LT8580 datasheet(14/32 Pages) LINER

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LT8580

8580fa

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applicaTions inForMaTion

Using the circuit in Figure 4 as an example, Table 3 shows

the parameters used to generate the bode plot shown in

Figure 6.

In Figure 6, the phase is –126° when the gain reaches 0dB

giving a phase margin of 54°. The crossover frequency is

14kHz, which is more than three times lower than the fre-

quency of the RHP zero to achieve adequate phase margin.

Diode Selection

Schottky diodes, with their low forward-voltage drops and

fast switching speeds, are recommended for use with the

LT8580. For applications where VR (see Tables 4, 5 and

6) < 40V, the Diodes, Inc. SBR1V40LP is a good choice.

Where VR > 40V, the Diodes Inc. DFLS1100 works well.

These diodes are rated to handle an average forward

current of 1A.

Oscillator

The operating frequency of the LT8580 can be set by the

internal free-running oscillator. When the SYNC pin is

driven low (< 0.4V), the frequency of operation is set by a

resistor from RT to ground. An internally trimmed timing

capacitor resides inside the IC. The oscillator frequency

is calculated using the following formula:

fOSC =

85.5

(RT + 1)

where fOSC is in MHz and RT is in kΩ. Conversely, RT

(in kΩ) can be calculated from the desired frequency

(in MHz) using:

RT =

85.5

fOSC

−1

Clock Synchronization

TheoperatingfrequencyoftheLT8580canbesynchronized

to an external clock source. To synchronize to the external

source, simply provide a digital clock signal into the SYNC

pin. The LT8580 will operate at the SYNC clock frequency.

The LT8580 will revert to the internal free-running oscilla-

tor clock after SYNC is driven low for a few free-running

clock periods.

Figure 6. Bode Plot for Example Boost Converter

FREQUENCY (Hz)

100

120

100

10k

100k

8580 F06

–20

140

–180

–135

–90

–45

–225

–270

–315

–360

54° AT

14kHz

PHASE

GAIN

Table 3. Bode Plot Parameters

PARAMETER

VALUE

UNITS

COMMENT

Application Specific

COUT

4.7

µF

Application Specific

RESR

Application Specific

300

Not Adjustable

3300

Adjustable

Optional/Adjustable

CPL

Optional/Adjustable

6.04

Adjustable

130

Adjustable

14.6

Not Adjustable

VOUT

Application Specific

VIN

Application Specific

gma

200

µmho

Not Adjustable

gmp

mho

Not Adjustable

µH

Application Specific

1.5

MHz

Adjustable

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