MIC2142 Datasheet, PDF (8/17 Pages) Micrel Semiconductor – Micropower Boost Converter

MIC2142
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
an miniumum 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 Equa-
tion 3 or the graph in Figure 13.
V_OUT = 12V
I_OUT = 5mA
V_IN = 2.5V to 4.7V
F_max = 360kHz
η = 0.8 = efficiency
D_nom = 0.55
T_S(min)=
1
F_max
=
1
360kHz
= 2.78µsec
t_ON(min)=
D_nom
f_max
=
0.55 _1.53µsec
360kHz
L_max =
V_IN(min)2 × t_ON(min)2
I_O(max) × 2 × T_S(min)
×
V_O
η
1
− V_IN(min)
L_max =
2.5² × 1.53µsec² _×
5mA × 2 × 2.78µsec
12
1 _{= 42µH}
− 2.5
0.8
Select 39µH ±10%.
t_ON(max)=
1.1× D_nom
F_min
=
1.1× 0.55
300kHz
= 2µsec
I_peak =
t_ON(max) × V_IN(max)
L_min
=
2.0µsec × 4.7V
35µH
=
270mA
Micrel
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 impoves the power capa-
bility of the MIC2142 by increasing the gate drive voltage
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 V_CC pin when in the bootstrap configu-
ration. Reducing the ripple at the V_CC pin can improve output
ripple in some applications.
+3.0V to +4.2V
V_IN
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
1µF
GND
Figure 1. Bootstrap V_CC with V_CC Low Pass Filter
MIC2142
8
December 2000