LP3936 Datasheet, PDF (13/30 Pages) National Semiconductor (TI) – Lighting Management System for Six White LEDs and One RGB or FLASH LED

LP3936
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SNVS259D – NOVEMBER 2003 – REVISED MAY 2013
Examples for optical sensor are photodiode SHF2400 and phototransistor SFH3410 from Osram or BSC 3216
G1 optical sensor from TDK.
ADC can be used for temperature measurement with a thermistor. It enables temperature compensated LED
driving.
If ADC is not used, it should be disabled by writing en_ambadc bit low. AIN and AREF pins can be left
unconnected.
Ambient
light
Vx
Phototransistor
or
Photodiode
en_ambadc
LP3936
en_ambave
Input range
[Vref...2*Vref]
R=
Vref
I(sensor max)
Max sink current
~ 200uA
AIN
AREF
Dual slope
8-bit ADC
ain[7:0]
Digital
64 sample
average
Buffer
en_ambadc
Vref = 1.23V
ain[7:0]
Figure 12. A/D Converter – Ambient Light Measurement Circuitry
Magnetic Boost DC/DC Converter
The LP3936 Boost DC/DC Converter generates a 4.1V–5.3V supply voltage for the LEDs from single Li-Ion
battery (3V … 4.5V). The output voltage is controlled with an 8-bit register in 9 steps. The converter is a
magnetic switching PFM/PWM mode DC/DC converter with a current limit. The converter has a 1 MHz switching
frequency when timing resistor RT is 82 kΩ.
The topology of the magnetic boost converter is called CPM control, current programmed mode, where the
inductor current is measured and controlled with the feedback. The user can program the output voltage of the
boost converter. The control changes the resistor divider in the feedback loop.
Figure 13 shows the boost topology with the protection circuitry. Three different protection schemes are
implemented:
1) Over voltage protection, limits the maximum output voltage
a. Keeps the output below breakdown voltage.
b. Prevents boost operation if battery voltage is much higher than desired output.
2) Over current protection, limits the maximum inductor current
a. Voltage over switching NMOS is monitored; too high voltages turn the switch off.
3) Duty cycle limiting, done with digital control.
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