LTC3613
18
3613fa
APPLICATIONS INFORMATION
be placed close to each ITH pin. If a compensation scheme
is stable on a single phase application, a polyphase ap-
plication with N phases should be compensated as:
C
ITH1
= N • C
ITH(SINGLE)
, C
ITH2
= N • C
ITH2(SINGLE)
and
R
ITH
= R
ITH(SINGLE)
/N.
The TRACK/SS pins should be connected together so
that all LTC3613s start up with the same slew rate. The
V
OSENSE
+
pins of paralleled LTC3613s should be connected
together to prevent any false triggering of overvoltage and
short circuit protection. Only one divider is necessary. The
remote output and ground traces should be routed together
as differential pairs and terminated at the same remote
sensing location (preferably Kelvin connected across the
bulk capacitors at the remote output point). The smaller
value ceramic input and output capacitors, however, should
be in close proximity to the ICs.
C
IN
and C
OUT
Selection
In continuous mode, the current into PV
IN
is a square wave
of duty cycle V
OUT
/V
IN
. To prevent large voltage transients,
a low ESR input capacitor sized for the maximum RMS
current must be used. The maximum RMS capacitor cur-
rent is given by:
I
RMS
≅I
OUT(MAX)
•
V
OUT
V
IN
•
V
IN
V
OUT
–1
This formula has a maximum at V
IN
= 2V
OUT
, where I
RMS
= I
OUT(MAX)
/2. This simple worst-case condition is com-
monly used for design because even significant deviations
do not offer much relief. Note that capacitor manufactur-
ers’ ripple current ratings for electrolytic and conductive
polymer capacitors are often based on only 2000 hours of
life. This makes it advisable to further derate the capacitor
or to choose a capacitor rated at a higher temperature
than required.
The selection of C
OUT
is primarily determined by the effec-
tive series resistance, ESR, to minimize voltage ripple. The
output ripple, ΔV
OUT
, in continuous mode is determined by:
ΔV
OUT
≤ΔI
L
R
ESR
+
1
8• f•C
OUT
⎛
⎝
⎜
⎞
⎠
⎟
The output ripple is highest at maximum input voltage
since ΔI
L
increases with input voltage. Typically, once the
ESR requirement for C
OUT
has been met, the RMS current
rating generally far exceeds the peak-to-peak current ripple
requirement. The choice of using smaller output capaci-
tance increases the ripple voltage due to the discharging
term but can be compensated for by using capacitors of
very low ESR to maintain the ripple voltage.
Multiple capacitors placed in parallel may be needed to
meet the ESR and RMS current handling requirements.
Dry tantalum, special polymer, aluminum electrolytic and
ceramic capacitors are all available in surface mount pack-
ages. Special polymer capacitors offer very low ESR but
have lower capacitance density than other types. Tantalum
capacitors have the highest capacitance density but it is
important to only use types that have been surge tested
for use in switching power supplies. Aluminum electrolytic
capacitors have significantly higher ESR, but can be used
in cost-sensitive applications provided that consideration
is given to ripple current ratings and long-term reliability.
Ceramic capacitors have excellent low ESR characteristics
but can have a high voltage coefficient and audible piezo-
electric effects. The high Q of ceramic capacitors with trace
inductance can also lead to significant ringing. When using
ceramic input capacitors, care must be taken to ensure
that ringing from inrush currents and switching does not
pose an overvoltage hazard to the regulator.
For high switching frequencies, reducing output ripple and
better EMI filtering may require small-value capacitors that
have low ESL (and correspondingly higher self resonant
frequencies) to be placed in parallel with larger value
capacitors that have higher ESL. This will ensure good
noise and EMI filtering in the entire frequency spectrum
of interest. Even though ceramic capacitors generally
have good high frequency performance, small ceramic
capacitors may still have to be parallel connected with
large ones to optimize performance.
