LP8555 datasheet(12/64 Pages) TI1

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OVP

Light

Load

OCP

Switch

Driver

Osc/

Ramp

VREF

Boost Output

Voltage

Adjustment

Startup

LP8555

SNVS857 – FEBRUARY 2014

www.ti.com

8.3 Features Description

8.3.1 Boost Converter Overview

8.3.1.1 Operation

The boost DC/DC converters generate 7-V to 28-V boost output voltage from a 2.7-V to 20-V boost input voltage

(input voltage must be lower than VBOOST). The maximum boost output voltage can be set digitally by pre-

configuring EPROM memory (VMAX field).

The converter is a magnetic switching PWM mode DC/DC boost converter with a current limit. It uses CPM

(current programmed mode) control, where the inductor current is measured and controlled with the feedback.

During start-up, the soft-start function reduces the peak inductor current. Figure 20 shows the boost block

diagram.

Figure 20. Boost Converter Functional Block Diagram

Both boost converters are operating at 180° phase shift to reduce current spikes from the input rail and EMI.

8.3.1.2 Protection

Three different protection schemes are implemented:

1. Overvoltage protection, limits the maximum output voltage:

–

Overvoltage protection limit changes dynamically based on output voltage setting. If the boost voltage is

over 1.6 V higher than the adaptive control set value, the boost will stop switching.

–

Keeps the output below breakdown voltage. The output voltage control limits the boost maximum voltage

to 18...28 V (EPROM programmable).

–

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

2. Overcurrent protection, limits the maximum inductor current to 3.1 A (EPROM programmable).

3. Duty cycle limiting.

8.3.1.3 Setting Boost Switching Frequency

The LP8555 boost converter switching frequency can be set by pre-configuring EPROM memory with the choice

of boost frequency (BFREQ field). Table 1 summarizes setting of the switching frequency.

Table 1. Setting Boost Switching Frequency

BFREQ

ƒSW [kHz]

500

1000

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