main[7:0]

en_main

8-Bit IDAC

en_main_pwm

External PWM

&

MAIN

VBOOST

LP3958

SNVS423C – JANUARY 2006 – REVISED MARCH 2013

www.ti.com

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

Min

Typ

Max

Unit

ILEAKAGE

KEY1, KEY2, KEY3 pin leakage current

1

µA

IMAX(KEY)

Maximum recommended sink current(1)

CC mode

15

mA

SW mode

60

mA

Accuracy at 15mA

CC mode

5

%

Current mirror ratio

CC mode

1:100

KEY current matching error

IKEY set to 15mA, CC mode

3

%

RSW

Switch resistance

SW mode

3.5

Ω

ƒKEY

KEY internal PMW switching frequency

Accuracy same as internal clock

20

kHz

frequency accuracy

VSAT

Saturation voltage (current drop 10%)

IKEY set to 15mA

100

500

mV

(1)

KEY current should be limited as follows:

constant current mode – limited by external RKEY resistor

switch mode – limited by external ballast resistors

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.

Figure 16. MAIN output for 4 LEDs (DISPL = 0)

16

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