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LT3493E-3

1.2A, 750kHz Step-Down Switching Regulator in 2mm × 3mm DFN

Linear Technology 

LT3493-3

APPLICATIO S I FOR ATIO

Minimum On Time

The part will still regulate the output at input voltages that

exceed VIN(PS) (up to 40V), however, the output voltage

ripple increases as the input voltage is increased. Figure 1

illustrates switching waveforms in continuous mode for a

3V output application near VIN(PS) = 33V.

As the input voltage is increased, the part is required to

switch for shorter periods of time. Delays associated with

turning off the power switch dictate the minimum on time

of the part. The minimum on time for the LT3493-3 is

130ns. Figure 2 illustrates the switching waveforms when

the input voltage is increased to VIN = 35V.

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 ~200mV. 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 40V.

As the input voltage increases, the inductor current ramps

up quicker, the number of skipped pulses increases and

the output voltage ripple increases. For operation above

VIN(MAX) the only component requirement is that the

components be adequately rated for operation at the

intended voltage levels.

The part is robust enough to survive prolonged operation

under these conditions as long as the peak inductor

current does not exceed 2.2A. Inductor current saturation

may further limit performance in this operating regime.

Inductor Selection and Maximum Output Current

A good first choice for the inductor value is:

L = 1.6 (VOUT + VD)

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

VSW

20V/DIV

IL

0.5A/DIV

VOUT

200mV/DIV

AC COUPLED

COUT = 10µF

VOUT = 3V

VIN = 35V

ILOAD = 0.75A

L = 10µH

2µs/DIV

Figure 2

3493-3 F02

VSW

20V/DIV

IL

0.5A/DIV

VOUT

200mV/DIV

AC COUPLED

COUT = 10µF

VOUT = 3V

VIN = 40V

ILOAD = 0.75A

L = 10µH

2µs/DIV

Figure 3

3493-3 F03

3493-3f

9

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