LTC3810-5
22
38105fd
The safe operating area (SOA) for the external NMOS
should be chosen so that capacitor charging does not
damage the NMOS. Excessive values of capacitor are
unnecessary and should be avoided. Typically values in
the 1µF to 10µF work well.
One more design requirement for this mode is the minimum
soft-start capacitor value. The fault timeout is enabled
when SS/TRACK voltage is greater than 4V. This gives the
power supply time to bring the output up before it starts
the timeout sequence. To prevent timeout sequence from
starting prematurely during start-up, a minimum C
SS
value
is necessary to ensure that V
SS/TRACK
< 4V until V
EXTVCC
> 4.7V. To ensure this, choose:
C
SS
> C
OUT
• (2.3 • 10
–6
)/I
OUT(MAX)
Mode 2 should be used if V
OUT
is outside of the 4.7V <
EXTV
CC
< 15V operating range and the extra complexity
of a charge pump or extra inductor winding is not wanted
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
TH
)/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 1Meg resistor
from SS/TRACK pin 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
and C
DRVCC
through R
PULLUP
(t
DELAY
= 0.77
• R
PULLUP
• C
DRVCC
) and usually requires larger INTV
CC
/
DRV
CC
capacitor values to hold up the supply voltage dur-
ing start-up. Once the INTV
CC
/DRV
CC
voltage reaches the
trickle charge UV threshold of 9V, the drivers will turn on
and start discharging C
INTVCC
/C
DRVCC
at a rate determined
by the driver current I
G
. In order to ensure proper start-
up, C
INTVCC
/C
DRVCC
must be chosen large enough so that
the EXTV
CC
voltage reaches the switchover threshold of
4.7V before C
INTVCC
/C
DRVCC
discharges below the falling
UV threshold of 4V. This is ensured if:
C
INTVCC
+
C
DRVCC
>
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
/
DRV
CC
Pins
If an external supply is available between 4.5V 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 LTC3810-5 contains an internal low dropout regulator
to produce the 5.5V INTV
CC
/DRV
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 significant efficiency 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 configured 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
applicaTions inForMaTion