LTC3425
18
3425f
voltage from exceeding its maximum rating during the
break-before-make time. Surface mount diodes, such as
the MBR0520L or equivalent, must be used and must be
located very close to the pins to minimize stray inductance.
Two example application circuits are shown in Figures 7
and 8, one with output disconnect and one without.
Operating Frequency Selection
T
here are several considerations in selecting the operat-
ing frequency of the converter. The first is, which are the
sensitive frequency bands that cannot tolerate any spec-
tral noise? For example, in products incorporating RF
communications, the 455kHz IF frequency is sensitive to
any noise, therefore switching above 600kHz is desired.
Some communications have sensitivity to 1.1MHz, and
in that case, a 1.5MHz converter frequency may be
employed.
The second consideration is the physical size of the
converter. As the operating frequency goes up, the induc-
tor and filter capacitors go down in value and size. The
trade off is in efficiency, since the switching losses in-
crease proportionally with frequency.
Thermal Considerations
To deliver the power that the LTC3425 is capable of, it is
imperative that a good thermal path be provided to dissi-
pate the heat generated within the package. This can be
accomplished by taking advantage of the large thermal
pad on the underside of the IC. It is recommended that
multiple vias in the printed circuit board be used to
conduct heat away from the IC and into a copper plane with
as much area as possible. In the event that the junction
temperature gets too high, the peak current limit will
automatically be decreased. If the junction temperature
continues to rise, the part will go into thermal shutdown,
and all switching will stop until the temperature drops.
Closing the Feedback Loop
The LTC3425 uses current mode control with internal
adaptive slope compensation. Current mode control elimi-
nates the 2nd order filter, due to the inductor and output
capacitor exhibited in voltage mode controllers, and sim-
plifies it to a single pole filter response. The product of the
APPLICATIO S I FOR ATIO
WUUU
The ESR (equivalent series resistance) is usually the most
dominant factor for ripple in most power converters. The
ripple due to capacitor ESR is given by:
V
RCESR
= I
P
• C
ESR
where C
ESR
= Capacitor Series Resistance
The ESL (equivalent series inductance) is also an impor-
tant factor for high frequency converters. Using small,
surface mount ceramic capacitors, placed as close as
possible to the V
OUT
pins, will minimize ESL.
Low ESR/ESL capacitors should be used to minimize
output voltage ripple. For surface mount applications, AVX
TPS Series tantalum capacitors, Sanyo POSCAP or X5R
type ceramic capacitors are recommended.
In all applications, a minimum of 1µF, low ESR ceramic
capacitor should be placed as close to each of the four
V
OUT
pins as possible, and grounded to a local ground
plane.
Input Capacitor Selection
The input filter capacitor reduces peak currents drawn
from the input source and reduces input switching noise.
Since the IC can operate at voltages below 0.5V once the
output is regulated (as long as SHDN is above 0.65V), the
demand on the input capacitor to lower ripple is much less.
Taiyo Yuden offers very low ESR capacitors, for example
the 2.2µF in a 0603 case (JMK107BJ22MA). See Table 3
for a list of capacitor manufacturers for input and output
capacitor selection.
Table 3. Capacitor Vendor Information
SUPPLIER PHONE FAX WEB SITE
AVX (803) 448-9411 (803) 448-1943 www.avxcorp.com
Sanyo (619) 661-6322 (619) 661-1055 www.sanyovideo.com
TDK (847) 803-6100 (847) 803-6296 www.component.tdk.com
Murata USA: USA: www.murata.com
(814) 237-1431 (814) 238-0490
(800) 831-9172
Taiyo Yuden (408) 573-4150 (408) 573-4159 www.t-yuden.com
Applications Where V
OUT
> 4.3V
Due to the very high slew rates associated with the switch
nodes, Schottky diode clamps are required in any applica-
tion where V
OUT
can exceed 4.3V to prevent the switch
