11

LT1521/LT1521-3

LT1521-3.3/LT1521-5

is required to prevent oscillations. The LT1521 is a

micropower device and output transient response will be

a function of output capacitance. See the Transient

Response curves in the Typical Performance Characteris-

tics. Larger values of output capacitance will decrease the

peak deviations and provide improved output transient

response for larger load current deltas. Bypass capaci-

tors, used to decouple individual components powered by

the LT1521, will increase the effective value of the output

capacitor.

Protection Features

The LT1521 incorporates several protection features which

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

addition to the normal protection features associated with

monolithic regulators, such as current limiting and ther-

mal limiting, the device is protected against reverse input

voltages, reverse output voltages and reverse voltages

from output to input.

Current limit protection and thermal overload protection

are intended to protect the device against current overload

conditions at the output of the device. For normal opera-

tion, the junction temperatures should not exceed 125

°C.

The input of the device will withstand reverse voltages of

20V. Current flow into the device will be limited to less

than 1mA (typically less than 100

µA) and no negative

voltage will appear at the output. The device will protect

both itself and the load. This provides protection against

batteries that can be plugged in backward.

For fixed voltage versions of the device, the output can be

pulled below ground without damaging the device. If the

input is left open circuit or grounded, the output can be

pulled below ground by 20V. The output will act like an

open circuit, no current will flow out of the pin. If the input

is powered by voltage source, the output will source the

short-circuit current of the device and will protect itself by

thermal limiting. For the adjustable version of the device,

the output pin is internally clamped at one diode drop

below ground. Reverse current for the adjustable device

must be limited to 5mA.

Table 3. SOT-223 Package

(Thermal Resistance Junction-to-Tab 20

°C/W)

COPPER AREA

THERMAL RESISTANCE

TOPSIDE*

BACKSIDE

BOARD AREA

(JUNCTION-TO-AMBIENT)

50

°C/W

50

°C/W

58

°C/W

64

°C/W

57

°C/W

0

60

°C/W

* Tab of device attached to topside copper.

Calculating Junction Temperature

Example: Given an output voltage of 3.3V, an input voltage

range of 4.5V to 7V, an output current range of 0mA to

150mA and a maximum ambient temperature of 50

°C,

what will the maximum junction temperature be?

The power dissipated by the device will be equal to:

Where,

So,

P = 150mA(7V – 3.3V) + (2.1mA)(7V) = 0.57W

If we use a SOT-223 package, then the thermal resistance

will be in the range of 50

°C/W to 65°C/W depending on the

copper area. So the junction temperature rise above

ambient will be approximately equal to:

0.57W(60

°C/W) = 34.2°C

The maximum junction temperature will then be equal to the

maximum junction temperature rise above ambient plus the

maximum ambient temperature or:

Output Capacitance and Transient Performance

The LT1521 is designed to be stable with a wide range of

output capacitors. A minimum output capacitor of 1.5

µF