LTC3810-5 Datasheet, PDF (21/36 Pages) Linear Technology – 60V Current Mode Synchronous Switching Regulator Controller

LTC3810-5
APPLICATIONS INFORMATION
V_IN is typically much higher than 14V a separate supply
for the IC power (INTV_CC) and driver power (DRV_CC) must
be used. The LTC3810-5 has integrated bias supply con-
trol 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 simplified schematics of Figure 3 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 5.5V 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
4.7V, the LTC3810-5 turns off the external NMOS and the
LTC3810-5 regulates the 5.5V 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 4.7V < EXTV_CC
< 15V. If V_OUT < 4.7V, 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 4.5V threshold, the MOSFET connected to V_IN
is turned back on to maintain the 5.5V 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
270μA
−
V_TH
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 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
LTC3810-5 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 TRACK/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 INTV_CC from 0V to 5.5V is controlled
by the LTC3810-5 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
FAULT TIMEOUT
DRIVER OFF THRESHOLD
ENABLED _{DRIVER POWER}
FROM V_OUT
SS/TRACK
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
38105 F10
Figure 10. Fault Timeout Operation
38105fc
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