LTC3727LX-1_15 Datasheet, PDF (17/28 Pages) Linear Technology – High Efficiency, 2-Phase Synchronous Step-Down Switching Regulator

LTC3727LX-1
APPLICATIO S I FOR ATIO
tion (7.2V < V_OUT < 8.5V) and from the internal regulator
when the output is out of regulation (start-up, short-
circuit). If more current is required through the EXTV_CC
switch than is specified, an external Schottky diode can be
added between the EXTV_CC and INTV_CC pins. Do not apply
greater than 8.5V to the EXTV_CC pin and ensure that
EXTV_CC < V_IN.
Significant efficiency gains can be realized by powering
INTV_CC from the output, since the V_IN current resulting
from the driver and control currents will be scaled by a
factor of (Duty Cycle)/(Efficiency). For 7.5V 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 7.5V 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 7.5V regulator and provides the high-
est efficiency.
3. EXTV_CC Connected to an External supply. If an external
supply is available in the 7.5V to 8.5V range, it may be
OPTIONAL EXTV_CC
CONNECTION
7.5V < V_SEC < 8.5V
V_IN
+
C_IN
V_IN
LTC3727LX-1
TG1
N-CH
EXTV_CC
SW
T1
1:N
R6
FCB
BG1
R5
SGND
PGND
N-CH
3727LX1 F06
V_SEC
+
R_SENSE
1µF
V_OUT
+
C_OUT
Figure 6. Secondary Output Loop & EXTV_CC Connection
used to power EXTV_CC providing it is compatible with
the MOSFET gate drive requirements.
4. EXTV_CC Connected to an Output-Derived Boost Net-
work. For 3.3V and other low voltage regulators, effi-
ciency gains can still be realized by connecting EXTV_CC
to an output-derived voltage that has been boosted to
greater than 7.5V. This can be done with the inductive
boost winding as shown in Figure 6.
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 LTC3727LX-1 output voltages are each set by an
external feedback resistive divider carefully placed across
the output capacitor. The resultant feedback signal is
compared with the internal precision 0.800V voltage
reference by the error amplifier. The output voltage is
given by the equation:
V_OUT
=
0.8V
⎛
⎝⎜
1+
R2 ⎞
R1⎠⎟
where R1 and R2 are defined in Figure 2.
3727lx1fa
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