lp3856et-2.5 National Semiconductor Corporation, lp3856et-2.5 Datasheet - Page 14

• Manufacturer Part Number: lp3856et-2.5
• Description: 3a Fast Response Ultra Low Dropout Linear Regulators
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Hints
remote load will provide regulation at the remote load, as
SHUTDOWN OPERATION
A CMOS Logic level signal at the shutdown ( SD) pin will
turn-off the regulator. Pin SD must be actively terminated
through a 10kΩ pull-up resistor for a proper operation. If this
pin is driven from a source that actively pulls high and low
(such as a CMOS rail to rail comparator), the pull-up resistor
is not required. This pin must be tied to Vin if not used.
DROPOUT VOLTAGE
The dropout voltage of a regulator is defined as the minimum
input-to-output differential required to stay within 2% of the
nominal output voltage. For CMOS LDOs, the dropout volt-
age is the product of the load current and the Rds(on) of the
internal MOSFET.
REVERSE CURRENT PATH
The internal MOSFET in LP3853 and LP3856 has an inher-
ent parasitic diode. During normal operation, the input volt-
age is higher than the output voltage and the parasitic diode
is reverse biased. However, if the output is pulled above the
input in an application, then current flows from the output to
the input as the parasitic diode gets forward biased. The
output can be pulled above the input as long as the current
in the parasitic diode is limited to 200mA continuous and 1A
peak.
POWER DISSIPATION/HEATSINKING
LP3853 and LP3856 can deliver a continuous current of 3A
over the full operating temperature range. A heatsink may be
required depending on the maximum power dissipation and
maximum ambient temperature of the application. Under all
possible conditions, the junction temperature must be within
the range specified under operating conditions. The total
power dissipation of the device is given by:
P
where I
(specified under Electrical Characteristics).
D
= (V
GND
IN
−V
OUT
is the operating ground current of the device
)I
OUT
+ (V
IN
)I
GND
(Continued)
FIGURE 4. Improving remote load regulation using LP3856
14
shown in Figure 4. If the sense option pin is not required, the
sense pin must be connected to the V
The maximum allowable temperature rise (T
on the maximum ambient temperature (T
cation, and the maximum allowable junction temperature
(T
T
The maximum allowable value for junction to ambient Ther-
mal Resistance, θ
θ
LP3853 and LP3856 are available in TO-220 and TO-263
packages. The thermal resistance depends on amount of
copper area or heat sink, and on air flow. If the maximum
allowable value of θ
TO-220 package and ≥ 60 ˚C/W for TO-263 package no
heatsink is needed since the package can dissipate enough
heat to satisfy these requirements. If the value for allowable
θ
HEATSINKING TO-220 PACKAGE
The thermal resistance of a TO220 package can be reduced
by attaching it to a heat sink or a copper plane on a PC
board. If a copper plane is to be used, the values of θ
be same as shown in next section for TO263 package.
The heatsink to be used in the application should have a
heatsink to ambient thermal resistance,
θ
In this equation, θ
to the surface of the heat sink and θ
tance from the junction to the surface of the case. θ
about 3˚C/W for a TO220 package. The value for θ
pends on method of attachment, insulator, etc. θ
between 1.5˚C/W to 2.5˚C/W. If the exact value is unknown,
2˚C/W can be assumed.
JA
JA
HA
Rmax
Jmax
≤ θ
= T
falls below these limits, a heat sink is required.
):
JA
= T
Rmax
− θ
Jmax
CH
/ P
− T
D
− θ
CH
Amax
JA
JC
, can be calculated using the formula:
JA
is the thermal resistance from the case
.
calculated above is ≥ 60 ˚C/W for
JC
20030908
OUT
is the thermal resis-
Amax
pin.
Rmax
) of the appli-
) depends
CH
CH
JA
varies
JC
de-
will
is