LT1175 查看數據表（PDF） - Linear Technology

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LT1175

500mA Negative Low Dropout Micropower Regulator

Linear Technology 

LT1175

Applications Information

TA = Maximum ambient temperature

TMAX = Maximum LT1175 die temperature (125°C for

commercial and industrial grades)

θJA = LT1175 thermal resistance, junction to ambient

VIN = Maximum continuous input voltage at maximum

load current

ILOAD = Maximum load current

Example: LT1175S8 with ILOAD = 200mA, VOUT = 5V,

VIN = 7V, TA = 60°C. Maximum die temperature for the

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 Power Dissipation = 125 – 60 = 0.81W

80

( ) Maximum Continuous

Input Voltage

= 125 – 60 + 5 = 9V

(for Thermal Considerations) 80 0.2

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

voltage will become slightly reversed.

High Frequency Ripple Rejection

The LT1175 will sometimes be powered from switching

regulators that generate the unregulated or quasi-regulated

input voltage. This voltage will contain high frequency 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.

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

OUTPUT

20mV/DIV

INPUT

RIPPLE

100mV/DIV

OUTPUT

100mV/DIV

INPUT

RIPPLE

100mV/DIV

12

COUT = 4.7µF TANT

COUT = 1µF TANT

f = 50kHz

5µs/DIV

Figure 4a.

1175 F04a

COUT = 4.7µF TANT

COUT = 1µF TANT

f = 100kHz

2µs/DIV

Figure 4b.

1175 F04b

1175fe

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