Datasheet

LTC3727A-1
17
3727a1fa
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 LTC3727A-1.
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 di-
rectly 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
maximum junction temperature rating for the LTC3727A-1
to be exceeded. The system supply current is normally
dominated 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
effi ciency is lowered. The gate charge current is dependent
on operating frequency as discussed in the Effi ciency
Considerations section. The junction temperature can be
estimated by using the equations given in Note 2 of the
Electrical Characteristics. For example, the LTC3727A-1
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 tem-
perature to:
T
J
= 70°C + (24mA)(7.5V)(95°C/W) = 87°C
APPLICATIONS INFORMATION
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
exceeded, the input supply current must be checked
operating in continuous mode at maximum V
IN
.
EXTV
CC
Connection
The LTC3727A-1 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 sup-
plying 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 specifi ed,
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
.
Signifi cant effi ciency 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)/(Effi ciency). For 7.5V regulators
this supply means connecting the EXTV
CC
pin directly to
V
OUT
. However, for 3.3V and other lower voltage regula-
tors, 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 effi ciency penalty of up to 10% at high
input voltages.