LP3958 datasheet(13/28 Pages) NSC

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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:

AVG=(CKEY1xIKEY1+CKEY2xIKEY2+CKEY3xIKEY3)xB

where

C = Balance [%] (see table of balance control earlier)

B = Brightness [%] (see table of Brightness Control)

LED ON/OFF CONTROL WITH KEYPAD CONTROL

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

Ω. 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

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:

MAX = 100 x 1.23V / (RKEY +50

Ω)

where

MAX = maximum KEY current in any KEY output (during

constant current mode)

1.23V = reference voltage

100 = internal current mirror multiplier

KEY = resistor value in Ohms

Ω = Internal resistor in the I

KEY input

Table with example resistance values and corresponding

output currents:

KEY resistor R

KEY (k

Ω)

Maximum current / output

MAX (mA)

8.2

14.9

9.1

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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