Datasheet

LP3875-ADJ
SNVS247D –SEPTEMBER 2003–REVISED APRIL 2013 www.ti.com
SHORT-CIRCUIT PROTECTION
The LP3875-ADJ is short circuit protected and in the event of a peak over-current condition, the short-circuit
control loop will rapidly drive the output PMOS pass element off. Once the power pass element shuts down, the
control loop will rapidly cycle the output on and off until the average power dissipation causes the thermal
shutdown circuit to respond to servo the on/off cycling to a lower frequency. Please refer to the section on
thermal information for power dissipation calculations.
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 voltage is the product of the load current and
the Rds(on) of the internal MOSFET.
REVERSE CURRENT PATH
The internal MOSFET in the LP3875-ADJ has an inherent parasitic diode. During normal operation, the input
voltage 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
The LP3875-ADJ can deliver a continuous current of 1.5A 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
D
= (V
IN
−V
OUT
)I
OUT
+ (V
IN
)I
GND
(3)
where I
GND
is the operating ground current of the device (specified under Electrical Characteristics).
The maximum allowable temperature rise (T
Rmax
) depends on the maximum ambient temperature (T
Amax
) of the
application, and the maximum allowable junction temperature (T
Jmax
):
T
Rmax
= T
Jmax
− T
Amax
(4)
The maximum allowable value for junction to ambient Thermal Resistance, θ
JA
, can be calculated using the
formula:
θ
JA
= T
Rmax
/ P
D
(5)
The LP3875-ADJ is available in TO-220 and DDPAK/DDPAK/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
JA
calculated above
is ≥ 60 °C/W for TO-220 package and ≥ 60 °C/W for DDPAK/TO-263 package no heatsink is needed since the
package can dissipate enough heat to satisfy these requirements. If the value for allowable θ
JA
falls below these
limits, a heat sink is required.
HEATSINKING TO-220 PACKAGE
The thermal resistance of a TO-220 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
JA
will be same as shown in next section for
DDPAK/TO-263 package.
The heatsink to be used in the application should have a heatsink to ambient thermal resistance,
θ
HA
≤ θ
JA
− θ
CH
− θ
JC
. (6)
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