LT1129IST-3.3TRPBF Просмотр технического описания (PDF) - Linear Technology

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LT1129IST-3.3TRPBF

Micropower Low Dropout Regulators with Shutdown

Linear Technology 

LT1129/LT1129-3.3/LT1129-5

APPLICATIONS INFORMATION

Adjustable Operation

The adjustable version of the LT1129 has an output voltage

range of 3.75V to 30V. The output voltage is set by the

ratio of two external resistors as shown in Figure 2. The

device servos the output voltage to maintain the voltage

at the adjust pin at 3.75V. The current in R1 is then equal

to 3.75V/R1. The current in R2 is equal to the sum of the

current in R1 and the adjust pin bias current. The adjust

pin bias current, 150nA at 25°C, flows through R2 into the

adjust pin. The output voltage can be calculated according

to the formula in Figure 2. The value of R1 should be less

than 400k to minimize errors in the output voltage caused

by the adjust pin bias current. Note that in shutdown the

output is turned off and the divider current will be zero.

Curves of Adjust Pin Voltage vs Temperature and Adjust

Pin Bias Current vs Temperature appear in the Typical

Performance Characteristics. The reference voltage at

the adjust pin has a positive temperature coefficient of

approximately 15ppm/°C. The adjust pin bias current has

a negative temperature coefficient. These effects are small

and will tend to cancel each other.

The adjustable device is specified with the adjust pin tied

to the output pin. This sets the output voltage to 3.75V.

Specifications for output voltages greater than 3.75V will

be proportional to the ratio of the desired output voltage

to 3.75V (VOUT/3.75V). For example: load regulation for an

output current change of 1mA to 700mA is – 6mV typical at

VOUT = 3.75V. At VOUT = 12V, load regulation would be:

12V 

 3.75V 

• (–6mV) = (–19mV)

IN

OUT

LT1129

SHDN

ADJ

GND

VOUT

R2 +

R1

( ) ( ) VOUT

=

3.75V

1

+

R2

R1

+

IADJ • R2

VADJ = 3.75V

IADJ = 150nA at 25°C

OUTPUT RANGE = 3.75V to 30V

112935 F02

Figure 2. Adjustable Operation

10

Thermal Considerations

The power handling capability of the device will be limited

by the maximum rated junction temperature (125°C). 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. Ground pin current multiplied by the input voltage:

IGND • VIN.

The ground pin current can be found by examining the

Ground Pin Current curves in the Typical Performance

Characteristics. Power dissipation will be equal to the

sum of the two components listed above.

The LT1129 series regulators have internal thermal

limiting designed to protect the device during overload

conditions. For continuous normal load conditions the

maximum junction temperature rating of 125°C must not

be exceeded. It is important to give careful consideration

to all sources of thermal resistance from junction to ambi-

ent. Additional heat sources mounted nearby must also be

considered.

For surface mount devices heat sinking is accomplished

by using the heat spreading capabilities of the PC board

and its copper traces. Experiments have shown that the

heat spreading copper layer does not need to be electri-

cally connected to the tab of the device. The PC material

can be very effective at transmitting heat between the pad

area, attached to the tab of the device, and a ground or

power plane layer either inside or on the opposite side of

the board. Although the actual thermal resistance of the

PC material is high, the length/area ratio of the thermal

resistor between layers is small. Copper board stiffeners

and plated through holes can also be used to spread the

heat generated by power devices.

The following tables list thermal resistances for each

package. For the TO-220 package, thermal resistance

is given for junction-to-case only since this package

is usually mounted to a heat sink. Measured values of

thermal resistance for several different board sizes and

copper areas are listed for each package. All measure-

ments were taken in still air on 3/32" FR-4 board with 1-oz

112935ff

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