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Fabricante

LT1934IS6

Micropower Step-Down Switching Regulators in ThinSOT and DFN

Linear Technology 

LT1934/LT1934-1

APPLICATIONS INFORMATION

Table 2. Inductor Vendors

VENDOR PHONE

Murata

(404) 426-1300

Sumida

(847) 956-0666

Coilcraft (847) 639-6400

Würth

(866) 362-6673

Electronics

URL

www.murata.com

www.sumida.com

www.coilcraft.com

www.we-online.com

PART SERIES

LQH3C

CR43

CDRH4D28

CDRH5D28

DO1607C

DO1608C

DT1608C

WE-PD1, 2, 3, 4

COMMENTS

Small, Low Cost, 2mm Height

the average inductor current equals the load current, the

maximum load current is:

IOUT(MAX) = IPK – ΔIL/2

where IPK is the peak inductor current and ΔIL is the

peak-to-peak ripple current in the inductor. The ripple

current is determined by the off time, tOFF = 1.8μs, and

the inductor value:

ΔIL = (VOUT + VD) • tOFF/L

IPK is nominally equal to ILIM. However, there is a slight

delay in the control circuitry that results in a higher peak

current and a more accurate value is:

IPK = ILIM + 150ns • (VIN – VOUT)/L

These expressions are combined to give the maximum

load current that the LT1934 will deliver:

IOUT(MAX) = 350mA + 150ns • (VIN – VOUT)/L – 1.8μs

• (VOUT + VD)/2L (LT1934)

IOUT(MAX) = 90mA + 150ns • (VIN – VOUT)/L – 1.8μs

• (VOUT + VD)/2L (LT1934-1)

The minimum current limit is used here to be conservative.

The third term is generally larger than the second term,

so that increasing the inductor value results in a higher

output current. This equation can be used to evaluate

a chosen inductor or it can be used to choose L for a

given maximum load current. The simple, single equation

rule given above for choosing L was found by setting

ΔIL = ILIM/2.5. This results in IOUT(MAX) ~0.8ILIM (ignoring

the delay term). Note that this analysis assumes that the

inductor current is continuous, which is true if the ripple

current is less than the peak current or ΔIL < IPK.

The inductor must carry the peak current without satu-

rating excessively. When an inductor carries too much

current, its core material can no longer generate ad-

ditional magnetic flux (it saturates) and the inductance

drops, sometimes very rapidly with increasing current.

This condition allows the inductor current to increase

at a very high rate, leading to high ripple current and

decreased overload protection.

Inductor vendors provide current ratings for power induc-

tors. These are based on either the saturation current or

on the RMS current that the inductor can carry without

dissipating too much power. In some cases it is not clear

which of these two determine the current rating. Some data

sheets are more thorough and show two current ratings,

one for saturation and one for dissipation. For LT1934 ap-

plications, the RMS current rating should be higher than

the load current, while the saturation current should be

higher than the peak inductor current calculated above.

Input Capacitor

Step-down regulators draw current from the input sup-

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

capacitor is required to reduce the resulting voltage ripple

at the LT1934 and to force this switching current into

a tight local loop, minimizing EMI. The input capacitor

must have low impedance at the switching frequency to

do this effectively. A 2.2μF ceramic capacitor (1μF for the

LT1934-1) satisfies these requirements.

If the input source impedance is high, a larger value ca-

pacitor may be required to keep input ripple low. In this

case, an electrolytic of 10μF or more in parallel with a 1μF

ceramic is a good combination. Be aware that the input

1934fe

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