LTC3728 Datasheet, PDF (17/32 Pages) Linear Technology – Dual, 550kHz, 2-Phase Synchronous Step-Down Switching Regulator

LTC3728
APPLICATIO S I FOR ATIO
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 LTC3728. 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 LTC3728 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 5V internal 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 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 LTC3728 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)(5V)(95°C/W) = 81°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
exceeded, the input supply current must be checked
operating in continuous mode at maximum V_IN.
EXTV_CC Connection
The LTC3728 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 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 7V 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 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 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 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.
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
than 4.7V. This can be done with either the inductive boost
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