+

-

+

-

+

-

+

-

FB

SW

R

S

R

R

R

Duty control

2 MHz clock

OVPCOMP

ERRORAMP

OLPCOMP

RESETCOMP

LOOPC

+

-

+

-

R

FBNCCOMP

ACTIVE

LOAD

SWITCH

VREF

SLOPER

LP55281

www.ti.com

SNVS458B – JUNE 2007 – REVISED JUNE 2007

Magnetic Boost DC/DC Converter

The LP55281 Boost DC/DC Converter generates a 4.0 - 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 PWM mode DC/DC converter with a current limit. The converter has three options for switching

frequency, 1 MHz, 1.67 MHz and 2 MHz (default), when timing resistor RT is 82 k

Ω. Timing resistor defines the

internal oscillator frequency and thus directly affects boost frequency and all circuit's internally generated timing

(RGB, ALED).

The LP55281 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. At very light load and when input and output voltages

are very close to each other, the pulse skipping is not completely eliminated. Output voltage should be at least

0.5V higher than input voltage to avoid pulse skipping. Reducing the switching frequency will also reduce the

required voltage difference.

Active load can be disabled with the EN_AUTOLOAD bit. Disabling will increase the efficiency at light loads, but

the downside is that pulse skipping will occur. The Boost Converter should be stopped when there is no load to

minimise 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 4. Boost Converter Topology

BOOST STANDBY MODE

User can stop the Boost Converter operation by writing the Enables register bit EN_BOOST low. When

EN_BOOST is written high, the converter starts for 10 ms in PFM mode and then goes to PWM mode.

Copyright © 2007, Texas Instruments Incorporated

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