lm2706 National Semiconductor Corporation, lm2706 Datasheet - Page 13

• Manufacturer Part Number: lm2706
• Description: Miniature, Variable, Step-down Dc-dc Converter With Bypass For Rf Power Amplifiers
• Manufacturer: National Semiconductor Corporation

Application Information
The input filter capacitor supplies current to the PFET switch
of the LM2706 in the first part of each cycle and reduces
voltage ripple imposed on the input power source. The out-
put filter capacitor smooths out current flow from the inductor
to the load, helps maintain a steady output voltage during
transient load changes and reduces output voltage ripple.
These capacitors must be selected with sufficient capaci-
tance and sufficiently low ESR to perform these functions.
The ESR, or equivalent series resistance, of the filter capaci-
tors is a major factor in voltage ripple. The contribution from
ESR to voltage ripple is around 75%–95% for most types of
electrolytic capacitors, and less for ceramic capacitors. The
remainder of the ripple is from charge storage due to capaci-
tance.
OPERATING MODE SELECTION
Drive the ABD and BYP pins using the system controller to
set the operating mode of the LM2706. See Table 1 for the
pin settings and corresponding operating modes. Use a
comparator, Schmitt trigger or logic gate to drive the ABD
and BYP pins. Drive the pins below 0.5V for a low logic level
and above 1.2V for a high logic level. In systems where
operation in Automatic Bypass mode is desired and digital
control of operating modes is unnecessary, connect ABD
and BYP to SGND. ABD and BYP have pull-down currents
and will default to Automatic Bypass mode if left floating.
EN PIN CONTROL
Drive the EN pin using the system controller to turn the
LM2706 ON and OFF. Use a comparator, Schmitt trigger or
logic gate to drive the EN pin. Set EN high (
normal operation and low (
down mode.
Set EN low to turn off the LM2706 during power-up and
under voltage conditions when the supply is less than the
2.7V minimum operating voltage. The LM2706 is designed
for mobile phones where the system controller controls op-
eration mode for maximizing battery life and requirements
for small package size outweigh the additional size required
for inclusion of UVLO (Under Voltage Lock-Out) circuitry.
micro SMD PACKAGE ASSEMBLY AND USE
Use of the micro-SMD package requires specialized board
layout, precision mounting and careful reflow techniques, as
detailed in National Semiconductor Application Note 1112.
Refer to the section Surface Mount Technology (SMT) As-
sembly Considerations. For best results in assembly, align-
ment ordinals on the PC board should be used to facilitate
placement of the device.
The pad style used with Micro SMD package must be the
NSMD (non-solder mask defined) type. This means that the
solder-mask opening is larger than the pad size. This pre-
vents a lip that otherwise forms if the solder-mask and pad
4.7 µF for C1, C2 (Input or Output Capacitor)
JMK212BJ475MG
LMK316BJ475ML
C2012X5R0J475K
Model
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Ceramic
Ceramic
Ceramic
0.5V) for a 0.1 µA (typ) shut-
TABLE 4. Suggested Capacitors and Their Suppliers
Type
(Continued)
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1.2V) for
Taiyo-Yuden
Taiyo-Yuden
TDK
Vendor
13
overlap, from holding the device off the surface of the board
and interfering with mounting. See Applications Note 1112
for specific instructions how to do this.
The 10-Bump package used for the LM2706 has 300 micron
solder balls and requires 10.82 mil pads for mounting on the
circuit board. The trace to each pad should enter the pad
with a 90˚ entry angle to prevent debris from being caught in
deep corners. Initially, the trace to each pad should be
6–7 mil wide, for a section approximately 6 mil long, as a
thermal relief. Then each trace should neck up or down to its
optimal width. The important criterion is symmetry. This en-
sures the solder bumps on the LM2706 re-flow evenly and
that the device solders level to the board. In particular,
special attention must be paid to the pads for bumps 6–8.
Because PGND and PV
copper planes, inadequate thermal relief’s can result in late
or inadequate reflow of these bumps.
The micro SMD package is optimized for the smallest pos-
sible size in applications with red or infrared opaque cases.
Because the micro SMD package lacks the plastic encapsu-
lation characteristic of larger devices, it is vulnerable to light.
Backside metallization and/or epoxy coating, along with
front-side shading by the printed circuit board, reduce this
sensitivity. However, the package has exposed die edges. In
particular, micro SMD devices are sensitive to light, in the
red and infrared range, shining n the package’s exposed die
edges.
Do not use or power-up the LM2706 while subjecting it to red
or infrared light otherwise degraded, unpredictable or erratic
operation may result. Examples of light sources with high red
or infrared content include the sun and halogen lamps. Pack-
age the circuit in a case opaque to red or infrared light.
BOARD LAYOUT CONSIDERATIONS
PC board layout is an important part of DC-DC converter
design. Poor board layout can disrupt the performance of a
DC-DC converter and surrounding circuitry by contributing to
EMI, ground bounce, and resistive voltage loss in the traces.
These can send erroneous signals to the DC-DC converter
IC, resulting in poor regulation or instability. Poor layout can
also result in reflow problems leading to poor solder joints
between the Micro SMD package and board pads. Poor
solder joints can result in erratic or degraded performance.
Good layout for the LM2706 can be implemented by follow-
ing a few simple design rules.
1. Place the LM2706 on 10.82 mil (10.82/1000 in.) pads. As
2. Place the LM2706, inductor and filter capacitors close
a thermal relief, connect to each pad with a 7 mil wide,
approximately 7 mil long traces, and then incrementally
increase each trace to its optimal width. The important
criterion is symmetry to ensure the solder bumps on the
LM2706 re-flow evenly (see Micro SMD Package Assem-
bly and Use).
together and make the traces short. The traces between
these components carry relatively high switching currents
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