LT3684EDD Просмотр технического описания (PDF) - Linear Technology

Номер в каталоге

Компоненты Описание

Список матч

LT3684EDD

36V, 2A, 2.8MHz Step-Down Switching Regulator

Linear Technology 

LT3684

APPLICATIONS INFORMATION

Soft-Start

The RUN/SS pin can be used to soft-start the LT3684,

reducing the maximum input current during start-up.

The RUN/SS pin is driven through an external RC filter to

create a voltage ramp at this pin. Figure 7 shows the start-

up and shut-down waveforms with the soft-start circuit.

By choosing a large RC time constant, the peak start-up

current can be reduced to the current that is required to

regulate the output, with no overshoot. Choose the value

of the resistor so that it can supply 20µA when the RUN/SS

pin reaches 2.3V.

RUN

15k

0.22µF

RUN/SS

GND

IL

1A/DIV

VRUN/SS

2V/DIV

VOUT

2V/DIV

2ms/DIV

3481 F06

Figure 6. To Soft-Start the LT3684, Add a Resisitor

and Capacitor to the RUN/SS Pin

Shorted and Reversed Input Protection

If the inductor is chosen so that it won’t saturate exces-

sively, an LT3684 buck regulator will tolerate a shorted

output. There is another situation to consider in systems

where the output will be held high when the input to the

LT3684 is absent. This may occur in battery charging ap-

plications or in battery backup systems where a battery

or some other supply is diode OR-ed with the LT3684’s

output. If the VIN pin is allowed to float and the RUN/SS

pin is held high (either by a logic signal or because it is

tied to VIN), then the LT3684’s internal circuitry will pull

its quiescent current through its SW pin. This is fine if

your system can tolerate a few mA in this state. If you

ground the RUN/SS pin, the SW pin current will drop to

essentially zero. However, if the VIN pin is grounded while

the output is held high, then parasitic diodes inside the

D4

MBRS140

VIN

VIN

BOOST

LT3684

RUN/SS

SW

VC

GND FB

VOUT

BACKUP

3684 F07

Figure 7. Diode D4 Prevents a Shorted Input from

Discharging a Backup Battery Tied to the Output. It Also

Protects the Circuit from a Reversed Input. The LT3684

Runs Only When the Input is Present

LT3684 can pull large currents from the output through

the SW pin and the VIN pin. Figure 7 shows a circuit that

will run only when the input voltage is present and that

protects against a shorted or reversed input.

PCB Layout

For proper operation and minimum EMI, care must be

taken during printed circuit board layout. Figure 8 shows

the recommended component placement with trace,

ground plane and via locations. Note that large, switched

currents flow in the LT3684’s VIN and SW pins, the catch

diode (D1) and the input capacitor (C1). The loop formed

by these components should be as small as possible. These

components, along with the inductor and output capacitor,

should be placed on the same side of the circuit board,

and their connections should be made on that layer. Place

a local, unbroken ground plane below these components.

The SW and BOOST nodes should be as small as possible.

Finally, keep the FB and VC nodes small so that the ground

traces will shield them from the SW and BOOST nodes.

The Exposed Pad on the bottom of the package must be

soldered to ground so that the pad acts as a heat sink. To

keep thermal resistance low, extend the ground plane as

much as possible, and add thermal vias under and near

the LT3684 to additional ground planes within the circuit

board and on the bottom side.

3684f

16

Share Link: