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LT1683

Slew Rate Controlled Ultralow Noise Push-Pull DC/DC Controller

Linear Technology 

LT1683

APPLICATIONS INFORMATION

Reducing EMI from switching power supplies has tradition-

ally invoked fear in designers. Many switchers are designed

solely on efficiency and as such produce waveforms filled

with high frequency harmonics that then propagate through

the rest of the system.

The LT1683 provides control over two of the more impor-

tant variables for controlling EMI with switching inductive

loads: switch voltage slew rate and switch current slew

rate. The use of this part will reduce noise and EMI over

conventional 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 encountering problems during production.

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

fundamentals. Application Note 70 contains much informa-

tion concerning noise in switching regulators and should

be consulted.

Oscillator Frequency

The oscillator determines the switching frequency and

therefore the fundamental positioning of all harmonics.

The use of good quality external components is important

to ensure oscillator frequency stability. The oscillator is of

a sawtooth design. A current defined by external resistor,

RT, is used to charge and discharge the capacitor, CT . The

discharge rate is approximately ten times the charge rate.

By allowing the user to have control over both compo-

nents, trimming of oscillator frequency can be more easily

achieved.

The external capacitance CT is chosen by:

CT (nF)

=

2180

f(kHz) •RT

(kΩ)

where f is the desired oscillator frequency in kHz. For RT

equal to 16.9k, this simplifies to:

CT (nF)

=

129

f(kHz)

Nominally RT should be 16.9k. Since it sets up current, its

temperature coefficient should be selected to compliment

the capacitor. Ideally, both should have low temperature

coefficients.

Oscillator frequency is important for noise reduction in

two ways. First the lower the oscillator frequency the

lower the waveform’s harmonics, making it easier to filter

them. Second the oscillator will control the placement of

the output voltage harmonics which can aid in specific

problems where you might be trying to avoid a certain

frequency bandwidth.

Oscillator Sync

If a more precise frequency is desired (e.g., to accurately

place harmonics) the oscillator can be synchronized to

an external clock. Set the RC timing components for an

oscillator frequency 10% lower than the desired sync

frequency.

Drive the SYNC pin with a square wave (with greater than

1.4V amplitude). The rising edge of the sync square wave

will initiate clock discharge. The sync pulse should have

a minimum pulse width of 0.5µs.

Be careful in sync’ing to frequencies much different

from the part since the internal oscillator charge slope

determines slope compensation. It would be possible to

get into subharmonic oscillation if the sync doesn’t al-

low for the charge cycle of the capacitor to initiate slope

compensation. In general, this will not be a problem until

the sync frequency is greater than 1.5 times the oscillator

free-run frequency.

Slew Rate Setting

The primary reason to use this part is to gain advantage

of lower EMI and noise due to slew control. The rolloff in

higher frequency harmonics has its theoretical basis with

two primary components. First, the clock frequency sets

the fundamental positioning of harmonics and second, the

associated normal frequency rolloff of harmonics.

e.g., CT = 1.29nF for f = 100kHz

1683fd

13

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