LTC3878_15 Datasheet, PDF (14/26 Pages) Linear Technology – Fast, Wide Operating Range No RSENSE Step-Down DC/DC Controller

LTC3878
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 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 volt­age is
determined by:
V_OUT
=
0.8V


1+
R_B
R_A


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.
V_OUT
LTC3878
V_FB
R_B
C_FF
R_A
3878 F05
Figure 5. Setting Output Voltage
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_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_OUT2(MIN)
=
0.8V


1+
R4
R3


LTC3878
V_IN
TG
R4
SW
FCB
R3
BG
SGND PGND
V_IN
C_IN
1N4148
Si4884
•
•
Si4874
V_OUT2
C_OUT2
V_OUT
C_OUT
3878 F06
Figure 6. Secondary Output Loop
3878fa
14