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參數(shù)資料
| 型號： | LX1742EVALKIT |
| 英文描述： | DC:DC Converter |
| 中文描述： | 直流：DC轉(zhuǎn)換器 |
| 文件頁數(shù)： | 8/15頁 |
| 文件大?。?/td> | 459K |
| 代理商： | LX1742EVALKIT |
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 8
Copyright
2000
Rev. 1.1, 2002-11-21
W
M
.
C
LX1741
High Efficiency High Voltage Boost Controller
P
RODUCTION
D
ATA
S
HEET
I N T E G R A T E D P R O D U C T S
A P P L I C A T I O N I N F O R M A T I O N ( C O N T I N U E D )
O
UTPUT
R
IPPLE
C
APACITOR
S
ELECTION
Output voltage ripple is a function of the inductor value
(L), the output capacitor value (C
OUT
), the peak switch
current setting (I
PEAK
), the load current (I
OUT
), the input
voltage (V
IN
) and the output voltage (V
OUT
) for a this
boost converter regulation scheme. When the switch is
first turned on, the peak-to-peak voltage ripple is a
function of the output droop (as the inductor current
charges to I
PEAK
), the feedback transition error (i.e.,
typically 10mV), and the output overshoot (when the
stored energy in the inductor is delivered to the load at the
end of the charging cycle). Therefore the total ripple
voltage is
V
RIPPLE
=
V
DROOP
+
V
OVERSHOOT
+ 10mV
The initial droop can be estimated as follows where the
0.5v value in the denominator is an estimate of the voltage
drop across the inductor and the FET’s R
DS_ON
:
(
)
(
)
0.5
V
I
I
C
L
V
IN
OUT
PK
OUT
DROOP
×
×
=
The output overshoot can be estimated as follows
where the 0.5 value in the denominator is an estimate of
the voltage drop across the diode:
(
)
(
)
IN
OUT
V
2
OUT
PK
OUT
2
1
OVERSHOOT
V
V
0.5
I
I
C
L
+
×
×
=
D
ESIGN
E
XAMPLE
:
Determine the V
RIPPLE
where I
PK
equals 200mA, I
OUT
equals 35mA, L equals 47
μ
H, C
OUT
equals 4.7
μ
F, V
IN
equals 3.0V, and V
OUT
equals 18.0V:
(
)
(
)
mV
8
2
0.5
3.0
35mA
200mA
F
4.7
H
47
V
DROOP
=
×
×
μ
μ
=
(
)
(
)
mV
4
0
0.5
0
1
mA
5
3
mA
200
F
4.7
H
μ
47
V
2
2
1
OVERSHOOT
=
+
×
μ
×
=
Therefore, for C
OUT
equals 4.7
μ
F:
V
RIPPLE
= 28mV + 9.4mV + 10mV = 47.4mV
Increasing the output capacitor value results in the
reduction of the output voltage ripple voltage. Low ESR
capacitors are recommended to reduce ripple caused by the
switching current. Multi-layer ceramic capacitors with
X5R or X7R dielectric are a superior choice featuring small
size, very low ESR, and a temperature stable dielectric.
Low ESR electrolytic capacitors such as solid tantalum or
OS-CON types are also acceptable. Moreover, adding a
capacitor from the output to the feedback pin (C2) allows
the internal feedback circuitry to respond faster which
further minimizes output voltage ripple and reduces noise
coupling into the high impedance feedback input.
D
IODE
S
ELECTION
A Schottky diode is recommended for most applications
(e.g. Microsemi UPS5819). The low forward voltage drop
and fast recovery time associated with this device supports
the switching demands associated with this circuit topology.
The designer is encouraged to consider the diode’s average
and peak current ratings with respect to the application’s
output and peak inductor current requirements. Further, the
diode’s reverse breakdown voltage characteristic must be
capable of withstanding a negative voltage transition that is
greater than V
OUT
.
T
RANSISTOR
S
ELECTION
The LX1741 can drive up to 100mA of gate drive
current. An N-channel MOSFET with a relatively low
threshold voltage, low gate charge and low R
DS(ON)
is
required to optimize overall circuit performance. The
LXE1741 Evaluation Board uses a Fairchild FDV303. This
NMOS device was chosen because it demonstrates an
R
DS_ON
of 0.33
and a total gate charge Q
g
of 1.64nC (typ.).
PCB L
AYOUT
The LX1741 produces high slew-rate voltage and current
waveforms hence; the designer should take this into
consideration when laying out the circuit. Minimizing trace
lengths from the IC to the inductor, transistor, diode, input
and output capacitors, and feedback connection (i.e., pin 6)
are typical considerations. Moreover, the designer should
maximize the DC input and output trace widths to
accommodate peak current levels associated with this
circuit.
A
P
P
L
I
C
A
T
I
O
N
S
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