Copyright © 2016, Texas Instruments Incorporated

+

-

+

-

+

-

+

-

FB

SW

R

S

R

R

R

Duty control

2 MHz clock

OVPCOMP

ERRORAMP

OLPCOMP

RESETCOMP

LOOPC

+

-

+

-

R

FBNCCOMP

ACTIVE

LOAD

SWITCH

VOUT

VIN

VREF

SLOPER

13

LP55281

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SNVS458D – JUNE 2007 – REVISED OCTOBER 2016

Product Folder Links: LP55281

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Copyright © 2007–2016, Texas Instruments Incorporated

7 Detailed Description

7.1 Overview

The LP55281 boost DC-DC converter generates a 4-V to 5.3-V supply voltage for the LEDs from single Li-Ion

battery (3 V...4.5 V). 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. When timing resistor RT is 82 k

Ω, the converter has

three options for switching frequency: 1 MHz, 1.67 MHz, and 2 MHz (default). Timing resistor defines the internal

oscillator frequency and thus directly affects boost frequency and all internally generated timing (RGB, ALED) of

the circuit.

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 must be at least

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

required voltage difference.

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

downside is that pulse skipping will occur. The boost converter must be stopped when there is no load to

minimize the current consumption.

The topology of the magnetic boost converter is called current programmed mode (CPM) control, 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.

Figure 10 shows the boost topology with the protection circuitry. Four different protection schemes are

implemented:

1. Overvoltage 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. Overcurrent 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 10. Boost Converter Topology