MIC2142_07 Datasheet, PDF(7/17 Page) Micrel Semiconductor

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MIC2142_07 Datasheet, PDF (7/17 Pages) Micrel Semiconductor – Micropower Boost Converter

◁

Micrel, Inc.

Application Information

Pre-designed circuit information is at the end of this

section.

Component Selection

Resistive Divider (Adjustable Version)

The external resistive divider should divide the output

volt-age 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 2Mâ¦ limits the additional load on the output

to 20ÂµA 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)

VOUT

ââ

R1+ R2

ââ âVREF

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 maximum inductor value. The largest

inductor possible is preferable 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; ï¬nd 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.

MIC2142

(2)

IO(max)

(VIN(min) t ON )2

2LMAX TS

eff

â VIN(min)

(3)

IPK

t ON(max)VIN(max)

L MIN

Table 1 lists common inductors suitable for most

applications. Due to the internal transistor peak current

limitation at low input voltages, inductor values less than

10ÂµH are not recommended. 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 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

LC4/C3/C1HQ

CR32

78F

Device Type

surface mount

axial leaded

Table 1. Inductor Examples

Boost Output Diode

Speed, forward voltage, and reverse current are very

important in selecting the output diode. In the boost

conï¬guration 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 âOn Semiconductorâ and âPhillipsâ

respectively. They are suitable for peak repetitive

currents of 300mA or less with good reverse current

characteristics. 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 Semiconductorâs MBR0530 or

Vishayâs 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.

October 2007

M9999-102507

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