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LTC1871

1871fe

APPLICATIONS INFORMATION

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 = 5V and

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

and the maximum ambient temperature is 70°C. The power

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 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-greatest low QG, low

RDS(ON) devices. Power MOSFET manufacturing tech-

nologies 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+

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 LTC1871 contains an independent, micropower voltage

reference and comparator detection circuit that remains

active even when the device is shut down, as shown in

Figure 8. This allows users to accurately program 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 comparator 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 con-

nected to the input voltage through an external 1M resistor,

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

Application Circuits

A basic LTC1871 application circuit is shown in Figure 1.

External component selection is driven by the character-

istics 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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