LTC3784_15 Datasheet, PDF (14/38 Pages) Linear Technology – 60V PolyPhase Synchronous Boost Controller

LTC3784
Operation
Overvoltage Mode Selection
The OVMODE pin is used to select how the LTC3784
operates during an overvoltage event, defined as when
the output feedback voltage (V_FB) is greater than 110%
of its normal regulated point of 1.2V. It is also used to
determine the light-load mode of operation when the
LTC3784 is synchronized to an external clock through the
PLLIN/MODE pin.
The OVMODE pin is a logic input that should normally be
tied to INTV_CC or grounded. Alternatively, the pin can be
left floating, which allow a weak internal resistor to pull
it down to ground.
OVMODE = INTV_CC: An overvoltage event causes the error
amplifier to pull the ITH pin low. In Burst Mode operation,
this causes the LTC3784 to go to sleep and TG1/TG2 and
BG1/BG2 are held off. In pulse-skipping mode, BG1/BG2 are
held off and TG1/TG2 will turn on if the inductor current is
positive. In forced continuous mode, TG1/TG2 (and BG1/
BG2) will switch on and off as the LTC3784 will regulate the
inductor current to a negative peak value (corresponding
to ITH = 0V) to discharge the output.
When OVMODE is tied to INTV_CC, the LTC3784 operates
in pulse-skipping mode when synchronized.
In summary, with OVMODE = INTV_CC, the inductor cur-
rent is not allowed to go negative (reverse from output to
input) except in forced continuous mode, where it does
reverse current but in a controlled manner with a regulated
negative peak current. OVMODE should be tied to INTV_CC
in applications where the output voltage may sometimes
be above its regulation point (for example, if the output
is a battery or if there are other power supplies driving
the output) and no reverse current flow from output to
input is desired.
OVMODE Grounded or Left Floating: When OVMODE is
grounded or left floating, overvoltage protection is enabled
and the TG1/TG2 are turned on continuously until the
overvoltage condition is cleared, regardless of whether
Burst Mode operation, pulse-skipping mode, or forced
continuous mode is selected by the PLLIN/MODE pin.
This can cause large negative inductor currents to flow
from the output to the input if the output voltage is higher
than the input voltage.
Note however that in Burst Mode operation, the LTC3784
is in sleep during an overvoltage condition, which disables
the internal oscillator and BOOST-SW charge pump. So the
BOOST-SW voltage may discharge (due to leakage) if the
overvoltage conditions persists indefinitely. If BOOST-SW
discharges, then by definition TG would turn off.
When OVMODE is grounded or left floating, the LTC3784
operates in forced continuous mode when synchronized.
OVMODE should be tied to ground or left floating in circuits,
such as automotive applications, where the input voltage
can often be above the regulated output voltage and it is
desirable to turn on TG1/TG2 to “pass through” the input
voltage to the output.
Operation at Low SENSE Pin Common Mode Voltage
The current comparator in the LTC3784 is powered directly
from the SENSE⁺ pin. This enables the common mode
voltage of the SENSE⁺ and SENSE^– pins to operate at as
low as 2.3V, which is below the UVLO threshold. The figure
on the first page shows a typical application in which the
controller’s VBIAS is powered from V_OUT while the V_IN
supply can go as low as 2.3V. If the voltage on SENSE⁺
drops below 2.3V, the SS pin will be held low. When the
SENSE voltage returns to the normal operating range, the
SS pin will be released, initiating a new soft-start cycle.
BOOST Supply Refresh and Internal Charge Pump
Each top MOSFET driver is biased from the floating bootstrap
capacitor, C_B, which normally recharges during each cycle
through an external diode when the bottom MOSFET turns
on. There are two considerations for keeping the BOOST
supply at the required bias level. During start-up, if the
bottom MOSFET is not turned on within 200μs after UVLO
goes low, the bottom MOSFET will be forced to turn on for
~400ns. This forced refresh generates enough BOOST-SW
voltage to allow the top MOSFET ready to be fully enhanced
instead of waiting for the initial few cycles to charge up.
There is also an internal charge pump that keeps the required
bias on BOOST. The charge pump always operates in both
forced continuous mode and pulse-skipping mode. In Burst
Mode operation, the charge pump is turned off during sleep
and enabled when the chip wakes up. The internal charge
pump can normally supply a charging current of 55μA.
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