Texas Instruments 36V, 2A PowerWise® Adjustable Frequency Synchronous Buck Regulator Evaluation Module LM20242EVAL/NOPB LM20242EVAL/NOPB 데이터 시트
제품 코드
LM20242EVAL/NOPB
R
FB1
=
- 1
V
OUT
0.8
x R
FB2
I
IN-RMS
= I
OUT
D(1 - D)
V
DROOP
=
'
I
OUTSTEP
x R
ESR
+
L x
'
I
OUTSTEP
2
C
OUT
x (V
IN
- V
OUT
)
SNVS534E – OCTOBER 2007 – REVISED MARCH 2013
Where,
Δ
V
OUT
(V) is the amount of peak to peak voltage ripple at the power supply output, R
ESR
(
Ω
) is the series
resistance of the output capacitor, f
SW
(Hz) is the switching frequency, and C
OUT
(F) is the output capacitance
used in the design. The amount of output ripple that can be tolerated is application specific; however a general
recommendation is to keep the output ripple less than 1% of the rated output voltage. Keep in mind ceramic
capacitors are sometimes preferred because they have very low ESR; however, depending on package and
voltage rating of the capacitor the value of the capacitance can drop significantly with applied voltage. The output
capacitor selection will also affect the output voltage droop during a load transient. The peak droop on the output
voltage during a load transient is dependent on many factors; however, an approximation of the transient droop
ignoring loop bandwidth can be obtained using
recommendation is to keep the output ripple less than 1% of the rated output voltage. Keep in mind ceramic
capacitors are sometimes preferred because they have very low ESR; however, depending on package and
voltage rating of the capacitor the value of the capacitance can drop significantly with applied voltage. The output
capacitor selection will also affect the output voltage droop during a load transient. The peak droop on the output
voltage during a load transient is dependent on many factors; however, an approximation of the transient droop
ignoring loop bandwidth can be obtained using
(5)
Where, C
OUT
(F) is the minimum required output capacitance, L (H) is the value of the inductor, V
DROOP
(V) is the
output voltage drop ignoring loop bandwidth considerations,
Δ
I
OUTSTEP
(A) is the load step change, R
ESR
(
Ω
) is
the output capacitor ESR, V
IN
(V) is the input voltage, and V
OUT
(V) is the set regulator output voltage. Both the
tolerance and voltage coefficient of the capacitor should be examined when designing for a specific output ripple
or transient droop target.
or transient droop target.
INPUT CAPACITOR SELECTION
Good quality input capacitors are necessary to limit the ripple voltage at the VIN pin while supplying most of the
switch current during the on-time. In general it is recommended to use a ceramic capacitor for the input as they
provide both a low impedance and small footprint. One important note is to use a good dielectric for the ceramic
capacitor such as X5R or X7R. These provide better over temperature performance and also minimize the DC
voltage derating that occurs on Y5V capacitors. The input capacitors should be placed as close as possible to
the VIN and GND pins on both sides of the device.
switch current during the on-time. In general it is recommended to use a ceramic capacitor for the input as they
provide both a low impedance and small footprint. One important note is to use a good dielectric for the ceramic
capacitor such as X5R or X7R. These provide better over temperature performance and also minimize the DC
voltage derating that occurs on Y5V capacitors. The input capacitors should be placed as close as possible to
the VIN and GND pins on both sides of the device.
Non-ceramic input capacitors should be selected for RMS current rating and minimum ripple voltage. A good
approximation for the required ripple current rating is given by the relationship:
approximation for the required ripple current rating is given by the relationship:
(6)
As indicated by the RMS ripple current equation, highest requirement for RMS current rating occurs at 50% duty
cycle. For this case, the RMS ripple current rating of the input capacitor should be greater than half the output
current. For best performance, low ESR ceramic capacitors should be placed in parallel with higher capacitance
capacitors to provide the best input filtering for the device.
cycle. For this case, the RMS ripple current rating of the input capacitor should be greater than half the output
current. For best performance, low ESR ceramic capacitors should be placed in parallel with higher capacitance
capacitors to provide the best input filtering for the device.
SETTING THE OUTPUT VOLTAGE (R
FB1
, R
FB2
)
The resistors R
FB1
and R
FB2
are selected to set the output voltage for the device.
provides suggestions
for R
FB1
and R
FB2
for common output voltages.
Table 1. Suggested Values for R
FB1
and R
FB2
R
FB1
(k
Ω
)
R
FB2
(k
Ω
)
V
OUT
short
open
0.8
4.99
10
1.2
8.87
10.2
1.5
12.7
10.2
1.8
21.5
10.2
2.5
31.6
10.2
3.3
If different output voltages are required, R
FB2
should be selected to be between 4.99 k
Ω
to 49.9 k
Ω
and R
FB1
can
be calculated using
(7)
Copyright © 2007–2013, Texas Instruments Incorporated
15
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