LT1175 查看數據表（PDF） - Linear Technology

零件编号

产品描述 (功能)

比赛名单

LT1175

500mA Negative Low Dropout Micropower Regulator

Linear Technology 

LT1175

Applications Information

capacitors do not fail during a “shorting out” surge, only

during a “charge up” surge.

The output capacitor should be located within several

inches of the regulator. If remote sensing is used, the output

capacitor can be located at the remote sense node, but the

GND pin of the regulator should also be connected to the

remote site. The basic rule is to keep SENSE and GND pins

close to the output capacitor, regardless of where it is.

Operating at very large input-to-output differential volt-

ages (>15V) with load currents less than 5mA requires an

output capacitor with an ESR greater than 1Ω to prevent

low level output oscillations.

Input Capacitor

The LT1175 requires a separate input bypass capacitor

only if the regulator is located more than six inches from

the raw supply output capacitor. A 1µF or larger tantalum

capacitor is suggested for all applications, but if low ESR

capacitors such as ceramic or film are used for the out-

put and input capacitors, the input capacitor should be

at least three times the value of the output capacitor. If a

solid tantalum or aluminum electrolytic output capacitor

is used, the input capacitor is very noncritical.

High Temperature Operation

The LT1175 is a micropower design with only 45µA qui-

escent current. This could make it perform poorly at high

temperatures (>125°C), where power transistor leakage

might exceed the output node loading current (5µA to

15µA). To avoid a condition where the output voltage

drifts uncontrolled high during a high temperature no-load

condition, the LT1175 has an active load which turns on

when the output is pulled above the nominal regulated

voltage. This load absorbs power transistor leakage and

maintains good regulation. There is one downside to this

feature, however. If the output is pulled high deliberately, as

it might be when the LT1175 is used as a backup to a slightly

higher output from a primary regulator, the LT1175 will act

as an unwanted load on the primary regulator. Because of

this, the active pull-down is deliberately “weak.” It can be

modeled as a 2k resistor in series with an internal clamp

voltage when the regulator output is being pulled high. If

a 4.8V output is pulled to 5V, for instance, the load on the

primary regulator would be (5V – 4.8V)/2kΩ = 100µA.

This also means that if the internal pass transistor leaks

50µA, the output voltage will be (50µA)(2kΩ) = 100mV

high. This condition will not occur under normal operating

conditions, but could occur immediately after an output

short circuit had overheated the chip.

Thermal Considerations

The LT1175 is available in a special 8-pin surface mount

package which has Pins 1 and 8 connected to the die attach

paddle. This reduces thermal resistance when Pins 1 and 8

are connected to expanded copper lands on the PC board.

Table 2 shows thermal resistance for various combinations

of copper lands and backside or internal planes. Table 2

also shows thermal resistance for the 5-pin DD surface

mount package and the 8-pin DIP and package.

Table 2. Package Thermal Resistance (°C/W)

LAND AREA

DIP

ST

SO

Q

Minimum

140

90

100

60

Minimum with

Backplane

110

70

80

50

1cm2 Top Plane with 100

64

75

35

Backplane

10cm2 Top Plane

80

50

60

27

with Backplane

To calculate die temperature, maximum power dissipation

or maximum input voltage, use the following formulas

with correct thermal resistance numbers from Table 2.

For through-hole TO-220 applications use θJA = 50°C/W

without a heat sink and θJA = 5°C/W + heat sink thermal

resistance when using a heat sink.

( )( ) Die Temp = TA + θJA VIN − VOUT ILOAD

Maximum Power Dissipation = TMAX − TA

θJA

( ) Maximum Input Voltage

for Thermal Considerations

= TMAX − TA

θJA ILOAD

+ VOUT

1175fe

11

Share Link: