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参数资料
| 型号： | LT1683IG |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | Ultralow Noise Push-Pull DC/DC Controller |
| 中文描述： | SWITCHING CONTROLLER, 250 kHz SWITCHING FREQ-MAX, PDSO20 |
| 封装： | 5.30 MM, PLASTIC, SSOP-20 |
| 文件页数： | 14/24页 |
| 文件大小： | 237K |
| 代理商： | LT1683IG |
14
LT1683
1683f
C1
MOSFET DRAIN
C2
CAP A OR B
C3
1683 F02
Figure 2
APPLICATIU
This part creates a second higher frequency rolloff of
harmonics that inversely depends on the slew time, the
time that voltage or current spends between the off state
and on state. This time is adjustable through the choice of
the slew resistors, the external resistors to ground on the
R
VSL
and R
CSL
pins and the external components used for
the external voltage feedback capacitors C
AV
, C
BV
(from
CAP A or CAP B to their respective MOSFET drains) and the
sense resistor. Lower slew rates (longer slew times, lower
frequency for harmonics rolloff) is created with higher
values of R
VSL
, R
CSL
, C
AV
, C
BV
and the current sense
resistor
Setting the voltage and current slew rates should be done
empirically. The most practical way of determining these
components is to set C
AV
, C
BV
and the sense resistor
value. Then, start by making R
VSL
, R
CSL
each a 50k
resistor pot in series with 3.3k. Starting from the lowest
resistor setting (fast slew) adjust the pots until the noise
level meets your guidelines. Note that slower slewing
waveforms will dissipate more power so that efficiency
will drop. You can monitor this as you make your slew
adjustment by measuring input and output voltage and
their respective currents. Monitor the MOSFET tempera-
ture as slew rates are slowed. These components will heat
up as efficiency decreases.
Measuring noise should be done carefully. It is easy to
introduce noise by poor measurement techniques. Con-
sult AN70 for recommended measurement techniques.
Keeping probe ground leads very short is essential.
Usually it will be desirable to keep the voltage and current
slew resistors approximately the same. There are circum-
stances where a better optimization can be found by
adjusting each separately, but as these values are sepa-
rated further, a loss of independence of control may occur.
It is possible to use a single slew setting resistor. In this
case the R
VSL
and R
CSL
pins are tied together. A resistor
with a value of 1.8k to 34k (one half the individual resis-
tors) can then be tied from these pins to ground.
In general only the R
CSL
value will be available for adjust-
ment of current slew. The current slew time does also
depend on the current sense resistor but this resistor is
W
U
U
normally set with consideration of the maximum current
in the MOSFETs.
Setting the voltage slew also involves selection of the
capacitors C
AV
, C
BV
. The voltage slew time is proportional
to the output voltage swing (basically input voltage), the
external voltage feedback capacitor and the R
VSL
value.
Thus at higher input voltages smaller capacitors will be
used with lower R
VSL
values. For a starting point use
Table2.
Table 2
INPUT VOLTAGE
< 25V
50V
100V
CAPACITOR VALUE
5pF
2.5pF
1pF
Smaller value capacitors can be made in two ways. The
first is simply combining two capacitors in series. The
equivalent capacitance is then (C1 C2)/(C1 + C2).
The second method makes use of a capacitor divider. Care
should be taken that the voltage ratings of the capacitors
satisfy the full voltage swing (2x input voltage for push-
pull configurations) thus essentially the same rating as the
MOSFETs.
The equivalent slew capacitance for Figure 2 is (C1 C2)/
(C1 + C2 + C3).
Positive Output Voltage Setting
Sensing of a positive output voltage is usually done using
a resistor divider from the output to the FB pin. The
positive input to the error amp is connected internally to a
1.25V bandgap reference. The FB pin will regulate to this
voltage.
Referring to Figure 3, R1 is determined by:
R
R
V
OUT
1 25
.
1
2
1
=
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