Datasheet

LTC3810
22
3810fc
The external NMOS for the linear regulator should be a
standard 3V threshold type (i.e., not a logic level threshold).
The rate of charge of INTV
CC
from 0V to 10V is controlled
by the LTC3810 to be approximately 75μs regardless of
the size of the capacitor connected to the INTV
CC
pin. The
charging current for this capacitor is approximately:
I
C
=
10V
75μs
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C
INTVCC
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
> 6.7V. 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 6.7V <
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 6.7V. 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
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To operate properly, the fault timeout operation must be
disabled by choosing
R
NDRV
> (V
IN(MAX)
– 10V – 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 2M resistor
from the 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 12V, 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
6.7V before C
INTVCC
/C
DRVCC
discharges below the falling
UV threshold of 6V. This is ensured if:
C
INTVCC
+ C
DRVCC
>I
G
•
Largerof
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)
–12V)/I
Q,SHUTDOWN
APPLICATIONS INFORMATION
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