LP3958 Datasheet, PDF (16/36 Pages) National Semiconductor (TI) – Lighting Management Unit with High Voltage Boost Converter

LP3958
SNVS423C – JANUARY 2006 – REVISED MARCH 2013
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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.
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 outputs 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
Test Conditions
I_LEAKAGE
I_MAX(KEY)
KEY1, KEY2, KEY3 pin leakage current
Maximum recommended sink current⁽¹⁾ CC mode
SW mode
Accuracy at 15mA
CC mode
Current mirror ratio
CC mode
R_SW
ƒ_KEY
KEY current matching error
Switch resistance
KEY internal PMW switching frequency
I_KEY set to 15mA, CC mode
SW mode
Accuracy same as internal clock
frequency accuracy
V_SAT
Saturation voltage (current drop 10%) I_KEY set to 15mA
(1) KEY current should be limited as follows:
constant current mode – limited by external R_KEY resistor
switch mode – limited by external ballast resistors
Min
Typ
Max
Unit
1
µA
15
mA
60
mA
5
%
1:100
3
%
3.5
Ω
20
kHz
100
500
mV
Backlight Drivers
LP3958 has 2 independent backlight drivers. Both drivers are regulated constant current sinks. LED current for
both LED strings are controlled by the 8-bit current mode DACs with 0.1 mA step. MAIN and SUB LEDs can be
also controlled with one DAC (MAIN) for better matching allowing the use of larger displays having up to 8 white
LEDs by setting DISPL bit to 1.
V_BOOST
External PWM
en_main_pwm
main[7:0]
en_main
&
8-Bit IDAC
MAIN
Figure 16. MAIN output for 4 LEDs (DISPL = 0)
16
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