LTC3422
12
3422fa
APPLICATIO S I FOR ATIO
WUUU
Output Capacitor Selection
The output voltage ripple has two components to it. The
bulk value of the capacitor is set to reduce the ripple due
to charge into the capacitor each cycle. The maximum
ripple due to charge is given by:
V
IV
CV
R BULK
PIN
OUT OUT
()
•
••
=
ƒ
where I
P
= peak inductor current
The ESR (equivalent series resistance) is usually the most
dominant factor for ripple in most power converters. The
ripple due to capacitor ESR is simply given by:
V
RCESR
= I
P
• C
ESR
where C
ESR
= capacitor equivalent series resistance.
Low ESR capacitors should be used to minimize output
voltage ripple. For most applications, Murata or Taiyo
Yuden X5R ceramic capacitors are recommended.
Input Capacitor Selection
The input filter capacitor reduces peak currents drawn
from the input source and reduces input switching noise.
Since the LTC3422 can operate at voltages below 0.5V
once the output is regulated, the demand on the input
capacitor is much less. In most applications 1µF per Amp
of peak input current is recommended. Taiyo Yuden offers
very low ESR ceramic capacitors, for example the 1µF in
a 0603 case (JMK107BJ105MA).
Table 2. 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
Operating Frequency Selection
There are several considerations in selecting the operating
frequency of the converter, such as, what are the sensitive
frequency bands that cannot tolerate any spectral noise.
Another consideration is the physical size of the converter.
As the operating frequency goes up, the inductor and filter
capacitors go down in value and size. The trade off is in
efficiency since the switching losses due to gate charge
are proportionally increasing with frequency. For example,
as shown in Figure 2, for a 2.4V to 3.3V converter, the
efficiency at 160mA is 9% less at 3MHz versus 300kHz.
OUTPUT CURRENT (mA)
1
40
EFFICIENCY (%)
50
60
70
80
10 100 1000
3422 F02
30
20
10
0
90
100
V
IN
= 2.4V
V
OUT
= 3.3V
f
OSC
= 300kHz
f
OSC
= 3MHz
Figure 2. 2.4V to 3.3V Efficiency vs Frequency of Operation
The final consideration is whether the application can
allow “pulse skipping.” In this mode, the minimum on time
of the converter cannot support the duty cycle, so the
converter ripple will go up and there will be a low frequency
component of the output ripple. In many applications
where physical size is the main criterion, running the
converter in this mode is acceptable. In applications where
it is preferred not to enter this mode, the maximum
operating frequency is given by:
ƒ
MAX NOSKIP
OUT IN
OUT ON MIN
VV
Vt
Hz
_
()
–
•
=
where t
ON(MIN)
= minimum on time = 120ns.
Thermal Considerations
To deliver the power that the LTC3422 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 LTC3422. It is recommended
that multiple vias in the printed circuit board be used to
