ltc1876eg-trpbf Linear Technology Corporation, ltc1876eg-trpbf Datasheet - Page 19

• Manufacturer Part Number: ltc1876eg-trpbf
• Description: High Efficiency, 2-phase, Dual Synchronous Step-down Switching Controller And Step-up Regulator
• Manufacturer: Linear Technology Corporation

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 V
current is limited to less than 24mA from a 24V supply
when not using the EXTV
Use of the EXTV
ture to:
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 V
EXTV
The LTC1876 contains an internal P-channel MOSFET
switch connected between the EXTV
When the voltage applied to EXTV
internal regulator is turned off and the switch closes,
connecting the EXTV
supplying internal power. The switch remains closed as
long as the voltage applied to EXTV
This allows the MOSFET driver and control power to be
derived from the output during normal operation (4.7V <
V
is out of regulation (start-up, short-circuit). If more cur-
rent is required through the EXTV
fied, an external Schottky diode can be added between the
EXTV
the EXTV
Significant efficiency gains can be realized by powering
INTV
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
However, for 3.3V and other lower voltage regulators,
additional circuitry is required to derive INTV
from the output.
OUT
T
T
J
J
CC
< 7V) and from the internal regulator when the output
CC
CC
= 70 C + (24mA)(24V)(95 C/W) = 125 C
= 70 C + (24mA)(5V)(95 C/W) = 81 C
from the output, since the V
and INTV
Connection
CC
pin and ensure that EXTV
CC
CC
input pin reduces the junction tempera-
U
pins. Do not apply greater than 7V to
CC
pin to the INTV
CC
U
pin as follows:
IN
.
CC
CC
CC
CC
W
rises above 4.7V, the
switch than is speci-
CC
remains above 4.5V.
IN
pin directly to V
CC
and INTV
current resulting
< V
CC
IN
pin thereby
.
U
CC
CC
power
pins.
OUT
IN
.
The following list summarizes the four possible connec-
tions for EXTV
EXTV
1. EXTV
to be powered from the internal 5V regulator resulting in
an efficiency penalty of up to 10% at high input voltages.
2. EXTV
connection for a 5V regulator and provides the highest
efficiency.
3. EXTV
If the LTC1876 auxillary boost regulator is set up for
output voltage between 4.7V and 7V, the EXTV
connected to this output.
4. EXTV
work. For 3.3V and other low voltage regulators, efficiency
gains can still be realized by connecting EXTV
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. EXTV
supply is available in the 5V to 7V range, it may be used to
power EXTV
gate drive requirements.
Figure 6a. Secondary Output Loop and EXTV
R6
R5
CC
OPTIONAL EXTV
CONNECTION
5V < V
EXTV
FCB
SGND
CC
CC
CC
CC
CC
does not exceed 7V .
Left Open (or Grounded). This will cause INTV
Connected directly to V
Connected to the output of the boost regulator.
Connected to an External supply. If an external
CC
Connected to an Output-Derived Boost Net-
SEC
LTC1876
CC
< 7V
providing it is compatible with the MOSFET
CC
PGND
. Make sure the voltage applied to the
CC
BG1
TG1
SW
V
IN
N-CH
N-CH
V
IN
+
OUT
C
IN
1:N
T1
. This is the normal
LTC1876
CC
R
Connection
SENSE
V
SEC
CC
+
+
CC
19
can be
1876 F06a
to an
1 F
C
V
1876fa
OUT
OUT
CC