LT1616 Просмотр технического описания (PDF) - Linear Technology

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LT1616

600mA, 1.4MHz Step-Down Switching Regulator in SOT-23

Linear Technology 

LT1616

APPLICATIO S I FOR ATIO

Inductor Selection and Maximum Output Current

The duty cycle of the internal switch is:

DC = (VOUT + VD)/(VIN – VSW + VD)

where VD is the forward voltage drop of the catch diode

(D1) and VSW is the voltage drop of the internal switch.

Usually one is interested in DC at full load current, so you

can use VD = VSW = 0.4V. Note that the LT1616 has a

maximum guaranteed duty cycle of 0.8. This will limit the

minimum input voltage for a particular output voltage.

When the switch is off, the inductor sees the output

voltage plus the catch diode drop. This gives the peak-to-

peak ripple current in the inductor:

∆IL = (1 – DC)(VOUT + VD)/(L • f)

where f is the switching frequency of the LT1616 and L is

the value of the inductor. The average inductor current is

equal to the output current, so the peak inductor current

will be the output current plus one half of the ripple

current:

ILPK = IOUT + ∆IL/2.

To maintain output regulation, this peak current must be

less than the LT1616’s switch current limit ILIM. ILIM is at

least 630mA at low duty cycles, decreasing to 430mA at

80% duty cycle. The maximum output current is a function

of the chosen inductor value:

IOUT(MAX) = ILIM – ∆IL/2.

If the inductor value is chosen so that the ripple current is

small, then the available output current will be near the

switch current limit. A good approach is to choose the

inductor so that the peak-to-peak inductor ripple is equal

to one third of the switch current limit. This leads to:

L = 3(1 – DC)(VOUT + VD)/(ILIM • f)

and

IOUT(MAX) = (5/6)ILIM.

These expressions depend on duty cycle and therefore on

input voltage. Pick a nominal input voltage to calculate L,

then check the maximum available output current at the

minimum and maximum input voltages.

If your application calls for output current less than

400mA, you may be able to relax the value of the inductor

and operate with higher ripple current. This may allow you

to pick a physically smaller inductor or one with a lower DC

resistance. Be aware that these equations assume con-

tinuous inductor current. If the inductor value is low or the

load current is light, then the inductor current may become

discontinuous. This occurs when ∆IL = 2IOUT. For details

of discontinuous mode operation, see Linear Technology

Application Note AN44. Also, high duty cycle operation

may require slightly higher inductor values to avoid sub-

harmonic oscillations. See AN19.

The maximum load current as a function of input voltage

is plotted in the Typical Performance Characteristics sec-

tion of this data sheet. Maximum load current for 3.3V and

5V outputs is shown for several values of L. At the highest

input voltages, the load current is limited by power dissi-

pation in the LT1616.

Choose an inductor that is intended for power applica-

tions. Table 1 lists several manufacturers and inductor

series. The saturation current of the inductor should be

above 0.5A. The RMS current rating should be equal to or

greater than output current. For indefinite operation into a

short circuit, the RMS current rating should be greater

than 0.7A. The DC resistance should be less than 0.5Ω in

order maintain circuit efficiency.

Capacitor Selection

A Buck regulator draws from its input a square wave of

current with peak-to-peak amplitude as high as the switch

current limit. The input capacitor (C1) must supply the AC

component of this current. An RMS current rating of

250mA is adequate for LT1616 circuits. The input capaci-

tor must bypass the LT1616 internal control circuitry and

any other circuitry that operates from the input source. A

1µF ceramic capacitor will satisfy both of these require-

ments. If the impedance of the input source is high (due to

long wires or filter components), additional bulk input

capacitance may be required. In high duty cycle applica-

tions (5VIN to 3.3VOUT, for example), increase the input

capacitor to 2.2µF. It may be possible to achieve lower cost

by using an electrolytic capacitor (tantalum or aluminum)

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