Datasheet LT8365 (Analog Devices) - 10

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APPLICATIONS INFORMATION ACHIEVING ULTRALOW QUIESCENT CURRENT
To enhance efficiency at light loads the LT8365 uses a low ripple Burst Mode architecture. This keeps the IL1+ IL2 100mA/DIV output capacitor charged to the desired output voltage while minimizing the input quiescent current and output ripple. In Burst Mode operation, the LT8365 delivers single VOUT 20mA/DIV small pulses of current to the output capacitor followed by sleep periods where the output power is supplied by 2µs/DIV 8365 F02 the output capacitor. While in sleep mode, the LT8365 consumes only 9µA.
Figure 2. Burst Mode Operation
As the output load decreases, the frequency of single cur- RT pin as shown in Figure 1. The output load at which rent pulses decreases (see Figure 1) and the percentage of the LT8365 reaches the fixed frequency varies based time the LT8365 is in sleep mode increases, resulting in on input voltage, output voltage, and inductor choice. much higher light load efficiency than for typical convert- ers. To optimize the quiescent current performance at light
PROGRAMMING INPUT TURN-ON AND TURN-OFF
loads, the current in the feedback resistor divider must be
THRESHOLDS WITH EN/UVLO PIN
minimized as it appears to the output as load current. In ad- dition, all possible leakage currents from the output should The EN/UVLO pin voltage controls whether the LT8365 is also be minimized as they all add to the equivalent output enabled or is in a shutdown state. A 1.6V reference and a load. The largest contributor to leakage current can be due comparator A6 with built-in hysteresis (typical 80mV) allow to the reverse biased leakage of the Schottky diode (see the user to accurately program the system input voltage Diode Selection in the Applications Information section). at which the IC turns on and off (see the Block Diagram). The typical input falling and rising threshold voltages can While in Burst Mode operation, the current limit of be calculated by the following equations: the switch is approximately 325mA resulting in the output voltage ripple shown in Figure 2. Increasing VIN(FALLING,UVLO(–)) = 1.60 • R3 + R4 the output capacitance will decrease the output ripple R4 proportionally. As the output load ramps upward from V zero the switching frequency will increase but only up IN(RISING, UVLO(+)) = 1.68 • R3 + R4 R4 to the fixed frequency defined by the resistor at the VIN current is reduced below 1µA when the EN/UVLO pin 600 voltage is less than 0.2V. The EN/UVLO pin can be con- nected directly to the input supply VIN for always-enabled operation. A logic input can also control the EN/UVLO pin. 400 When operating in Burst Mode operation for light load currents, the current through the R3 and R4 network can easily be greater than the supply current consumed by the 200 LT8365. Therefore, R3 and R4 should be large enough to SWITCHING FREQUENCY (kHz) minimize their effect on efficiency at light loads. VIN = 50V, VOUT = 100V, BOOST 0 0 10 20 30 40 50 LOAD CURRENT (mA) 8365 F01
Figure 1. Burst Frequency vs Load Current
Rev. 0 10 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Typical Application Related Parts