説明

The LTC3631 is a high voltage, high efficiency step-down DC/DC converter with internal high side and synchronous power switches that draws only 12μA typical DC supply current at no load while maintaining output voltage regulation.

The LTC3631 can supply up to 100mA load current and features a programmable peak current limit that provides a simple method for optimizing efficiency in lower current applications. The LTC3631’s combination of Burst Mode operation, integrated power switches, low quiescent current, and programmable peak current limit provides high efficiency over a broad range of load currents.

With its wide 4.5V to 45V input range and internal over voltage monitor capable of protecting the part from 60V surges, the LTC3631 is a robust converter suited for regulating a wide variety of power sources. Additionally, the LTC3631 includes a precise run threshold and a soft-start feature to guarantee that power system start-up is well-controlled in any environment.

The LTC3631 is available in the thermally enhanced 3mm × 3mm DFN and MS8E packages.

 

Operation

The LTC3631 is a step-down DC/DC converter with internal power switches that uses Burst Mode control, combining low quiescent current with high switching frequency, which results in high efficiency across a wide range of load currents. Burst Mode operation functions by using short “burst” cycles to ramp the inductor current through the internal power switches, followed by a sleep cycle where the power switches are off and the load current is supplied by the output capacitor. During the sleep cycle, the LTC3631 draws only 12µA of supply current. At light loads, the burst cycles are a small percentage of the total cycle time which minimizes the average supply current, greatly improving efficiency.

 

Main Control Loop

The feedback comparator monitors the voltage on the VFB pin and compares it to an internal 800mV reference. If this voltage is greater than the reference, the comparator activates a sleep mode in which the power switches and current comparators are disabled, reducing the VIN pin

supply current to only 12µA. As the load current discharges the output capacitor, the voltage on the VFB pin decreases.

When this voltage falls 5mV below the 800mV reference, the feedback comparator trips and enables burst cycles.

At the beginning of the burst cycle, the internal high side power switch (P-channel MOSFET) is turned on and the inductor current begins to ramp up. The inductor current increases until either the current exceeds the peak current comparator threshold or the voltage on the VFB pin exceeds 800mV, at which time the high side power switch is turned off, and the low side power switch (N-channel MOSFET) turns on. The inductor current ramps down until the reverse current comparator trips, signaling that the current is close to zero. If the voltage on the VFB pin is still less than the 800mV reference, the high side power switch is turned on again and another cycle commences.

The average current during a burst cycle will normally begreater than the average load current. For this architecture, the maximum average output current is equal to half of the peak current.

The hysteretic nature of this control architecture results in a switching frequency that is a function of the input voltage, output voltage and inductor value. This behavior provides inherent short-circuit protection. If the output is shorted to ground, the inductor current will decay very slowly during a single switching cycle. Since the high side switch turns on only when the inductor current is near zero, the LTC3631 inherently switches at a lower frequency during start-up or short-circuit conditions.

 

Start-Up and Shutdown

If the voltage on the RUN pin is less than 0.7V, the LTC3631 enters a shutdown mode in which all internal circuitry is disabled, reducing the DC supply current to 3µA. When the voltage on the RUN pin exceeds 1.21V, normal operation of the main control loop is enabled. The RUN pin comparator has 110mV of internal hysteresis, and therefore must fall below 1.1V to disable the main control loop. The HYST pin provides an added degree of flexibility for the RUN pin operation. This open-drain output is pulled to ground whenever the RUN comparator is not tripped, signaling that the LTC3631 is not in normal operation. In applications where the RUN pin is used to monitor the VIN voltage through an external resistive divider, the HYST pin can be used to increase the effective RUN comparator hysteresis.