LT3580EMS8E-TRPBF(RevC) Ver la hoja de datos (PDF) - Linear Technology

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LT3580EMS8E-TRPBF
(Rev.:RevC)

Boost/Inverting DC/DC Converter with 2A Switch, Soft-Start, and Synchronization

Linear Technology 

LT3580

APPLICATIONS INFORMATION

Using the circuit in Figure 14 as an example, Table 3 shows

the parameters used to generate the bode plot shown in

Figure 5. Note that R2 is 14.6k ||14.6k = 7.3k which is the

effective small signal resistance looking into the FB pin

of the LT3580.

Table 3. Bode Plot Parameters

PARAMETER

VALUE

UNITS

COMMENT

RL

21.8

Ω

Application Specific

COUT

10

μF

Application Specific

RESR

10

mΩ

Application Specific

RO

305

kΩ

Not Adjustable

CC

1000

pF

Adjustable

CPL

0

pF

Optional/Adjustable

RC

10

kΩ

Adjustable

R1

130

kΩ

Adjustable

R2

7.3

kΩ

Not Adjustable

VREF

1.215

V

Not Adjustable

VOUT

12

V

Application Specific

VIN

5

V

Application Specific

gma

230

μmho

Not Adjustable

gmp

7

mho

Not Adjustable

L

4.2

μH

Application Specific

fS

1.2

MHz

Adjustable

In Figure 5, the phase is –140° when the gain reaches 0dB

giving a phase margin of 40°. The crossover frequency

is 10kHz, which is more than three times lower than the

frequency of the RHP zero to achieve adequate phase

margin.

180

0

160

–20

140

–40

120

–60

PHASE

100

–80

80

60

40

GAIN

20

40° AT

10kHz

–100

–120

–140

–160

0

–180

–20

10

–200

100 1k 10k 100k 1M

FREQUENCY (Hz)

3580 F05

Figure 5. Bode Plot for Example Boost Converter

Diode Selection

Schottky diodes, with their low forward voltage drops and

fast switching speeds, are recommended for use with the

LT3580. The Microsemi UPS120 is a very good choice.

Where the input-to-output voltage differential exceeds 20V,

use the UPS140 (a 40V diode). These diodes are rated to

handle an average forward current of 1A.

Oscillator

The operating frequency of the LT3580 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

=

91.9

(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

=

91.9

fOSC

1

Clock Synchronization

The operating frequency of the LT3580 can be synchro-

nized to an external clock source. To synchronize to the

external source, simply provide a digital clock signal into

the SYNC pin. The LT3580 will operate at the SYNC clock

frequency. The LT3580 will revert to the internal free-run-

ning oscillator clock after SYNC is driven low for a few

free-running clock periods.

Driving SYNC high for an extended period of time effec-

tively stops the operating clock and prevents latch SR1

from becoming set (see the Block Diagram). As a result,

the switching operation of the LT3580 will stop.

The duty cycle of the SYNC signal must be between 35%

and 65% for proper operation. Also, the frequency of the

SYNC signal must meet the following two criteria:

3580fc

13

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