MIC2570 datasheet(8/15 Pages) MIC

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MIC2570

Micrel, Inc.

MIC2570

August 2007

at each cycle. At the threshold between continuous and dis-

continuous operation we can use the fact that I1 = I2 to get:

V1 × t1 = V2 × t2

This relationship is useful for ﬁnding the desired oscillator

duty cycle based on input and output voltages. Since input

voltages typically vary widely over the life of the battery, care

must be taken to consider the worst case voltage for each

parameter. For example, the worst case for t1 is when VIN

is at its minimum value and the worst case for t2 is when VIN

is at its maximum value (assuming that VOUT, VDIODE and

VSAT do not change much).

To select an inductor for a particular application, the worst

case input and output conditions must be determined. Based

on the worst case output current we can estimate efﬁciency

and therefore the required input current. Remember that

this is power conversion, so the worst case average input

current will occur at maximum output current and minimum

input voltage.

Average IIN(max) =

VOUT × IOUT(max)

VIN(min) × Efficiency

Referring to Figure 1, it can be seen the peak input current

will be twice the average input current. Rearranging the

inductor equation to solve for L:

L =

× t1

L =

VIN(min)

2 × Average IIN(max)

× t1

where t1 =

duty cycle

fOSC

To illustrate the use of these equations a design example

will be given:

Assume:

MIC2570-1 (ﬁxed oscillator)

VOUT = 5V

IOUT(max) =50mA

VIN(min) = 1.8V

efﬁciency = 75%.

L =

1.8V × 0.7

2 × 185.2mA × 20kHz

Average IIN(max) =

5V × 50mA

1.8V × 0.75

× 185.2mA

L = 170µH

Use the next lowest standard value of inductor and verify

that it does not saturate at a current below about 400mA

(< 2 × 185.2mA).

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