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LT1939

1939f

APPLICATIONS INFORMATION

Discontinuous operation occurs when IOUT is less than

IL/2 as calculated above.

Figure 4 illustrates the inductance value needed for a 3.3V

output with a maximum load capability of 2A. Referring

to Figure 4, an inductor value between 3.3μH and 4.7μH

will be sufﬁcient for a 15V input voltage and a switch

frequency of 750kHz. There are several graphs in the

Typical Performance Characteristics section of this data

sheet that show inductor selection as a function of input

voltage and switch frequency for several popular output

voltages and output ripple currents. Also, low inductance

may result in discontinuous mode operation, which is

okay, but further reduces maximum load current. For

details of maximum output current and discontinuous

mode operation, see Linear Technology Application Note

44. Finally, for duty cycles greater than 50% (VOUT/VIN

> 0.5), there is a minimum inductance required to avoid

subharmonic oscillations. See Application Note 19 for

more information.

capacitor is required to reduce the resulting voltage

ripple at the LT1939 and to force this very high frequency

switching current into a tight local loop, minimizing EMI.

The input capacitor must have low impedance at the

switching frequency to do this effectively, and it must

have an adequate ripple current rating.

A conservative value is the RMS input current is given

by:

ICIN(RMS) =

IOUT1 VOUT1 •VIN VOUT1

() 0.5

VIN

IOUT1

and is largest when VIN = 2VOUT1 (50% duty cycle).

The frequency, VIN to VOUT ratio, and maximum load

current requirement of the LT1939 along with the input

supply source impedance, determine the energy storage

requirements of the input capacitor. Determine the worst-

case condition for input ripple current and then size the

input capacitor such that it reduces input voltage ripple to

an acceptable level. Typical values for input capacitors run

from 10μF at low frequencies to 2.2μF at higher frequencies.

The combination of small size and low impedance (low

equivalent series resistance or ESR) of ceramic capacitors

make them the preferred choice. The low ESR results in

very low voltage ripple and the capacitors can handle plenty

of ripple current. They are also comparatively robust and

can be used in this application at their rated voltage. X5R

and X7R types are stable over temperature and applied

voltage, and give dependable service. Other types (Y5V and

Z5U) have very large temperature and voltage coefﬁcients

of capacitance, so they may have only a small fraction of

their nominal capacitance in your application. While they

will still handle the RMS ripple current, the input voltage

ripple may become fairly large, and the ripple current may

end up ﬂowing from your input supply or from other by-

pass capacitors in your system, as opposed to being fully

sourced from the local input capacitor. An alternative to a

high value ceramic capacitor is a lower value along with

a larger electrolytic capacitor, for example a 1μF ceramic

capacitor in parallel with a low ESR tantalum capacitor.

For the electrolytic capacitor, a value larger than 10μF will

be required to meet the ESR and ripple current require-

ments. Because the input capacitor is likely to see high

Figure 4. Inductor Values for 2A Maximum Load Current

(VOUT1 = 3.3V, IRIPPLE = 1A)

Input Capacitor Selection

Bypass the input of the LT1939 circuit with a 4.7μF or

higher ceramic capacitor of X7R or X5R type. A lower

value or a less expensive Y5V type can be used if there

is additional bypassing provided by bulk electrolytic or

tantalum capacitors. The following paragraphs describe

the input capacitor considerations in more detail.

Step-down regulators draw current from the input sup-

ply in pulses with very fast rise and fall times. The input

INPUT VOLTAGE (V)

250

500

1000

1250

1500

2500

1939 F04

750

1750

2000

2250

L = 1μH

L = 1.5μH

L = 2.2μH

L = 3.3μH

L = 4.7μH

L = 6.8μH

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