LT3680 データシートの表示（PDF） - Linear Technology

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LT3680

36V, 3.5A, 2.4MHz Step-Down Switching Regulator with 75µA Quiescent Current

Linear Technology 

LT3680

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 LT3680 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)

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

RT VALUE (kΩ)

215

140

100

78.7

63.4

53.6

45.3

39.2

34

26.7

22.1

18.2

15

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:

( ) fSW(MAX)

=

VD +

tON(MIN) VD

VOUT

+ VIN

–

VSW

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 LT3680 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:

DCMIN = fSWtON(MIN)

DCMAX = 1– fSWtOFF(MIN)

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 LT3680 applications

depends on switching frequency and Absolute Maxi-

mum Ratings of the VIN and BOOST pins (36V and 56V

respectively).

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

overload conditions, the switching frequency should be

chosen according to the following equation:

VIN(MAX)

=

VOUT + VD

fSW tON(MIN)

–

VD

+

VSW

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

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

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

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

tON(MIN) is the minimum switch on time (~100ns). Note that

a higher switching frequency will depress the maximum

3680fb

9

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