LT1963 datasheet(20/28 Pages) LINER

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LT3089

3089f

For more information www.linear.com/LT3089

APPLICATIONS INFORMATION

The second technique for reducing power dissipation,

shown in Figure 13, uses a resistor in parallel with the

LT3089. This resistor provides a parallel path for current

flow, reducing the current flowing through the LT3089.

This technique works well if input voltage is reasonably

constant and output load current changes are small. This

technique also increases the maximum available output

current at the expense of minimum load requirements.

RP dissipates 0.85W of power. As with the first technique,

choose appropriate wattage resistors to handle and dis-

sipate the power properly. With this configuration, the

LT3089 supplies only 0.43A. Therefore, load current can

increase by 0.37A to a total output current of 1.17A while

keeping the LT3089 in its normal operating range.

Protection Features

The LT3089 incorporates several protection features ideal

for harsh industrial and automotive environments, among

otherapplications.Inadditiontonormalmonolithicregula-

torprotectionfeaturessuchascurrentlimitingandthermal

limiting, the LT3089 protects itself against reverse-input

voltages, reverse-output voltages, and large OUT-to-SET

pin voltages.

Current limit protection and thermal overload protection

protect the IC against output current overload conditions.

For normal operation, do not exceed the rated absolute

maximum junction temperature. The thermal shutdown

circuit’s temperature threshold is typically 165°C and

incorporates about 5°C of hysteresis.

TheLT3089’sINpinwithstands±40Vvoltageswithrespect

to the OUT and SET pins. Reverse current flow, if OUT is

greater than IN, is less than 1mA (typically under 100µA),

protecting the LT3089 and sensitive loads.

Clamping diodes and 400Ω limiting resistors protect the

LT3089’s SET pin relative to the OUT pin voltage. These

protection components typically only carry current under

transient overload conditions. These devices are sized to

handle ±10V differential voltages and ±25mA crosspin

current flow without concern. Relative to these applica-

tion concerns, note the following two scenarios. The first

scenario employs a noise-reducing SET pin bypass

capacitorwhileOUTisinstantaneouslyshortedtoGND.The

second scenario follows improper shutdown techniques

in which the SET pin is reset to GND quickly while OUT

is held up by a large output capacitance with light load.

3089 F13

SET

OUT

–

LT3089

50µA

RSET

VOUT

VIN

Figure 13. Reducing Power Dissipation Using a Parallel Resistor

As an example, assume: VIN = 5V, VIN(MAX) = 5.5V,

VOUT = 3.3V, VOUT(MIN) = 3.2V, IOUT(MAX) = 0.8A and

IOUT(MIN) = 0.4A. Also, assuming that RP carries no more

than 90% of IOUT(MIN) = 360mA.

Calculating RP yields:

RP =

5.5V – 3.2V

0.36A

= 6.39Ω

(5% Standard value = 6.2Ω)

The maximum total power dissipation is:

(5.5V – 3.2V) • 0.8A = 1.84W

However, the LT3089 supplies only:

0.8A –

5.5V – 3.2V

6.2Ω

= 0.43A

Therefore, the LT3089’s power dissipation is only:

PDISS = (5.5V – 3.2V) • 0.43A = 0.99W

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