17
LTC3727/LTC3727-1
3727fc
INTV
CC
Regulator
An internal P-channel low dropout regulator produces
7.5V at the INTV
CC
pin from the V
IN
supply pin. INTV
CC
powers the drivers and internal circuitry within the
LTC3727. The INTV
CC
pin regulator can supply a peak
current of 50mA and must be bypassed to ground with a
minimum of 4.7μF tantalum, 10μF special polymer, or low
ESR type electrolytic capacitor. A 1μF ceramic capacitor
placed directly adjacent to the INTV
CC
and PGND IC pins
is highly recommended. Good bypassing is necessary to
supply the high transient currents required by the MOSFET
gate drivers and to prevent interaction between channels.
Higher input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the maxi-
mum junction temperature rating for the LTC3727 to be
exceeded. The system supply current is normally domi-
nated by the gate charge current. Additional external
loading of the INTV
CC
and 3.3V linear regulators also
needs to be taken into account for the power dissipation
calculations. The total INTV
CC
current can be supplied by
either the 7.5V internal linear regulator or by the EXTV
CC
input pin. When the voltage applied to the EXTV
CC
pin is
less than 7.3V, all of the INTV
CC
current is supplied by the
internal 7.5V linear regulator. Power dissipation for the IC
in this case is highest: (V
IN
)(I
INTVCC
), and overall efficiency
is lowered. The gate charge current is dependent on
operating frequency as discussed in the Efficiency Consid-
erations section. The junction temperature can be esti-
mated by using the equations given in Note 2 of the
Electrical Characteristics. For example, the LTC3727 V
IN
current is limited to less than 24mA from a 24V supply
when not using the EXTV
CC
pin as follows:
T
J
= 70°C + (24mA)(24V)(95°C/W) = 125°C
Use of the EXTV
CC
input pin reduces the junction tempera-
ture to:
T
J
= 70°C + (24mA)(7.5V)(95°C/W) = 87°C
Dissipation should be calculated to also include any added
current drawn from the internal 3.3V linear regulator. To
prevent maximum junction temperature from being ex-
ceeded, the input supply current must be checked operat-
ing in continuous mode at maximum V
IN
.
EXTV
CC
Connection
The LTC3727 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
7.3V, the
internal regulator is turned off and the switch closes,
connecting the EXTV
CC
pin to the INTV
CC
pin thereby
supplying internal power. The switch remains closed as
long as the voltage applied to EXTV
CC
remains above 7.0V.
This allows the MOSFET driver and control power to be
derived from the output during normal operation (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 be-
tween 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 highest
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 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, efficiency
gains can still be realized by connecting EXTV
CC
to an
output-derived voltage that has been boosted to greater
APPLICATIO S I FOR ATIO
WUUU