LTC1628CG-SYNC View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1628CG-SYNC' PDF : 32 Pages View PDF
LTC1628-SYNC
APPLICATIO S I FOR ATIO
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 LTC1628-
SYNC. The INTVCC pin regulator can supply a peak current
of 50mA and must be bypassed to ground with a mini-
mum of 4.7µF tantalum, 10µF special polymer, or low
ESR type electrolytic capacitor. A 1µF ceramic capacitor
placed directly adjacent to the INTVCC 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 LTC1628-SYNC
to be exceeded. 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 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 LTC1628-SYNC
VIN current is limited to less than 24mA from a 24V supply
when not using the EXTVCC pin as follows:
TJ = 70°C + (24mA)(24V)(95°C/W) = 125°C
Use of the EXTVCC input pin reduces the junction tempera-
ture to:
TJ = 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 ex-
ceeded, the input supply current must be checked operat-
ing in continuous mode at maximum VIN.
EXTVCC Connection
The LTC1628-SYNC contains an internal P-channel MOS-
FET switch connected between the EXTVCC and INTVCC
pins. When the voltage applied to EXTVCC rises above
4.7V, the internal regulator is turned off and the switch
closes, connecting 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 opera-
tion (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 between 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.
4. EXTVCC Connected to an Output-Derived Boost Net-
work. For 3.3V and other low voltage regulators, efficiency
gains can still be realized by connecting EXTVCC to an
output-derived voltage that has been boosted to greater
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