LP3856ESX-3.3/NOPB National Semiconductor, LP3856ESX-3.3/NOPB Datasheet - Page 12

• Manufacturer Part Number: LP3856ESX-3.3/NOPB
• Description: IC REG LDO 3A 3.3V TO-263-5
• Manufacturer: National Semiconductor

Specifications of LP3856ESX-3.3/NOPB

• Regulator Topology: Positive Fixed
• Voltage - Output: 3.3V
• Voltage - Input: Up to 7V
• Voltage - Dropout (typical): 0.39V @ 3A
• Number Of Regulators: 1
• Current - Output: 3A (Max)
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: TO-263-5, D²Pak (5 leads + Tab), TO-263BA
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Limit (min): -
• Other names: LP3856ESX-3.3

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voltage applied to it. The Z5U and Y5V also exhibit a severe
temperature effect, losing more than 50% of nominal capac-
itance at high and low limits of the temperature range.
X7R and X5R dielectric ceramic capacitors are strongly rec-
ommended 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 typi-
cally larger and more costly than Z5U/Y5U types for a given
voltage and capacitance.
TANTALUM: Solid Tantalum capacitors are typically recom-
mended 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 qual-
ity 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 oscil-
lations 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 fre-
quency (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 tempera-
ture.
Any applications using aluminum electrolytics should be thor-
oughly 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 pro-
grammed 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 in-
put 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 con-
nect).
The best way to do this is to lay out C
device with short traces to the V
regulator ground pin should be connected to the external cir-
cuit ground so that the regulator and its capacitors have a
"single point ground".
IN
, V
OUT
, and ground pins. The
IN
and C
OUT
near the
12
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
coming from V
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 perfor-
mance because of the small dimensions of the geometries
inside the device. In applications where circuit sources are
present which generate signals with significant high frequen-
cy 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 capac-
itors must be used at the input pin of the IC.
If a load is connected to the IC output which switches at high
speed (such as a clock), the high-frequency current pulses
required by the load must be supplied by the capacitors on
the IC output. Since the bandwidth of the regulator loop is less
than 100 kHz, the control circuitry cannot respond to load
changes above that frequency. This means the effective out-
put impedance of the IC at frequencies above 100 kHz is
determined only by the output capacitor(s).
In applications where the load is switching at high speed, the
output of the IC may need RF isolation from the load. It is
recommended that some inductance be placed between the
output capacitor and the load, and good RF bypass capacitors
be placed directly across the load.
PCB layout is also critical in high noise environments, since
RFI/EMI is easily radiated directly into PC traces. Noisy cir-
cuitry should be isolated from "clean" circuits where possible,
and grounded through a separate path. At MHz frequencies,
ground planes begin to look inductive and RFI/EMI can cause
ground bounce across the ground plane.
In multi-layer PCB applications, care should be taken in layout
so that noisy power and ground planes do not radiate directly
into adjacent layers which carry analog power and ground.
OUTPUT NOISE
Noise is specified in two ways-
Spot Noise or Output noise density is the RMS sum of all
noise sources, measured at the regulator output, at a specific
frequency (measured with a 1Hz bandwidth). This type of
noise is usually plotted on a curve as a function of frequency.
Total output Noise or Broad-band noise is the RMS sum of
spot noise over a specified bandwidth, usually several
decades of frequencies.
Attention should be paid to the units of measurement. Spot
noise is measured in units µV/
noise is measured in µV(rms).
The primary source of noise in low-dropout regulators is the
internal reference. In CMOS regulators, noise has a low fre-
quency component and a high frequency component, which
depend strongly on the silicon area and quiescent current.
Noise can be reduced in two ways: by increasing the transis-
tor area or by increasing the current drawn by the internal
OUT
, Kelvin connect the capacitor leads to these
√
Hz or nV/
√
Hz and total output
IN
and