LTC3728LCGN-PBF Datasheet, PDF (20/38 Pages) Linear Technology – Dual, 550kHz, 2-Phase Synchronous Regulators

LTC3728L/LTC3728LX
Applications Information
Panasonic SP, NEC Neocap, Cornell Dubilier ESRE and
Sprague 595D series. Consult manufacturers for other
specific 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 efficiency is lowered. The
gate charge current is dependent on operating frequency
as discussed in the Efficiency 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
20
current drawn from the internal 3.3V linear regulator. To
prevent maximum junction temperature from being ex-
ceeded, 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 specified, 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.
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.
3728lxff