LP8556 datasheet(13/67 Pages) TI2

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ramp

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VBOOST

VFB

Active Load

Startup

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Edge Rate

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LP8556

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SNVS871M – JULY 2012 – REVISED JUNE 2020

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(1)

See Table 5.

8.3 Feature Description

8.3.1 Boost Converter

8.3.1.1 Boost Converter Operation

The LP8556 boost DC-DC converter generates a 7-V to approximately 43-V of boost output voltage from a 2.7-V

to 36-V boost input voltage. The boost output voltage minimum, maximum value and range can be set digitally by

pre-configuring EPROM memory (VBOOST_RANGE, VBOOST, and VBOOST_MAX fields).

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. The LP8556 has an internal 20-MHz

oscillator which is used for clocking the boost. Figure 4 shows the boost block diagram.

Figure 4. LP8556 Boost Converter Block Diagram

8.3.1.2 Setting Boost Switching Frequency

The LP8556 boost converter switching frequency can be set either by an external resistor (BOOST_FSET_EN =

1 selection), RFSET, or by pre-configuring EPROM memory with the choice of boost frequency (BOOST_FREQ

field). Table 1 summarizes setting of the switching frequency. Note that the RFSET is shared for setting the PWM

dimming frequency in addition to setting the boost switching frequency. Setting the boost switching frequency

and PWM dimming frequency using an external resistor is separately shown in Table 5.

Table 1. Configuring Boost Switching Frequency via EPROM

RFSET [Ω]

BOOST_FSET_EN

BOOST_FREQ[1:0]

ƒSW [kHz]

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312

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625

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1250

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