+

-

+

-

+

-

+

-

FB

SW

R

S

R

R

R

Duty control

2 MHz clock

OVPCOMP

ERRORAMP

SLOPER

OLPCOMP

RESETCOMP

LOOPC

+

-

+

-

R

FBNCCOMP

ACTIVE

LOAD

SWITCH

V

OUT

LP3950

www.ti.com

SNVS331C – NOVEMBER 2004 – REVISED APRIL 2013

Magnetic Boost DC/DC Converter

The boost DC/DC converter generates a 4.1V–5.3V output voltage to drive LEDs from a single Li-Ion battery

(3.0V to 4.5V). The output voltage is controlled with an eight-bit register in nine steps. The converter is a

magnetic switching PWM mode DC/DC converter with a current limit. The converter has three options for

switching frequency, 1.0 MHz, 1.67 MHz and 2.0 MHz (default), when the timing resistor RT is 82 k

Ω.

The LP3950 boost converter uses an unique pulse-skipping elimination method to stabilize the noise spectrum.

Even with light load or no load a minimum length current pulse is fed to the inductor. An internal active load is

used to remove the excess charge from the output capacitor when needed (see NOTE below). The boost

converter should be disabled when there is no load to avoid idle current consumption.

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 output voltage control changes the resistor

divider in the feedback loop.

Figure 12 shows the boost topology with the protection circuitry. Four different protection schemes are

implemented:

1. Over voltage protection, limits the maximum output voltage

–

Keeps the output below breakdown voltage

–

Prevents boost operation if the battery voltage is much higher than desired output

2. Over current protection, limits the maximum inductor current

–

Voltage over switching NMOS is monitored; too high voltages turn the switch off

3. Feedback (FB) protection for no connection

4. Duty cycle limit function, done with digital control

NOTE

When the battery voltage is close to the output voltage, the output voltage may rise slightly

over programmed value if the load on output is small and pulse-skipping elimination is

active.

Figure 12. Boost Converter Functional Block Diagram

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