LX1734 查看數據表（PDF） - Microsemi Corporation
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LX1734 Datasheet PDF : 8 Pages
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LX1734
TM
®
1.0MHz Inverting DC/DC Converter
APPLICATION NOTE (CONTINUED)
( ) D =
VOUT
VOUT − VIN
where VOUT < 0
For example, the duty cycle for +5V = VIN and –3.3V = VOUT
is 40%.
The duty cycle for the Inverting Charge Pump Output topology
(assuming continuous inductor current mode operation) is
approximately:
D
=
1+
⎜⎜⎝⎛
VIN
VOUT
⎟⎟⎠⎞
where VOUT < 0
For example, the duty cycle for +5V = VIN and –12V = VOUT is
58%.
The peak-to-peak ripple current in the input inductor is
approximately:
( ( ) ) I = RIPPLEpp
VIN× D
LIN× Fsw
where Fsw =1.0MHz (the switching frequency)
For example, with a +5V input and a-12V output in an Inverting
Charge Pump Output topology with a 47µH inductor, the peak-to-
peak input ripple is 52mA.
In the dual inductor topology with separate inductors, the peak-
to-peak ripple current in the output inductor is approximately:
[ ] ( ( ) ) I = RIPPLEpp
− VOUT× 1− D
LOUT× Fsw
For example, with a +5V input and a -3.3V output in a dual
inductor topology with a 47µH output inductor, the peak-to-peak
output ripple is 35mA.
There are many inductor models from many different
manufacturers that work well with the LX1734. Some sources
are listed in Table 5. Ferrite core inductors are recommended to
reduce core losses due to the high operating frequency of the
LX7134. Using inductors with low DC resistance will further
reduce efficiency losses.
Vendor Phone
URL
Sumida
(847)
956-0666
www.sumida.com
Murata
(404)
436-1300
www.murata.com
Coiltronics
(407)
241-7876
www.coiltronics.com
Table 5 – List of Inductor Vendors
Part
CLS62-22022
CD43-470
LQH3C-220
CTX20-1
Comments
22µH
Coupled
47µH
22µH, 2mm
Height
20µH,
Coupled,
Low DCR
Capacitor Selection
To minimize ripple voltage, only capacitors with low series
resistance (ESR) are recommended. Mutli-layer ceramic capacitors
with X5R or X7R dielectric are an excellent choice featuring small
size, very low ESR, and a temperature stable dielectric. The level
shifting capacitor, C2 (of Figure 1), should have a value of 1µF and
a voltage difference between the input and output voltages. The
input and output capacitors (C1 and C2, respectively) should have
values in the range of 1µF or larger. If the inductor ripple current is
known, the ripple voltage can be estimated by the following
equation:
( ( ) ) V = PP(RIPPLE)
I2PPRIPPLE× L
2× C× V
Since ripple voltage is inversely proportional to the capacitor value,
larger value ceramic capacitors will result in lower ripple voltages.
When using a ceramic capacitor for the output capacitor, it is
recommended that a phase lead network be inserted in the feedback
loop to improve the transient response. This can be accomplished
by placing a capacitor in parallel with resistor R1 (see Figure 1).
The corner frequency for the phase lead zero is between 20KHz and
60KHz. C4 can be calculated using the following equation:
fz
=
(2
π×
1
R1×
C
4)
Electrolytic capacitors such as solid tantalum or OS-CON types can
also be used with consideration for the ESR. Since ESR adds to the
capacitor reactive impedance, ESR will increase the ripple voltage.
The electrolytic output capacitor impedance has a built in zero, so
adding C4 is usually not required when using an electrolytic
capacitor.
Diode Selection
A Schottky diode is recommended for use with the LX1734. The
Microsemi UPS530 (30V @ 0.5A) or Microsemi UPS5817 (20V @
1A) are good choices.
Layout Considerations
In operation, current is transferred between the LX1734 and D1 so
to minimize ground noise it is recommended that the D1 cathode be
connected directly to the ground pin pad for the LX1734 (refer to
figure 1). When laying out the converter, to minimize EMI, it is
important to minimize the area enclosed within the main current
loops. It is also important to minimize the length of etch connecting
to pin 3 (NFB) and to minimize the total trace area on both sides of
C2. A ceramic bypass capacitor should be connected between pin 5
(VIN) and pin 2 (GND) and located in close proximity to the
LX1734.
Copyright © 2002
Rev. 3.0a, 2005-03-14
Microsemi
Microsemi Integrated Products
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 6
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