LP38500ASD-ADJSLASHNOPB datasheet(13/23 Pages) TI

13 / 23 page

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 RDS(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 (PD(MAX)) of the LP38500/2-ADJ is limited by the maximum junction temperature

of 125°C, along with the maximum ambient temperature (TA(MAX)) of the application, and the thermal resistance

(

θJA) of the package. Under all possible conditions, the junction temperature (TJ) must be within the range

specified in the Operating Ratings. The total power dissipation of the device is given by:

PD = ((VIN − VOUT) x IOUT) + (VIN x IGND)

where

•

IGND is the operating ground current of the device (specified under Electrical Characteristics)

(3)

The maximum allowable junction temperature rise (

ΔTJ) depends on the maximum expected ambient

temperature (TA(MAX)) of the application, and the maximum allowable junction temperature (TJ(MAX)):

ΔTJ = TJ(MAX)− TA(MAX)

(4)

The maximum allowable value for junction to ambient Thermal Resistance,

θJA, can be calculated using the

formula:

θJA = ΔTJ / PD(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

θJA 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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