LTC3728L-1
17
3728l1fc
APPLICATIONS INFORMATION
in cost-driven applications providing that consideration
is given to ripple current ratings, temperature and long
term reliability. A typical application will require several
to many aluminum electrolytic capacitors in parallel. A
combination of the above mentioned capacitors will often
result in maximizing performance and minimizing overall
cost. Other capacitor types include Nichicon PL series,
Panasonic SP, NEC Neocap, Cornell Dubilier ESRE and
Sprague 595D series. Consult manufacturers for other
specifi c recommendations.
INTV
CC
Regulator
An internal P-channel low dropout regulator produces
5V at the INTV
CC
pin from the V
IN
supply pin. INTV
CC
powers the drivers and internal circuitry within the IC.
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 maxi-
mum junction temperature rating for the IC 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 5V in-
ternal linear regulator or by the EXTV
CC
input pin. When
the voltage applied to the EXTV
CC
pin is less than 4.7V, all
of the INTV
CC
current is supplied by the internal 5V linear
regulator. Power dissipation for the IC in this case is high-
est: (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 IC V
IN
current is thermally limited to less
than 67mA from a 24V supply when not using the EXTV
CC
pin as follows:
T
J
= 70°C + (67mA)(24V)(34°C/W) = 125°C
Use of the EXTV
CC
input pin reduces the junction tem-
perature to:
T
J
= 70°C + (67mA)(5V)(34°C/W) = 81°C
The absolute maximum rating for the INTV
CC
Pin is 40mA.
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 IC 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
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 regu-
lation (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 7V 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 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
effi ciency penalty of up to 10% at high input voltages.
