Datasheet LT8361 (Analog Devices) - 10

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APPLICATIONS INFORMATION ACHIEVING ULTRALOW QUIESCENT CURRENT
To enhance efficiency at light loads the LT8361 uses a low ripple Burst Mode architecture. This keeps the IL 200mA/DIV output capacitor charged to the desired output voltage while minimizing the input quiescent current and output ripple. In Burst Mode operation, the LT8361 delivers single VOUT small pulses of current to the output capacitor followed 2mV/DIV by sleep periods where the output power is supplied by 10µs/DIV 8361 F02 the output capacitor. While in sleep mode, the LT8361 consumes only 9µA.
Figure 2. Burst Mode Operation
As the output load decreases, the frequency of single cur- The output load at which the LT8361 reaches the fixed rent pulses decreases (see Figure 1) and the percentage of frequency varies based on input voltage, output voltage, time the LT8361 is in sleep mode increases, resulting in 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 LT8361 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 the be calculated by the following equations: switch is approximately 400mA resulting in the output voltage ripple shown in Figure 2. Increasing the output VIN(FALLING,UVLO(–)) = 1.60 • R3 + R4 capacitance will decrease the output ripple proportionally. R4 As the output load ramps upward from zero the switching V frequency will increase but only up to the fixed frequency IN(RISING, UVLO(+)) = 1.68 • R3 + R4 R4 defined by the resistor at the RT pin as shown in Figure 1. VIN current is reduced below 1µA when the EN/UVLO pin 2.5 voltage is less than 0.2V. The EN/UVLO pin can be con- FRONT PAGE APPLICATION VIN = 12V nected directly to the input supply VIN for always-enabled VOUT = 48V 2.0 operation. A logic input can also control the EN/UVLO pin. When operating in Burst Mode operation for light load 1.5 currents, the current through the R3 and R4 network can 1.0 easily be greater than the supply current consumed by the LT8361. Therefore, R3 and R4 should be large enough to 0.5 minimize their effect on efficiency at light loads. SWITCHING FREQUENCY (MHz) 0 0 20 40 60 80 100 LOAD CURRENT (mA) 8361 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