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參數(shù)資料
| 型號： | LT1683 |
| 廠商： | Linear Technology Corporation |
| 英文描述： | Ultralow Noise Push-Pull DC/DC Controller |
| 中文描述： | 超低噪聲推挽式DC / DC控制器 |
| 文件頁數(shù)： | 13/24頁 |
| 文件大?。?/td> | 237K |
| 代理商： | LT1683 |
13
LT1683
1683f
APPLICATIU
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 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 infor-
mation concerning noise in switching regulators and
should be consulted.
W
U
U
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 nF
f kHz R k
(
)
(
)
(
)
=
2180
where f is the desired oscillator frequency in kHz. For R
T
equal to 16.9k, this simplifies to:
C nF
f kHz
(
(
)
)
=
129
e.g., C
T
= 1.29nF for f = 100kHz
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 prob-
lems where you might be trying to avoid a certain fre-
quency 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 0.5
μ
s.
Be careful in sync’ing to frequencies much different from
the part since the internal oscillator charge slope deter-
mines slope compensation. It would be possible to get into
subharmonic oscillation if the sync doesn’t allow for the
charge cycle of the capacitor to initiate slope compensa-
tion. 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.
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