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 3.5

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 (RKEY) and the

maximum 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 IMAX

01

0.50 x IMAX

10

0.75 x IMAX

11

1.00 x IMAX

External ballast resistors are not needed in this mode. The maximum current for all keypad LED drivers is set

with RKEY. The equation for calculating the maximum current is:

IMAX = 100 × 1.23V / (RKEY + 50 Ω)

where

•

IMAX = maximum KEY current in any KEY output (during constant current mode)

•

1.23V = reference voltage

•

100 = internal current mirror multiplier

•

RKEY = resistor value in Ohms

•

50

Ω = Internal resistor in the IKEY input

(3)

Table with example resistance values and corresponding output currents:

Maximum current / output IMAX

KEY resistor RKEY (kΩ)

(mA)

8.2

14.9

9.1

13.4

10

12.2

12

10.2

15

8.2

18

6.8

24

5.1

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