LP3958TL datasheet(14/28 Pages) NSC

14 / 28 page

Functionality of Keypad LED Outputs (KEY1, KEY2, KEY3) (Continued)

External PWM Control

The GPIO[0]/PWM pin can be used to control the KEY

output. PWM function for the pin is selected by writing EN-

_PWM_PIN high in GPIO control register (address 06H).

Note, that EN_KEYP bit must be set high. Each LED output

can be enabled with K1SW, K2SW and K3SW bits.

EN_EXT_K1_PWM,

EN_EXT_K2_PWM

and

EN_EXT_K3_PWM bits are used to select, which LED out-

puts are controlled with the external PWM input. Note that

polarity of external PWM control is active high i.e. when high,

then LED output is enabled. If KEYP_PWM is set low, then

each selected LED output is controlled directly with external

PWM input. If KEYP_PWM is set high, then internal PWM

control is modulated by the external PWM input. In latter

case, internal PWM control is passed to LED when external

PWM input is high.

Keypad LEDs Driver Performance Characteristics

Symbol

Parameter

Condition

Min

Typ

Max Units

LEAKAGE

KEY1, KEY2, KEY3 pin leakage current

µA

MAX(KEY)

Maximum recommended sink current

CC mode

SW mode

Accuracy @ 15mA

CC mode

Current mirror ratio

CC mode

1:100

KEY current matching error

KEY set to 15mA, CC mode

Switch resistance

SW mode

3.5

Ω

KEY

KEY internal PMW switching frequency

Accuracy same as internal clock

frequency accuracy

kHz

SAT

Saturation voltage (current drop 10%)

KEY set to 15mA

100

500

Note: KEY current should be limited as follows:

constant current mode – limited by external RKEY resistor

switch mode – limited by external ballast resistors

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