参数资料
| 型号： | LT1185IT#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 8/16页 |
| 文件大小： | 0K |
| 描述： | IC REG LDO NEG ADJ 3A TO220 |
| 产品培训模块： | More Information on LDOs |
| 标准包装： | 50 |
| 稳压器拓扑结构： | 负，可调式 |
| 输出电压： | -2.5 V ~ -25 V |
| 输入电压： | -4.2 V ~ -35 V |
| 电压 - 压降（标准）： | 0.67V @ 3A |
| 稳压器数量： | 1 |
| 电流 - 输出： | 3A |
| 电流 - 限制（最小）： | 可调式 |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 通孔 |
| 封装/外壳： | TO-220-5 成形引线 |
| 供应商设备封装： | TO-220-5 |
| 包装： | 管件 |
| 产品目录页面： | 1326 (CN2011-ZH PDF) |
LT1185
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.
Ground Pin Current
Ground pin current for the LT1185 is approximately 2mA
plus I OUT /40. At I OUT = 3A, ground pin current is typically
2mA + 3/40 = 77mA. Worst case guarantees on the ratio of
I OUT 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 V IN = 8V, V OUT = 5V and I OUT = 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 “I OUT /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
External Current Limit
The LT1185 requires a resistor to set current limit. The
+
PARASITIC
LEAD RESISTANCES
– r b +
r a
+
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
V IN
–
V IN
R LIM
REF
LT1185
GND
FB
I GND
R1*
2.37k
R2
LOAD
V OUT
current must be lower than full load current (see Typical
Applications).
V OUT
–
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 volt-
age, not output voltage per se (see Typical Performace
Characteristics).
LT1185 ? F02
*R1 SHOULD BE CONNECTED DIRECTLY TO GROUND LEAD, NOT TO THE LOAD,
SO THAT r a ≈ 0 ? . THIS LIMITS THE OUTPUT VOLTAGE ERROR TO (I GND )(r b ).
ERRORS CREATED BY r a ARE MULTIPLIED BY (1 + R2/R1). NOTE THAT V OUT
INCREASES WITH INCREASING GROUND PIN CURRENT. R2 SHOULD BE CONNECTED
DIRECTLY TO LOAD FOR REMOTE SENSING
Figure 2. Proper Connection of Positive Sense Lead
1185ff
8
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