13
LTC3778
3778f
commonly used for design because even significant
deviations do not offer much relief. Note that ripple
current ratings from capacitor manufacturers are often
based on only 2000 hours of life which makes it advisable
to derate the capacitor.
The selection of C
OUT
is primarily determined by the ESR
required to minimize voltage ripple and load step
transients. The output ripple ∆V
OUT
is approximately
bounded by:
∆≤∆ +
V I ESR
fC
OUT L
OUT
1
8
Since ∆I
L
increases with input voltage, the output ripple is
highest at maximum input voltage. Typically, once the ESR
requirement is satisfied, the capacitance is adequate for
filtering and has the necessary RMS current rating.
Multiple capacitors placed in parallel may be needed to
meet the ESR and RMS current handling requirements.
Dry tantalum, special polymer, POSCAP aluminum elec-
trolytic and ceramic capacitors are all available in surface
mount packages. Special polymer capacitors offer very
low ESR but have lower capacitance density. 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 providing 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
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 control-
ler. When necessary, adding a small 5µF to 50µF alumi-
num electrolytic capacitor with an ESR in the range of
0.5Ω to 2Ω dampens input voltage transients. High per-
formance through-hole capacitors may also be used, but
an additional ceramic capacitor in parallel is recommended
to reduce the effect of their lead inductance.
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 DRV
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
+ DRV
CC
. The boost capacitor needs
to store about 100 times the gate charge required by the
top MOSFET. In most applications a 0.1µF to 0.47µF, X5R
or X7R dielectric ceramic capacitor is adequate.
Discontinuous Mode Operation and FCB Pin
The FCB pin determines whether the bottom MOSFET
remains on when current reverses in the inductor. Tying
this pin above its 0.6V threshold enables discontinuous
operation where the bottom MOSFET turns off when
inductor current reverses. The load current at which
inductor current reverses and discontinuous operation
begins depends on the amplitude of the inductor ripple
current and will vary with changes in V
IN
. Tying the FCB pin
below the 0.6V threshold forces continuous synchronous
operation, allowing current to reverse at light loads and
maintaining high frequency operation.
In addition to providing a logic input to force continuous
operation, the FCB pin provides a means to maintain a
flyback winding output when the primary is operating in
discontinuous mode. The secondary output V
OUT2
is nor-
mally set as shown in Figure 4 by the turns ratio N of the
transformer. However, if the controller goes into discon-
tinuous mode and halts switching due to a light primary
load current, then V
OUT2
will droop. An external resistor
divider from V
OUT2
to the FCB pin sets a minimum voltage
V
OUT2(MIN)
below which continuous operation is forced
until V
OUT2
has risen above its minimum.
VV
R
R
OUT MIN2
08 1
4
3
()
.=+
APPLICATIO S I FOR ATIO
WUUU
