LT3511HMS-TRPBF データシートの表示（PDF） - Linear Technology

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LT3511HMS-TRPBF

Monolithic High Voltage Isolated Flyback Converter

Linear Technology 

LT3511

Applications Information

BIAS Pin Considerations

The BIAS pin powers the internal circuitry of the LT3511.

Three unique configurations exist for regulation of the BIAS

pin. In the first configuration, the internal LDO drives the

BIAS pin internally from the VIN supply. In the second setup,

the VIN supply directly drives the BIAS pin through a direct

connection bypassing the internal LDO. This configuration

will allow the part to operate down to 4.5V and up to 15V.

In the third configuration, an external supply or third wind-

ing drives the BIAS pin. Use this option when a voltage

supply exists lower than the input supply. Drive the BIAS

pin with a voltage supply higher than 3.3V to disable the

internal LDO. The lower voltage supply provides a more

efficient source of power for internal circuitry.

LT3511

VIN

6V TO 100V

LDO

3V

BIAS

LT3511

VIN

4.5V TO 15V

LDO

BIAS

OPTIONAL

LT3511

VIN

6V TO 100V

LDO

3.3V < BIAS < 20V EXTERNAL

BIAS

SUPPLY

3511 F08

Figure 8. BIAS Pin Configurations

Overdriving the BIAS Pin with a Third Winding

The LT3511 provides excellent output voltage regulation

without the need for an opto-coupler, or third winding, but

for some applications with higher input voltages (>20V),

an additional winding (often called a third winding) im-

proves overall system efficiency. Design the third winding

to output a voltage between 3.3V and 12V. For a typical

48VIN application, overdriving the BIAS pin improves

efficiency 4% to 5%.

Loop Compensation

An external resistor-capacitor network compensates the

LT3511 on the VC pin. Typical compensation values are in

the range of RC = 20k and CC = 2.2nF (see the numerous

schematics in the Typical Applications section for other pos-

sible values). Proper choice of both RC and CC is important

to achieve stability and acceptable transient response. For

example, vulnerability to high frequency noise and jitter

result when RC is too large. On the other hand, if RC is

too small, transient performance suffers. The inverse is

true with respect to the value of CC. Transient response

suffers with too large of a CC, and instability results from

too small a CC. The specific value for RC and CC will vary

based on the application and transformer choice. Verify

specific choices with board level evaluation and transient

response performance.

DESIGN PROCEDURE/DESIGN EXAMPLE

Use the following design procedure as a guide to design-

ing applications for the LT3511. Remember, the unique

sampling architecture requires an iterative process for

choosing correct resistor values.

The design example involves designing a 15V output with

a 100mA load current and an input range from 36V to 72V.

VIN(MIN) = 36V, VIN(NOM) = 48V, VIN(MAX) = 72V, VOUT =

15V and IOUT = 100mA

Step 1: Select the transformer turns ratio.

NPS

<

VSW (MAX )

– VIN(MAX) –

VOUT + VF

VLEAKAGE

VSW(MAX) = Max rating of internal switch = 150V

VLEAKAGE = Margin for transformer leakage spike = 40V

VF = Forward voltage of output diode = assume approxi-

mately ~ 0.5V

3511fa

15

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