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LT1940

1940i

APPLICATIO S I FOR ATIO

(VOUT/VIN < 0.5), there is a minimum inductance required

to avoid subharmonic oscillations. See AN19. The discus-

sion below assumes continuous inductor current.

The current in the inductor is a triangle wave with an

average value equal to the load current. The peak switch

current is equal to the output current plus half the peak-to-

peak inductor ripple current. The LT1940 limits its switch

current in order to protect itself and the system from

overload faults. Therefore, the maximum output current

that the LT1940 will deliver depends on the current limit,

the inductor value, and the input and output voltages. L is

chosen based on output current requirements, output

voltage ripple requirements, size restrictions and effi-

ciency goals.

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 LT1940 and L is

the value of the inductor. The peak inductor and switch

current is

ISWPK = ILPK = IOUT + ∆IL/2.

To maintain output regulation, this peak current must be

less than the LT1940’s switch current limit ILIM. ILIM is at

least 1.8A at low duty cycle and decreases linearly to 1.5A

at DC = 0.8. The maximum output current is a function of

the chosen inductor value:

IOUTMAX = ILIM – ∆IL/2 = 1.8A • (1 – 0.21 • DC) – ∆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.

One approach to choosing the inductor is to start with the

simple rule given above, look at the available inductors,

and choose one to meet cost or space goals. Then use

these equations to check that the LT1940 will be able to

deliver the required output current. Note again that these

equations assume that the inductor current is continuous.

Discontinuous operation occurs when IOUT is less than

∆IL/2 as calculated above.

The maximum input voltage is determined by the absolute

maximum ratings of the VIN and BOOST pins and by the

minimum duty cycle DCMIN = 0.15:

VINMAX = (VOUT + VD)/DCMIN – VD + VSW.

This limits the maximum input voltage to ~14V with

VOUT = 1.8V and ~19V with VOUT = 2.5V. Note that this is

a restriction on the operating input voltage; the circuit will

tolerate transient inputs up to the absolute maximum

rating.

Inductor Selection and Maximum Output Current

A good first choice for the inductor value is:

L = (VOUT + VD)/1.2

where VD is the voltage drop of the catch diode (~0.4V) and

L is in

µH. With this value the maximum load current will

be ~1.4A, independent of input voltage. The inductor’s

RMS current rating must be greater than your maximum

load current and its saturation current should be about

30% higher. To keep efficiency high, the series resistance

(DCR) should be less than 0.1

Ω. Table 1 lists several

vendors and types that are suitable.

Of course, such a simple design guide will not always

result in the optimum inductor for your application. A

larger value provides a slightly higher maximum load

current, and will reduce the output voltage ripple. If your

load is lower than 1.4A, then you can decrease the value of

the inductor and operate with higher ripple current. This

allows you to use a physically smaller inductor, or one with

a lower DCR resulting in higher efficiency. Be aware that if

the inductance differs from the simple rule above, then the

maximum load current will depend on input voltage. There

are several graphs in the Typical Performance Character-

istics section of this data sheet that show the maximum

load current as a function of input voltage and inductor

value for several popular output voltages. 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%

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