LT3710 データシートの表示（PDF） - Linear Technology

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LT3710

Secondary Side Synchronous Post Regulator

Linear Technology 

LT3710

APPLICATIO S I FOR ATIO

Output Inductor Selection

The key parameters for choosing the inductor include

inductance, RMS and saturation current ratings and DCR.

The inductance must be selected to achieve a reasonable

value of ripple current, which is determined by:

∆IL

=

VOUT2 • (1− D2)

f•L

Typically, the inductor ripple current is designed to be

20% to 40% of the maximum output current.

The RMS current rating must be high enough to deliver the

maximum output current. A sufficient saturation current

rating should prevent the inductor core from saturating.

These two current ratings can be determined by:

IRMS ≥

IO2

+

∆ILMAX2

12

ISAT

≥ IO

+

∆ILMAX

2

where IO is the maximum output current and ∆ILMAX is the

maximum peak-to-peak inductor ripple current.

To optimize the efficiency, we usually choose the inductor

with the minimum DCR if the inductance and current

ratings are the same.

Power MOSFET Selection

The LT3710 drives two external N-channel MOSFETs to

deliver high currents at high efficiency. The gate drive

voltage is typically 6.5V. The key parameters for choos-

ing MOSFETs include drain to source voltage rating VDSS

and RDS(ON) at 6.5V gate drive. Note that the transformer

secondary voltage waveform will overshoot at its rising

edge due to the ringing between transformer leakage

inductance and parasitic capacitance. The VDSS of both

top and bottom MOSFETs must be sufficiently higher

than the maximum overshoot. It is recommended that an

RC snubber or a voltage clamping circuitry be placed

across the transformer secondary winding to limit the VS

overshoot.

The RDS(ON) of the MOSFETs should be selected to deliver

the required current at the desired efficiency as well as to

meet the thermal requirement of the MOSFET package.

The conduction power losses of the MOSFETs are:

PM1 ≅ IO2 • RDS(ON)M1 • D2

PM2 ≅ IO2 • RDS(ON)M2 • (1 – D2)

where IO is the maximum output current of LT3710 circuit,

RDS(ON)M1 and RDS(ON)M2 are the on-resistance for the top

and bottom MOSFETs, respectively. The RDS(ON) must be

determined with 6.5V gate drive and the expected operat-

ing temperature.

A good number of high performance power MOSFET

selections are available from Siliconix, International Rec-

tifier and Fairchild. If the VDSS and RDS(ON) ratings are the

same, the MOSFETs with the lowest gate charge QG should

be chosen to minimize the power loss associated with the

MOSFET gate drives, the switching transitions and the

controller bias supply.

Output Capacitor Selection

The selection of the output capacitor is determined by the

output ripple and load transient requirements. In low

output voltage applications, always choose capacitors

with low ESR. The output ripple voltage is approximated

by:

∆VOUT

≈

∆IL





ESR

+

1

8fCOUT 

where ∆IL is the inductor peak-to-peak ripple current.

A partial list of low ESR high performance capacitor types

includes SP capacitors from Panasonic and Cornell Dubilier,

POSCAPs and OS-CON capacitors from Sanyo, T510 and

T520 surface mount capacitors from Kemet.

Design Example

Figure 3 shows an application example for the LT3710. It

is a dual output, high efficiency, isolated DC/DC power

supply with 36V to 72V input, 3.3V/10A and 1.8V/10A

outputs. The basic power stage topology is a 2-transistor

3710f

9

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