LP3958 Datasheet, PDF (14/28 Pages) National Semiconductor (TI) – Lighting Management Unit with High Voltage Boost Converter

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
I_LEAKAGE
I_MAX(KEY)
Parameter
KEY1, KEY2, KEY3 pin leakage current
Maximum recommended sink current
R_SW
ƒ_KEY
Accuracy @ 15mA
Current mirror ratio
KEY current matching error
Switch resistance
KEY internal PMW switching frequency
V_SAT
Saturation voltage (current drop 10%)
Note: KEY current should be limited as follows:
constant current mode – limited by external R_KEY resistor
switch mode – limited by external ballast resistors
Condition
CC mode
SW mode
CC mode
CC mode
I_KEY set to 15mA, CC mode
SW mode
Accuracy same as internal clock
frequency accuracy
I_KEY set to 15mA
Min Typ Max Units
1 µA
15 mA
60 mA
5
%
1:100
3
%
3.5
Ω
20
kHz
100 500 mV
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