LTC3812-5_15 Datasheet, PDF (20/34 Pages) Linear Technology – 60V Current Mode Synchronous Switching Regulator Controller

LTC3812-5
APPLICATIONS INFORMATION
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
270μA
−
V_T
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
FAULT TIMEOUT
ENABLED
DRIVER OFF THRESHOLD
DRIVER POWER
FROM V_OUT
RUN/SS
DRIVER POWER
DRIVER POWER
FROM V_IN
FROM V_IN
START-UP
EXTV_CC UV THRESHOLD
I_SS/TRACK = 1.4μA (SOURCE)
I_SS/TRACK = 0.1μA (SINK)
V_OUT
SHORT-CIRCUIT EVENT
START-UP INTO SHORT CIRCUIT
TG/BG
38125 F09
Figure 9. Fault Timeout Operation
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