MIC2141-BM5 MICREL [Micrel Semiconductor], MIC2141-BM5 Datasheet - Page 8

• Manufacturer Part Number: MIC2141-BM5
• Description: Micropower Boost Converter Preliminary Information
• Manufacturer: MICREL [Micrel Semiconductor]

MIC2141
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 10 F 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 10 F to 47 F can be used for the output
capacitor.
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.
MIC2141
M
a
P
V
I
V
n
OUT
m
a
V
OUT
IN
u
n
u
s i
a f
a
R
h
= 4.8V to 2.5V
s
t c
a
a
= 1mA
o
= 12V
a t
y
n
r u
c i
Table 4. Capacitor Examples
r e
M
S
G
s -
r e
5
R
9
r e
+2.7V to +4.7V
e i
4
M
e i
s
s
Return
+2.7V to +12V
+2.7V to +12V
c
e
e
a t
V
e l
a r
V
IN
C
T
n
t c
m
y
a t
Return
Return
o r
c i
p
u l
1N4148
e
y l
V
10 F
10 F
Y
V
10 F
CR2
m
V
V
c i t
25V
25V
25V
IN
IN
5
C
C
C2
C2
C2
V
Figure 2. Gain-Boost Configuration
Figure 3. Bootstrap Configuration
s
s
0.1 F
r u
r u
0.1 F
Figure 1. Basic Configuration
0.1 F
C4
P
a f
a f
C4
e l
C4
a
c
c
c
a
e
e
k
d
a
m
m
e
g
d
o
o
e
u
u
t n
t n
1
5
4
5
1
5
IN
VC
IN
VC
IN
VC
MIC2141
MIC2141
MIC2141
4.7 H
33 H
22 H
L1
L1
L1
8
GND
GND
GND
SW
SW
Select 15 H 10%.
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, V
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.
SW
FB
FB
FB
3
4
2
3
2
1
3
4
2
BAT54HT1
BAT54HT1
MBR0530
L
L
I
I
1N4148
34.8k
PEAK
PEAK
121k
CR1
CR1
MAX
MAX
CR1
CR3
I
R1
R2
FB
17 H
I
272mA
t
C1
10 F
25V
O(max)
ON(max)
C1
10 F
25V
C1
10 F
25V
V
+12V
Return
OUT
V
+5V to +20V
Return
V
+5V to +15V
Return
OUT
OUT
V
L
IN(min)
V
eff
MIN
OUT
I
O
V
FB(typ)
V
OUT
IN(max)
will be 20V.
V
2
IN(min)
6V 1
15 A for V
t
C
ON(min)
0.767 s
R2
R1
2
OUT
2
T
I
S(min)
FB
15V
R1
13.5 H
4.8V
June 2000
Micrel