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LT3972

3972f

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:

OUT

079

= ⎛

⎝⎜

⎞

⎠⎟

–

Reference designators refer to the Block Diagram.

Setting the Switching Frequency

The LT3972 uses a constant frequency PWM architecture

that can be programmed to switch from 200kHz to 2.4MHz

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)

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

215

140

100

78.7

63.4

53.6

45.3

39.2

26.7

22.1

18.2

12.7

10.7

9.09

Figure 1. Switching Frequency vs. RT Value

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 output

voltage, VD is the catch diode drop (~0.5V) and 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 allows a lower dropout

voltage. The reason input voltage range depends on the

switching frequency is because the LT3972 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.

Typical minimum on time at 25°C is 80ns. 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 LT3972 applications de-

pendsonswitchingfrequency,AbsoluteMaximumRatings

of the VIN and BOOST pins, and the operating mode.

The LT3972 can operate from input voltages of up to 33V,

and withstand voltages up to 62V. Note that while VIN is

above 35V typical (33V minimum and 37V maximum)

the part will keep the switch off and the output will not

be in regulation.

To safely allow inputs of up to 62V, be sure to choose a

Schottky diode, inductor size, and switching frequency

to allow safe operation at 37V according to the following

discussions.

While the output is in start-up, short-circuit, or other

overload conditions, the switching frequency should be

chosen according to the following equation:

APPLICATIONS INFORMATION

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