LT3796 데이터 시트보기 (PDF) - Linear Technology

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LT3796

100V Constant-Current and Constant-Voltage Controller with Dual Current Sense

Linear Technology 

LT3796

APPLICATIONS INFORMATION

Power MOSFET Selection

For applications operating at high input or output voltages,

the power N-channel MOSFET switch is typically chosen

for drain voltage VDS rating and low gate charge QG.

Consideration of switch on-resistance, RDS(ON), is usually

secondary because switching losses dominate power loss.

The INTVCC regulator on the LT3796 has a fixed current

limit to protect the IC from excessive power dissipation

at high VIN, so the MOSFET should be chosen so that

the product of QG at 7.7V and switching frequency does

not exceed the INTVCC current limit. For driving LEDs be

careful to choose a switch with a VDS rating that exceeds

the threshold set by the FB pin in case of an open load

fault. Several MOSFET vendors are listed in Table 4. The

MOSFETs used in the application circuits in this data sheet

have been found to work well with the LT3796. Consult

factory applications for other recommended MOSFETs.

Table 4. MOSFET Manufacturers

VENDOR

WEB

Vishay Siliconix

www.vishay.com

Fairchild

www.fairchildsemi.com

International Rectifier

www.irf.com

Infineon

www.infineon.com

High Side PMOS Disconnect Switch Selection

A high side PMOS disconnect switch with a minimum

VTH of –1V to –2V is recommended in most LT3796 ap-

plications to optimize or maximize the PWM dimming

ratio and protect the LED string from excessive heating

during fault conditions as well. The PMOS disconnect

switch is typically selected for drain-source voltage VDS,

and continuous drain current ID. For proper operations,

VDS rating must exceed the open LED regulation voltage

set by the FB1 pin, and ID rating should be above ILED.

Schottky Rectifier Selection

The power Schottky diode conducts current during the

interval when the switch is turned off. Select a diode rated

for the maximum SW voltage. If using the PWM feature for

dimming, it is important to consider diode leakage, which

increases with the temperature, from the output during the

PWM low interval. Therefore, choose the Schottky diode

with sufficiently low leakage current. Table 5 has some

recommended component vendors.

Table 5. Schottky Rectifier Manufacturers

VENDOR

WEB

On Semiconductor

www.onsemi.com

Diodes, Inc

www.diodes.com

Central Semiconductor

www.centralsemi.com

Rohm Semiconductor

www.rohm.com

Sense Resistor Selection

The resistor, RSENSE, between the source of the external

N-channel MOSFET and GND should be selected to provide

adequate switch current to drive the application without

exceeding the 113mV (typical) current limit threshold on

the SENSE pin of LT3796. For buck mode applications,

select a resistor that gives a switch current at least 30%

greater than the required LED current. For buck mode,

select a resistor according to:

RSENSE(BUCK

)

≤

0.07V

ILED

For buck-boost mode, select a resistor according to:

RSENSE(BUCK

−

BOOST

)

≤

VIN • 0.07V

(VIN +VLED)ILED

For boost, select a resistor according to:

RSENSE(BOOST )

≤

VIN • 0.07V

VLED • ILED

The placement of RSENSE should be close to the source of

the NMOS FET and GND of the LT3796. The SENSE input

to LT3796 should be a Kelvin connection to the positive

terminal of RSENSE.

70mV is used in the equations above to give some margin

below the 113mV (typical) sense current limit threshold.

3796f

20

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