LP3875EMP-18 Datasheet, PDF (9/25 Pages) Texas Instruments – LP3872/LP3875 1.5A Fast Ultra Low Dropout Linear Regulators

LP3872, LP3875
www.ti.com
SNVS227F – FEBRUARY 2003 – REVISED APRIL 2013
TANTALUM: Solid Tantalum capacitors are recommended for use on the output because their typical ESR is
very close to the ideal value required for loop compensation. They also work well as input capacitors if selected
to meet the ESR requirements previously listed.
Tantalums also have good temperature stability: a good quality Tantalum will typically show a capacitance value
that varies less than 10-15% across the full temperature range of 125°C to −40°C. ESR will vary only about 2X
going from the high to low temperature limits.
The increasing ESR at lower temperatures can cause oscillations when marginal quality capacitors are used (if
the ESR of the capacitor is near the upper limit of the stability range at room temperature).
ALUMINUM: This capacitor type offers the most capacitance for the money. The disadvantages are that they are
larger in physical size, not widely available in surface mount, and have poor AC performance (especially at
higher frequencies) due to higher ESR and ESL.
Compared by size, the ESR of an aluminum electrolytic is higher than either Tantalum or ceramic, and it also
varies greatly with temperature. A typical aluminum electrolytic can exhibit an ESR increase of as much as 50X
when going from 25°C down to −40°C.
It should also be noted that many aluminum electrolytics only specify impedance at a frequency of 120 Hz, which
indicates they have poor high frequency performance. Only aluminum electrolytics that have an impedance
specified at a higher frequency (between 20 kHz and 100 kHz) should be used for the LP387X. Derating must be
applied to the manufacturer's ESR specification, since it is typically only valid at room temperature.
Any applications using aluminum electrolytics should be thoroughly tested at the lowest ambient operating
temperature where ESR is maximum.
TURN-ON CHARACTERISTICS FOR OUTPUT VOLTAGES PROGRAMMED TO 2.0V OR BELOW
As Vin increases during start-up, the regulator output will track the input until Vin reaches the minimum operating
voltage (typically about 2.2V). For output voltages programmed to 2.0V or below, the regulator output may
momentarily exceed its programmed output voltage during start up. Outputs programmed to voltages above 2.0V
are not affected by this behavior.
PCB LAYOUT
Good PC layout practices must be used or instability can be induced because of ground loops and voltage drops.
The input and output capacitors must be directly connected to the input, output, and ground pins of the regulator
using traces which do not have other currents flowing in them (Kelvin connect).
The best way to do this is to lay out C_IN and C_OUT near the device with short traces to the V_IN, V_OUT, and ground
pins. The regulator ground pin should be connected to the external circuit ground so that the regulator and its
capacitors have a "single point ground".
It should be noted that stability problems have been seen in applications where "vias" to an internal ground plane
were used at the ground points of the IC and the input and output capacitors. This was caused by varying ground
potentials at these nodes resulting from current flowing through the ground plane. Using a single point ground
technique for the regulator and it's capacitors fixed the problem.
Since high current flows through the traces going into V_IN and coming from V_OUT, Kelvin connect the capacitor
leads to these pins so there is no voltage drop in series with the input and output capacitors.
RFI/EMI SUSCEPTIBILITY
RFI (radio frequency interference) and EMI (electromagnetic interference) can degrade any integrated circuit's
performance because of the small dimensions of the geometries inside the device. In applications where circuit
sources are present which generate signals with significant high frequency energy content (> 1 MHz), care must
be taken to ensure that this does not affect the IC regulator.
If RFI/EMI noise is present on the input side of the regulator (such as applications where the input source comes
from the output of a switching regulator), good ceramic bypass capacitors must be used at the input pin of the IC.
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