+

-

+

-

+

-

+

-

V

OUT

V

IN

FB

SW

R

S

R

R

R

Duty control

1 MHz clock

OVPCOMP

ERRORAMP

SLOPER

OLPCOMP

RESETCOMP

LOOPC

+

-

ACTIVE

LOAD

SWITCH

+

-

UVCOMP

2V

+

-

R

OCPCOMP

R

0V

0V

LP5526

SNVS431C – FEBRUARY 2006 – REVISED JULY 2009

www.ti.com

Magnetic Boost DC/DC Converter

The LP5526 Boost DC/DC Converter generates an 8…20V supply voltage for the LEDs from single Li-Ion battery

(3V…4.5V). The output voltage is controlled with an 8-bit register in 12 steps. The converter is a magnetic

switching PWM mode DC/DC converter with a current limit. Switching frequency is 1MHz, when timing resistor

RT is 82k

Ω. Timing resistor defines the internal oscillator frequency and thus directly affects boost frequency and

RGB timings.

components should be as near as possible to the SW pin to ensure reliable operation. The LP5526 Boost

Converter uses pulse-skipping elimination to stabilize the noise spectrum. Even with light load or no load a

minimum length current pulse is fed to the inductor. An active load is used to remove the excess charge from the

output capacitor at very light loads. Active load can be disabled by writing the EN_AUTOLOAD bit low. Disabling

active load will increase slightly the efficiency at light loads, but the downside is that pulse skipping will occur.

The Boost Converter should be stopped and set to 8V when there is no load to minimize the 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 user can program the output voltage of the

boost converter. The output voltage control changes the resistor divider in the feedback loop. The following figure

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 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 break protection. Prevents uncontrolled operation if FB pin gets disconnected.

4. Duty cycle limiting, done with digital control.

Figure 3. Boost Converter Topology

8

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