LP3958 Datasheet, PDF (13/28 Pages) National Semiconductor (TI) – Lighting Management Unit with High Voltage Boost Converter

Functionality of Keypad LED Outputs (KEY1, KEY2, KEY3) (Continued)
Non-overlapping Mode
20175588
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_KEY1xI_KEY1+C_KEY2xI_KEY2+C_KEY3xI_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 3.5Ω. 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 maxi-
mum current control register (address 02H).
KEYPAD MAX CURRENT REGISTER (02H)
Name Bit
Description
IK1[1:0] 5:4
KEY1 maximum current
IK2[1:0] 3:2
KEY2 maximum current
IK3[1:0] 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] Maximum current / output
00
0.25 x I_MAX
01
0.50 x I_MAX
10
0.75 x I_MAX
11
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:
where
I_MAX = 100 x 1.23V / (R_KEY + 50 Ω)
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
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
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.
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