Datasheet

LTC3834
17
3834fb
APPLICATIONS INFORMATION
INTV
CC
Regulators
The LTC3834 features two separate internal P-channel low
dropout linear regulators (LDO) that supply power at the
INTV
CC
pin from either the V
IN
supply pin or the EXTV
CC
pin, respectively, depending on the connection of the
EXTV
CC
pin. INTV
CC
powers the gate drivers and much of
the LTC3834’s internal circuitry. The V
IN
LDO regulates
the voltage at the INTV
CC
pin to 5.25V and the EXTV
CC
LDO regulates it to 7.5V. Each of these can supply a peak
current of 50mA and must be bypassed to ground with a
minimum of 4.7μF ceramic capacitor. The ceramic capacitor
placed directly adjacent to the INTV
CC
and PGND IC pins is
highly recommended. Good bypassing is needed to supply
the high transient currents required by the MOSFET gate
drivers and to prevent interaction between the channels.
High input voltage applications in which large MOSFETs are
being driven at high frequencies may cause the maximum
junction temperature rating for the LTC3834 to be exceeded.
The INTV
CC
current, which is dominated by the gate charge
current, may be supplied by either the 5V 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 dissipa-
tion for the IC in this case is highest and is equal to V
IN
•
I
INTVCC
. The gate charge current is dependent on operating
frequency as discussed in the Effi ciency Considerations
section. The junction temperature can be estimated by
using the equations given in Note 3 of the Electrical Char-
acteristics. For example, the LTC3834 INTV
CC
current is
limited to less than 41mA from a 24V supply when in the
G package and not using the EXTV
CC
supply:
T
J
= 70°C + (41mA)(36V)(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 the LTC3834 switch-
ing regulator output (4.7V ≤ V
OUT
≤ 10V) during normal
operation and from the V
IN
LDO when the output is out
of regulation (e.g., start-up, short circuit). If more current
is required through the EXTV
CC
LDO than is specifi 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 that 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
4.7V 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 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 7V range, it may be used
to power EXTV
CC
providing it is compatible with the
MOSFET gate drive requirements.
4. EXTV
CC
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 6.