LTC3442 Datasheet, PDF (14/20 Pages) Linear Integrated Systems – Micropower Synchronous Buck-Boost DC/DC Converter with Automatic Burst Mode Operation

LTC3442
APPLICATIO S I FOR ATIO
The output capacitance is usually many times larger than
the minimum value in order to handle the transient re-
sponse 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 calcula-
tions in order to maintain the desired transient response.
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 2 for
contact information.
Input Capacitor Selection
Since V_IN is the supply voltage for the IC, as well as the
input to the power stage of the converter, it is recom-
mended to place at least a 4.7µF, low ESR ceramic bypass
capacitor close to the V_IN and SGND pins. It is also
important to minimize any stray resistance from the con-
verter to the battery or other power source.
Optional Schottky Diodes
The Schottky diodes across the synchronous switches B
and D are not required (V_OUT < 4.3V), but provide a lower
drop during the break-before-make time (typically 15ns)
improving efficiency. Use a surface mount Schottky diode
such as an MBRM120T3 or equivalent. Do not use ordi-
nary rectifier diodes, since the slow recovery times will
compromise efficiency. For applications with an output
voltage above 4.3V, a Schottky diode is required from
SW2 to V_OUT.
Output Voltage < 2.4V
The LTC3442 can operate as a buck converter with output
voltages as low as 0.4V. The part is specified at 2.4V
minimum to allow operation without the requirement of a
Schottky diode. Synchronous switch D is powered from
V_OUT and the R_DS(ON) will increase at low output voltages,
therefore a Schottky diode is required from SW2 to V_OUT
to provide the conduction path to the output. Note that
Burst Mode operation is inhibited at output voltages below
1.6V typical.
Output Voltage > 4.3V
A Schottky diode from SW2 to V_OUT is required for output
voltages over 4.3V. The diode must be located as close to
the pins as possible in order to reduce the peak voltage on
SW2 due to the parasitic lead and trace inductance.
Input Voltage > 4.5V
For applications with input voltages above 4.5V which
could exhibit an overload or short-circuit condition, a
2Ω/1nF series snubber is required between SW1 and
GND. A Schottky diode from SW1 to V_IN should also be
added as close to the pins as possible. For the higher input
voltages, V_IN bypassing becomes more critical; therefore,
a ceramic bypass capacitor as close to the V_IN and SGND
pins as possible is also required.
Operating Frequency Selection
Higher operating frequencies allow the use of a smaller
inductor and smaller input and output filter capacitors,
thus reducing board area and component height. How-
ever, higher operating frequencies also increase the IC’s
total quiescent current due to the gate charge of the four
switches, as given by:
Buck:
Iq = (0.8 • V_IN • f) mA
Boost: Iq = [0.4 • (V_IN + V_OUT) • f] mA
Buck/Boost: Iq = [f • (1.2 • V_IN + 0.4 • V_OUT)] mA
Table 2. Capacitor Vendor Information
SUPPLIER
PHONE
AVX
[phone]
Murata
[phone]
[phone]
Sanyo
[phone]
Taiyo Yuden
[phone]
TDK
[phone]
FAX
[phone]
[phone]
[phone]
[phone]
[phone]
WEB SITE
www.avxcorp.com
www.murata.com
www.sanyovideo.com
www.t-yuden.com
www.component.tdk.com
3442f
14