Datasheet LT8608, LT8608B (Analog Devices) - 10

link to page 11 LT8608/LT8608B
OPERATION
The LT8608 is a monolithic constant frequency current switch is shut down, reducing the input supply current to mode step-down DC/DC converter. An oscillator with 1.7μA. In a typical application, 2.5μA will be consumed frequency set using a resistor on the RT pin turns on from the input supply when regulating with no load. The the internal top power switch at the beginning of each SYNC pin is tied low to use Burst Mode operation and clock cycle. Current in the inductor then increases until can be floated to use pulse-skipping mode. If a clock is the top switch current comparator trips and turns off the applied to the SYNC pin the part will synchronize to an top power switch. The peak inductor current at which the external clock frequency and operate in pulse-skipping top switch turns off is controlled by the voltage on the mode. While in pulse-skipping mode the oscillator oper- internal VC node. The error amplifier servos the VC node ates continuously and positive SW transitions are aligned by comparing the voltage on the VFB pin with an internal to the clock. During light loads, switch pulses are skipped 0.778V reference. When the load current increases it to regulate the output and the quiescent current will be causes a reduction in the feedback voltage relative to the several mA. The SYNC pin may be tied high for spread reference leading the error amplifier to raise the VC volt- spectrum modulation mode, and the LT8608 will oper- age until the average inductor current matches the new ate similar to pulse-skipping mode but vary the clock load current. When the top power switch turns off the frequency to reduce EMI. The LT8608 DFN has no SYNC synchronous power switch turns on until the next clock pin and will always operate in Burst Mode operation. The cycle begins or inductor current falls to zero. If overload LT8608B DFN has no SYNC pin and will always operate conditions result in excess current flowing through the in pulse-skipping mode. bottom switch, the next clock cycle will be delayed until Comparators monitoring the FB pin voltage will pull the switch current returns to a safe level. PG pin low if the output voltage varies more than ±8.5% If the EN/UV pin is low, the LT8608 is shut down and draws (typical) from the set point, or if a fault condition is present. 1µA from the input. When the EN/UV pin is above 1.05V, The oscillator reduces the LT8608’s operating frequency the switching regulator becomes active. when the voltage at the FB pin is low. This frequency fold- To optimize efficiency at light loads, the LT8608 enters back helps to control the inductor current when the output Burst Mode operation during light load situations. Between voltage is lower than the programmed value which occurs bursts, all circuitry associated with controlling the output during start-up. When a clock is applied to the SYNC pin the frequency foldback is disabled.
APPLICATIONS INFORMATION Achieving Ultralow Quiescent Current
As the output load decreases, the frequency of single cur- To enhance efficiency at light loads, the LT8608 enters into rent pulses decreases (see Figure 1) and the percentage of low ripple Burst Mode operation, which keeps the output time the LT8608 is in sleep mode increases, resulting in capacitor charged to the desired output voltage while much higher light load efficiency than for typical convert- minimizing the input quiescent current and minimizing ers. By maximizing the time between pulses, the converter output voltage ripple. In Burst Mode operation the LT8608 quiescent current approaches 2.5µA for a typical application delivers single small pulses of current to the output capaci- when there is no output load. Therefore, to optimize the tor followed by sleep periods where the output power is quiescent current performance at light loads, the current supplied by the output capacitor. While in sleep mode the in the feedback resistor divider must be minimized as it LT8608 consumes 1.7μA. appears to the output as load current. Rev C 10 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts