LTC3728_15 Datasheet, PDF (20/36 Pages) Linear Technology – Dual, 550kHz, 2-Phase Synchronous Step-Down Switching Regulator

LTC3728
APPLICATIONS INFORMATION
factor of (Duty Cycle)/(Efficiency). For 5V regulators this
supply means connecting the EXTV_CC pin directly to V_OUT.
However, for 3.3V and other lower voltage regulators,
additional circuitry is required to derive INTV_CC power
from the output.
The following list summarizes the four possible connec-
tions for EXTV_CC:
1. EXTV_CC Left Open (or Grounded). This will cause INTV_CC
to be powered from the internal 5V regulator resulting in
an efficiency penalty of up to 10% at high input voltages.
2. EXTV_CC Connected Directly to V_OUT. This is the normal
connection for a 5V regulator and provides the highest
efficiency.
3. EXTV_CC Connected to an External Supply. If an external
supply is available in the 5V to 7V range, it may be used
to power EXTV_CC providing it is compatible with the
MOSFET gate drive requirements.
4. EXTV_CC Connected to an Output-Derived Boost Network.
For 3.3V and other low voltage regulators, efficiency
gains can still be realized by connecting EXTV_CC to an
output-derived voltage that has been boosted to greater
than 4.7V. This can be done with either the inductive
boost winding as shown in Figure 6a or the capacitive
charge pump shown in Figure 6b. The charge pump
has the advantage of simple magnetics.
Topside MOSFET Driver Supply (C_B, D_B)
External bootstrap capacitors C_B connected to the BOOST
pins supply the gate drive voltages for the topside MOS-
FETs. Capacitor C_B in the Functional Diagram is charged
though external diode D_B from INTV_CC when the SW pin
is low. When one of the topside MOSFETs is to be turned
on, the driver places the C_B voltage across the gate-source
of the desired MOSFET. This enhances the MOSFET and
turns on the topside switch. The switch node voltage, SW,
rises to V_IN and the BOOST pin follows. With the topside
MOSFET on, the boost voltage is above the input supply:
V_BOOST = V_IN + V_INTVCC. The value of the boost capacitor
C_B needs to be 100 times that of the total input capacitance
of the topside MOSFET(s). The reverse breakdown of the
external Schottky diode must be greater than V_IN(MAX).
When adjusting the gate drive level, the final arbiter is the
total input current for the regulator. If a change is made
and the input current decreases, then the efficiency has
improved. If there is no change in input current, then there
is no change in efficiency.
Output Voltage
The LTC3728 output voltages are each set by an exter-
nal feedback resistive divider carefully placed across
the output capacitor. The resultant feedback signal is
compared with the internal precision 0.800V voltage
OPTIONAL EXTV_CC
CONNECTION
5V < V_SEC < 7V
V_IN
+
C_IN
V_IN
LTC3728
TG1
N-CH
EXTV_CC
SW
T1
1:N
R6
FCB
BG1
R5
SGND
N-CH
PGND
V_SEC
+
R_SENSE
1MF
V_OUT
+
C_OUT
3728 F06a
Figure 6a. Secondary Output Loop and EXTV_CC Connection
20
V_IN
+
C_IN
V_IN
LTC3728
TG1
EXTV_CC
N-CH
SW
BG1
N-CH
PGND
+
1MF
BAT85
0.22MF
BAT85
VN2222LL
R_SENSE
L1
BAT85
V_OUT
+
C_OUT
3728 F06b
Figure 6b. Capacitive Charge Pump for EXTV_CC
3728fg