20081416

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.

The following figure 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.

20081417

www.national.com

12