LTC3729L-6 Datasheet, PDF (16/28 Pages) Linear Technology – PolyPhase, Synchronous Step-Down Switching Regulator

LTC3729L-6
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. The INTV_CC
regulator powers the drivers and internal circuitry of the
LTC3729L-6. The INTV_CC pin regulator can supply up to
50mA peak and must be bypassed to power ground with
a minimum of 4.7µF tantalum or electrolytic capacitor. An
additional 1µF ceramic capacitor placed very close to the
IC is recommended due to the extremely high instanta-
neous currents required by the MOSFET gate drivers.
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the maxi-
mum junction temperature rating for the LTC3729L-6 to
be exceeded. The supply current is dominated by the gate
charge supply current, in addition to the current drawn
from the differential amplifier output. The gate charge is
dependent on operating frequency as discussed in the
Efficiency Considerations section. The supply current can
either be supplied by the internal 5V regulator or via the
EXTV_CC pin. When the voltage applied to the EXTV_CC pin
is less than 4.7V, all of the INTV_CC load current is supplied
by the internal 5V linear regulator. Power dissipation for
the IC is higher in this case by (I_IN)(V_IN – INTV_CC) and
efficiency is lowered. The junction temperature can be
estimated by using the equations given in Note 1 of the
Electrical Characteristics. For example, the LTC3729L-6
V_IN current is thermally limited to less than 54mA from a
30V supply:
T_J = 70°C + (54mA)(30V)(34°C/W) = 125°C
Use of the EXTV_CC pin reduces the junction temperature
to:
T_J = 70°C + (54mA)(5V)(34°C/W) = 79.2°C
The input supply current should be measured while the
controller is operating in continuous mode at maximum
V_IN and the power dissipation calculated in order to pre-
vent the maximum junction temperature from being ex-
ceeded.
EXTV_CC Connection
The LTC3729L-6 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
16
internal regulator is turned off and the switch closes,
connecting the EXTV_CC pin to the INTV_CC pin thereby
supplying internal and MOSFET gate driving 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_EXTVCC < 7V) and from
the internal regulator when the output is out of regulation
(start-up, short-circuit). Do not apply greater than 7V to
the EXTV_CC pin and ensure that EXTV_CC < V_IN + 0.3V when
using the application circuits shown. If an external voltage
source is applied to the EXTV_CC pin when the V_IN supply is
not present, a diode can be placed in series with the
LTC3729L-6’s V_IN pin and a Schottky diode between the
EXTV_CC and the V_IN pin, to prevent current from backfeeding
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 the
ratio: (Duty Factor)/(Efficiency). For 5V regulators this
means connecting the EXTV_CC pin directly to V_OUT. How-
ever, for 3.3V and other lower voltage regulators, addi-
tional 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
a significant efficiency penalty 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. V_IN must be greater than or equal
to the voltage applied to the EXTV_CC pin.
4. EXTV_CC connected to an output-derived boost network.
For 3.3V and other low voltage regulators, efficiency gains
can still be realized by connecting EXTV_CC to an output-
derived voltage which has been boosted to greater than
4.7V but less than 7V. This can be done with either the
sn3729l6 3729l6fs