LP3855-ADJ_15 Datasheet, PDF (10/22 Pages) Texas Instruments – 1.5A Fast Response Ultra Low Dropout Linear Regulators

LP3855-ADJ
SNVS244F – SEPTEMBER 2003 – REVISED APRIL 2013
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Z5U and Y5V dielectric ceramics have capacitance that drops severely with applied voltage. A typical Z5U or
Y5V capacitor can lose 60% of its rated capacitance with half of the rated voltage applied to it. The Z5U and Y5V
also exhibit a severe temperature effect, losing more than 50% of nominal capacitance at high and low limits of
the temperature range.
X7R and X5R dielectric ceramic capacitors are strongly recommended if ceramics are used, as they typically
maintain a capacitance range within ±20% of nominal over full operating ratings of temperature and voltage. Of
course, they are typically larger and more costly than Z5U/Y5U types for a given voltage and capacitance.
TANTALUM: Solid Tantalum capacitors are typically recommended for use on the output because their ESR is
very close to the ideal value required for loop compensation.
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 LP385X. 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.
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 LP385X
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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