LT1777CS datasheet(10/24 Pages) LINER

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LT1777

APPLICATIONS INFORMATION

Deciding upon a value for the sense inductor involves

evaluating the trade-off between overall efficiency (POUT/

PIN) and switch current slew rate. Larger sense inductors

yield lower current slew rates which offer reduced high

frequency RFI emissions, but at the expense of poorer

efficiency.

The question is “What is the allowed range of values for a

sense inductor in a given application?” There is really no

minimum limit to the sense inductor, i.e., its value is

allowed to be zero. (In other words, the physical sense

inductor ceases to exist and is replaced by a short circuit.)

This will yield the highest efficiency possible in a given

situation. Although an explicit current slew rate no longer

exists, the naturally less aggressive nature of the LT1777

will often yield quieter supply operation than other stan-

dard switching regulators.

As far as the

maximum allowable value for the sense

inductor, this is dictated by the current ramp rate in the

main inductor during the conventional part of the switch-

ing cycle. It is generally overconservative to limit the

switch current slew rate to that exhibited by the main

inductor. This would potentially yield a triangular current

waveform. Efficiency would be greatly reduced at little

further gain in noise performance. Stated mathematically,

maximum slew rate in the main inductor occurs at maxi-

mum input voltage as:

Max V

OUT

MAIN

–

The sense inductor experiences 2VBE of applied voltage.

This is perhaps 1.0V at a maximum hot condition. If we use

an additional factor of two to be conservative, this yields

a maximum sense inductor value as follows:

–

Max V

Max L

Max V

SENSE

OUT

MAIN

SENSE

MAIN

OUT









As an example, a maximum input voltage of 36V, an output

voltage of 5V and a main inductor value of 220

µH yields a

maximum suggested sense inductor value of 3.5

µH.

Circuit behavior versus sense inductor value is shown in

the oscilloscope photos in Figure 2. The circuit and oper-

ating conditions are similar to the Typical Application on

the first page of this data sheet with the exception that the

sense inductor is allowed to assume the series of values:

µH, 0.47µH, 1µH and 2.2µH. Figure 2a shows a close-up

of the leading edge (turn-on) of the current waveform.

Values of 0

µH and 0.47µH are found to yield a dI/dt of

about 2.2A/

µs, while 1µH yields 1.4A/µs and 2.2µH yields

0.6A/

µs. Figure 2b shows the trailing edge (turn-off) of the

Figure 2. VSW Node Current Behavior vs LSENSE Value.

LSENSE = 0µH, 0.47µH, 1.0µH and 2.2µH

200ns/DIV

1777 F02a

(a) Leading Edge

200ns/DIV

1777 F02b

(b) Trailing Edge

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