12

LT3020/LT3020-1.2/

LT3020-1.5/LT3020-1.8

3020fc

Table 2. Measured Thermal Resistance for MS8 Package

COPPER AREA

THERMAL RESISTANCE

TOPSIDE*

BACKSIDE

BOARD AREA

(JUNCTION-TO-AMBIENT)

2500mm

2

2500mm

2

2500mm

2

110

°C/W

1000mm

2

2500mm

2

2500mm

2

115

°C/W

225mm

2

2500mm

2

2500mm

2

120

°C/W

100mm

2

2500mm

2

2500mm

2

130

°C/W

50mm

2

2500mm

2

2500mm

2

140

°C/W

*Device is mounted on topside.

Calculating Junction Temperature

Example: Given an output voltage of 1.8V, an input voltage

range of 2.25V to 2.75V, an output current range of 1mA

to 100mA, and a maximum ambient temperature of 70

°C,

what will the maximum junction temperature be for an

application using the DD package?

The power dissipated by the device is equal to:

where

so

P = 100mA(2.75V – 1.8V) + 3mA(2.75V) = 0.103W

The thermal resistance is in the range of 35

°C/W to

70

°C/W depending on the copper area. So the junction

temperature rise above ambient is approximately equal to:

0.103W(52.5

°C/W) = 5.4°C

The maximum junction temperature equals the maximum

junction temperature rise above ambient plus the maxi-

mum ambient temperature or:

Protection Features

The LT3020 incorporates several protection features that

make it ideal for use in battery-powered circuits. In addi-

tion to the normal protection features associated with

monolithic regulators, such as current limiting and ther-

mal limiting, the device also protects against reverse-

input voltages, reverse-output voltages and reverse

output-to-input voltages.

APPLICATIO S I FOR ATIO

Current limit protection and thermal overload protection

protect the device against current overload conditions at

the output of the device. For normal operation, do not

exceed a junction temperature of 125

°C.

The IN pins of the device withstand reverse voltages of

10V. The LT3020 limits current flow to less than 1

µA and

no negative voltage appears at OUT. The device protects

both itself and the load against batteries that are plugged

in backwards.

The LT3020 incurs no damage if OUT is pulled below

ground. If IN is left open circuit or grounded, OUT can be

pulled below ground by 10V. No current flows from the

pass transistor connected to OUT. However, current flows

in (but is limited by) the resistor divider that sets the output

voltage. Current flows from the bottom resistor in the

divider and from the ADJ pin’s internal clamp through the

top resistor in the divider to the external circuitry pulling

OUT below ground. If IN is powered by a voltage source,

OUT sources current equal to its current limit capability

and the LT3020 protects itself by thermal limiting. In this

case, grounding SHDN turns off the LT3020 and stops

OUT from sourcing current.

The LT3020 incurs no damage if the ADJ pin is pulled

above or below ground by 10V. If IN is left open circuit or

grounded and ADJ is pulled above ground, ADJ acts like a

25k resistor in series with a 1V clamp (one Schottky diode

in series with one diode). ADJ acts like a 25k resistor in

series with a Schottky diode if pulled below ground. If IN

is powered by a voltage source and ADJ is pulled below its

reference voltage, the LT3020 attempts to source its

current limit capability at OUT. The output voltage in-

load current the LT3020 supports. This condition can

potentially damage external circuitry powered by the

LT3020 if the output voltage increases to an unregulated

high voltage. If IN is powered by a voltage source and ADJ

is pulled above its reference voltage, two situations can

occur. If ADJ is pulled slightly above its reference voltage,

the LT3020 turns off the pass transistor, no output current

is sourced and the output voltage decreases to either the

voltage at ADJ or less. If ADJ is pulled above its no load

recovery threshold, the no load recovery circuitry turns on

and attempts to sink current. OUT is actively pulled low