參數(shù)資料
型號(hào)： AN-37
英文描述： LinkSwitch-TN Design Guide
中文描述： 的LinkSwitch - TN設(shè)計(jì)指南
文件頁(yè)數(shù)： 12/16頁(yè)
文件大?。?/td> 155K
代理商： AN-37
AN-37
12
A
1/04
Figure 7. Inductor Voltage and Inductor Current of a Buck
Converter in DCM.
PI-3778-121803
I
Limit
V
IN
-V
O
V
O
V
L
I
L
t
ON
t
OFF
t
IDLE
I
O
t
t
required output current. Capacitor C
filters the voltage
across R
, which is modulated by inductor ripple current.
The value of C
should be large enough to minimize the
ripple voltage, especially in MDCM designs. A value of C
SENSE
is selected such that the time constant (t) of R
and C
greater than 20 times that of the switching period (15
μ
s). The
peak voltage seen by C
SENSE
is equal to R
SENSE
I
LIMIT(MAX)
.
The output capacitor is optional; however with no output
capacitor the load will see the full peak current (I
) of the
selected
LinkSwitch-TN
. Increase the value of C
(typically in
the range of 100 nF to 10 uF) to reduce the peak current to an
acceptable level for the load.
If the load is disconnected, feedback is lost and the large output
voltage which results may cause circuit failure. To prevent this,
a second voltage control loop, D
and VR
, can be added as
shown if Figure 6. This also requires that C
is fitted. The
voltage of the Zener is selected as the next standard value above
the maximum voltage across the LED string when it is in
constant current operation.
The same design equations / design spreadsheet can be used as
for a standard buck-boost design, with the following additional
considerations.
1. V
O
= LED V
Number of LEDs per string
2. I
= LED I
Number of strings
3. Lower efficiency estimate due to R
losses (enter
R
into design spreadsheet as inductor resistance)
4. Set R
BIAS
= 2 k
and R
FB
= 300
5. R
SENSE
O
6. C
= 20
(15
μ
s/R
)
7. Select C
based on acceptable output ripple current
through the load
8. If the load can be disconnected or for additional fault
protection, add voltage feedback components D
FB
and
VR
FB
, in addition to C
O
.
Thermal Environment
To ensure good thermal performance, the SOURCE pin
temperature should be maintained below 100
°
C, by providing
adequate heatsinking.
For applications with high ambient temperature (>50
°
C), it is
recommended to build and test the power supply at the maximum
operating ambient temperature, and ensure that there is adequate
thermal margin. The figures for maximum output current
provided in the data sheet correspond to an ambient temperature
of 50
°
C, and may need to be thermally derated. Also, it is
recommended to use ultra fast (
≤
35 ns) low reverse recovery
diodes at higher operating temperatures (>70
°
C).
Recommended Layout Considerations
Traces carrying high currents should be as short in length and
thick in width, as possible. These are the traces which connect
the input capacitor,
LinkSwitch-TN
, inductor, freewheeling
diode and the output capacitor.
Most off-the-shelf inductors are drum core inductors or dog-
bone inductors. These inductors do not have a good closed
magnetic path, and are a source of significant magnetic coupling.
They are a source of differential mode noise and for this reason,
they should be placed as far away as possible from the AC input
lines.
Appendix A
Calculations for Inductor Value for Buck and Buck-
Boost Topologies
There is a minimum value of inductance that is required to
deliver the specified output power, regardless of line voltage
and operating mode.
As a general case, Figure 7 shows the inductor current in
discontinuous conduction mode (DCM). The following
expressions are valid for both CCM as well as DCM operation.
There are three unique intervals in DCM as can be seen from
Figure 7. Interval t
is when the
LinkSwitch-TN
is ON and the
freewheeling diode is OFF. Current ramps up in the inductor
from an initial value of zero. The peak current is the current limit
I
of the device. Interval t
is when the
LinkSwitch-TN
is
OFF and the freewheeling diode is ON. Current ramps down to
zero during this interval. Interval t
is when both the
LinkSwitch-TN
and freewheeling diode are OFF, and the inductor
current is zero.
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