LT1930 datasheet(12/32 Pages) LINER

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LTC1871X

1871xf

For more information www.linear.com/LTC1871X

APPLICATIONS INFORMATION

tion 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

dischargethegateofthepowerMOSFET.The 10-pinMSOP

package has a thermal resistance of RTH(JA) = 120°C/W.

As an example, consider a power supply with VIN = 5V and

VO = 12V at IO = 1A. The switching frequency is 500kHz,

andthemaximumambienttemperatureis70°C.Thepower

MOSFET chosen is the IRF7805, which has a maximum

RDS(ON) of 11mΩ (at room temperature) and a maximum

total gate charge of 37nC (the temperature coefficient of

the gate charge is low).

IQ(TOT) = 600µA + 37nC • 500kHz = 19.1mA

PIC = 5V • 19.1mA = 95mW

TJ = 70°C + 120°C/W • 95mW = 81.4°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 be-

ing exceeded, the input supply current must be checked

when operating in a continuous mode at high VIN. A trad-

eoff 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

operatingfrequency,however,besuretocheckwithpower

MOSFET manufacturers for their latest-and-greatest low

QG, low RDS(ON) devices. Power MOSFET manufacturing

technologies are continually improving, with newer and

betterperformancedevicesbeingintroducedalmostyearly.

Output Voltage Programming

The output voltage is set by a resistor divider according

to the following formula:

VO =1.230V • 1+

⎛

⎝⎜

⎞

⎠⎟

The external resistor divider is connected to the output

as shown in Figure 1, allowing remote voltage sensing.

The resistors R1 and R2 are typically chosen so that the

error caused by the current flowing into the FB pin dur-

ing normal operation is less than 1% (this translates to a

maximum value of R1 of about 250k).

Programming Turn-On and Turn-Off Thresholds with

the RUN Pin

The LTC1871X contains an independent, micropower

voltage reference and comparator detection circuit that

remains active even when the device is shut down, as

showninFigure8.Thisallowsuserstoaccuratelyprogram

an input voltage at which the converter will turn on and

off. The falling threshold voltage on the RUN pin is equal

to the internal reference voltage of 1.248V. The compara-

tor has 100mV of hysteresis to increase noise immunity.

The turn-on and turn-off input voltage thresholds are

programmed using a resistor divider according to the

following formulas:

VIN(OFF) =1.248V • 1+

⎛

⎝⎜

⎞

⎠⎟

VIN(ON) =1.348V • 1+

⎛

⎝⎜

⎞

⎠⎟

The resistor R1 is typically chosen to be less than 1M.

For applications where the RUN pin is only to be used

as a logic input, the user should be aware of the 7V

Absolute Maximum Rating for this pin! The RUN pin can

be connected to the input voltage through an external 1M

resistor, as shown in Figure 8c, for “always on” operation.

Application Circuits

AbasicLTC1871XapplicationcircuitisshowninFigure1.

The circuit in Figure 1 was tested at 175°C to verify circuit

operation. External component selection is driven by

the characteristics of the load and the input supply. The

first topology to be analyzed will be the boost converter,

followed by SEPIC (single ended primary inductance

converter).

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