LT3751EUFD datasheet(21/28 Pages) LINER

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LT3751

3751f

Figure 12. Effects of RC Snubber

APPLICATIONS INFORMATION

Figure 12 shows the effect of the RC snubber resulting in

a lower voltage spike and faster settling time.

LOW NOISE REGULATION

The LT3751 has the option to provide low noise regulated

output voltage when using a resistive voltage divider

from the output node to the FB pin. Refer to the Selecting

Component Parameters section to design the transformer,

NMOS power switch, output diode, and sense resistor.

Use the following equations to select the feedback resis-

tor values based on the power dissipation and desired

output voltage:

RFBH =

VOUT − 1.22

()2

; Top Feedback Resistor

RFBL =

1.22

VOUT − 1.22

⎛

⎝⎜

⎞

⎠⎟

•RFBH ; Bottom Feedback Resistor

RFBH, depending on output voltage and type used, may

require several smaller values placed in series. This will

reduce the risk of arcing and damage to the feedback resis-

tors. Consult the manufacturer’s rated voltage speciﬁcation

for safe operation of the feedback resistors.

The LT3751 has a minimum periodic refresh frequency

limit of 23kHz. This drastically reduces switching frequency

components in the audio spectrum. The LT3751 can oper-

ate with no-load, but the regulation scheme switches to

overvoltage Burst Mode Operation and audible noise and

output voltage ripple increase. This can be avoided by

operating with a minimum load current.

Minimum Load Current

Periodic refresh circuitry requires an average minimum

load current to avoid entering overvoltage burst mode.

Usually, the feedback resistors should be adequate to

provide this minimum load current.

ILOAD(MIN) ≥

LPRI •I

PK •23kHz

100 • VOUT

IPK is the peak primary current at maximum power de-

livery. The LT3751 will enter overvoltage burst mode if

the minimum load current is not met. Overvoltage burst

mode will prevent the application from entering a runaway

condition; however, the output voltage will increase 10%

over the nominal regulated voltage.

3751 F12

VDRAIN

(WITH

SNUBBER)

VDRAIN

(WITHOUT

SNUBBER)

IPRI

NMOS DIODE

CONDUCTS

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