LT3500 Ver la hoja de datos (PDF) - Linear Technology

Número de pieza

componentes Descripción

Fabricante

LT3500

Monolithic 2A Step-Down Regulator Plus Linear Regulator/Controller

Linear Technology 

LT3500

OPERATION

VIN undervoltage detection or thermal shutdown will

set the soft-start latch, resulting in a complete soft-start

sequence.

The switch driver operates from either the VIN or BST volt-

age. An external diode and capacitor are used to generate

a drive voltage higher than VIN to saturate the output NPN

and maintain high efficiency.

In addition to the switching regulator, the LT3500 contains

a NPN linear regulator with a 0.8V reference, and 13mA

current capability. The 0.8 reference will track the SS pin

in the same manner as the switching regulator. The linear

output can also be configured to drive an external NPN to

provide a linear regulator with higher current capability.

A power good comparator with 30mV of hysteresis trips

when both FB and LFB are above 90% of the 0.8V refer-

ence. The PG output is an open collector NPN that is off

when the output is in regulation allowing a resistor to pull

the PG pin to a desired voltage. The PG output is an open-

collector NPN that is on when the output is in regulation

providing either drive for an output disconnect transistor

or inverted power good logic.

APPLICATIONS INFORMATION

Choosing the Output Voltage

The output voltage is programmed with a resistor divider

between the output and the FB pin. Choose the 1% resis-

tors according to:

R1=

R2


VOUT1

0.8V

–

1

R2 should be 10.0k or less to avoid bias current errors.

Reference designators refer to the Block Diagram in

Figure 1.

Choosing the Switching Frequency

The LT3500 switching frequency is set by resistor R5 in

Figure 1. The RT/SYNC pin is internally regulated at 1V.

Setting resistor R5 sets the current in the RT/SYNC pin

which determines the oscillator frequency as illustrated

in Figure 2.

The switching frequency is typically set as high as pos-

sible to reduce overall solution size. The LT3500 employs

techniques to enhance dropout at high frequencies but

efficiency and maximum input voltage decrease due to

switching losses and minimum switch on times. The

maximum recommended frequency can be approximated

by the equation:

Frequency (Hz) = VOUT1 + VD • 1

VIN − VSW + VD tON(MIN)

where VD is the forward voltage drop of the catch diode

(D1 Figure 1), VSW is the voltage drop of the internal

switch, and tON(MIN) is the minimum on time of the

switch, all at maximum load current.

2500

2250

2000

1750

1500

1250

1000

750

500

250

0

0 20 40 60 80 100 120 140 160 180 200

RRT/SYNC (kΩ)

3500 F02

Figure 2. Frequency vs RT/SYNC Resistance

3500fc

11

Share Link: