LP38502SDX-ADJ Datasheet, PDF (13/23 Pages) Texas Instruments – LP38500/2-ADJ, LP38500A/2A-ADJ 1.5A FlexCap Low Dropout Linear Regulator for 2.7V to 5.5V Inputs

LP38500-ADJ, LP38502-ADJ
www.ti.com
SNVS539F – NOVEMBER 2007 – REVISED APRIL 2013
DROPOUT VOLTAGE
The dropout voltage of a regulator is defined as the input-to-output differential required by the regulator to keep
the output voltage within 2% of the nominal value. For CMOS LDOs, the dropout voltage is the product of the
load current and the R_DS(on) of the internal MOSFET pass element.
Since the output voltage is beginning to “drop out” of regulation when it drops by 2%, electrical performance of
the device will be reduced compared to the values listed in the Electrical Characteristics table for some
parameters (line and load regulation and PSRR would be affected).
REVERSE CURRENT PATH
The internal MOSFET pass element in the LP38500/2-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 200 mA continuous and 1A peak. The regulator output pin should not be taken below
ground potential. If the LP38500/2-ADJ is used in a dual-supply system where the regulator load is returned to a
negative supply, the output must be diode-clamped to ground.
POWER DISSIPATION/HEATSINKING
The maximum power dissipation (P_D(MAX)) of the LP38500/2-ADJ is limited by the maximum junction temperature
of 125°C, along with the maximum ambient temperature (T_A(MAX)) of the application, and the thermal resistance
(θ_JA) of the package. Under all possible conditions, the junction temperature (T_J) must be within the range
specified in the Operating Ratings. The total power dissipation of the device is given by:
P_D = ((V_IN − V_OUT) x I_OUT) + (V_IN x I_GND)
where
• I_GND is the operating ground current of the device (specified under Electrical Characteristics)
(3)
The maximum allowable junction temperature rise (ΔT_J) depends on the maximum expected ambient
temperature (T_A(MAX)) of the application, and the maximum allowable junction temperature (T_J(MAX)):
ΔT_J = T_J(MAX)− T_A(MAX)
(4)
The maximum allowable value for junction to ambient Thermal Resistance, θ_JA, can be calculated using the
formula:
θ_JA = ΔT_J / P_D(MAX)
(5)
The LP38500/2-ADJ is available in the DDPAK/TO-263 and WSON-8 packages. The thermal resistance depends
on the amount of copper area allocated to heat transfer.
HEATSINKING DDPAK/TO-263 and PFM PACKAGES
The DDPAK/TO-263 package and PFM package use the copper plane on the PCB as a heatsink. The DAP of
the package is soldered to the copper plane for heat sinking. Figure 26 shows a typical curve for the θ_JA of the
DDPAK/TO-263 package for different copper area sizes (the thermal performance of both DDPAK/TO-263 and
PFM are the same). The tests were done using a PCB with 1 ounce copper on top side only, with copper
patterns which were square in shape.
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