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LT1110

APPLICATI

I FOR ATIO

Picking an inductor value of 47

µH with 0.2Ω DCR results

in a peak switch current of

emA

PEAK

=−













−•

862

)

Substituting IPEAK into Equation 05 results in

L =

()(

) =

0 862

17 5

µµ

(

)

Since 17.5

µJ > 13.7µJ, the 47µH inductor will work. This

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

mum 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 such

that 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.

Inductor Selection — Step-Down Converter

The step-down case (Figure 5) 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 ~800mA

in this mode. Higher current can be obtained by using an

external switch (see Figure 6). 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 (0.69)

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 (10µs).

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 250mA is to be generated

from a 9V to 18V input. Recalling Equation (10),

PEAK =

()











 =

2 250

069

50 5

91 5

0 5

498

–.

)

Next, inductor value is calculated using Equation (11)

=•

91 5

498

–. –

)

µµ

Use the next lowest standard value (47

µH).

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

RLIM = 82Ω.

Inductor Selection — Positive-to-Negative Converter

Figure 7 shows hookup for positive-to-negative conver-

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

In this case,

OUT

()( )

(

)

In this mode the switch is arranged in common collector

or step-down mode. The switch drop can be modeled as

a 0.75V source in series with a 0.65

Ω resistor. When the

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