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LTC4091

4091fa

For more information www.linear.com/LTC4091

When using the SYNC function, there is the potential for

inductor current runaway during short-circuit or overload

conditions, if the following condition is violated:

fSW(MAX) =

VD+ VOUT

tON(MIN) VD+ VIN – VSW

(

)

where VIN(MAX) is the maximum operating input voltage,

VHVOUT is the switching regulator output voltage, VD is the

catch diode drop (~0.5V), VSW is the internal switch drop

(~0.5V at max load), fSW is the synchronized switching

frequency, and tON(MIN) is the minimum switch on time

(~100ns).Obviously,withashortedoutput,itisnotdifficult

to violate this condition.

Alternate NTC Thermistors and Biasing

The LTC4091 provides temperature qualified charging if

a grounded thermistor and a bias resistor are connected

to NTC (see Figure 8). By using a bias resistor whose

value is equal to the room temperature resistance of the

thermistor (R25C) the upper and lower temperatures are

pre-programmed to approximately 50°C and 0°C, respec-

tively (assuming a Vishay Curve 2 thermistor).

APPLICATIONS INFORMATION

–

RNOM

10k

RNTC

10k

NTC

VNTC

0.1V

NTC_ENABLE

4091 F09

NTC BLOCK

TOO_COLD

TOO_HOT

0.738 • VNTC

0.29 • VNTC

–

The upper and lower temperature thresholds can be ad-

justed by either a modification of the bias resistor value

or by adding a second adjustment resistor to the circuit.

If only the bias resistor is adjusted, then either the upper

or the lower threshold can be modified but not both. The

other trip point will be determined by the characteristics

of the thermistor. Using the bias resistor in addition to an

adjustmentresistor,boththeupperandthelowertempera-

ture trip points can be independently programmed with

the constraint that the difference between the upper and

lower temperature thresholds cannot decrease. Examples

of each technique are given below.

NTC thermistors have temperature characteristics which

areindicatedonresistance-temperatureconversiontables.

The Vishay-Dale thermistor NTHS0603N02N1002J, used

in the following examples, has a nominal value of 10k

and follows the Vishay Curve 2 resistance-temperature

characteristic. The LTC4091 trip points were designed

to work with thermistors whose resistance-temperature

characteristics follow Vishay Dale’s R-T Curve 2. The

Vishay NTHS0603N02N1002J is an example of such a

thermistor. However, Vishay Dale has many thermistor

products that follow the R-T Curve 2 characteristic in a

variety of sizes. Furthermore, any thermistor whose ratio

of RCOLD to RHOT is about 7.0 will also work (Vishay Dale

R-T Curve 2 shows a ratio of 2.815 / 0.409 = 6.89).

In the explanation below, the following notation is used.

R25C = Value of the Thermistor at 25°C

RNTC|COLD = Value of Thermistor at the Cold Trip Point

RNTC|HOT = Value of the Thermistor at the Hot Trip Point

rcold = Ratio of RNTC|COLD to R25C

rHOT= Ratio of RNTC|HOT to R25C

RNOM=PrimaryThermistorBiasResistor(seeFigure9)

R1 = Optional Temperature Range Adjustment Resistor

(see Figure 9)

Figure 8. Typical NTC Thermistor Circuit

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