LTC3728 Linear Technology, LTC3728 Datasheet - Page 17

• Manufacturer Part Number: LTC3728
• Description: 2-Phase Synchronous Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
INTV
An internal P-channel low dropout regulator produces 5V
at the INTV
the drivers and internal circuitry within the LTC3728. The
INTV
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
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 LTC3728 to be
exceeded. The system supply current is normally domi-
nated by the gate charge current. Additional external
loading of the INTV
needs to be taken into account for the power dissipation
calculations. The total INTV
either the 5V internal linear regulator or by the EXTV
input pin. When the voltage applied to the EXTV
less than 4.7V, all of the INTV
internal 5V linear regulator. Power dissipation for the IC in
this case is highest: (V
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 LTC3728 V
current is limited to less than 24mA from a 24V supply
when not using the EXTV
Use of the EXTV
ture to:
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
T
T
J
J
CC
CC
= 70°C + (24mA)(24V)(95°C/W) = 125°C
= 70°C + (24mA)(5V)(95°C/W) = 81°C
Regulator
pin regulator can supply a peak current of 50mA
CC
pin from the V
CC
input pin reduces the junction tempera-
U
CC
IN
and 3.3V linear regulators also
)(I
CC
U
INTVCC
CC
IN
CC
pin as follows:
CC
supply pin. INTV
current can be supplied by
and PGND IC pins is highly
current is supplied by the
), and overall efficiency
W
IN
U
CC
.
CC
powers
pin is
CC
IN
EXTV
The LTC3728 contains an internal P-channel MOSFET
switch connected between the EXTV
When the voltage applied to EXTV
internal regulator is turned off and the switch closes,
connecting the EXTV
supplying internal power. The switch remains closed as
long as the voltage applied to EXTV
This allows the MOSFET driver and control power to be
derived from the output during normal operation (4.7V <
V
is out of regulation (start-up, short-circuit). If more
current is required through the EXTV
specified, an external Schottky diode can be added be-
tween the EXTV
than 7V to the EXTV
Significant efficiency gains can be realized by powering
INTV
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
However, for 3.3V and other lower voltage regulators,
additional circuitry is required to derive INTV
from the output.
The following list summarizes the four possible connec-
tions for EXTV
1. EXTV
to be powered from the internal 5V regulator resulting in
an efficiency penalty of up to 10% at high input voltages.
2. EXTV
connection for a 5V regulator and provides the highest
efficiency.
3. EXTV
supply is available in the 5V to 7V range, it may be used to
power EXTV
gate drive requirements.
4. EXTV
work. For 3.3V and other low voltage regulators, efficiency
gains can still be realized by connecting EXTV
output-derived voltage that has been boosted to greater
than 4.7V. This can be done with either the inductive boost
OUT
CC
< 7V) and from the internal regulator when the output
CC
from the output, since the V
CC
CC
CC
CC
Connection
Left Open (or Grounded). This will cause INTV
Connected directly to V
Connected to an External supply. If an external
Connected to an Output-Derived Boost Net-
CC
CC:
providing it is compatible with the MOSFET
CC
and INTV
CC
CC
pin and ensure that EXTV
pin to the INTV
CC
pins. Do not apply greater
w w w . D a t a S h e e t 4 U . c
CC
CC
OUT
CC
rises above 4.7V, the
CC
remains above 4.5V.
IN
pin directly to V
. This is the normal
CC
LTC3728
and INTV
current resulting
CC
switch than is
pin thereby
CC
CC
CC
CC
17
power
< V
to an
pins.
OUT
3728fb
IN
CC
.
.