Datasheet

LTC3810
23
3810fc
Using an External Supply Connected to the INTV
CC
/
DRV
CC
Pins
If an external supply is available between 6.2V and 14V,
the supply can be connected directly to the INTV
CC
/DRV
CC
pins. In this mode, INTV
CC
, EXTV
CC
and NDRV must be
shorted together.
INTV
CC
/DRV
CC
Supply and the EXTV
CC
Connection
The LTC3810 contains an internal low dropout regulator to
produce the 10V INTV
CC
/DRV
CC
supply from the EXTV
CC
pin voltage. This regulator turns on when the EXTV
CC
pin
is above 6.7V and remains on until EXTV
CC
drops below
6.4V. This allows the IC/MOSFET power to be derived from
the output or an output derived boost network during
normal operation and from the external NMOS from V
IN
during start-up or short-circuit. Using the EXTV
CC
pin in
this way results in signifi cant effi ciency gains compared
to what would be possible when deriving this power
continuously from the typically much higher V
IN
voltage.
The EXTV
CC
connection also allows the power supply to
be confi gured in trickle charge mode in which it starts up
with a high valued “bleed” resistor connected from V
IN
to INTV
CC
to charge up the INTV
CC
capacitor. As soon as
the output rises above 6.7V the internal EXTV
CC
regulator
takes over before the INTV
CC
capacitor discharges below
the UV threshold. When the EXTV
CC
regulator is active,
the EXTV
CC
pin can supply up to 50mA RMS. Do not ap-
ply more than 15V to the EXTV
CC
pin. The following list
summarizes the possible connections for EXTV
CC
:
1. EXTV
CC
grounded. This connection will require INTV
CC
to be powered continuously from an external NMOS
from V
IN
resulting in an effi ciency penalty as high as
10% at high input voltages.
2. EXTV
CC
connected directly to V
OUT
. This is the normal
connection for 6.7V < V
OUT
< 15V and provides the
highest effi ciency. The power supply will start up using
an external NMOS or a bleed resistor until the output
supply is available.
3. EXTV
CC
connected to an output-derived boost network.
If V
OUT
< 6.7V. The low voltage output can be boosted
using a charge pump or fl yback winding to greater
than 6.7V.
4. EXTV
CC
connected to INTV
CC
. This is the required
connection for EXTV
CC
if INTV
CC
is connected to an
external supply where the external supply is 6.2V <
V
EXT
< 15V.
Applications using large MOSFETs with a high input volt-
age and high frequency of operation may result in a large
EXTV
CC
pin current. Therefore, it is good design practice
to verify that the maximum junction temperature rating
and RMS current rating are within the maximum limits.
Typically, most of the EXTV
CC
current consists of the
MOSFET gates current. In continuous mode operation,
this EXTV
CC
current is:
I
EXTVCC
= fQ
G(TOP)
+ Q
G(BOTTOM)
()
+ 3mA < 50mA
The junction temperature can be estimated from the
equations given in Note 2 of the Electrical Characteristics
as follows:
T
J
= T
A
+ I
EXTVCC
• (V
EXTVCC
– V
INTVCC
)(100°C/W)
< 125°C
If absolute maximum ratings are exceeded, consider
using an external supply connected directly to the
INTV
CC
pin.
APPLICATIONS INFORMATION
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