LTC3704 datasheet(26/36 Pages) LINER

部件名	LTC3704

功能描述	Wide Input Range, No RSENSE Current Mode Boost, Flyback and SEPIC Controller

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制造商	LINER [Linear Technology]

网页	http://www.linear.com

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LTC1871

26

1871fe

APPLICATIONS INFORMATION

During the MOSFET’s on-time, the control circuit limits

the maximum voltage drop across the power MOSFET to

about 150mV (at low duty cycle). The peak inductor current

is therefore limited to 150mV/RDS(ON). The relationship

between the maximum load current, duty cycle and the

RDS(ON) of the power MOSFET is:

RDS(ON)

VSENSE(MAX)

IO(MAX)

•

1

+

2

• T

•

1

VO + VD

VIN(MIN)

+ 1

The VSENSE(MAX) term is typically 150mV at low duty cycle

and is reduced to about 100mV at a duty cycle of 92% due

to slope compensation, as shown in Figure 8. The constant

‘χ’ in the denominator represents the ripple current in the

inductors relative to their maximum current. For example,

if 30% ripple current is chosen, then χ = 0.30. The ρT term

accounts for the temperature coefﬁcient of the RDS(ON) of

the MOSFET, which is typically 0.4%/°C. Figure 9 illustrates

the variation of normalized RDS(ON) over temperature for

a typical power MOSFET.

Another method of choosing which power MOSFET to

use is to check what the maximum output current is for a

given RDS(ON) since MOSFET on-resistances are available

in discrete values.

IO(MAX)

VSENSE(MAX)

RDS(ON)

•

1

+

2

• T

•

1

VO + VD

VIN(MIN)

+ 1

Calculating Power MOSFET Switching and Conduction

Losses and Junction Temperatures

In order to calculate the junction temperature of the

power MOSFET, the power dissipated by the device must

be known. This power dissipation is a function of the

duty cycle, the load current and the junction temperature

itself. As a result, some iterative calculation is normally

required to determine a reasonably accurate value. Since

the controller is using the MOSFET as both a switching

and a sensing element, care should be taken to ensure

that the converter is capable of delivering the required

load current over all operating conditions (load, line and

temperature) and for the worst-case speciﬁcations for

VSENSE(MAX) and the RDS(ON) of the MOSFET listed in the

manufacturer’s data sheet.

The power dissipated by the MOSFET in a SEPIC converter

is:

PFET = IO(MAX) •

DMAX

1– DMAX

2

•RDS(ON) •DMAX • T

+ k• VIN(MIN) + VO

()1.85•I

O(MAX) •

DMAX

1– DMAX

•CRSS •f

The ﬁrst term in the equation above represents the I2R

losses in the device and the second term, the switching

losses. The constant k = 1.7 is an empirical factor inversely

related to the gate drive current and has the dimension

of 1/current.

From a known power dissipated in the power MOSFET, its

junction temperature can be obtained using the following

formula:

TJ = TA + PFET •RTH(JA)

The RTH(JA) to be used in this equation normally includes

the RTH(JC) for the device plus the thermal resistance from

the board to the ambient temperature in the enclosure.

This value of TJ can then be used to check the original

assumption for the junction temperature in the iterative

calculation process.

SEPIC Converter: Output Diode Selection

To maximize efﬁciency, a fast-switching diode with low

forward drop and low reverse leakage is desired. The output

diode in a SEPIC converter conducts current during the

switch off-time. The peak reverse voltage that the diode

must withstand is equal to VIN(MAX) + VO. The average

forward current in normal operation is equal to the output

current, and the peak current is equal to:

ID(PEAK) = 1+

2

•IO(MAX) •

VO + VD

VIN(MIN)

+ 1

The power dissipated by the diode is:

PD = IO(MAX) • VD

and the diode junction temperature is:

TJ = TA + PD • RTH(JA)

The RTH(JA) to be used in this equation normally includes

the RTH(JC) for the device plus the thermal resistance from

the board to the ambient temperature in the enclosure.

类似零件编号 - LTC3704

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Linear Technology	LTC3704	350Kb / 28P	Wide Input Range, No RSENSE Positive-to-Negative DC/DC Controller
	LTC3704	764Kb / 32P	Wide Input Range, No RSENSE??Current Mode Boost, Flyback and SEPIC Controller
	LTC3704EMS	350Kb / 28P	Wide Input Range, No RSENSE Positive-to-Negative DC/DC Controller
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制造商	部件名	数据表	功能描述
Linear Technology	LTC1871-1	517Kb / 36P	Wide Input Range, No RSENSE??Current Mode Boost, Flyback and SEPIC Controller
	LTC1871	411Kb / 36P	Wide Input Range, No RSENSE Current Mode Boost, Flyback and SEPIC Controller
	LTC1871IMS	527Kb / 36P	Wide Input Range, No RSENSE Current Mode Boost, Flyback and SEPIC Controller
	LTC1871	527Kb / 36P	Wide Input Range, No RSENSE Current Mode Boost, Flyback and SEPIC Controller
	LTC1871-1	530Kb / 36P	Wide Input Range, No RSENSE Current Mode Boost, Flyback and SEPIC Controller
	LTC1871X	764Kb / 32P	Wide Input Range, No RSENSE??Current Mode Boost, Flyback and SEPIC Controller
	LTC1871EMS-7	477Kb / 36P	Wide Input Range, No RSENSE Current Mode Boost,
	LTC1871IMS	527Kb / 36P	Current Mode Boost Flyback and SEPIC Controller
	LTC1871-7	393Kb / 34P	High Input Voltage, Current Mode Boost, Flyback and SEPIC Controller
	LTC1871-7	455Kb / 32P	High Input Voltage,Current Mode Boost, Flyback and SEPIC Controller

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LTC3704 数据表(PDF) 26 Page - Linear Technology

LTC3704 数据表(HTML) 26 Page - Linear Technology

类似零件编号 - LTC3704

类似说明 - LTC3704

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LTC3704 数据表(PDF) 26 Page - Linear Technology

LTC3704 数据表(HTML) 26 Page - Linear Technology

类似零件编号 - LTC3704

类似说明 - LTC3704

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