MBRS340 データシートの表示（PDF） - Linear Technology

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MBRS340

42V, 2.5A, 2MHz Step-Down Switching Regulator with 2.7µA Quiescent Current

Linear Technology 

LT3975

APPLICATIONS INFORMATION

light loads, the part will go to sleep between groups of

pulses, so the quiescent current of the part will still be low,

but not as low as in Burst Mode operation. The quiescent

current in a typical application when synchronized with an

external clock is 11µA at no load. Holding the SYNC pin

DC high yields no advantages in terms of output ripple or

minimum load to full frequency, so is not recommended.

FB Resistor Network

The output voltage is programmed with a resistor divider

between the output and the FB pin. Choose the resistor

values according to:

R1=

R2





VOUT

1.197V

–

1

Reference designators refer to the Block Diagram. 1%

resistors are recommended to maintain output voltage

accuracy.

The total resistance of the FB resistor divider should be

selected to be as large as possible to enhance low current

performance. The resistor divider generates a small load

on the output, which should be minimized to optimize the

low supply current at light loads.

When using large FB resistors, a 10pF phase lead capacitor

should be connected from VOUT to FB.

Setting the Switching Frequency

The LT3975 uses a constant frequency PWM architecture

that can be programmed to switch from 200kHz to 2MHz

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 Table 1.

To estimate the necessary RT value for a desired switching

frequency, use the equation:

( ) RT =

51.1

fSW 1.09

– 9.27

where RT is in kΩ and fSW is in MHz.

Table 1. Switching Frequency vs RT Value

SWITCHING FREQUENCY (MHz)

RT VALUE (kΩ)

0.2

294

0.3

182

0.4

130

0.6

78.7

0.8

54.9

1.0

41.2

1.2

32.4

1.4

26.1

1.6

21.5

1.8

17.8

2.0

14.7

2.2

12.4

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, and

lower maximum input voltage. The highest acceptable

switching frequency (fSW(MAX)) for a given application

can be calculated as follows:

( ) fSW(MAX)

=

VOUT

tON(MIN) VIN

+

–

VD

VSW

+

VD

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 (~0.22V at max load). This equa-

tion shows that slower switching frequency is necessary

to safely accommodate high VIN/VOUT ratio. This is due

to the limitation on the LT3975’s minimum on-time. The

minimum on-time is a strong function of temperature.

Use the typical minimum on-time curve to design for an

application’s maximum temperature, while adding about

30% for part-to-part variation. The minimum duty cycle that

can be achieved taking minimum on time into account is:

DCMIN = fSW • tON(MIN)

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

minimum switch on-time.

3975f

12

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