Datasheet

LTC3833
18
3833f
APPLICATIONS INFORMATION
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, ∆V
OUT
, in continuous mode is determined by:
∆V
OUT
≤ ∆I
L
R
ESR
+
1
8 • f • C
OUT






The output ripple is highest at maximum input voltage
since ∆I
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 characteris-
tics but can have a high voltage coefficient and audible
piezoelectric effects. The high Q of ceramic capacitors with
trace inductance can also lead to significant ringing. When
used as input capacitors, care must be taken to ensure
that ringing from inrush currents and switching does not
pose an overvoltage hazard to the power switches and
controller.
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.
Top MOSFET Driver Supply (C
B
, D
B
)
An external bootstrap capacitor, C
B
, connected to the BOOST
pin supplies the gate drive voltage for the topside MOSFET.
This capacitor is charged through diode D
B
from INTV
CC
when the switch node is low. When the top MOSFET turns
on, the switch node rises to V
IN
and the BOOST pin rises to
approximately V
IN
+ INTV
CC
. The boost capacitor needs to
store approximately 100 times the gate charge required by
the top MOSFET. In most applications a 0.1μF to 0.47μF, X5R
or X7R dielectric capacitor is adequate. It is recommended
that the BOOST capacitor be no larger than 10% of the
INTV
CC
capacitor, C
VCC
, to ensure that the C
VCC
can supply
the upper MOSFET gate charge and BOOST capacitor under
all operating conditions. Variable frequency in response
to load steps offers superior transient performance but
requires higher instantaneous gate drive. Gate charge
demands are greatest in high frequency low duty factor
applications under high dI/dt load steps and at start-up.
In order to minimize SW node ringing and EMI, connect a
5Ω to 10Ω resistor in series with the BOOST pin. Make the
C
B
and D
B
connections on the other side of the resistor. This
series resistor helps to slow down the TG rise time, limiting
the high dI/dt current through the top MOSFET that causes
SW node ringing.
INTV
CC
Regulator and EXTV
CC
Power
The LTC3833 features a PMOS low dropout linear regu-
lator (LDO) that supplies power to INTV
CC
from the V
IN
supply. INTV
CC
powers the gate drivers and much of the
LTC3833’s internal circuitry. The LDO regulates the voltage
at the INTV
CC
pin to 5.3V.
The LDO can supply a maximum current of 50mA
RMS
and
must be bypassed to ground with a minimum of 4.7μF
ceramic capacitor. Good bypassing is needed to supply