Datasheet

LTC3714
15
3714f
applicaTions inForMaTion
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:
ΔV
OUT
≤ ΔI
L
ESR +
1
8fC
OUT
⎛
⎝
⎜
⎞
⎠
⎟
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 electrolytic
and ceramic capacitors are all available in surface mount
packages. 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 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 piezoelectric effects. The high Q of ceramic
capacitors with trace inductance can also lead to signifi-
cant 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. High performance 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 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 about 100 times the gate charge required by the top
MOSFET. In most applications 0.1µF to 0.47µF 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 (typically to INTV
CC
) en-
ables discontinuous operation where the bottom MOSFET
turns off when inductor current reverses. The load current
at which current reverses and discontinuous operation
begins, depends on the amplitude of the inductor ripple
current. The ripple current depends on the choice of in-
ductor value and operating frequency as well as the input
and output voltages.
Tying the FCB pin below the 0.6V threshold forces continu-
ous synchronous operation, allowing current to reverse
at light loads.
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
SEC
is
normally set as shown in Figure 5 by the turns ratio N
of the transformer. However, if the controller goes into
discontinuous mode and halts switching due to a light
primary load current, then V
SEC
will droop. An external
resistor divider from V
SEC
to the FCB pin sets a minimum
voltage V
SEC(MIN)
below which continuous operation is
forced until V
SEC
has risen above its minimum.
V
SEC(MIN)
= 0.6V 1+
R4
R3
⎛
⎝
⎜
⎞
⎠
⎟
Figure 5. Secondary Output Loop and EXTV
CC
Connection
V
IN
SENSE
LTC3714
SGND
FCB
EXTV
CC
TG
SW
OPTIONAL
EXTV
CC
CONNECTION
5V < V
SEC
< 7V
R3
R4
3714 F05
T1
1:N
BG
PGND
+
C
SEC
1µF
V
OUT
V
SEC
V
IN
+
C
IN
1N4148
•
•
+
C
OUT