LTC3810EG LINER [Linear Technology], LTC3810EG Datasheet - Page 21

• Manufacturer Part Number: LTC3810EG
• Description: 100V Current Mode Synchronous Switching Regulator Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
is typically much higher than 14V a separate supply for
the IC power (INTV
be used. The LTC3810 has integrated bias supply control
circuitry that allows the IC/driver supply to be easily
generated from V
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
In Mode 1, a small external SOT23 MOSFET, controlled by
the NDRV pin, is used to generate a 10V start-up supply
from V
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
to V
internal low dropout regulator. For this mode to work
properly, EXTV
15V. If V
be used to raise EXTV
natively, Mode 2 should be used as explained later in this
section. If V
minimum 6.5V threshold, the MOSFET connected to V
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
OUT
IN
OUT
. The small SOT23 package can be used because
or a V
OUT
< 6.7V, a charge pump or extra winding can
CC
OUT
SS/TRACK
is shorted or otherwise goes below the
must be in the range 6.7V < EXTV
IN
TG/BG
V
CC
derived boost network) through an
OUT
and/or V
) and driver power (DRV
FAULT TIMEOUT
ENABLED
CC
DRIVER POWER
FROM V
START-UP
to the proper voltage, or alter-
IN
SHORT-CIRCUIT EVENT
OUT
with minimal external
DRIVER POWER
FROM V
CC
CC
/DRV
OUT
pin (connected
Figure 10. Fault Timeout Operation
CC
DRIVER POWER
FROM V
CC
Supply
EXTV
) must
IN
DRIVER OFF THRESHOLD
CC
CC
UV THRESHOLD
IN
<
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
current I
ing formulas:
where
and V
The value of R
follows:
where V
MOSFETs. If minimum V
reducing R
an NMOS to reduce V
When using R
LTC3810 will enable the low duty cycle soft-start retries
only when the desired maximum power dissipation,
P
drivers on continuously otherwise. The shutoff/restart
times are a function of the TRACK/SS capacitor value.
MOSFET(MAX)
V
I
R
START-UP INTO SHORT-CIRCUIT
CC
IN(MIN)
NDRV
TH
= (f) Q
INTVCC(MIN)
is the threshold voltage of the MOSFET.
NDRV
I
SS/TRACK
NDRV
(
= V
P
MOSFET(MAX)
G(TOP)
, in the MOSFET is exceeded and leave the
is greater than 270μA by using the follow-
INTVCC(MIN)
NDRV
= 1.4 A (SOURCE)
and/or using a darlington NPN instead of
NDRV
is normally 6V for driving 60V to 100V
+ Q
270μA
equal to the computed value, the
T
to ~1.4V.
I
SS/TRACK
G(BOTTOM)
also affects the V
IN
+ (40μA) R
/ I
is not low enough, consider
CC
NDRV
= 0.1 A (SINK)
V
)
TH
such that the resistor
3810 F10
+ 3mA
NDRV
LTC3810
+V
T
IN(MIN)
21
3810fb
(1)
as