Datasheet LTC3769 (Analog Devices) - 14

LTC3769
OPERATION
is a battery or if there are other power supplies driving
Operation at Low SENSE Pin Common Mode Voltage
the output) and no reverse current flow from output to The current comparator in the LTC3769 is powered directly input is desired. from the SENSE+ pin. This enables the common mode
OVMODE Grounded or Left Floating:
When OVMODE is voltage of the SENSE+ and SENSE– pins to operate at as grounded or left floating, overvoltage protection is enabled low as 2.3V, which is below the UVLO threshold. Figure 10 and TG is turned on continuously until the overvoltage shows a typical application in which the controller’s VBIAS condition is cleared, regardless of whether Burst Mode is powered from VOUT while the VIN supply can go as low operation, pulse-skipping mode, or forced continuous as 2.3V. If the voltage on SENSE+ drops below 2.3V, the mode is selected by the PLLIN/MODE pin. This can cause SS pin will be held low. When the SENSE voltage returns large negative inductor currents to flow from the output to the normal operating range, the SS pin will be released, to the input if the output voltage is higher than the input initiating a new soft-start cycle. voltage. Note however that in Burst Mode operation, the LTC3769
BOOST Supply Refresh and Internal Charge Pump
is in sleep during an overvoltage condition, which disables The top MOSFET driver is biased from the floating bootstrap the internal oscil ator and BOOST-SW charge pump. So the capacitor, CB, which normally recharges during each cycle BOOST-SW voltage may discharge (due to leakage) if the through an external diode when the bottom MOSFET turns overvoltage conditions persists indefinitely. If BOOST-SW on. There are two considerations for keeping the BOOST discharges, then by definition TG would turn off. supply at the required bias level. During start-up, if the When OVMODE is grounded or left floating, the LTC3769 bottom MOSFET is not turned on within 200μs after UVLO operates in forced continuous mode when synchronized. goes low, the bottom MOSFET will be forced to turn on for ~400ns. This forced refresh generates enough BOOST-SW OVMODE should be tied to ground or left floating in cir- voltage to allow the top MOSFET ready to be fully enhanced cuits, such as automotive applications, where the input instead of waiting for the initial few cycles to charge up. voltage can often be above the regulated output voltage There is also an internal charge pump that keeps the required and it is desirable to turn on TG to “pass through” the bias on BOOST. The charge pump always operates in both input voltage to the output. 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. 3769fa 14 For more information www.linear.com/LTC3769 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Revision History Typical Application Related Parts