Data Sheet

LT8615

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Rev 0

19 of 31

resulting in 3.7μA no-load current from the 12V supply. Note that the IQ equation implies that the no-load current is

a function of VIN; this is plotted in the Achieving Ultralow Quiescent Current section.

When using large FB resistors, connect a 4.7pF to 22pF phase-lead capacitor from VOUT to FB.

Setting the Switching Frequency

The LT8615 uses a constant frequency PWM architecture that can be programmed to switch from 400kHz to 2.5MHz

by using a resistor tied from the RT pin to ground. Table 4 shows the necessary RT value for a desired switching

frequency. When in spread-spectrum modulation mode, the frequency is modulated upwards of the frequency set

by RT.

Table 4. Switching Frequency vs RTValue

fSW (MHz)

RT (kΩ)

0.4

105

0.5

76.8

0.6

68.1

0.8

51.1

1.0

40.2

1.2

33.2

1.4

27.4

1.6

23.7

1.8

20.5

2.0

18.2

2.2

16.2

2.5

13.7

Operating Frequency Selection and Trade-Offs

Selection of the operating frequency is a trade-off between efficiency, component size, and input voltage range. The

advantage of high frequency operation is that smaller inductor and capacitor values may be used. The disadvantages

are lower efficiency and a smaller input voltage range.

The highest switching frequency (fSW(MAX)) for a given application can be calculated as follows:

fSW(MAX) =

VOUT + VSW(BOT)

tON(MIN)(VIN − VSW(TOP) + VSW(BOT))

where, VIN is the typical input voltage, VOUT is the output voltage, VSW(TOP) and VSW(BOT) are the internal switch drops

(~0.28V, ~0.28V, respectively, at max. load) and

tON(MIN) is the minimum top switch on-time (see Typical Applications).

This equation shows that slower switching frequency is necessary to accommodate a high VIN/VOUT ratio.

For transient operation, VIN may go as high as the Abs Max rating regardless of the RT value. However, the LT8615

reduces switching frequency as necessary to maintain control of the inductor current to assure safe operation.