LT1683
13
1683fd
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,
R
T
, is used to charge and discharge the capacitor, C
T
. 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 C
T
is chosen by:
C
T
(nF)=
2180
f(kHz)• R
T
(kΩ)
where f is the desired oscillator frequency in kHz. For R
T
equal to 16.9k, this simplifies to:
C
T
(nF)=
129
f(kHz)
e.g., C
T
= 1.29nF for f = 100kHz
APPLICATIONS INFORMATION
Nominally R
T
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.
