LTC3826
21
3826fc
APPLICATIONS INFORMATION
gate charge current, may be supplied by either the 5.25V
V
IN
LDO or the 7.5V EXTV
CC
LDO. When the voltage on
the EXTV
CC
pin is less than 4.7V, the V
IN
LDO is enabled.
Power dissipation for the IC in this case is highest and is
equal to V
IN
• INTV
CC
. The gate charge current is depen-
dent on operating frequency as discussed in the Effi ciency
Considerations section. The junction temperature can be
estimated by using the equations given in Note 2 of the
Electrical Characteristics. For example, the LTC3826 INTV
CC
current is limited to less than 24mA from a 24V supply when
in the G package and not using the EXTV
CC
supply:
T
J
= 70°C + (24mA)(24V)(95°C/W) = 125°C
To prevent the maximum junction temperature from being
exceeded, the input supply current must be checked while
operating in continuous conduction mode (PLLIN/MODE
= INTV
CC
) at maximum V
IN
.
When the voltage applied to EXTV
CC
rises above 4.7V, the
V
IN
LDO is turned off and the EXTV
CC
LDO is enabled. The
EXTV
CC
LDO remains on as long as the voltage applied to
EXTV
CC
remains above 4.5V. The EXTV
CC
LDO attempts
to regulate the INTV
CC
voltage to 7.5V, so while EXTV
CC
is less than 7.5V, the LDO is in dropout and the INTV
CC
voltage is approximately equal to EXTV
CC
. When EXTV
CC
is greater than 7.5V up to an absolute maximum of 10V,
INTV
CC
is regulated to 7.5V.
Using the EXTV
CC
LDO allows the MOSFET driver and
control power to be derived from one of the LTC3826’s
switching regulator outputs (4.7V ≤ V
OUT
≤ 10V) during
normal operation and from the V
IN
LDO when the output
is out of regulation (e.g., startup, short-circuit). If more
current is required through the EXTV
CC
LDO than is spec-
ifi ed, an external Schottky diode can be added between the
EXTV
CC
and INTV
CC
pins. Do not apply more than 10V to
the EXTV
CC
pin and make sure than EXTV
CC
≤ V
IN
.
Signifi cant effi ciency and thermal gains can be realized
by powering INTV
CC
from the output, since the V
IN
cur-
rent resulting from the driver and control currents will be
scaled by a factor of (Duty Cycle)/(Switcher Effi ciency). For
5V to 10V regulator outputs, this means connecting the
EXTV
CC
pin directly to V
OUT
. Tying the EXTV
CC
pin to a 5V
supply reduces the junction temperature in the previous
example from 125°C to:
T
J
= 70°C + (24mA)(5V)(95°C/W) = 81°C
However, for 3.3V and other low voltage outputs, addi-
tional 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 5.25V regulator
resulting in an effi ciency 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 to 10V regulator and provides the
highest effi ciency.
3. EXTV
CC
Connected to an External supply. If an external
supply is available in the 5V to 10V range, it may be
used to power EXTV
CC
providing it is compatible with
the MOSFET gate drive requirements.
4. EXTVCC Connected to an Output-Derived Boost Network.
For 3.3V and other low voltage regulators, effi ciency
gains can still be realized by connecting EXTV
CC
to an
output-derived voltage that has been boosted to greater
than 4.7V. This can be done with the capacitive charge
pump shown in Figure 8.
EXTV
CC
V
IN
TG1
SW
BG1
PGND
LTC3826
R
SENSE
V
OUT
VN2222LL
+
C
OUT
3826 F08
N-CH
N-CH
+
C
IN
1μF
V
IN
L1
BAT85 BAT85
BAT85
0.22μF
Figure 8. Capacitive Charge Pump for EXTV
CC
