Datasheet LT8607, LT8607B (Analog Devices) - 10

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APPLICATIONS INFORMATION Achieving Ultralow Quiescent Current
programmed for Burst Mode operation and cannot enter To enhance efficiency at light loads, the LT8607 enters pulse-skipping mode. The LT8607B DFN is programmed into low ripple Burst Mode operation, which keeps the for pulse-skipping mode and cannot enter Burst Mode output capacitor charged to the desired output voltage operation. while minimizing the input quiescent current and mini- 2500 mizing output voltage ripple. In Burst Mode operation the L = 2.2µH 2250 V LT8607 delivers single small pulses of current to the out- IN = 12V 2000 VOUT = 3.3V put capacitor followed by sleep periods where the output SYNC = 0V OR LT8607 DFN 1750 power is supplied by the output capacitor. While in sleep 1500 mode the LT8607 consumes 1.7µA. 1250 As the output load decreases, the frequency of single cur- 1000 rent pulses decreases (see Figure 1) and the percentage 750 of time the LT8607 is in sleep mode increases, result- SWITHCING FREQUENCY (kHz) 500 ing in much higher light load efficiency than for typical 250 0 converters. By maximizing the time between pulses, the 0 25 50 75 100 125 converter quiescent current approaches 3.0µA for a typi- OUTPUT CURRENT (mA) 8607 F01 cal application when there is no output load. Therefore,
Figure 1. Burst Frequency vs Output Current
to optimize the quiescent current performance at light loads, the current in the feedback resistor divider must 150 be minimized as it appears to the output as load current. L = 2.2µH VIN = 12V 125 V While in Burst Mode operation the current limit of the OUT = 3.3V RT = 18.2kΩ top switch is approximately 250mA resulting in output 100 voltage ripple shown in Figure 3. Increasing the output 75 capacitance will decrease the output ripple proportionally. As load ramps upward from zero the switching frequency 50 will increase but only up to the switching frequency OUTPUT CURRENT (mA) programmed by the resistor at the RT pin as shown in 25 Table 1. The output load at which the LT8607 reaches the 0 programmed frequency varies based on input voltage, 0 5 10 15 20 25 30 35 40 45 INPUT VOLTAGE (V) output voltage, and inductor choice. 8607 F02 For some applications it is desirable for the LT8607 to
Figure 2. Minimum Load to Full Frequency (SYNC Float to 1.9V) (MSOP or LT8607B DFN)
operate in pulse-skipping mode, offering two major differ- ences from Burst Mode operation. First is the clock stays awake at all times and all switching cycles are aligned to VOUT the clock. In this mode much of the internal circuitry is 20mV/DIV awake at all times, increasing quiescent current to several ILOAD 200mA/DIV hundred µA. Second is that full switching frequency is reached at lower output load than in Burst Mode operation VSW 10V/DIV as shown in Figure 2. To enable pulse-skipping mode the 2µs/DIV SYNC pin is floated. To achieve spread spectrum modula- tion with pulse-skipping mode, the SYNC pin is tied high. 8607 F03 While a clock is applied to the SYNC pin the LT8607 will
Figure 3. Burst Mode Operation
also operate in pulse-skipping mode. The LT8607 DFN is Rev. C 10 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts