LTC3879 Linear Technology Corporation, LTC3879 Datasheet - Page 15

• Manufacturer Part Number: LTC3879
• Description: Wide Operating Range No RSENSE Step-Down Controller
• Manufacturer: Linear Technology Corporation

www.datasheet4u.com
APPLICATIONS INFORMATION
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 LTC3879 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.
V
OUT
VCC
=
can supply the upper MOSFET gate charge and
FF
0 6
FB
, may be used. Great care should be taken to
.
V
line away from noise sources, such as the
Figure 5. Setting Output Voltage
⎛
⎝ ⎜
LTC3879
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
3879 F05
, connected to the BOOST
B
A
B
)
VCC
C
FF
, to ensure that
B
from INTV
CC
Discontinuous Mode Operation and MODE Pin
The MODE pin determines whether the LTC3879 operates in
forced continuous mode or allows discontinuous conduc-
tion 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
discontinuous conduction mode occurs for DC load cur-
rents less than 1/2 the peak-to-peak ripple current. Tying
the MODE 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 MODE 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
V
R4
R3
OUT MIN
2
(
OUT2(MIN)
SGND
MODE
LTC3879
)
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 MODE pin sets a minimum
R
R
IN
3
OUT2
4
. In steady-state operation,
C
⎞
⎠ ⎟
IN
V
•
IN
will droop. An external
1N4148
•
LTC3879
3879 F06
C
C
OUT
OUT2
V
V
OUT2
OUT
15
OUT2
3879f
is