MIC2142 Datasheet, PDF (7/17 Pages) Micrel Semiconductor – Micropower Boost Converter

MIC2142
Micrel
Application Information
Predesigned circuit information is at the end of this section.
Component Selection
Resistive Divider (Adjustable Version)
The external resistive divider should divide the output voltage
down to the nominal reference voltage. Current drawn through
this resistor string should be limited in order to limit the effect
on the overall efficiency. The maximum value of the resistor
string is limited by the feedback input bias current and the
potential for noise being coupled into the feedback pin. A
resistor string on the order of 2MΩ limits the additional load
on the output to 20µA for a 20V output. In addition, the
feedback input bias current error would add a nominal 60mV
error to the expected output. Equation 1 can be used for
determining the values for R2 and R1.
(1)
V_OUT
=

R1+ R2
R1

V_REF
Boost Inductor
Maximum power is delivered to the load when the oscillator
is gated on 100% of the time. Total output power and circuit
efficiency must be considered when determining the maxi-
mum inductor value. The largest inductor possible is prefer-
able in order to minimize the peak current and output ripple.
Efficiency can vary from 80% to 90% depending upon input
voltage, output voltage, load current, inductor, and output
diode.
Equation 2 solves for the output current capability for a given
inductor value and expected efficiency. Figures 7 through 12
show estimates for maximum output current assuming the
minimum duty and maximum frequency and 80% efficiency.
To determine the necessary inductance, find the intersection
between the output voltage and current, and then select the
value of the inductor curve just above the intersection. If the
efficiency is expected to be different than the 85% used for the
graph, Equation 2 can then be used to better determine the
maximum output capability.
The peak inductor/switch current can be calculated from
Equation 3 or read from the graph in Figure 13. The peak
current shown in the graph in Figure 13 is derived assuming
a max duty cycle and a minimum frequency. The selected
inductor and diode peak current capability must be greater
than this. The peak current seen by the inductor is calculated
at the maximum input voltage. A wide ranging input voltage
will result in a higher worst case peak current in the inductor
than a narrow input range.
(2)
( )²
I_O(max) =
V_IN(min) t_ON
2L_MAX T_S
×
V_O
eff
1
− V_IN(min)
(3)
I_PK
=
t_ON(max) V_IN(max)
L_MIN
Table 1 lists common inductors suitable for most applica-
tions. Due to the internal transistor peak current limitation at
low input voltages, inductor values less than 10µH are not
recommmended. Table 6 lists minimum inductor sizes versus
input and output voltage. In low-cost, low-peak-current appli-
cations, RF-type leaded inductors may sufficient. All induc-
tors listed in Table 5 can be found within the selection of
CR32- or LQH4C-series inductors from either Sumida or
muRata.
Manufacturer
MuRata
Sumida
J.W. Miller
Coilcraft
Series
LQH1C/3C/4C
CR32
78F
90
Device Type
surface mount
surface mount
axial leaded
axial leaded
Table 1. Inductor Examples
Boost Output Diode
Speed, forward voltage, and reverse current are very impor-
tant in selecting the output diode. In the boost configuration
the average diode current is the same as the average load
current and the peak is the same as the inductor and switch
current. The peak current is the same as the peak inductor
current and can be derived from Equation 3 or the graph in
Figure 13. Care must be taken to make sure that the peak
current is evaluated at the maximum input voltage.
The BAT54 and BAT85 series are low current Shottky diodes
available from “On Semiconductor” and “Phillips” respec-
tively. 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 1 mA at very high
junction temperatures. Table 2 summarizes some typical
performance characteristics of various suitable diodes.
Diode
75°C
V_FWD
at
100mA
25°C
V_FWD
at
100mA
Room
Temp.
Leakage
at 15V
75°C
Leakage
at 15V
Package
MBR0530 0.275V 0.325V 2.5µA
90µA
SOD123
SMT
1N4148
0.6V
(175°C)
0.95V
25nA
(20V)
0.2µA leaded
(20V) and SMT
BAT54
0.4V
(85°C)
0.45V
10nA
(25V)
1µA
(20V)
SMT
BAT85
0.54
(85°C)
0.56V
0.4µA
2µA DO-34
(85°C) 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 10uF @ 25V with a Y5V temperature
coefficient in a 1210 surface mount package. Low cost
applications can use the M series leaded electrolytic capaci-
tor from Panasonic. In general, ceramic, electrolytic, or
tantalum values ranging from 1µF to 22µF can be used for the
output capacitor.
December 2000
7
MIC2142