LT1185(Rev_D) Просмотр технического описания (PDF) - Linear Technology

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LT1185
(Rev.:Rev_D)

Low Dropout Regulator

Linear Technology 

LT1185

UU W U

APPLICATIO S I FOR ATIO

factor is that local ambient temperature may be somewhat

higher because of the point source of heat. The conse-

quences of excess junction temperature include poor

reliability, especially for plastic packages, and the possi-

bility of thermal shutdown or degraded electrical charac-

teristics. The final design should be checked in situ with a

thermocouple attached to the regulator case under worst-

case conditions of high ambient, high input voltage and

full load.

What About Overloads?

IC regulators with thermal shutdown, like the LT1185,

allow heat sink designs which concentrate on worst-case

“normal” conditions and ignore “fault” conditions. An

output overload or short may force the regulator to exceed

its maximum junction temperature rating, but thermal

shutdown is designed to prevent regulator failure under

these conditions. A word of caution however; thermal

shutdown temperatures are typically 175°C in the control

portion of the die and 180°C to 225°C in the power

transistor section. Extended operation at these tempera-

tures can cause permanent degradation of plastic encap-

sulation. Designs which may be subjected to extended

periods of overload should either use the hermetic TO-3

package or increase heat sink size. Foldback current

limiting can be implemented to minimize power levels

under fault conditions.

External Current Limit

The LT1185 requires a resistor to set current limit. The

value of this resistor is 15k divided by the desired current

limit (in amps). The resistor for 2A current limit would be

15k/2A = 7.5k. Tolerance over temperature is ±10%, so

current limit is normally set 15% above maximum load

current. Foldback limiting can be employed if short-circuit

current must be lower than full load current (see Typical

Applications).

The LT1185 has internal current limiting which will over-

ride external current limit if power in the pass transistor is

excessive. The internal limit is ≈ 3.6A with a foldback

characteristic which is dependent on input-output voltage,

not output voltage per se (see Typical Performace Charac-

teristics).

Ground Pin Current

Ground pin current for the LT1185 is approximately 2mA

plus IOUT/40. At IOUT = 3A, ground pin current is typically

2mA + 3/40 = 77mA. Worst case guarantees on the ratio of

IOUT to ground pin current are contained in the Electrical

Specifications.

Ground pin current can be important for two reasons. It

adds to power dissipation in the regulator and it can affect

load/line regulation if a long line is run from the ground pin

to load ground. The additional power dissipation is found

by multiplying ground pin current by input voltage. In a

typical example, with VIN = 8V, VOUT = 5V and IOUT = 2A, the

LT1185 will dissipate (8V – 5V)(2A) = 6W in the pass

transistor and (2A/40)(8V) = 0.4W in the internal drive

circuitry. This is only a 1.5% efficiency loss, and a 6.7%

increase in regulator power dissipation, but these values

will increase at higher output voltages.

Ground pin current can affect regulation as shown in

Figure 2. Parasitic resistance in the ground pin lead will

create a voltage drop which increases output voltage as

load current is increased. Similarly, output voltage can

decrease as input voltage increases because the “IOUT/40”

component of ground pin current drops significantly at

higher input-output differentials. These effects are small

enough to be ignored for local regulation applications, but

for remote sense applications, they may need to be consid-

ered. Ground lead resistance of 0.4Ω would cause an

output voltage error of up to (3A/40)(0.4Ω) = 30mV, or

0.6% at VOUT = 5V. Note that if the sense leads are

connected as shown in Figure 2, with ra ≈ 0Ω, this error is

a fixed number of millivolts, and does not increase as a

function of DC output voltage.

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