ltc3810 Linear Technology Corporation, ltc3810 Datasheet - Page 22

• Manufacturer Part Number: ltc3810
• Description: 100v Current Mode Synchronous Switching Regulator Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LTC3810
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
by the LTC3810 to be approximately 75μs regardless of
the size of the capacitor connected to the INTV
charging current for this capacitor is approximately:
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
is necessary to ensure that V
> 6.7V. To ensure this, choose:
Mode 2 should be used if V
EXTV
of a charge pump or extra inductor winding is not wanted
to boost this voltage above 6.7V. In this mode, EXTV
grounded and the NMOS is chosen to handle the worst-
case power dissipation:
To operate properly, the fault timeout operation must be
disabled by choosing
22
C
R
I
P
C
MOSFET
SS
NDRV
=
CC
> C
< 15V operating range and the extra complexity
75μs
10V
> (V
OUT
= V
(
IN(MAX)
IN(MAX)
• (2.3 x 10
C
INTVCC
)
– 10V – V
( )
f
-6
(
CC
Q
)/I
G(TOP)
OUT
from 0V to 10V is controlled
OUT(MAX)
SS/TRACK
TH
is outside of the 6.7V <
+ Q
)/270μA
G(BOTTOM)
< 4V until V
)
CC
+ 3mA
pin. The
SS
EXTVCC
value
CC
is
If the required R
value for V
can also be disabled by connecting a 500k to 2M resistor
from the SS/TRACK pin to INTV
Using Trickle Charge Mode
Trickle charge mode is selected by shorting NDRV and
INTV
mode has the advantage of not requiring an external
MOSFET but takes longer to start up due to slow charge
up of C
• R
DRV
ing start-up. Once the INTV
trickle charge UV threshold of 12V, the drivers will turn on
and start discharging C
by the driver current I
up, C
the EXTV
6.7V before C
UV threshold of 6V. This is ensured if:
Where I
Q
selected by V
For R
to ensure proper start-up:
G(BOTTOM)
Min R
Max R
C
PULLUP
INTVCC
L arger of
CC
CC
INTVCC
PULLUP
INTVCC
capacitor values to hold up the supply voltage dur-
and connecting EXTV
G
PULLUP
CC
PULLUP
IN(MIN)
• C
/C
+ C
is the gate drive current = (f)(Q
) and I
, the value should fall in the following range
voltage reaches the switchover threshold of
RNG
DRVCC
and C
DRVCC
INTVCC
DRVCC
C
I
> (V
MAX
< (V
.
(see Equation 1), fault timeout operation
OUT
NDRV
MAX
DRVCC
) and usually requires larger INTV
must be chosen large enough so that
/C
IN(MAX)
IN(MIN)
>I
G
DRVCC
or
INTVCC
is the maximum inductor current
value results in an unacceptable
. In order to ensure proper start-
G
through R
•
5.5 • 10
CC
–12V)/I
–14V)/I
discharges below the falling
V
/C
/DRV
OUT(REG)
CC
DRVCC
CC
to V
5
.
CC
• C
PULLUP
CCSR
Q,SHUTDOWN
voltage reaches the
at a rate determined
OUT
SS
. Trickle charge
(t
DELAY
G(TOP)
= 0.77
3810fa
CC
+
/