LT3980IMSE-TRPBF データシートの表示（PDF） - Linear Technology

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LT3980IMSE-TRPBF

58V, 2A, 2.4MHz Step-Down Switching Regulator with 85µA Quiescent Current

Linear Technology 

LT3980

Applications Information

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:

R1=

R2





VOUT

0.79V

–



1

Reference designators refer to the Block Diagram.

Setting the Switching Frequency

The LT3980 uses a constant frequency PWM architecture

that can be programmed to switch from 100kHz 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)

0.1

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

RT VALUE (kΩ)

432

215

137

97.6

76.8

60.4

51.1

43.2

35.7

32.4

24.9

20

16.2

14

11

Figure 1. Switching Frequency vs RT Value

Operating Frequency Trade-Offs

Selection of the operating frequency is a trade-off 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:

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 LT3980 switch has

finite minimum on and off times. The switch can turn on

for a minimum of ~200ns and turn off for a minimum of

~200ns. This means that the minimum and maximum

duty cycles are:

DCMIN = fSWtON(MIN)

DCMAX = 1– fSWtOFF(MIN)

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

minimum switch on time (~200ns), and the tOFF(MIN) is

the minimum switch off time (~200ns). These equations

show that duty cycle range increases when switching

frequency is decreased.

A good choice of switching frequency should allow ade-

quate input voltage range (see next section) and keep the

inductor and capacitor values small.

Input Voltage Range

The maximum input voltage for LT3980 applications de-

pends on switching frequency , Absolute Maximum Ratings

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

The LT3980 can operate from input voltages of up to 58V,

and withstand voltages up to 80V. Note that while VIN is

above 61V typical (58V minimum and 64V maximum)

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

be in regulation.

The switching frequency should be chosen according to

the following equation:

VIN(MAX)

=

VOUT + VD

fSW tON(MIN)

–

VD

+

VSW

( ) fSW(MAX)

=

VD +

tON(MIN) VD

VOUT

+ VIN

–

VSW

where VIN(MAX) is the maximum typical operating input

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

3980fa

For more information www.linear.com/LT3980

9

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