LT8310
17
8310f
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INTV
CC
Regulator Bypassing and Operation
The GATE and SOUT pin drivers and other chip loads
are powered from the INTV
CC
pin, which is an internally
regulated supply. The internal low dropout regulator re-
quires a
capacitor from the IN
TV
CC
pin to GND for stable
operation and to store the charge for the large GATE and
SOUT switching currents; a 4.7μF capacitor is adequate
for most applications. Choose a 16V rated low ESR, X7R
ceramic capacitor for best performance. Place the capaci
-
tor close
to the LT8310 to minimize the trace length both
to
the INTV
CC
pin and to the chip ground. In shutdown,
the INTV
CC
pin sinks 125μA (typical) until the pin voltage
falls below 4.75V.
An internal current limit on the INTV
CC
output protects
the LT8310 from excessive on-chip power dissipation.
The minimum specified current limit should be consid
-
ered when choosing the switching N-channel MOSFET
and the operating frequency. Careful selection of a lower
Q
G
MOSFET allows higher GATE switching frequencies,
which leads to smaller magnetics. SOUT switching current
must be accounted for when that pin drives a MOSFET
gate, but in typical applications where SOUT is unused or
drives an
AC-coupled pulse transformer, GATE switching
dominates
the steady-state regulator load and the SOUT
current may be ignored. The MOSFET gate drive switching
current required may be calculated using Equation 7, see
the Thermal Considerations section for further information.
I
DRIVE
= Q
G
• f
SW
[7]
The INTV
CC
voltage tracks a few hundred millivolts below
the supply voltage until the regulation loop closes when
V
IN
exceeds about 10.5V. The INTV
CC
pin has its own
undervoltage disable set to 4.75V (typical) that protects
the external MOSFET from excessive power dissipation
caused by not being fully enhanced. If the INTV
CC
pin
drops below its undervoltage threshold, the GATE pin will
be forced to GND, the SOUT pin will follow the INTV
CC
voltage, and the soft-start pin will be reset.
The regulator may be overdriven from external circuitry to
reduce switching power dissipation in the LT8310 package,
or to drive a MOSFET switch with a high threshold. The
overdriven INTV
CC
pin voltage must be less than the IC
supply to avoid back-driving the V
IN
pin. The INTV
CC
pin
applicaTions inForMaTion
has its own overvoltage threshold set to 17.4V (typical) that
disables the system to protect MOSFETs rated for V
GS(MAX)
= 20V, a common specification. As with undervoltage
shutdown, the GATE pin will be forced to GND, the SOUT
pin will follow the INTV
CC
voltage, and the soft-start pin
will be reset. A 4.7μF 25V rated low ESR, X7R capacitor
is recommended when INTV
CC
is overdriven.
Programming the System Turn-On and Turn-Off
Thresholds
The system undervoltage and overvoltage thresholds are
programmed by a resistive voltage divider from V
IN
to
UVLO and OVLO, respectively (Figure 4). The falling UVLO
threshold,1.22V (nom), accurately sets the minimum op
-
erating V
IN
(Equation 8), below which the system goes into
low power mode. A 5.7μA (typical) pull-down current that
is active when the UVLO pin is below its falling threshold
provides rising hysteresis that sets the minimum start-
up V
IN
(Equation 9). The built-in comparator hysteresis
contributes a small amount to the rising threshold as well.
V
IN(UVLO FALLING)
= 1.22V •
+
+
R2+R1
⎛
⎝
⎜
⎞
⎠
⎟
[8]
IN(UVLO RISING)
IN(UVLO FALLING)
+
+ 40mV •
R3+ R2+ R1
R2+ R1
⎛
⎝
⎜
⎞
⎠
⎟
[9]
The rising OVLO threshold, 1.25V (nom), accurately sets
the maximum operating V
IN
(Equation 10), above which
the system stops switching and awaits soft-start. The
built-in comparator hysteresis provides falling hysteresis
that sets the maximum restart V
IN
(Equation 11).
Figure 4. Resistor Connections for System UVLO
and OVLO Threshold Programming
GND
LT8310
UVLO
OVLO
V
IN
V
IN
R3
R2
R1