LTC3728LX(RevA) View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC3728LX Datasheet PDF : 32 Pages
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LTC3728L/LTC3728LX
APPLICATIO S I FOR ATIO
NEC Neocap, Cornell Dubilier ESRE and Sprague 595D
series. Consult manufacturers for other specific recom-
mendations.
INTVCC Regulator
An internal P-channel low dropout regulator produces 5V
at the INTVCC pin from the VIN supply pin. INTVCC powers
the drivers and internal circuitry within the IC. The INTVCC
pin regulator can supply a peak current of 50mA and must
be bypassed to ground with a minimum of 4.7µF tanta-
lum, 10µF special polymer, or low ESR type electrolytic
capacitor. A 1µF ceramic capacitor placed directly adja-
cent to the INTVCC and PGND IC pins is highly recom-
mended. 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 ex-
ceeded. The system supply current is normally dominated
by the gate charge current. Additional external loading of
the INTVCC and 3.3V linear regulators also needs to be
taken into account for the power dissipation calculations.
The total INTVCC current can be supplied by either the 5V
internal linear regulator or by the EXTVCC input pin. When
the voltage applied to the EXTVCC pin is less than 4.7V, all
of the INTVCC current is supplied by the internal 5V linear
regulator. Power dissipation for the IC in this case is
highest: (VIN)(IINTVCC), and overall efficiency is lowered.
The gate charge current is dependent on operating fre-
quency 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 VIN current is ther-
mally limited to less than 67mA from a 24V supply when
not using the EXTVCC pin as follows:
TJ = 70°C + (67mA)(24V)(34°C/W) = 125°C
Use of the EXTVCC input pin reduces the junction tempera-
ture to:
TJ = 70°C + (67mA)(5V)(34°C/W) = 81°C
The absolute maximum rating for the INTVCC 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 VIN.
EXTVCC Connection
The IC contains an internal P-channel MOSFET switch
connected between the EXTVCC and INTVCC pins. When
the voltage applied to EXTVCC rises above 4.7V, the inter-
nal regulator is turned off and the switch closes, connect-
ing the EXTVCC pin to the INTVCC pin thereby supplying
internal power. The switch remains closed as long as the
voltage applied to EXTVCC remains above 4.5V. This
allows the MOSFET driver and control power to be derived
from the output during normal operation (4.7V < VOUT <
7V) and from the internal regulator when the output is out
of regulation (start-up, short-circuit). If more
current is required through the EXTVCC switch than is
specified, an external Schottky diode can be added be-
tween the EXTVCC and INTVCC pins. Do not apply greater
than 7V to the EXTVCC pin and ensure that EXTVCC␣ <␣ VIN.
Significant efficiency gains can be realized by powering
INTVCC from the output, since the VIN 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 EXTVCC pin directly to VOUT.
However, for 3.3V and other lower voltage regulators,
additional circuitry is required to derive INTVCC power
from the output.
The following list summarizes the four possible connec-
tions for EXTVCC:
1. EXTVCC Left Open (or Grounded). This will cause INTVCC
to be powered from the internal 5V regulator resulting in
an efficiency penalty of up to 10% at high input voltages.
2. EXTVCC Connected directly to VOUT. This is the normal
connection for a 5V regulator and provides the highest
efficiency.
3. EXTVCC Connected to an External supply. If an external
supply is available in the 5V to 7V range, it may be used to
power EXTVCC providing it is compatible with the MOSFET
gate drive requirements.
3728lxfa
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