LM26484SQEV National Semiconductor, LM26484SQEV Datasheet - Page 9

• Manufacturer Part Number: LM26484SQEV
• Description: BOARD EVALUATION FOR LM2684
• Manufacturer: National Semiconductor

Specifications of LM26484SQEV

• Main Purpose: DC/DC, Step Down with LDO
• Outputs And Type: 3, Non-Isolated
• Voltage - Output: 1V, 1.8V, 1V
• Current - Output: 2A, 2A, 1A
• Voltage - Input: 3 ~ 5.5V
• Regulator Topology: Buck
• Frequency - Switching: 2MHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM26484
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

LDO Functional Description
The LDO is a linear regulator which targets analog loads
characterized by low noise requirements. The LDO is enabled
through the ENLDO pin. The output voltage is determined by
the configuration of the external feedback resistors, as seen
in the typical application circuit (page 1), R5 and R6.
RESISTOR SELECTION FOR LDO
The output voltage of the LDO on the LM26484 is established
by the feed back resistor divider R5 and R6 shown on the
typical application circuit (page 1). The equation for deter-
mining V
voltage on the LDO_FB pin.
The LDO control loop will force the voltage on V
Table 1
from 0.8V to 1.5V along with common resistor values to es-
tablish these voltages. Common resistors do not always pro-
duce the target value. The resulting output voltage using
common resistors is also found in
consumed by the feedback network low it is recommended
that R6 be established as about 200 kΩ. Lesser values of R6
are OK and can be used at the user’s discretion.
NFET SELECTION
There are a few major concerns when selecting an NFET for
the LM26484 controller. The most important factor to consider
is the maximum power rating. It is important for the NFET to
have a maximum power rating larger than the application will
need. The LM26484 has the ability to drive the gate voltage
very close to VIN and down to approximately 1.5V. Selecting
an NFET where the guaranteed operation of the V
is important.
EXTERNAL CAPACITORS
The LDO on the LM26484 requires external capacitors for
regulator stability. These are specifically designed for
portable applications requiring minimum board space and
smallest components. These capacitors must be correctly se-
lected for good performance. The tolerance and temperature
coefficient must be considered when selecting the capacitor
to ensure that the capacitance will remain close to ideal over
the entire operating temperature range.
Part
Number
Si1450DH
V
Target
OUT
shows ideal resistor values to establish LDO voltages
0.8
0.9
1.1
1.2
1.3
1.4
1.5
OUT
1
(V)
is: V
Vendor
OUT
Vishay
Recommended NFET
= V
R5 (KΩ)
FB
Ideal Resistor Values
120
160
200
240
280
320
360
400
*(R5+R6)/R6, where V
V
TABLE 1. LDO Configuration and Component Selection Guide
GS
1.5V
Table
1. To keep the power
R6 (KΩ)
P
DISSIPATION
200
200
200
200
200
200
200
200
FB
2.78W
to be 0.50V.
GS
FB
is
≥
is the
1.5V
R5 (KΩ)
120
162
200
240
280
324
357
402
Common R Values
9
NO-LOAD STABILITY
The LDO will remain stable and in regulation with no external
load. This is an important consideration in some circuits, for
example CMOS RAM keep-alive applications.
FEEDBACK CAPACITOR
A Feedback capacitor is required for stability; recommended
values can be seen in
a distance of not more than 1 cm from the LDO_FB pin and
LDO_OUT. Any good quality ceramic or film capacitor should
be used.
OUTPUT CAPACITOR
The LDO on the LM26484 is designed specifically to work with
very small ceramic output capacitors. A 10.0 µF ceramic ca-
pacitor, marked as C10 in the Typical Application Circuit on
page 1, temperature types Z5U, Y5V or X7R with ESR be-
tween 5 mΩ to 500 mΩ, is suitable for proper operation.
It is also possible to use tantalum or film capacitors, but these
are not as attractive for reasons of size and cost. The output
capacitor must meet the requirement for the minimum value
of capacitance and also have an ESR value that is within the
range 50 mΩ to 500 mΩ for stability.
CAPACITOR CHARACTERISTICS
The LDO is designed to work with ceramic capacitors on the
output to take advantage of the benefits they offer. For ca-
pacitance values in the range of 0.47 μF to 44 μF, ceramic
capacitors are the smallest, least expensive and have the
lowest ESR values, thus making them best for eliminating
high frequency noise. The ESR of a typical 10 μF ceramic
capacitor is in the range of 20 mΩ to 40 mΩ, which easily
meets the ESR requirement for stability for the LDO.
For both input and output capacitors, careful interpretation of
the capacitor specification is required to ensure correct device
operation. The capacitor value can change greatly, depend-
ing on the operating conditions and capacitor type.
In particular, the output capacitor selection should take ac-
count of all the capacitor parameters, to ensure that the
specification is met within the application. The capacitance
can vary with DC bias conditions as well as temperature and
frequency of operation. Capacitor values will also show some
decrease over time due to aging. The capacitor parameters
are also dependent on the particular case size, with smaller
sizes giving poorer performance figures in general.
R6 (KΩ)
200
200
200
200
200
200
200
200
Table
Actual V
Com R (V)
1. This capacitor must be located
0.905
1.393
1.505
1.31
0.8
1.1
1.2
1
OUT
with
Feedback
Capacitor
C11 (pF)
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