ltc3810 Linear Technology Corporation, ltc3810 Datasheet - Page 21

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

APPLICATIONS INFORMATION
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
from overheating. Soft-start cycles are then attempted at
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
STARTUP
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
<
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
= 1.4μA (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
= 0.1μA (SINK)
V
)
TH
such that the resistor
3810 F10
+ 3mA
NDRV
LTC3810
+V
T
IN(MIN)
21
3810fa
(1)
as