A8603KESTR-R datasheet(23/42 Pages) ALLEGRO

23 / 42 page

Multiple-Output Regulator for

Automotive LCD Displays

A8603

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

Boost Frequency Dithering

The A8603 has an optional dithering function for the boost

switching frequency. When enabled, the switching frequency is

varied linearly within a certain frequency range, as modulated by

a triangular ramp signal. By spreading the frequency of the boost

converter, the overall system noise magnitude can be greatly

reduced. Note that the frequency dithering function is not avail-

able when an external synchronization signal is used at the FSET

pin.

The dithering feature is controlled by Register 0x10. Frequency

of the dithering modulation ramp signal is 12 kHz typical.

Table 15: Register 0x10 Dithering Feature

Reg0x10 Content

(in binary)

Dithering Feature

(frequency range)

‘00’

‘01’

±5%

‘10’

±10%

‘11’

±15%

Frequency

Time

~80 µs

2.2 MHz

2.0 MHz

1.8 MHz

Figure 11: Switching Frequency Varied Linearly

In this example, Reg0x10 = ‘10’ and the central switching frequency

is fSW = 2.0 MHz. The actual frequency is varied linearly between

fSW –10% and fSW +10% by the modulating frequency at 12 kHz.

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