LTC1700_ 查看數據表（PDF） - Linear Technology

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LTC1700_

High Input Voltage, Current Mode Boost, Flyback and SEPIC Controller

Linear Technology 

U

OPERATIO

LTC1871-7

–

1.230V

+

R2

VIN

P-CH

R1

7V INTVCC

INPUT

SUPPLY

2.5V TO 30V

CIN

LOGIC

DRIVER

GATE

CVCC

4.7µF

X5R

6V-RATED

M1

POWER

MOSFET

GND

18717 F07

GND

PLACE AS CLOSE AS

POSSIBLE TO DEVICE PINS

Figure 7. Bypassing the LDO Regulator and Gate Driver Supply

cause the LTC1871-7 to exceed its maximum junction

temperature rating. The junction temperature can be

estimated using the following equations:

IQ(TOT) ≈ IQ + f • QG

PIC = VIN • (IQ + f • QG)

TJ = TA + PIC • RTH(JA)

The total quiescent current IQ(TOT) consists of the static

supply current (IQ) and the current required to charge and

discharge the gate of the power MOSFET. The 10-pin

MSOP package has a thermal resistance of RTH(JA) =

120°C/W.

As an example, consider a power supply with VIN =10V.

The switching frequency is 200kHz, and the maximum

ambient temperature is 70°C. The power MOSFET chosen

is the FDS3670(Fairchild), which has a maximum RDS(ON)

of 35mΩ (at room temperature) and a maximum total gate

charge of 80nC (the temperature coefficient of the gate

charge is low).

IQ(TOT) = 600µA + 80nC • 200kHz = 16.6mA

PIC = 10V • 16.6mA = 166mW

TJ = 70°C + 120°C/W • 166mW = 89.9°C

TJRISE = 19.9°C

This demonstrates how significant the gate charge current

can be when compared to the static quiescent current in

the IC.

To prevent the maximum junction temperature from being

exceeded, the input supply current must be checked when

operating in a continuous mode at high VIN. A tradeoff

between the operating frequency and the size of the power

MOSFET may need to be made in order to maintain a

reliable IC junction temperature. Prior to lowering the

operating frequency, however, be sure to check with

power MOSFET manufacturers for their latest-and-great-

est low QG, low RDS(ON) devices. Power MOSFET manu-

facturing technologies are continually improving, with

newer and better performance devices being introduced

almost yearly.

Output Voltage Programming

The output voltage is set by a resistor divider according to

the following formula:

VO

=

1.230V



• 1+

R2

R1

The external resistor divider is connected to the output as

shown in Figure 1, allowing remote voltage sensing. The

18717f

11

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