Datasheet

LTC3810
21
3810fc
is typically much higher than 14V a separate supply for
the IC power (INTV
CC
) and driver power (DRV
CC
) must
be used. The LTC3810 has integrated bias supply control
circuitry that allows the IC/driver supply to be easily
generated from V
IN
and/or V
OUT
with minimal external
components. There are four ways to do this as shown in
the simplifi ed schematics of Figure 4 and explained in the
following sections.
Using the Linear Regulator for INTV
CC
/DRV
CC
Supply
In Mode 1, a small external SOT23 MOSFET, controlled by
the NDRV pin, is used to generate a 10V start-up supply
from V
IN
. The small SOT23 package can be used because
the NMOS is on continuously only during the brief start-up
period. As soon as the output voltage reaches 6.7V, the
LTC3810 turns off the external NMOS and the LTC3810
regulates the 10V supply from the EXTV
CC
pin (connected
to V
OUT
or a V
OUT
derived boost network) through an
internal low dropout regulator. For this mode to work
properly, EXTV
CC
must be in the range 6.7V < EXTV
CC
<
15V. If V
OUT
< 6.7V, a charge pump or extra winding can
be used to raise EXTV
CC
to the proper voltage, or alter-
natively, Mode 2 should be used as explained later in this
section. If V
OUT
is shorted or otherwise goes below the
minimum 6.5V threshold, the MOSFET connected to V
IN
is turned back on to maintain the 10V supply. However if
the output cannot be brought up within a timeout period,
the drivers are turned off to prevent the SOT23 MOSFET
from overheating. Soft-start cycles are then attempted at
low duty cycle intervals to try to bring the output back up
(see Figure 10). This fault timeout operation is enabled
by choosing the choosing R
NDRV
such that the resistor
current I
NDRV
is greater than 270μA by using the follow-
ing formulas:
R
NDRV

P
MOSFET(MAX)
/I
CC
 V
TH
270μA
where
I
CC
= (f) Q
G(TOP)
+ Q
G(BOTTOM)
()
+ 3mA
and V
TH
is the threshold voltage of the MOSFET.
The value of R
NDRV
also affects the V
IN(MIN)
as
follows:
V
IN(MIN)
= V
INTVCC(MIN)
+ (40μA) R
NDRV
+V
T
(1)
where V
INTVCC(MIN)
is normally 6V for driving 60V to 100V
MOSFETs. If minimum V
IN
is not low enough, consider
reducing R
NDRV
and/or using a darlington NPN instead of
an NMOS to reduce V
T
to ~1.4V.
When using R
NDRV
equal to the computed value, the
LTC3810 will enable the low duty cycle soft-start retries
only when the desired maximum power dissipation,
P
MOSFET(MAX)
, in the MOSFET is exceeded and leave the
drivers on continuously otherwise. The shutoff/restart
times are a function of the TRACK/SS capacitor value.
Figure 10. Fault Timeout Operation
APPLICATIONS INFORMATION
SS/TRACK
V
OUT
TG/BG
FAULT TIMEOUT
ENABLED
EXTV
CC
UV THRESHOLD
DRIVER OFF THRESHOLD
DRIVER POWER
FROM V
IN
SHORT-CIRCUIT EVENT START-UP INTO SHORT-CIRCUIT
START-UP
DRIVER POWER
FROM V
OUT
DRIVER POWER
FROM V
IN
I
SS/TRACK
= 1.4MA (SOURCE)
I
SS/TRACK
= 0.1MA (SINK)
3810 F10
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