LTC3534
13
3534fb
For more information www.linear.com/LTC3534
applicaTions inForMaTion
Output Capacitor Selection
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:
∆V
P-P
Boost =
I
OUT
• V
OUT
– V
IN(MIN)
C
OUT
• V
OUT
• f
V
∆V
P-P
Buck =
1
8 •L •C
OUT
• f
2
•
V
IN(MAX)
– V
OUT
( )
• V
OUT
V
IN(MAX)
V
where f = switching frequency in Hz, typically 1MHz.
C
OUT
= output filter capacitor, F
I
OUT
= output load current, A
The output capacitance is usually many times larger than
the minimum value in order to handle the transient response
requirements of the converter. As 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. A 22µF or larger
ceramic capacitor is appropriate for most applications.
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 10µF, low ESR ceramic bypass
capacitor close to the PV
IN
/V
IN
and PGND/GND pins. It is
also
important to minimize any stray resistance from the
converter to the battery or other power source.
Optional Schottky Diodes
Schottky diodes across the synchronous switches B and
D are not required, but do provide a lower drop during the
break-before-make time (typically 15ns), thus improving
efficiency. Use a surface mount Schottky diode such as an
MBRM120T3 or equivalent. Do not use ordinary rectifier
diodes since their slow recovery times will compromise
efficiency.
Output Voltage < 1.8V
The LTC3534 can operate as a buck converter with output
voltages as low as 400mV. Since synchronous switch
D is powered from V
OUT
and the R
DS(ON)
will increase
significantly at output voltages below 1.8V typical, a
Schottky diode is required from SW2 to V
OUT
to provide
the conduction path to the output at low V
OUT
voltages.
The current limit is folded back to 800mA when V
OUT
<
0.9V typical which will significantly reduce the output
current capability of the application. Note that Burst Mode
operation is inhibited at output voltages below 1.6V typical.
Closing the Feedback Loop
The LTC3534 incorporates voltage mode PWM control. The
control to output gain varies with operation region (buck,
boost, buck-boost), but is
usually no greater than 15. The
output filter exhibits a double pole response, as given by:
f
FILTER _POLE
=
2• π • L1•C
OUT
Hz (in buck mode)
f
FILTER _POLE
=
V
IN
2• V
OUT
• π • L1•C
OUT
Hz (in boost mode)
where L1 is in Henries and C
OUT
is in Farads.
The output filter zero is given by:
Table 2. Capacitor Vendor Information
SUPPLIER PHONE FAX WEBSITE
AVX (803) 448-9411 (803) 448-1943 www.avxcorp.com
Sanyo (619) 661-6322 (619) 661-1055 www.sanyovideo.com
Taiyo
Yuden
(408) 573-4150 (408) 573-4159 www.t-yuden.com
TDK (847) 803-6100 (847) 803-6296 www.component.tdk.com
