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LT3681

3681f

FB Resistor Network

The output voltage is programmed with a resistor divider

between the output and the FB pin. Choose the 1% resis-

tors according to:

VOUT

1 265

= ⎛

⎝⎜

⎞

⎠⎟

–

Reference designators refer to the Block Diagram.

Setting the Switching Frequency

The LT3681 uses a constant frequency PWM architecture

that can be programmed to switch from 300kHz to 2.8MHz

by using a resistor tied from the RT pin to ground. A table

showing the necessary RT value for a desired switching

frequency is in Figure 1.

SWITCHING FREQUENCY (MHz)

RT VALUE (kΩ)

0.3

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

187

133

84.5

60.4

45.3

36.5

29.4

23.7

20.5

16.9

14.3

12.1

10.2

8.66

Operating Frequency Tradeoffs

Selection of the operating frequency is a tradeoff between

efﬁciency, component size, minimum dropout voltage, and

maximum input voltage. The advantage of high frequency

operation is that smaller inductor and capacitor values may

be used. The disadvantages are lower efﬁciency, lower

maximum input voltage, and higher dropout voltage. The

highest acceptable switching frequency (fSW(MAX)) for a

given application can be calculated as follows:

SW MAX

OUT

ON MIN

DIN

()

–

where VIN is the typical input voltage, VOUT is the out-

put voltage, VD is the power Schottky catch diode drop

(~0.55V), VSW is the internal switch drop (~0.5V at max

load). This equation shows that slower switching frequency

is necessary to safely accommodate high VIN/VOUT ratio.

Also, as shown in the next section, lower frequency al-

lows a lower dropout voltage. The reason input voltage

range depends on the switching frequency is because the

LT3681 switch has ﬁnite minimum on and off times. The

switch can turn on for a minimum of ~150ns and turn off

for a minimum of ~150ns. This means that the minimum

and maximum duty cycles are:

MIN

SW ON MIN

MAX

SW OFF MIN

()

1–

where fSW is the switching frequency, the tON(MIN) is the

minimum switch on time (~150ns), and the tOFF(MIN) is

the minimum switch off time (~150ns). These equations

show that duty cycle range increases when switching

frequency is decreased.

A good choice of switching frequency should allow ad-

equate input voltage range (see next section) and keep

the inductor and capacitor values small.

Input Voltage Range

The maximum input voltage for LT3681 applications de-

pends on switching frequency, the Absolute Maximum Rat-

ings on VIN and BOOST pins, and on operating mode.

If the output is in start-up or short-circuit operating modes,

then VIN must be below 34V and below the result of the

following equation:

IN MAX

OUT

SW ON MIN

DSW

()

–

where VIN(MAX) is the maximum operating input voltage,

VOUT is the output voltage, VD is the catch diode drop

(~0.55V), VSW is the internal switch drop (~0.5V at max

load), fSW is the switching frequency (set by RT), and

tON(MIN)istheminimumswitchontime(~150ns).Notethat

a higher switching frequency will depress the maximum

operating input voltage. Conversely, a lower switching

Figure 1. Switching Frequency vs. RT Value

APPLICATIONS INFORMATION

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