LP3936 Datasheet, PDF (18/30 Pages) National Semiconductor (TI) – Lighting Management System for Six White LEDs and One RGB or FLASH LED

LP3936
SNVS259D – NOVEMBER 2003 – REVISED MAY 2013
APPLICATION INFORMATION
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RGB LED Driver
The RGB driver has three outputs that can independently drive one RGB LED or three LEDs of any kind. User
has control over the following parameters separately for each LED:
• ON and OFF (start and stop time in blinking cycle)
• DUTY (PWM brightness control)
• SLOPE (dimming slope)
• ENABLE (direct enable control)
The main blinking cycle is controlled with 2-bit CYCLE control (0.25 / 0.5 / 1.0 / 2.0s).
LED
brightness
PWM
current
pulses
ON[6:0]
SLOPE[3:0]
Duty increases
OFF[6:0]
DUTY[3:0]
Duty constant
SLOPE[3:0]
Duty decreases
Blinking period
Figure 23. RGB PWM Operating Principle
RGB_START is the master enable control for the whole RGB function. The internal PWM and blinking control
can be disabled by setting the RGB_PWM control LOW. In this case the individual enable controls can be used
to switch outputs on and off. RGB_EN input can be used for external hardware PWM control. RGB_EN input can
be used as direct on/off or brightness (PWM) control. If RGB_EN input is not used, it must be tied to V_{DD_IO}.
Recommended maximum frequency of RGB LED external PWM control is 1 MHz.
In the normal PWM mode the R, G and B switches are controlled in 3 phases (one phase per driver). During
each phase the peak current set by external resistor is driven through the LED for the time defined by DUTY
setting (0 µs–50 µs). As a time averaged current this means 0%–33% of the peak current. The PWM period is
150 µs and the pulse frequency is 6.67 kHz in normal mode.
ROUT
1111
GOUT
0110
BOUT
1100
3
1
2
3
1
2
150 Ps / 6.7 kHz
Combined PWM cycle
Figure 24. Normal Mode PWM Waveforms at different duty settings
In the FLASH mode all the outputs are controlled in one phase and the PWM period is 50 µs. The time averaged
FLASH mode current is three times the normal mode current at the same DUTY value.
Blinking can be controlled separately for each output. On and OFF times determine, when a LED turns on and off
within the blinking cycle. When both ON and OFF are 0, the LED is on and doesn't blink. If ON equals OFF but is
not 0, the LED is permanently off.
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