MIC2141 Datasheet, PDF (7/16 Pages) Micrel Semiconductor – Micropower Boost Converter Preliminary Information

MIC2141
Application Information
Predesigned circuit information is at the end of this section.
Component Selection
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. The largest inductor possible is preferable in
order to minimize the peak current and output ripple. Effi-
ciency can vary from 80% to 90% depending upon input
voltage, output voltage, load current, inductor, and output
diode.
Equation 1 solves for the output current capability for a given
inductor value and expected efficiency. Figures 5 through 9
graph estimates for maximum output current, assuming the
minimum duty cycle, maximum frequency, and 85% effi-
ciency. To determine the required inductance, find the inter-
section between the output voltage and current and select the
value of the inductor curve just above the intersection. If the
efficiency is expected to be other than the 85% used for the
graph, Equation 1 can then be used to better determine the
maximum output capability.
(1)
( )²
I_O(max) =
V_IN(min) t_ON
2L_MAX T_S
×
V_O
eff
1
− V_IN(min)
The peak inductor and switch current can be calculated from
Equation 2 or read from the graph in Figure 10. The peak
current shown in Figure 10 is derived assuming a maximum
duty cycle and a minimum frequency. The selected inductor
and diode peak current capability must exceed this value.
The peak current seen by the inductor is calculated at the
maximum input voltage. A wider input voltage range will result
in a higher worst-case peak current in the inductor. This effect
can be seen in Table 4 by comparing the difference between
the peak current at V_IN(min) and V_IN(max).
(2)
I_PK
=
t_ON(max) V_IN(max)
L_MIN
DCM/CCM Boundary
Equation 3 solves for the point at which the inductor current
will transition from DCM (discontinuous conduction mode) to
V_OUT
3.3V
5.0V
9.0V
12.0V
15.0V
16.0V
20.0V
22.0V
V_IN(CCM)
3.04V
4.40V
7.60V
10.0V
12.4V
13.2V
16.4V
18.0V
Table 1. DCM/CCM Boundary
Micrel
CCM (continuous conduction mode). As the input voltage is
raised above this level the inductor has a potential for
developing a dc component while the oscillator is gated on.
Table 1 display the input points at which the inductor current
can possibly operate in the CCM region. Operation in this
region can result in a peak current slightly higher than
displayed Table 4.
( ) (3) ^V_IN(ccm) ^{= V}_OUT ^{+ V}_FWD ⁺ (^{1− D})
Table 2 lists common inductors suitable for most applica-
tions. Table 6 lists minimum inductor sizes versus input and
output voltage. In low-cost, low-peak-current applications,
RF-type leaded inductors may sufficient. All inductors listed
in Table 4 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 2. 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. (The peak current is the same as the peak inductor
current and can be derived from Equation 2 or Figure 10.)
Care must be take to make sure that the peak current is
evaluated at the maximum input voltage.
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 3. Diode Examples
As can be seen in the “Typical Characteristics: Efficiency”
graph, the output diode type can have an effect on circuit
efficiency. The BAT54- and BAT85-series diodes are low-
current Shottky diodes available from On Semiconductor and
Phillips, respectively. They are suitable for peak repetitive
currents of 300mA or less with good reverse current charac-
teristics. For applications 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 1mA at very high
junction temperatures. Table 3 summarizes some typical
performance characteristics of various suitable diodes.
June 2000
7
MIC2141