Datasheet

LTC3812-5
20
38125fc
the drivers are turned off to prevent the SOT-23 MOSFET
from overheating. Soft-start cycles are then attempted at
low duty cycle intervals to try to bring the output back
up (see Figure 9). 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
T
270μA
where
I
CC
= (f)(Q
G(TOP)
+ Q
G(BOTTOM)
) + 3mA
and V
T
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 4.5V for driving logic level
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
LTC3812-5 will enable the low duty cycle soft-start re-
tries 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 RUN/SS capacitor value.
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 V
CC
from 0V to 5.5V is controlled
by the LTC3812-5 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
=
5.5V
75µs
⎛
⎝
⎜
⎞
⎠
⎟
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 RUN/SS 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
RUN/SS
< 4V until V
EXTVCC
>
4.7V. 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 4.7V <
EXTV
CC
< 15V operating range and the extra complexity
of a charge pump or extra inductor winding is not wanted
Figure 9. Fault Timeout Operation
APPLICATIONS INFORMATION
RUN/SS
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
IN
I
SS/TRACK
= 1.4μA (SOURCE)
I
SS/TRACK
= 0.1μA (SINK)
38125 F09
DRIVER POWER
FROM V
OUT