LT3045 datasheet(25/44 Pages) AD

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Data Sheet

LT83401/LT83402

analog.com

Rev. 0

25 of 44

slopes, if the inductor is large enough to avoid subharmonic oscillations at the frequency set by RT, then the slope

compensation is sufficient for all synchronization frequencies.

Setting the Switching Frequency

The LT83401/LT83402 uses a constant-frequency Pulse width modulation (PWM) architecture that is programmed

to switch from 300kHz to 6MHz by using a resistor connected from the RT pin to GND.

The RT resistor required for the desired switching frequency is calculated by Equation 1.

RT =

118.41

fSW

− 11.7

(1)

where RT is in kΩ and fSW is the desired switching frequency in MHz. Table 4 shows the necessary RT value for the

desired switching frequency.

Table 4. SW Frequency vs. RT Value

fSW (MHz)

RT (kΩ)

0.3

412

0.4

301

0.5

237

0.6

191

0.7

162

0.8

140

0.9

121

1.0

107

1.2

88.7

1.4

73.2

1.6

61.9

1.8

53.6

47.5

2.5

35.7

27.4

3.5

22.1

17.8

8.25

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 by Equation 2.

fSW(MAX) =

VOUT + VSW(BOT)

tON(MIN) (VIN − VSW(TOP) + VSW(BOT))

(2)

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