LM2612 National Semiconductor Corporation, LM2612 Datasheet - Page 17

• Manufacturer Part Number: LM2612
• Description: LM2612 - Miniature, Programmable, Step-down DC-DC Converter For Ultra Low-voltage Circuits, Package: Microsmd, Pin Nb=10
• Manufacturer: National Semiconductor Corporation

Application Information
Capacitor Selection
Use a 10µF, 6.3V, X7R or X5R ceramic input filter capacitor
and a 22uF, X7R or X5R ceramic output filter capacitor.
These provide an optimal balance between small size, cost,
reliability and performance. Do not use Y5V ceramic capaci-
tors. Table 3 lists suggested capacitors and suppliers.
A 10µF ceramic capacitor can be used for the output filter
capacitor for smaller size in applications where the
worst-case transient load step is less than 200mA. Use of a
10µF output capacitor trades off smaller size for an increase
in output voltage ripple, and undershoot during line and load
transient response.
The input filter capacitor supplies current to the PFET switch
of the LM2612 in the first part of each cycle and reduces
voltage ripple imposed on the input power source. The out-
put filter capacitor smoothes out current flow from the induc-
tor 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 electrolytic
capacitors and considerably less for ceramic capacitors. The
remainder of the ripple is from charge storage due to capaci-
tance.
Diode Selection
An optional Schottky diode (D1 in Figure 1) can be added to
increase efficiency in PFM mode and light-load PWM mode.
This may be desired in applications where increased effi-
ciency for improving operational battery life takes prece-
dence over increased system size associated with the
Schottky diode. Typically, use of an external schottky diode
increases PFM mode efficiency from 72.7% to 85.0% (20
mA load, V
in the Typical Operating Characteristics .
Use a Schottky diode with a current rating higher than
850mA, such as an MBRM140T3. Use of a device rated for
30V or more reduces diode reverse leakage in high tempera-
ture applications.
Thermal Design
The LM2612 has a thermal overload protection feature
which activates when the junction temperature exceeds
around 155˚C, until the device cools to 130˚C. However,
running the device this hot continually may damage it and is
JMK325BJ226MM
ECJ4YB0J226M
GRM42-2X5R226K6.3
10µF, 6.3V, X7R or X5R Ceramic Capacitor for C1 (Input Filter Capacitor)
C2012X5R0J106M
JMK212BJ106MG
ECJ3YB0J106K
GRM40X5R106K6.3
OUT
Model
= 1.8V, V
IN
= 3.6V). See the efficiency curves
TABLE 3. Suggested Capacitors and Their Suppliers (Continued)
1210
1210
1210
0805
0805
1206
0805
Size
(Continued)
Taiyo-Yuden
Panasonic
muRata
TDK
Taiyo
Yuden
Panasonic
muRata
Vendor
17
poor practice. Sufficient thermal design should be done to
keep the device below the specified 125˚C maximum oper-
ating junction temperature.
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
AN-1112. Refer to the section Surface Mount Technology
(SMT) Assembly Considerations . For best results in assem-
bly, alignment ordinals on the PC board should be used to
facilitate placement of the device. Since micro SMD packag-
ing is a new technology, all layouts and assembly means
must be thoroughly tested prior to production. In particular,
proper placement, solder reflow and resistance to thermal
cycling must be verified.
The 10-Bump package used for the LM2612 has 170micron
solder balls and requires 6.7mil (6.7/1000 in.) 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 mil wide, for a section 6 mil long or longer, as a
thermal relief. Then each trace should neck up to its optimal
width over a span of 11 mils or more, so that the taper
extends beyond the edge of the package. The important
criterion is symmetry. This ensures the solder bumps on the
LM2612 re-flow evenly and that the device solders level to
the board. In particular, special attention must be paid to the
pads for bumps 6-9. Because PVIN and PGND are typically
connected to large copper planes, inadequate thermal reliefs
can result in late or inadequate reflow of these bumps.
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 or 9.7mils
for the LM2612. This prevents a lip that otherwise forms if
the solder-mask and pad overlap. This lip can hold the
device off the surface of the board and interfere with mount-
ing. See Applications Note AN-1112 for specific instructions.
The micro SMD package is optimized for the smallest pos-
sible size in applications with red or infra-red opaque cases.
Because the micro SMD package lacks the plastic encapsu-
lation characteristic of larger devices, it is vulnerable to light.
Back-side metalization 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 on the package’s exposed die
edges.
847-925-0888
714-373-7366
404-436-1300
847-803-6100
847-925-0888
714-373-7366
404-436-1400
Phone
847-925-0899
714-373-7323
404-436-3030
847-803-6296
847-925-0899
714-373-7323
404-436-3030
FAX
www.national.com