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LT1107

1107fa

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

switch closes, current in the inductor builds according to:

where R

′ = 0.65Ω + DCRL

VL = VIN – 0.75V

As an example, suppose –5V at 50mA is to be generated

from a 4.5V to 5.5V input. Recalling Equation (14),

Energy required from the inductor is:

Picking an inductor value of 100

µH with 0.2Ω DCR results

in a peak switch current of:

Substituting IPEAK into Equation (04) results in:

Since 5.28

µJ > 3.82µJ, the 100µH inductor will work.

With this relatively small input range, RLIM is not usually

necessary and the ILIM pin can be tied directly to VIN. As in

the step-down case, peak switch current should be limited

to ~650mA.

Step-Up (Boost Mode) Operation

A step-up DC/DC converter delivers an output voltage

higher than the input voltage. Step-up converters are

not

short-circuit protected since there is a DC path from input

to output.

kHz

OSC

275

.( )

()=

′

−













− ′

115

()

where DC = duty cycle (0.50 in step-down mode)

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:

where tON = switch ON time (7µs).

Next, the current limit resistor RLIM is selected to give

IPEAK from the Maximum Switch Current vs RLIM curve.

The addition of this resistor keeps maximum switch cur-

rent 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):

Next, inductor value is calculated using Equation (11):

−−

12 1 5

600

764

()

µµ

Use the next lowest standard value (56

µH).

Then pick RLIM from the curve. For IPEAK = 600mA, RLIM

= 56

Ω.

Inductor Selection –– 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,

(14)

L =+

()()

OUT

IN MIN

OUT

PEAK

−−

()

PEAK =

()

−+













2 300

050

50 5

12 15

600

()

= 275mW

(16 )

L =−

()()

50 5

PEAK

−

()

−













−

075

065

0 2

325

085

100

()

.•

ΩΩ

Ωµ

L =

()(

) =

100

0 325

5 28

µµ

( )

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