEY1, KEY2 and KEY3 are split in to non-
overlapping slots the output current through the keypad LED
can be calculated by following equation:
I
AVG
=(C
KEY1
xI
KEY1
+C
KEY2
xI
KEY2
+C
KEY3
xI
KEY3
)xB
where
C = Balance [%] (see table of balance control earlier)
B = Brightness [%] (see table of Brightness Control)
LED ON/OFF CONTROL WITH KEYPAD CONTROL
REGISTER
Each LED output can be set ON by writing the corresponding
bit high in the control register. K1SW controls KEY1, K2SW
controls KEY2 and K3SW controls KEY3 output. Note that
EN_KEYP bit must be high and KEYP_PWM bit low. In this
mode, the KEYPAD register does not have any effect.
CC_SW bit in control register defines the LED output mode.
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 3.5
.
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
) and the maxi-
mum current control register (address 02H).
KEYPAD MAX CURRENT REGISTER (02H)
Name
Bit
IK1[1:0]
5:4
IK2[1:0]
3:2
IK3[1:0]
1:0
Description
KEY1 maximum current
KEY2 maximum current
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] Maximum current / output
00
01
10
11
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 x 1.23V / (R
KEY
+ 50
)
where
I
= 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
= Internal resistor in the I
KEY
input
Table with example resistance values and corresponding
output currents:
KEY resistor R
KEY
(k
)
Maximum current / output
I
MAX
(mA)
14.9
13.4
12.2
10.2
8.2
6.8
5.1
8.2
9.1
10
12
15
18
24
Note that the LED output requires a minimum saturation
voltage in order to act as a true constant current sink. The
saturation voltage minimum is typically 100mV. If the LED
output voltage drops below 100mV, then the current will
decrease significantly.
L
www.national.com
13
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