16
LTC3729
sn3729 3729fas
maximum junction temperature rating for the LTC3729 to
be exceeded. The supply current is dominated by the gate
charge supply current, in addition to the current drawn
from the differential amplifier output. The gate charge is
dependent on operating frequency as discussed in the
Efficiency Considerations section. The supply current can
either be supplied by the internal 5V regulator or via the
EXTV
CC
pin. When the voltage applied to the EXTV
CC
pin
is less than 4.7V, all of the INTV
CC
load current is supplied
by the internal 5V linear regulator. Power dissipation for
the IC is higher in this case by (I
IN
)(V
IN
– INTV
CC
) and
efficiency is lowered. The junction temperature can be
estimated by using the equations given in Note 1 of the
Electrical Characteristics. For example, the LTC3729 V
IN
current is limited to less than 24mA from a 24V supply:
T
J
= 70
°
C + (24mA)(24V)(95
°
C/W) = 125
°
C
Use of the EXTV
CC
pin reduces the junction temperature
to:
T
J
= 70
°
C + (24mA)(5V)(95
°
C/W) = 81.4
°
C
The input supply current should be measured while the
controller is operating in continuous mode at maximum
V
IN
and the power dissipation calculated in order to pre-
vent the maximum junction temperature from being ex-
ceeded.
EXTV
CC
Connection
The LTC3729 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
supplying internal and MOSFET gate driving 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
EXTVCC
< 7V) and from
the internal regulator when the output is out of regulation
(start-up, short-circuit). Do not apply greater than 7V to
the EXTV
CC
pin and ensure that EXTV
CC
< V
IN
+ 0.3V when
using the application circuits shown.
If an external voltage
source is applied to the EXTV
CC
pin when the V
IN
supply is
not present, a diode can be placed in series with the
LTC3729’s V
IN
pin and a Schottky diode between the
EXTV
CC
and the V
IN
pin, to prevent current from backfeeding
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 the
ratio: (Duty Factor)/(Efficiency). For 5V regulators this
means connecting the EXTV
CC
pin directly to V
OUT
. How-
ever, for 3.3V and other lower voltage regulators, addi-
tional 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
a significant efficiency penalty 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. V
IN
must be greater than or equal
to the voltage applied to the EXTV
CC
pin.
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 which has been boosted to greater than
4.7V but less than 7V. This can be done with either the
inductive boost winding as shown in Figure 5a or the
capacitive charge pump shown in Figure 5b. The charge
pump has the advantage of simple magnetics.
Topside MOSFET Driver Supply (C
B
,D
B
) (Refer to
Functional Diagram)
External bootstrap capacitors C
B1
and C
B2
connected to
the BOOST1 and BOOST2 pins supply the gate drive
voltages for the topside MOSFETs. Capacitor C
B
in the
Functional Diagram is charged though diode D
B
from
INTV
CC
when the SW pin is low. When the topside MOSFET
turns 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
APPLICATIO S I FOR ATIO
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