Datasheet

LTC3878
14
3878fa
applicaTions inForMaTion
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 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 tran
-
sient per
formance 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.
Setting Output Voltage
The LTC3878 output voltage is set by an external feed
-
back resistive divider carefully placed across the output,
as shown in Figure 5. The regulated output voltage is
determined by:
V V
R
R
OUT
B
A
= +






0 8 1.
To improve the transient response, a feed-forward ca-
pacitor, C
FF
, may be used. Great care should be taken to
route the V
FB
line away from noise sources, such as the
inductor or the SW line.
Discontinuous Mode Operation and FCB Pin
The FCB (forced continuous bar) pin determines whether
the LTC3878 operates in forced continuous mode or al
-
l
o
ws discontinuous conduction mode. Tying this pin above
0.8V enables discontinuous operation, where the bottom
MOSFET turns off when the inductor current reverses
polarity. The load current at which current reverses and
discontinuous operation begins depends on the amplitude
of the inductor ripple current and will vary with changes in
V
IN
. In steady-state operation, discontinuous conduction
mode occurs for DC load currents less than 1/2 the peak-
to-peak ripple current. Tying the FCB pin below the 0.8V
threshold forces continuous switching, where inductor
current is allowed to reverse at light loads and maintain
synchronous switching.
In addition to providing a logic input to force continuous
operation, the FCB pin provides a means to maintain a
fly back winding output when the primary is operating
in discontinuous mode. The secondary output V
OUT2
is
normally set as shown in Figure 6 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
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.
V V
R
R
OUT MIN2
0 8 1
4
3
( )
.= +






Figure 6. Secondary Output Loop
Figure 5. Setting Output Voltage
LTC3878
V
FB
V
OUT
R
B
C
FF
R
A
3878 F05
FCB
SW
TG
Si4884
3878 F06
LTC3878
BG
R3
R4
PGNDSGND
Si4874
V
IN
C
IN
C
OUT
C
OUT2
V
IN
V
OUT2
V
OUT
1N4148
•
•