LT3685E 查看數據表（PDF） - Linear Technology

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LT3685E

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

Linear Technology 

LT3685

APPLICATIONS INFORMATION

6.0

5.5 TO START

(WORST CASE)

5.0

4.5

4.0

TO RUN

3.5

3.0 VOUT = 3.3V

TA = 25°C

2.5 L = 8.2μH

f = 700kHz

2.0

1

10

100

1000

LOAD CURRENT (A)

8.0

7.0 TO START

(WORST CASE)

6.0

10000

5.0 TO RUN

4.0

VOUT = 5V

3.0 TA = 25°C

L = 8.2μH

f = 700kHz

2.0

1

10

100

1000

LOAD CURRENT (A)

10000

3685 F05

Figure 5. The Minimum Input Voltage Depends on

Output Voltage, Load Current and Boost Circuit

At light loads, the inductor current becomes discontinu-

ous and the effective duty cycle can be very high. This

reduces the minimum input voltage to approximately

300mV above VOUT. At higher load currents, the inductor

current is continuous and the duty cycle is limited by the

maximum duty cycle of the LT3685, requiring a higher

input voltage to maintain regulation.

Soft-Start

The RUN/SS pin can be used to soft-start the LT3685,

reducing the maximum input current during start-up.

The RUN/SS pin is driven through an external RC filter to

create a voltage ramp at this pin. Figure 6 shows the start-

up and shut-down waveforms with the soft-start circuit.

RUN

15k

0.22μF

RUN/SS

GND

IL

1A/DIV

VRUN/SS

2V/DIV

VOUT

2V/DIV

2ms/DIV

3685 F06

Figure 6. To Soft-Start the LT3685, Add a Resisitor

and Capacitor to the RUN/SS Pin

By choosing a large RC time constant, the peak start-up

current can be reduced to the current that is required to

regulate the output, with no overshoot. Choose the value

of the resistor so that it can supply 20μA when the RUN/SS

pin reaches 2.5V.

Synchronization

Synchronizing the LT3685 oscillator to an external fre-

quency can be done by connecting a square wave (with

20% to 80% duty cycle) to the SYNC pin. The square

wave amplitude should have valleys that are below 0.3V

and peaks that are above 0.8V (up to 6V).

The LT3685 may be synchronized over a 250kHz to 2MHz

range. The RT resistor should be chosen to set the LT3685

switching frequency 20% below the lowest synchronization

input. For example, if the synchronization signal will be 250kHz

and higher, the RT should be chosen for 200kHz. To assure

reliable and safe operation the LT3685 will only synchronize

when the output voltage is near regulation as indicated by the

PG flag. It is therefore necessary to choose a large enough

inductor value to supply the required output current at the

frequency set by the RT resistor. See Inductor Selection sec-

tion. It is also important to note that slope compensation

is set by the RT value: When the sync frequency is much

higher than the one set by RT, the slope compensation will

be significantly reduced which may require a larger inductor

value to prevent subharmonic oscillation.

3685fb

15

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