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MIC2141 Datasheet, PDF (8/16 Pages) Micrel Semiconductor – Micropower Boost Converter Preliminary Information
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MIC2141
Micrel
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.
LMAX
=
VIN(min)2 ⋅ tON(min)2
IO(max)
VO
eff
− VIN(min) ⋅ 2 ⋅ TS(min)
LMAX = 17µH
Select 15µH ±10%.
IPEAK
=
tON(max) ⋅ VIN(max)
LMIN
=
0.767µs
4.8V
13.5µH
IPEAK = 272mA
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 Equa-
tion 2 or Figure 10.
VOUT = 12V
IOUT = 1mA
VIN = 4.8V to 2.5V
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
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
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) = 15µA for VOUT = 15V
Return
Figure 2. Gain-Boost Configuration
VIN
+2.7V to +4.7V
L1
4.7µH
CR1
MBR0530
VOUT
+12V
VC
Return
CR2
1N4148
C2
10µF
25V
C4
0.1µF
MIC2141
1 IN
SW 3
FB 4
5 VC GND 2
CR3
1N4148
C1
10µF
25V
Return
Figure 3. Bootstrap Configuration
MIC2141
8
June 2000
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