LT1108CS8 datasheet(7/12 Pages) LINER

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LT1108

APPLICATI

I FOR ATIO

Energy required from the inductor is

kHz

OSC

315

16 6

)

Picking an inductor value of 100

µH with 0.2Ω DCR results

in a peak switch current of

emA

PEAK











 =

605

100

–(

)

–.

Ω

Ωµ

Substituting IPEAK into Equation 04 results in

L =

()(

) =

100

6 605

18 3

µµ

(

)

Since 18.3

µJ > 16.6µJ, the 100µH inductor will work. This

trial-and-error approach can be used to select the optimum

inductor. Keep in mind the switch current maximum rating

of 1.5A. If the calculated peak current exceeds this, an

external power transistor can be used.

A resistor can be added in series with the ILIM pin to invoke

switch current limit. The resistor should be picked so the

calculated IPEAK at minimum VIN is equal to the Maximum

Switch Current (from Typical Performance Characteristic

curves). Then, as VIN increases, switch current is held

constant, resulting in increasing efficiency.

Step-Down Converter

The step-down case (Figure 2) differs from the step-up in

that the inductor current flows through the load during both

the charge and discharge periods of the inductor. Current

through the switch should be limited to ~650mA in this

mode. Higher current can be obtained by using an external

switch (see Figure 3). The ILIM pin is the key to successful

operation over varying inputs.

After establishing output voltage, output current and input

voltage range, peak switch current can be calculated by the

formula:

PEAK

OUT













–

()

where DC = duty cycle (0.60)

VSW = switch drop in step-down mode

VD = diode drop (0.5V for a 1N5818)

IOUT = output current

VOUT = output voltage

VIN = minimum input voltage

VSW is actually a function of switch current which is in turn

a function of VIN, L, time, and VOUT. To simplify, 1.5V can

be used for VSW as a very conservative value.

Once IPEAK is known, inductor value can be derived from

IN MIN

OUT

PEAK

−−

()

where tON = switch-ON time (36µs).

Next, the current limit resistor RLIM is selected to give IPEAK

from the RLIM Step-Down Mode curve. The addition of this

resistor keeps maximum switch current constant as the

input voltage is increased.

As an example, suppose 5V at 300mA is to be generated

from a 12V to 24V input. Recalling Equation (10),

PEAK =

()











 =

2 300

060

50 5

500

–.

()

Next, inductor value is calculated using Equation (11)

1 5

500

396

–. –

()

µµ

Use the next lowest standard value (330

µH).

Then pick RLIM from the curve. For IPEAK = 500mA,

RLIM = 220Ω.

Positive-to-Negative Converter

Figure 4 shows hookup for positive-to-negative conver-

sion. All of the output power must come from the inductor.

In this case,

PL = (VOUT+ VD)(IOUT)

(14)

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