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3439EFE

Slew Rate Controlled Ultralow Noise1A Isolated DC/DC Transformer Driver

Linear Technology 

LT3439

U

OPERATIO

vary from 2.8V to 17.5V with very little change in device

performance. When the part is in shutdown mode, the

internal regulator is turned off, drawing less than 20µA of

current from VIN.

Overcurrent Protection

A linearly controlled current limit circuit is provided to

protect the circuit from excessive currents and to facilitate

start-up into a highly capacitive load. Upon reaching cur-

rent limit, the switching cycle is not terminated, instead the

base drive to the output transistor is regulated to maintain

the maximum current over the entire switch cycle. Very high

power dissipation in the switches occurs during this mode

of operation. If the current limit is enabled for a long enough

period of time, over temperature protection shutdown will

be enabled to protect the device.

Overtemperature Protection

When the IC has exceeded the maximum temperature the

part will trigger the overtemperature protection circuit

where both output drivers are turned off.

Undervoltage Lockout Protection

When VIN is below 2.55V the part will go into undervoltage

lockout mode where both output drivers are turned off.

No Load Operation

The operation of the supply is stable all the way down to

zero load and a preload is not required.

APPLICATIO S I FOR ATIO

Reducing EMI from switching power supplies has tradi-

tionally invoked fear in designers. Many switchers are

designed solely on efficiency and, as such, produce wave-

forms filled with high frequency harmonics that propagate

through the rest of the supply.

The LT3439 provides control of two of the primary vari-

ables for controlling EMI while switching inductive loads:

switch voltage slew rate and switch current slew rate. The

use of this part will reduce noise and EMI over conven-

tional switch mode controllers. Because these variables

are under control, a supply built with this part will exhibit

far less tendency to create EMI and less chance of running

into problems during production.

It is beyond the scope of this data sheet to get into EMI

fundamentals. AN70, “A Monolithic Switching Regulator

with 100µV Output Noise” contains much information

concerning noise in switching regulators and should be

consulted.

Oscillator Frequency

The internal oscillator generates the switching frequency

that determines the fundamental positioning of the

harmonics. Using quality external components is impor-

tant to ensure oscillator frequency stability. A current

defined by external resistor RT charges and discharges

the capacitor CT creating a saw tooth waveform where the

outputs’ states change at the peak. The frequency of each

output is one half of the frequency of the oscillator.

By having both components external, the user has greater

flexibility in setting the frequency and the frequency is less

susceptible to any temperature variations in the device.

The external capacitance CT is chosen by:

CT(nF) = 1183/[fOSC(kHz) • RT(kΩ)]

where fOSC is the desired oscillator frequency.

For RT equal to 16.9k, this simplifies to:

CT(nF) = 70/fOSC(kHz)

e.g., CT = 1nF for fOSC = 70kHz

Nominally, RT should be set to 16.9k.

Low tolerance and low temperature coefficient compo-

nents are recommended.

6

sn3439 3439fs

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