LP3856ESX-ADJ/NOPB National Semiconductor, LP3856ESX-ADJ/NOPB Datasheet - Page 11

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

Specifications of LP3856ESX-ADJ/NOPB

• Regulator Topology: Positive Fixed
• Voltage - Output: Adjustable
• Voltage - Input: 2.5 ~ 7 V
• 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-ADJ

If the maximum load current is 2A and a 10µF ceramic input
capacitor is used, the regulator will be stable with ceramic
output capacitor values from 10µF up to about 50µF. At 3A of
load current, the ratio of input to output capacitance required
approaches 1:1, meaning that whatever amount of ceramic
output capacitance is used must also be provided at the input
for stable operation. For load currents between 1A, 2A, and
3A, interpolation may be used to approximate values on the
graph. When calculating the total ceramic output capacitance
present in an application, it is necessary to include any ce-
ramic bypass capacitors connected to the regulator output.
C
The capacitor C
prove loop compensation. The correct amount of capacitance
depends on the value selected for R1 (see Typical Application
Circuit). The capacitor should be selected such that the zero
frequency as given by the equation shown below is approxi-
mately 45 kHz:
A good quality ceramic with X5R or X7R dielectric should be
used for this capacitor.
SELECTING A CAPACITOR
It is important to note that capacitance tolerance and variation
with temperature must be taken into consideration when se-
lecting a capacitor so that the minimum required amount of
capacitance is provided over the full operating temperature
range. In general, a good Tantalum capacitor will show very
little capacitance variation with temperature, but a ceramic
may not be as good (depending on dielectric type). Aluminum
electrolytics also typically have large temperature variation of
capacitance value.
Equally important to consider is a capacitor's ESR change
with temperature: this is not an issue with ceramics, as their
ESR is extremely low. However, it is very important in Tanta-
lum and aluminum electrolytic capacitors. Both show increas-
ing ESR at colder temperatures, but the increase in aluminum
electrolytic capacitors is so severe they may not be feasible
for some applications (see Capacitor Characteristics Sec-
tion).
CAPACITOR CHARACTERISTICS
CERAMIC: For values of capacitance in the 10 to 100 µF
range, ceramics are usually larger and more costly than tan-
talums but give superior AC performance for bypassing high
frequency noise because of very low ESR (typically less than
10 mΩ). However, some dielectric types do not have good
capacitance characteristics as a function of voltage and tem-
perature.
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 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.
FF
(Feed Forward Capacitor)
Fz = 45,000 = 1 / ( 2 x
FF
is required to add phase lead and help im-
π
x R1 x C
FF
)
11
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.
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 LP3856-ADJ using
traces which do not have other currents flowing in them
(Kelvin connect).
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".
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
OUT
, Kelvin connect the capacitor leads to these
IN
, V
OUT
, and ground pins. The
IN
and C
OUT
www.national.com
near the
IN
and