LTC3707 Datasheet, PDF (18/32 Pages) Linear Technology – High Effi ciency, 2-Phase Synchronous Step-Down Switching Regulator

LTC3707
APPLICATIONS INFORMATION
EXTV_CC Connection
The LTC3707 contains an internal P-channel MOSFET
switch connected between the EXTV_CC and INTV_CC pins.
When the voltage applied to EXTV_CC rises above 4.7V,
the internal regulator is turned off and the switch closes,
connecting the EXTV_CC pin to the INTV_CC pin thereby sup-
plying internal power. The switch remains closed as long
as the voltage applied to EXTV_CC remains above 4.5V. This
allows the MOSFET driver and control power to be derived
from the output during normal operation (4.7V < V_OUT <
7V) 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 7V 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 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.
OPTIONAL EXTV_CC
CONNECTION
5V < V_SEC < 7V
V_IN
+
C_IN
V_IN
LTC3707
TG1
N-CH
EXTV_CC
SW
T1
1:N
R6
FCB
BG1
R5
SGND
PGND
N-CH
V_SEC
+
R_SENSE
1μF
V_OUT
+
C_OUT
3707 F06a
Figure 6a. Secondary Output Loop & EXTV_CC Connection
V_IN
+
C_IN
V_IN
LTC3707
TG1
N-CH
EXTV_CC
SW
BG1
PGND
N-CH
+
1μF
BAT85
0.22μF
BAT85
VN2222LL
R_SENSE
L1
BAT85
V_OUT
+
C_OUT
3707 F06b
Figure 6b. Capacitive Charge Pump for EXTV_CC
3707fb
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