Datasheet LT8331 (Analog Devices) - 9

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APPLICATIONS INFORMATION ACHIEVING ULTRALOW QUIESCENT CURRENT
frequency varies based on input voltage, output voltage, To enhance efficiency at light loads the LT8331 uses a low and inductor choice. ripple Burst Mode architecture. This keeps the output capaci-
PROGRAMMING INPUT TURN-ON AND TURN-OFF
tor charged to the desired output voltage while minimizing
THRESHOLDS WITH EN/UVLO PIN
the input quiescent current and output ripple. In Burst Mode operation, the LT8331 delivers single small pulses of current The EN/UVLO pin voltage controls whether the LT8331 is to the output capacitor followed by sleep periods where the enabled or is in a shutdown state. A 1.6V reference and output power is supplied by the output capacitor. While in a comparator A6 with built-in hysteresis (typical 140mV) sleep mode, the LT8331 consumes only 6µA. allow the user to accurately program the system input voltage at which the IC turns on and off (see the Block As the output load decreases, the frequency of single current Diagram). The typical input falling and rising threshold pulses decreases (see Figure 1) and the percentage of time voltages can be calculated by the following equations: the LT8331 is in sleep mode increases, resulting in much higher light load efficiency than for typical converters. To R3 + R4 V optimize the quiescent current performance at light loads, IN(FALLING,UVLO(–)) = 1.60 • R4 the current in the feedback resistor divider must be mini- R3 + R4 mized as it appears to the output as load current. In addition, V IN(RISING, UVLO(+)) = 1.74 • R4 all possible leakage currents from the output should also be minimized as they all add to the equivalent output load. VIN current is reduced below 1µA when the EN/UVLO pin The largest contributor to leakage current can be due to the voltage is less than 0.2V. The EN/UVLO pin can be con- reverse biased leakage of the Schottky diode (see Diode nected directly to the input supply VIN for always-enabled Selection in the Applications Information section). operation. A logic input can also control the EN/UVLO pin. While in Burst Mode operation, the current limit of the When operating in Burst Mode operation for light load switch is approximately 140mA resulting in the output currents, the current through the R3 and R4 network can voltage ripple shown in Figure 2. Increasing the output easily be greater than the supply current consumed by the capacitance will decrease the output ripple proportionally. LT8331. Therefore, R3 and R4 should be large enough to As the output load ramps upward from zero the switching minimize their effect on efficiency at light loads. frequency will increase but only up to the fixed frequency defined by the resistor at the RT pin as shown in Figure 1.
INTVCC REGULATOR
The output load at which the LT8331 reaches the fixed A low dropout (LDO) linear regulator, supplied from VIN, produces a 3.2V supply at the INTVCC pin. A minimum 1µF low ESR ceramic capacitor must be used to bypass 600 the INTVCC pin to ground to supply the high transient cur- FRONT PAGE APPLICATION VIN = 48V, VOUT = 48V rents required by the internal power MOSFET gate driver. 450 300 IL1+IL2 100mA/DIV 150 SWITCHING FREQUENCY (kHz) VOUT 20mV/DIV 0 0 20 40 60 80 100 LOAD CURRENT (mA) 8331 F02 8331 F01 5µs/DIV
Figure 1. Burst Frequency vs Load Current Figure 2. Burst Mode Operation
Rev. C For more information www.analog.com 9 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Electrical Characteristics Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts