Datasheet

LTC3812-5
21
38125fc
to boost this voltage above 4.7V. In this mode, EXTV
CC
is
grounded and the NMOS is chosen to handle the worst-
case power dissipation:
P
MOSFET
= (V
IN(MAX)
)[(f)(Q
G(TOP)
+ Q
G(BOTTOM)
+ 3mA]
To operate properly, the fault timeout operation must be
disabled by choosing
R
NDRV
> (V
IN(MAX)
– 5.5V – V
T
)/270μA
If the required R
NDRV
value results in an unacceptable
value for V
IN(MIN)
(see Equation 1), fault timeout operation
can also be disabled by connecting a 500k to 1M resistor
from RUN/SS to INTV
CC
.
Using Trickle Charge Mode
Trickle charge mode is selected by shorting NDRV and
INTV
CC
and connecting EXTV
CC
to V
OUT
. Trickle charge mode
has the advantage of not requiring an external MOSFET but
takes longer to start up due to slow charge up of C
INTVCC
through R
PULLUP
(t
DELAY
= 0.77 • R
PULLUP
• C
INTVCC
) and
usually requires a larger INTV
CC
capacitor value to hold
up the supply voltage during start-up. Once the INTV
CC
voltage reaches the trickle charge UV threshold of 9V, the
drivers will turn on and start discharging C
INTVCC
at a rate
determined by the driver current I
G
. In order to ensure
proper start-up, C
INTVCC
must be chosen large enough so
that the EXTV
CC
voltage reaches the switchover threshold
of 4.7V before C
INTVCC
discharges below the falling UV
threshold of 4V. This is ensured if:
C
INTVCC
>I
G
•Larger of
C
OUT
I
MAX
or
5.5 •10
5
•C
SS
V
OUT(REG)






where I
G
is the gate drive current = (f)(Q
G(TOP)
+ Q
G(BOTTOM)
)
and I
MAX
is the maximum inductor current selected by
V
RNG
.
For R
PULLUP
, the value should fall in the following range
to ensure proper start-up:
Min R
PULLUP
> (V
IN(MAX)
– 14V)/I
CCSR
Max R
PULLUP
< (V
IN(MIN)
– 9V)/I
Q,SHUTDOWN
Using an External Supply Connected to the INTV
CC
If an external supply is available between 4.2V and 14V,
the supply can be connected directly to the INTV
CC
pins.
In this mode, INTV
CC
, EXTV
CC
and NDRV must be shorted
together.
INTV
CC
Supply and the EXTV
CC
Connection
The LTC3812-5 contains an internal low dropout regula-
tor to produce the 5.5V INTV
CC
supply from the EXTV
CC
pin voltage. This regulator turns on when the EXTV
CC
pin
is above 4.7V and remains on until EXTV
CC
drops below
4.45V. 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 4.7V 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 4.7V < 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
< 4.7V. The low voltage output can be boosted
using a charge pump or fl yback winding to greater than
4.7V.
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 4.2V <
V
EXT
< 14V.
APPLICATIONS INFORMATION