Dual slope

8-bit ADC

Digital

64 sample

average

Vx

Phototransistor

or

Photodiode

Input range

[Vref...2*Vref]

Max sink current

~ 200uA

Ambient

light

Vref = 1.23V

LP3936

en_ambadc

AIN

AREF

en_ambadc

en_ambave

ain[7:0]

ain[7:0]

Buffer

R =

Vref

I(sensor max)

LP3936

www.ti.com

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.

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 k

Ω.

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.

Copyright © 2003–2013, Texas Instruments Incorporated

Submit Documentation Feedback

13

Product Folder Links: LP3936