LTC3878 Linear Technology Corporation, LTC3878 Datasheet - Page 14

• Manufacturer Part Number: LTC3878
• Description: Wide Operating Range No RSENSE Step-Down Controller
• Manufacturer: Linear Technology Corporation

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APPLICATIONS INFORMATION
LTC3878
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
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
the C
BOOST capacitor under all operating conditions. Variable
frequency in response to load steps offers superior tran-
sient 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.
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 volt age is
determined by:
To improve the transient response, a feed-forward ca-
pacitor, C
route the V
inductor or the SW line.
14
V
OUT
VCC
=
can supply the upper MOSFET gate charge and
FF
0 8
FB
, may be used. Great care should be taken to
.
V
line away from noise sources, such as the
Figure 5. Setting Output Voltage
⎛
⎝ ⎜
LTC3878
1
IN
+
R
R
+ INTV
V
B
A
CC
FB
⎞
⎠ ⎟
capacitor C
CC
V
IN
. The boost capacitor needs
OUT
B
B
, D
and the BOOST pin rises
R
R
3878 F05
, connected to the BOOST
B
A
B
)
VCC
C
FF
, to ensure that
B
from INTV
CC
Discontinuous Mode Operation and FCB Pin
The FCB (forced continuous bar) pin determines whether
the LTC3878 operates in forced continuous mode or al-
lows 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
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
fl y back winding output when the primary is operating
in discontinuous mode. The secondary output V
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
resistor divider from V
voltage V
forced until V
IN
V
. In steady-state operation, discontinuous conduction
R4
R3
OUT MIN
2
(
OUT2(MIN)
SGND
FCB
LTC3878
)
OUT2
Figure 6. Secondary Output Loop
=
PGND
SW
V
0 8
TG
BG
IN
.
has risen above its minimum.
below which continuous operation is
V
OUT2
⎛
⎝ ⎜
1
+
Si4884
Si4874
to the FCB pin sets a minimum
R
R
3
OUT2
4
C
⎞
⎠ ⎟
IN
V
IN
•
will droop. An external
1N4148
•
3878 F06
C
C
OUT
OUT2
V
V
OUT2
OUT
OUT2
3878f
is