LTC1876
APPLICATIO S I FOR ATIO
operating frequency as discussed in the Efficiency Consid-
erations section. The junction temperature can be esti-
mated by using the equations given in Note 3 of the
Electrical Characteristics. For example, the LTC1876 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 and added for 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 con-
tinuous mode at maximum VIN.
EXTVCC Connection
The LTC1876 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
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 operation (4.7V <
VOUT < 7V) and from the internal regulator when the output
is out of regulation (start-up, short-circuit). If more cur-
rent is required through the EXTVCC switch than is speci-
fied, 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. Make sure the voltage applied to the
EXTVCC does not exceed 7V.
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 the output of the boost regulator.
If the LTC1876 auxillary boost regulator is set up for
output voltage between 4.7V and 7V, the EXTVCC can be
connected to this output.
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
than 4.7V. This can be done with either the inductive boost
winding as shown in Figure 6a or the capacitive charge
pump shown in Figure 6b. The charge pump has the
advantage of simple magnetics.
5. 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.
OPTIONAL EXTVCC
CONNECTION
5V < VSEC < 7V
VIN
+
CIN
VIN
LTC1876
TG1
N-CH
EXTVCC
SW
T1
1:N
R6
FCB
BG1
R5
SGND
PGND
N-CH
VSEC
+
RSENSE
1µF
VOUT
+
COUT
1876 F06a
Figure 6a. Secondary Output Loop and EXTVCC Connection
1876fa
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