MIC2142 View Datasheet(PDF) - Micrel
Part Name
Description
Manufacturer
MIC2142 Datasheet PDF : 17 Pages
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Micrel, Inc.
Diode
MBR0530
1N4148
BAT54
BAT85
75°C
VFWD
at
100mA
0.275V
0.6V
(175°C)
0.4V
(85°C)
0.54V
(85°C)
25°C
VFWD
at
100mA
0.325V
0.95V
0.45V
0.56V
Room
Temp.
Leakage
at 15V
2.5µA
25nA
(20V)
10nA
(25V)
0.4µA
75°C
Leakage
at 15V
90µA
0.2µA
(20V)
1µA
(20V)
2µA
(85°C)
Package
SOD123
SMT
leaded
and SMT
SMT
DO-34
leaded
Table 2. Diode Examples
Output Capacitor
Due to the limited availability of tantalum capacitors,
ceramic capacitors and inexpensive electrolyics may be
preferred. Selection of the capacitor value will depend
upon the peak inductor current and inductor size.
MuRata offers the GRM series with up to 10µF @ 25V
with a Y5V temperature coefficient in a 1210 surface
mount package. Low cost applications can use the M-
series leaded electrolytic capacitor from Panasonic. In
general, ceramic, electrolytic, or tantalum values ranging
from 1µF to 22µF 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 3. Capacitor Examples
Design Example
Given a design requirement of 12V output and 1mA load
with a minimum input voltage of 2.5V, Equation 2 can be
used to calculate to maximum inductance or it can be
read from the graph in Figure 7. Once the maximum
inductance has been determined the peak current can
be determined using Equation 3 or the graph in Figure
13.
VOUT = 12V
IOUT = 5mA
VIN = 2.5V to 4.7V
Fmax = 360kHz
η = 0.8 = efficiency
Dnom = 0.55
TS(min)
=1
Fmax
=1
360kHz
= 2.78µsec
MIC2142
t ON(min)
= Dnom
fmax
= 0.55
360kHz
= 1.53µsec
L max
=
VIN(min) 2 × t ON(min) 2
IO(max) × 2 × TS(min)
×
VO
η
1
− VIN(min)
L max
=
2.5 2 × 1.53µsec 2
5mA × 2 × 2.78µsec
×
12
1
− 2.5
=
42µH
0.8
Select 39µH ±10%.
t ON(max)
= 1.1× Dnom
Fmin
= 1.1× 0.55
300kHz
= 2µsec
Ipeak
=
t ON(max) × VIN(max)
L min
=
2.0µsec × 4.7V
35µH
= 270mA
Bootstrap Configuration
For input voltages below 4.5V the bootstrap
configuration can increase the output power capability of
the MIC2142. Figure 2 shows the bootstrap configuration
where the output voltage is used to bias the MIC2142.
This improves the power capability of the MIC2142 by
increasing the gate drive volt-age hence the peak
current capability of the internal switch. This allows the
use of a smaller inductor which increases the output
power capability. Table 4 also summarizes the various
configurations and power capabilities using the
booststrap configuration. This bootstrap configuration is
limited to output voltage of 16V or less.
Figure 1 shows how a resistor (R3) can be added to
reduce the ripple seen at the VCC pin when in the
bootstrap configuration. Reducing the ripple at the VCC
pin can improve output ripple in some applications.
+3.0V to +4.2V
VIN
L1
33µH
CR1
MBR0530
+5V @80mA
R3
100
R2
36.5k
C3
270pF
C2
10µF
GND
U1 MIC2142
4 FB SW 3
GND 2
5 EN VCC 1
R1
12.4k
C1
22µF
C4
1F
GND
Figure 1. Bootstrap VCC with VCC Low Pass Filter
October 2007
8
M9999-102507
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