Datasheet LT8620 (Analog Devices) - 11

LT8620
APPLICATIONS INFORMATION Achieving Ultralow Quiescent Current
In order to achieve higher light load efficiency, more energy To enhance efficiency at light loads, the LT8620 operates must be delivered to the output during the single small in low ripple Burst Mode operation, which keeps the out- pulses in Burst Mode operation such that the LT8620 can put capacitor charged to the desired output voltage while stay in sleep mode longer between each pulse. This can be minimizing the input quiescent current and minimizing achieved by using a larger value inductor (i.e. 4.7µH), and output voltage ripple. In Burst Mode operation the LT8620 should be considered independent of switching frequency delivers single small pulses of current to the output capaci- when choosing an inductor. For example, while a lower tor followed by sleep periods where the output power is inductor value would typically be used for a high switch- supplied by the output capacitor. While in sleep mode the ing frequency application, if high light load efficiency is LT8620 consumes 1.7μA. desired, a higher inductor value should be chosen. As the output load decreases, the frequency of single cur- While in Burst Mode operation the current limit of the top rent pulses decreases (see Figure 1a) and the percentage switch is approximately 400mA resulting in output voltage of time the LT8620 is in sleep mode increases, resulting in ripple shown in Figure 2. Increasing the output capacitance much higher light load efficiency than for typical convert- will decrease the output ripple proportionally. As load ramps ers. By maximizing the time between pulses, the converter upward from zero the switching frequency will increase quiescent current approaches 2.5µA for a typical application but only up to the switching frequency programmed by when there is no output load. Therefore, to optimize the the resistor at the RT pin as shown in Figure 1a. The out- quiescent current performance at light loads, the current put load at which the LT8620 reaches the programmed in the feedback resistor divider must be minimized as it frequency varies based on input voltage, output voltage, appears to the output as load current. and inductor choice.
Burst Frequency Minimum Load to Full Frequency (SYNC DC High)
800 100 VIN = 12V VOUT = 5V VOUT = 5V 700 fSW = 700kHz 80 600 500 60 400 300 40 LOAD CURRENT (mA) 200 SWITCHING FREQUENCY (kHz) 20 100 0 0 0 50 100 150 200 5 15 25 35 45 55 65 LOAD CURRENT (mA) INPUT VOLTAGE (V) 8620 F01a 8620 F01b
(1a) (1b) Figure 1. SW Frequency vs Load Information in Burst Mode Operation (1a) and Pulse-Skipping Mode (1b)
8620fa For more information www.linear.com/LT8620 11 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 Package Description Typical Application Related Parts