LTC3714 Linear, LTC3714 Datasheet - Page 15

• Manufacturer Part Number: LTC3714
• Description: Wide Operating Range / Step-Down Controller with Internal Op Amp
• Manufacturer: Linear

APPLICATIO S I FOR ATIO
The selection of C
required to minimize voltage ripple and load step
transients. The output ripple V
bounded by:
Since I
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 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. Alumi-
num 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
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. 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
An external bootstrap capacitor C
pin supplies the gate drive voltage for the topside MOSFET.
This capacitor is charged through diode D
when the switch node is low. When the top MOSFET turns
on, the switch node rises to V
to approximately V
V
OUT
L
increases with input voltage, the output ripple is
I ESR
L
OUT
IN
U
+ INTV
is primarily determined by the ESR
8
U
fC
CC
1
OUT
IN
. The boost capacitor needs
B
B
, D
and the BOOST pin rises
connected to the BOOST
OUT
W
B
)
is approximately
B
from INTV
U
CC
to store about 100 times the gate charge required by the
top MOSFET. In most applications 0.1 F to 0.47 F 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
enables 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 induc-
tor ripple current. The ripple current depends on the
choice of inductor 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
mally set as shown in Figure 5 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
divider from V
V
until V
5V < V
SEC(MIN)
CONNECTION
V
OPTIONAL
Figure 5. Secondary Output Loop and EXTV
SEC MIN
SEC
EXTV
SEC
< 7V
(
CC
has risen above its minimum.
below which continuous operation is forced
)
R4
R3
SEC
. 0 6
EXTV
FCB
SGND
LTC3714
to the FCB pin sets a minimum voltage
V
CC
SEC
SENSE
PGND
1
SW
V
BG
TG
IN
will droop. An external resistor
R
R
4
3
+
•
V
C
IN
IN
1:N
T1
LTC3714
1N4148
CC
•
Connection
+
SEC
+
C
OUT
15
3714 F05
is nor-
C
1 F
SEC
V
V
3714f
CC
SEC
OUT
)