LT3580EDD-TRPBF Ver la hoja de datos (PDF) - Linear Technology

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LT3580EDD-TRPBF

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

Linear Technology 

LT3580

OPERATION

SEPIC Topology

The LT3580 can be configured as a SEPIC (single-ended

primary inductance converter). This topology allows for

the input to be higher, equal, or lower then the desired

output voltage. Output disconnect is inherently built into

the SEPIC topology, meaning no DC path exists between the

input and output. This is useful for applications requiring

the output to be disconnected from the input source when

the circuit is in shutdown.

Inverting Topology

The LT3580 can also work in a dual inductor inverting

topology. The part’s unique feedback pin allows for the

inverting topology to be built by simply changing the

connection of external components. This solution results

in very low output voltage ripple due to inductor L2 in

series with the output. Abrupt changes in output capacitor

current are eliminated because the output inductor deliv-

ers current to the output during both the off-time and the

on-time of the LT3580 switch.

Start-Up Operation

Several functions are provided to enable a very clean

start-up for the LT3580.

• First, the SHDN pin voltage is monitored by an internal

voltage reference to give a precise turn-on voltage level.

An external resistor (or resistor divider) can be connected

from the input power supply to the SHDN pin to provide

a user-programmable undervoltage lockout function.

VIN > VOUT

OR

VIN = VOUT

OR

VIN < VOUT

+

C1

SHUTDOWN

L1 •

C2

VIN

SW

LT3580

SHDN

FB

D1

•

L2

R1

RT

GND

+

VC

SYNC SS

RC

RT

CSS

CC

VOUT

C3

3580 F01

Figure 1. SEPIC Topology Allows for the Input to Span

the Output Voltage. Coupled or Uncoupled Inductors

Can Be Used. Follow Noted Phasing if Coupled

• Second, the soft-start circuitry provides for a gradual

ramp-up of the switch current. When the part is brought

out of shutdown, the external SS capacitor is first

discharged (providing protection against SHDN pin

glitches and slow ramping), then an integrated 275k

resistor pulls the SS pin up to ~2.2V. By connecting an

external capacitor to the SS pin, the voltage ramp rate

on the pin can be set. Typical values for the soft-start

capacitor range from 100nF to 1μF.

• Finally, the frequency foldback circuit reduces the

switching frequency when the FB pin is in a nominal range

of 350mV to 900mV. This feature reduces the minimum

duty cycle that the part can achieve thus allowing better

control of the switch current during start-up. When the

FB voltage is pulled outside of this range, the switching

frequency returns to normal.

Current Limit and Thermal Shutdown Operation

The LT3580 has a current limit circuit not shown in the

Block Diagram. The switch current is consistently moni-

tored and not allowed to exceed the maximum switch

current at a given duty cycle (see the Electrical Charac-

teristics table). If the switch current reaches this value,

the SR latch (SR1) is reset regardless of the state of the

comparator (A1/A2). Also not shown in the Block Diagram

is the thermal shutdown circuit. If the temperature of the

part exceeds approximately 165°C, the SR2 latch is set

regardless of the state of the comparator (A1/A2). A full

soft-start cycle will then be initiated. The current limit and

thermal shutdown circuits protect the power switch as well

as the external components connected to the LT3580.

VIN

+

C1

SHUTDOWN

L1 •

C2

VIN

SW

LT3580

SHDN

FB

• L2

D1

R1

VOUT

RT

GND

VC

SYNC SS

RC

RT

CSS

CC

C3

+

3580 F02

Figure 2. Dual Inductor Inverting Topology Results in

Low Output Ripple. Coupled or Uncoupled Inductors

Can Be Used. Follow Noted Phasing if Coupled

3580fg

7

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