LT1777CS datasheet(11/24 Pages) LINER

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LT1777

APPLICATIONS INFORMATION

current waveform. The four sense inductor values of 0

µH,

0.47

µH, 1µH and 2.2µH yield dI/dt rates of roughly

4.5A/

µs, 2.2A/µs, 1.4A/µs and 0.6A/µs, respectively.

These photos show that there is a minimum effective value

for sense inductance, which is 0.47

µH for a typical part at

room temperature as shown. This value inductor has a

small effect on the trailing edge rate, but essentially no

effect on the rising edge. Minimum effective sense induc-

tance value means that inductors much smaller than this

value will have substantially the same performance as zero

inductance, such that these inductors serve no useful

purpose.

In summary,

1. The LT1777 uses an external sense inductor to set a

theoretical limit for current ramp rate according to the

formula:

Max dI dt

SENSE

2. Allowable range for the sense inductor runs from a

minimum of 0 to a maximum of:

Max L

Max V

SENSE

MAIN

OUT









–

3. The minimum effective inductor size is typically 0.47

µH.

Harmonic Behavior

The LT1676 is a high efficiency “cousin” to the LT1777. An

additional set of oscilloscope photographs in Figure 3

show the leading edge and trailing edge of the current

waveform when this part is substituted for the LT1777.

(No sense inductor is used with the LT1676.) The leading

and trailing edges of the LT1676 current waveform are

much faster than that of the LT1777, even when the

LT1777 uses a sense inductor of 0

µH. The 10% to 90%

rise time/fall time is on the order of 10ns to 20ns, too fast

to measure accurately at the horizontal sweep rate of

200ns/DIV.

While this time-based analysis demonstrates that the

current waveform of the LT1777 is quieter than standard

high efficiency buck converters, some users may prefer to

see a direct comparison on a frequency domain basis.

Figures 4a, 4b, and 4c show a spectral analysis of the

current waveforms. The horizontal axis is 2MHz/DIV (0MHz

to 20MHz), and the vertical axis is 10dB/DIV. All photos

were taken with VIN = 24V and VOUT = 5V at 400mA. Figure

4a is of the LT1676 and is for comparison purposes.

Figures 4b and 4c are of the LT1777 with a sense inductor

of 0

µH and 2.2µH, respectively. A decrease in high fre-

quency energy is seen when going from the LT1676 to the

LT1777 with no sense inductor, and a further improve-

ment with a 2.2

µH sense inductor. For example, at 10MHz,

the LT1777 shows an improvement of about –10dB with

µH and perhaps –25dB with 2.2µH.

200ns/DIV

1777 F03a

(a) Leading Edge

Figure 3. LT1676 Current Behavior for Comparison Purposes Only

200ns/DIV

1777 F03b

(b) Trailing Edge

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