Functionality of Keypad LED

Outputs (KEY1, KEY2, KEY3)

LP3958 has three individual keypad LED output pins. Output

pins can be used in switch mode or constant current mode.

Output mode can be selected with the control register (ad-

dress 00H) bit CC_SW. If the bit is set high, then keypad

LED outputs are in switch mode, otherwise in constant cur-

rent mode. These modes are described later in separate

chapters.

Keypad LED output control can be done in three ways:

1.

Defining the expected balance and brightness in Keypad

register (address 01H)

2.

Direct setting each LED ON/OFF via Keypad control

register (address 00H)

3.

External PWM control

BRIGHTNESS CONTROL WITH KEYPAD REGISTER

If the keypad LED output is used by defining the balance and

brightness in the Keypad register, then one needs to set

EN_KEYP bit high and KEYP_PWM bit high in the Control

register (address 00H). K1SW, K2SW and K3SW are used

to enable each LED output, enabled when written high.

CC_SW defines the LED output mode. A single register is

used for defining the balance and brightness for keypad LED

output:

KEYPAD REGISTER (01H)

Name

Bit

Description

BALANCE[2:0]

6:4

Balance of KEY1, KEY2 and

KEY3 outputs

BRIGHT[2:0]

3:1

Brightness control

OVL

0

Overlapping mode selection:

0 = non-overlapping mode

1 = overlapping mode

Brightness control is logarithmic and is programmed as fol-

lows:

Bright[2:0]

Brightness [%]

Ratio to max

brightness

000

0

0

001

1.56

1/64

010

3.12

1/32

011

6.25

1/16

100

12.5

1/8

101

25

1/4

110

50

1/2

111

100

1/1

The LED balance can be selected as follows. This is valid

only in non-overlapping mode.

Balance

[2:0]

KEY1

active [%]

KEY2

active [%]

KEY3

active [%]

000

100

0

0

001

0

100

0

010

0

0

100

011

50

50

0

100

0

50

50

101

50

0

50

110

33

33

33

111

50

25

25

OVERLAPPING MODE

The brightness is controlled using PWM duty cycle based control method as the following figure shows.

20175597

Overlapping Mode

Since KEY outputs are on simuneltaneously, the maximum load peak current is:

I

NON-OVERLAPPING MODE

The timing diagram shows the splitted KEY1, KEY2 and KEY3 and brightness control effect to splitted parts. Full brightness is

at the last 25µs all LED outputs are OFF.

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