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LT3011

3011f

During normal operation, an internal glitch ﬁlter will ignore

short transients (<15μs). Longer transients below the 90%

threshold will reset the JK ﬂip-ﬂop. This ﬂip-ﬂop ensures

that the capacitor on the CT pin is quickly discharged all

the way to the VCT(LOW) threshold before restarting the

time delay. This provides a consistent time delay after the

ADJ pin is within 10% of the regulated voltage before the

PWRGD pin switches to high impedance.

Thermal Considerations

The power handling capability of the device will be limited by

the maximum rated junction temperature (125°C, LT3011E/

LT3011I or 150°C, LT3011H). The power dissipated by the

device will be made up of two components:

1. Output current multiplied by the input/output voltage

differential: IOUT • (VIN – VOUT) and,

2. GND pin current multiplied by the input voltage:

IGND • VIN

The GND pin current is found by examining the GND pin

current curves in the Typical Performance Characteristics

section. Power dissipation will be equal to the sum of the

two components listed above.

The LT3011 series regulators have internal thermal limiting

designed to protect the device during overload conditions.

For continuous normal conditions, the maximum junction

temperature rating of 125°C (LT3011E/ LT3011I) or 150°C

(LT3011H) must not be exceeded. It is important to give

careful consideration to all sources of thermal resistance

from junction to ambient. Additional heat sources mounted

nearby must also be considered.

VREF • 90%

ADJ

VCT(LOW)

0.1V

VCT(HIGH)

– VBE

( 1.1V)

ICT 3μA

3011 F04

–

PWRGD

For surface mount devices, heat sinking is accomplished

by using the heat spreading capabilities of the PC board

and its copper traces. Copper board stiffeners and plated

through-holes can also be used to spread the heat gener-

ated by power devices.

The following table lists thermal resistance for several

different board sizes and copper areas. All measurements

were taken in still air on 3/32" FR-4 board with one ounce

copper.

Table 1. MSOP Measured Thermal Resistance

COPPER AREA

BOARD AREA

THERMAL RESISTANCE

(JUNCTION-TO-AMBIENT)

TOPSIDE

BACKSIDE

2500 sq mm 2500 sq mm

2500 sq mm

52°C/W

1000 sq mm 2500 sq mm

2500 sq mm

54°C/W

225 sq mm

2500 sq mm

58°C/W

100 sq mm

2500 sq mm

64°C/W

Table 2. DFN Measured Thermal Resistance

COPPER AREA

BOARD AREA

THERMAL RESISTANCE

(JUNCTION-TO-AMBIENT)

TOPSIDE

BACKSIDE

2500 sq mm 2500 sq mm

2500 sq mm

52°C/W

1000 sq mm 2500 sq mm

2500 sq mm

54°C/W

225 sq mm

2500 sq mm

58°C/W

100 sq mm

2500 sq mm

64°C/W

The thermal resistance junction-to-case (

θJC), measured

at the Exposed Pad on the back of the die, is 16°C/W.

Continuous operation at large input/output voltage dif-

ferentials and maximum load current is not practical due

to thermal limitations. Transient operation at high input/

output differentials is possible. The approximate thermal

time-constant for a 2500sq mm 3/32" FR-4 board, with

maximum topside and backside area for one ounce cop-

per, is three seconds. This time-constant will increase as

more thermal mass is added (i.e., vias, larger board and

other components).

For an application with transient high power peaks, average

power dissipation can be used for junction temperature

calculations as long as the pulse period is signiﬁcantly less

than the thermal time constant of the device and board.

Figure 4. PWRGD Circuit Block Diagram

APPLICATIONS INFORMATION

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