LT3503 データシートの表示（PDF） - Linear Technology

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LT3503

1A, 2.2MHz Step-Down Switching Regulator in 2mm × 3mm DFN

Linear Technology 

LT3503

APPLICATIO S I FOR ATIO

Now the required on time has decreased below the mini-

mum on time of 130ns. Instead of the switch pulse width

becoming narrower to accommodate the lower duty cycle

requirement, the switch pulse width remains fixed at

130ns. In Figure 2 the inductor current ramps up to a value

exceeding the load current and the output ripple increases

to ~40mV. The part then remains off until the output

voltage dips below 100% of the programmed value before

it begins switching again.

Provided that the output remains in regulation and that the

inductor does not saturate, operation above VIN(PS) is safe

and will not damage the part. Figure 3 illustrates the

switching waveforms when the input voltage is increased

to its absolute maximum rating of 20V.

The part is robust enough to survive prolonged operation

under these conditions as long as the peak inductor

current does not exceed 2.2A. In Figure 3 the peak inductor

current of 2A suggests that the saturation current rating of

the inductor should be ~2.6A, which may require an

inductor of large physical size. The peak inductor current

value can be reduced by simultaneously increasing the

inductance and output capacitance. In Figure 4 the peak

inductor current is reduced to 1.3A by doubling the output

capacitor and inductor values. Now the required inductor

current saturation rating is ~1.7A, so that even though the

inductance value has increased, it may be possible to

achieve a physically smaller inductor size.

Note that inductor current saturation ratings often de-

crease with temperature and that inductor current satura-

tion may further limit performance in this operating regime.

Inductor Selection and Maximum Output Current

A good first choice for the inductor value is:

L = 0.6 (VOUT + VD)

VSW

10V/DIV

IL

1A/DIV

VOUT

20mV/DIV

COUT = 47µF

VOUT = 0.78V

VIN = 20V

ILOAD = 1.1A

L = 1.1µH

1µs/DIV

Figure 3

3503 F03

VSW

10V/DIV

IL

1A/DIV

VOUT

20mV/DIV

COUT = 2 × 47µF

VOUT = 0.78V

VIN = 20V

ILOAD = 1.1A

L = 2.7µH

1µs/DIV

Figure 4

3503 F04

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

L is in µH. With this value there will be no subharmonic

oscillation for applications with 50% or greater duty cycle.

The inductor’s RMS current rating must be greater than

your maximum load current and its saturation current

should be about 30% higher. For robust operation in fault

conditions, the saturation current should be above 2.2A.

To keep efficiency high, the series resistance (DCR) should

be less than 0.1Ω. 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 higher maximum load current and

reduces output voltage ripple at the expense of slower

3503f

9

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