10

LT1175

= Maximum ambient temperature

commercial and industrial grades)

= Maximum continuous input voltage at maximum

load current

LT1175S8 is 125

°C. Thermal resistance from Table 2 is

found to be 80

°C/W.

Die Temperature = 60 + 80 (0.2A)(8 – 5) = 108

°C

Maximum

W

V

Power Dissipation =

125 – 60

80

125 – 60

=

=

()

+=

081

80 0 2

59

.

.

Output Voltage Reversal

The LT1175 is designed to tolerate an output voltage

reversal of up to 2V. Reversal might occur, for instance, if

the output was shorted to a positive 5V supply. This would

almost surely destroy IC devices connected to the negative

output. Reversal could also occur during start-up if the

positive supply came up first and loads were connected

between the positive and negative supplies.

For these

reasons, it is always good design practice to add a reverse

biased diode from each regulator output to ground to limit

output voltage reversal. The diode should be rated to

handle full negative load current for start-up situations, or

the short-circuit current of the positive supply if supply-to-

supply shorts must be tolerated.

Input Voltage Lower Than Output

Linear Technology’s positive low dropout regulators

LT1121 and LT1129, will not draw large currents if the

input voltage is less than the output. These devices use a

lateral PNP power transistor structure that has 40V emitter

base breakdown voltage.

The LT1175, however, uses an

NPN power transistor structure that has a parasitic diode

between the input and output of the regulator. Reverse

voltages between input and output above 1V will damage

the regulator if large currents are allowed to flow. Simply

disconnecting the input source with the output held up will

not cause damage even though the input-to-output volt-

age will become slightly reversed.

High Frequency Ripple Rejection

The LT1175 will sometimes be powered from switching

regulators that generate the unregulated or quasi-regu-

lated input voltage. This voltage will contain high fre-

quency ripple that must be rejected by the linear regulator.

Special care was taken with the LT1175 to maximize high

frequency ripple rejection, but as with any micropower

design, rejection is strongly affected by ripple frequency.

The graph in the Typical Performance Characteristics

section shows 60dB rejection at 1kHz, but only 15dB

rejection at 100kHz for the 5V part. Photographs in Figures

4a and 4b show actual output ripple waveforms with

square wave and triwave input ripple.

Maximum Continuous

Input Voltage

(for Thermal Considerations)

5

µs/DIV

1175 F04

Figure 4a.

2

µs/DIV

1175 F04

Figure 4b.

INPUT

RIPPLE

100mV/DIV

OUTPUT

100mV/DIV

f = 100kHz

f = 50kHz

OUTPUT

20mV/DIV

INPUT

RIPPLE

100mV/DIV