LTC3444_15 Datasheet, PDF (15/20 Pages) Linear Technology – Micropower Synchronous Buck-Boost DC/DC Converter for WCDMA Applications

U
OPERATIO
Output Capacitor Selection
A 4.7μF, X5R or X7R type ceramic capacitor should be
used when using the internal loop compensation. When
using external compensation, larger values of output
capacitance can be used, however, larger output capaci-
tance will increase the time needed to slew the output
voltage as required in typical WCDMA applications. The
bulk value of the output filter capacitor is set to reduce the
ripple due to charge into the capacitor each cycle. The
steady state ripple due to charge is given by:
%RIPPLE _BOOST =
I_OUT • (V_OUT – V_IN(MIN)) • 100 _%
C_OUT • V_OUT2 • f
%RIPPLE _BUCK =
I_OUT(MAX) • (V_IN(MAX) – V_OUT) • 100 _%
C_OUT • V_IN(MAX) • V_OUT • f
where C_OUT = output filter capacitor in farads
f = switching frequency in Hz.
LTC3444
In a typical application the output capacitance may be
many times larger than that calculated above in order to
handle the transient load response requirements of the
converter. For a rule of thumb, the ratio of the operating
frequency to the unity-gain bandwidth of the converter is
the amount the output capacitance will have to increase
from the above calculations in order to maintain the
desired transient response. However, in WCDMA applica-
tions the output capacitance should be kept at a minimum
to maximize the output slew rate. Refer to the Loop
Compensation Networks section of this datasheet.
The other component of ripple is due to the ESR (equiva-
lent series resistance) of the output capacitor. Low ESR
capacitors should be used to minimize output voltage
ripple. For surface mount applications, Taiyo Yuden or
TDK ceramic capacitors, AVX TPS series tantalum capaci-
tors or Sanyo POSCAP are recommended. See Table 3 for
contact information.
Ceramic output capacitors should use case size 1206 or
larger. Smaller case sizes have a larger voltage coefficient
that can greatly reduce the output capacitance value at
higher output voltages.
Input Capacitor Selection
Since the V_IN pin is the supply voltage for the LTC3444, as
well as the input to the power stage of the converter, it is
recommended to place at least a 4.7μF, X5R or X7R
ceramic bypass capacitor close to the V_IN and GND pins.
It is also important to minimize any stray resistance from
the converter to the battery or other power source.
Table 3. Capacitor Vendor Information
SUPPLIER
PHONE
AVX
[phone]
Sanyo
[phone]
Taio Yuden
[phone]
TDK
[phone]
FAX
[phone]
[phone]
[phone]
[phone]
WEB SITE
www.avxcorp.com
www.sanyovideo.com
www.t-yuden.com
www.component.tdk.com
3444fb
15