LTC3769_15 Datasheet, PDF (14/32 Pages) Linear Technology – 60V Low IQ Synchronous Boost Controller

LTC3769
Operation
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 TG is 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 LTC3769
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 LTC3769
operates in forced continuous mode when synchronized.
OVMODE should be tied to ground or left floating in cir-
cuits, such as automotive applications, where the input
voltage can often be above the regulated output voltage
and it is desirable to turn on TG to “pass through” the
input voltage to the output.
Operation at Low SENSE Pin Common Mode Voltage
The current comparator in the LTC3769 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. Figure 10
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
The 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.
3769f
14
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