Datasheet LT8362 (Analog Devices) - 10

LT8362
APPLICATIONS INFORMATION ACHIEVING ULTRALOW QUIESCENT CURRENT
To enhance efficiency at light loads the LT8362 uses a low ripple Burst Mode architecture. This keeps the out- IL put capacitor charged to the desired output voltage while 500mA/DIV minimizing the input quiescent current and output ripple. In Burst Mode operation, the LT8362 delivers single small VOUT pulses of current to the output capacitor followed by sleep 10mV/DIV periods where the output power is supplied by the output 10µs/DIV 8362 F02 capacitor. While in sleep mode, the LT8362 consumes only 9µA.
Figure 2. Burst Mode Operation
As the output load decreases, the frequency of single cur- rent pulses decreases (see Figure 1) and the percentage of defined by the resistor at the RT pin as shown in Figure 1. time the LT8362 is in sleep mode increases, resulting in The output load at which the LT8362 reaches the fixed much higher light load efficiency than for typical convert- frequency varies based on input voltage, output voltage, ers. To optimize the quiescent current performance at light and inductor choice. loads, the current in the feedback resistor divider must be minimized as it appears to the output as load current.
PROGRAMMING INPUT TURN-ON AND TURN-OFF
In addition, all possible leakage currents from the output
THRESHOLDS WITH EN/UVLO PIN
should also be minimized as they all add to the equiva- The EN/UVLO pin voltage controls whether the LT8362 is lent output load. The largest contributor to leakage current enabled or is in a shutdown state. A 1.6V reference and can be due to the reverse biased leakage of the Schottky a comparator A6 with built-in hysteresis (typical 80mV) diode (see Diode Selection in the Applications Information allow the user to accurately program the system input section). voltage at which the IC turns on and off (see the Block While in Burst Mode operation, the current limit of the Diagram). The typical input falling and rising threshold switch is approximately 500mA resulting in the output voltages can be calculated by the following equations: voltage ripple shown in Figure 2. Increasing the output capacitance will decrease the output ripple proportionally. VIN(FALLING,UVLO(–)) = 1.60 • R3 + R4 As the output load ramps upward from zero the switching R4 frequency will increase but only up to the fixed frequency V IN(RISING, UVLO(+)) = 1.68 • R3 + R4 R4 2.5 FRONT PAGE APPLICATION VIN current is reduced below 1µA when the EN/UVLO pin VIN = 12V 2.0 V voltage is less than 0.2V. The EN/UVLO pin can be con- OUT = 48V nected directly to the input supply VIN for always-enabled 1.5 operation. A logic input can also control the EN/UVLO pin. When operating in Burst Mode operation for light load 1.0 currents, the current through the R3 and R4 network can easily be greater than the supply current consumed by the 0.5 SWITCHING FREQUENCY (MHz) LT8362. Therefore, R3 and R4 should be large enough to 0 minimize their effect on efficiency at light loads. 0 10 20 30 40 50 LOAD CURRENT (mA) 8362 F01
Figure 1. Burst Frequency vs Load Current
8362fa 10 For more information www.linear.com/LT8362 Document Outline FEATURES DESCRIPTION APPLICATIONS 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 REVISION HISTORY TYPICAL APPLICATION RELATED PARTS