MIC2141(2006) Просмотр технического описания (PDF) - Micrel
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MIC2141 Datasheet PDF : 16 Pages
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Micrel, Inc.
As can be seen in the “Typical Characteristics:
Efficiency” graph, the output diode type can have an
effect on circuit efficiency. The BAT54- and BAT85-
series diodes are low-current Shottky diodes available
from On Semiconductor and Phillips, respectively. They
are suitable for peak repetitive currents of 300mA or less
with good reverse current characteristics. For applica-
tions that are cost driven, the 1N4148, or equivalent, will
provide sufficient switching speed with greater forward
drop and reduced cost. Other acceptable diodes are On
Semiconductor’s MBR0530 or Vishay’s B0530, although
they can have reverse currents that exceed 1mA at very
high junction temperatures. Table 3 summarizes some
typical performance characteristics of various suitable
diodes.
Output Capacitor
If the availability of tantalum capacitors is limited,
ceramic capacitors and inexpensive electrolyics may be
necessary. Selection of the capacitor value will depend
upon on the peak inductor current and inductor size.
MuRata offers the GRM series with up to 10µF at 25V,
with a Y5V temperature coefficient, in a 1210 surface-
mount package. Low-cost applications can use M-series
leaded electrolytic capacitors from Panasonic. In
general, ceramic, electrolytic, or tantalum values ranging
from 10µF to 47µF can be used for the output capacitor.
Manufacturer
MuRata
Vishay
Panasonic
Series
GRM
594
M-series
Type
ceramic Y5V
tantalum
Electrolytic
Package
surface mount
surface mount
leaded
Table 4. Capacitor Examples
Design Example
Given a design requirement of 12V output and 1mA load
with a minimum input voltage of 2.5V, Equation 1 can be
used to calculate to maximum inductance or it can be
read from the graph in Figure 4. Once the maximum
inductance has been determined, the peak current can
be determined using Equation 2 or Figure 9.
VOUT = 12V
IOUT = 1mA
VIN = 4.8V to 2.5V
L MAX
=
VIN(min) 2 ⋅ t ON(min) 2
IO(max)
VO
eff
− VIN(min)
⋅ 2 ⋅ TS(min)
LMAX = 17µH
Select 15µH ±10%.
MIC2141
IPEAK
= t ON(max) ⋅ VIN(max)
L MIN
= 0.767µs 4.8V
13.5µH
IPEAK = 272mA
Select a BAT54 diode and CR32 inductor.
Always check the peak current to insure that it is within
the limits specified in the load line shown in Figure 10 for
all input and output voltages.
Gain Boost
Use Figure 2 to increase the voltage gain of the system.
The typical gain can easily be increased from the
nominal gain of 6 to a value of 8 or 10. Figure 2 shows a
gain of 8 so that with 2.5V applied to VC, VOUT will be
20V.
Bootstrap
The bootstrap configuration is used to increase the
maximum output current for a given input voltage. This is
most effective when the input voltage is less than 5V.
Output current can typically be tripled by using this
technique. See Table 4a. for bootstrap-ready-built
component values.
VIN
+2.7V to +12V
C2
10µF
25V
VC
Return
C4
0.1µF
L1
33µH
CR1
BAT54HT1
MIC2141
1 IN
SW 3
FB 4
5 VC GND 2
VOUT
+5V to +15V
C1
10µF
25V
Return
Figure 1. Basic Configuration
VIN
+2.7V to +12V
C2
10µF
25V
VC
Return
C4
0.1µF
L1
22µH
CR1
BAT54HT1
MIC2141
4 IN
SW 3
FB 2
5 VC GND 1
R1
34.8k
IFB
R2
121k
VOUT
+5V to +20V
C1
10µF
25V
VOUT = 6VC ⎛⎝1+ RR21⎞⎠⎟ + IFB - R1
IFB(typ) = 15mA for VOUT = 15V
Return
Figure 2. Gain-Boost Configuration
VIN
+2.7V to +4.7V
CR2
1N4148
C4
0.1µF
C2
10µF
25V
VC
Return
L1
4.7µH
CR1
MBR0530
VOUT
+12V
MIC2141
1 IN
SW 3
FB 4
5 VC GND 2
CR3
1N4148
C1
10µF
25V
Return
Figure 3. Bootstrap Configuration
December 2006
8
M9999-122006
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