LT3685E 데이터 시트보기 (PDF) - Linear Technology

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LT3685E

36V, 2A, 2.4MHz Step-Down Switching Regulator

Linear Technology 

LT3685

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 LT3685 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)

187

121

88.7

68.1

56.2

46.4

40.2

34

29.4

23.7

19.1

16.2

13.3

11.5

9.76

8.66

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 LT3685 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 LT3685 applications

depends on switching frequency, the Absolute Maximum

Ratings of the VIN and BOOST pins, and the operating

mode.

The LT3685 can operate from input voltages up to 38V,

and safely withstand input voltages up 60V. Note that

while VIN > 38V (typical), the LT3685 will stop switching,

allowing the output to fall out of regulation.

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

overload conditions, the switching frequency should be

chosen according to the following discussion.

For safe operation at inputs up to 60V the switching fre-

quency must be set low enough to satisfy VIN(MAX) ≥ 40V

according to the following equation. If lower VIN(MAX) is

desired, this equation can be used directly.

3685fb

9

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