Datasheet LT8494 (Analog Devices) - 10

LT8494
APPLICATIONS INFORMATION Low Ripple Burst Mode Operation
resistors and a low leakage Schottky diode in applications To enhance efficiency at light loads, the LT8494 regulator with ultralow Q current. enters low ripple Burst Mode operation keeping the output In Burst Mode operation, the burst frequency and the capacitor charged to the proper voltage while minimizing charge delivered with each pulse will not change with the input quiescent current. During Burst Mode operation, output capacitance. Therefore, the output voltage ripple the LT8494 regulator delivers single-cycle bursts of cur- will be inversely proportional to the output capacitance. rent to the output capacitor with each followed by a sleep In a typical application with a 47μF output capacitor, period where the output power is delivered to the load by the output ripple is about 10mV, and with two 47μF the output capacitor. The quiescent currents of VIN/BIAS output capacitors the output ripple is about 5mV (see are reduced to less than 3µA typically during the sleep Switching Waveforms, Burst Mode Operation in Typical time (see Electrical Characteristics table). Performance Characteristics section). The output voltage As the load current decreases towards a no-load con- ripple can continue to be decreased by increasing the dition, the frequency of single current pulses decreases output capacitance. (see Figure 1), therefore the percentage of time that the At higher output loads the LT8494 regulator runs at the LT8494 operates in sleep mode increases, resulting in frequency programmed by the RT resistor and operates as reduced average input current and thus high efficiency a standard current mode regulator. The transition between even at very low loads. high current mode and low ripple Burst Mode operation is By maximizing the time between pulses, the LT8494 qui- seamless, and will not disturb the output voltage. escent current is minimized. Therefore, to optimize the
Setting the Output Voltage
quiescent current performance at light loads, the current in the feedback resistor divider and the reverse current The output voltage is programmed with a resistor divider in the external diode must be minimized, as these appear from output to the FB pin (R2) and from the FB pin to to the output as load currents. More specifically, during ground (R1). Choose the 1% resistors according to: the sleep time, the boost converter has the reverse diode  V  leakage current conducting from output to input, while R2 =R1 OUT –1   the SEPIC converter has leakage current conducting 1.202 from output to ground. Use the largest possible feedback Note that choosing larger resistors decreases the quies- cent current of the application circuits. In low load appli- 500 FRONT PAGE APPLICATION cations, choosing larger resistors is more critical since the part enters Burst Mode operation with lower quiescent 400 current. 300
Power Switch Duty Cycle
200 In order to maintain loop stability and deliver adequate current to the load, the power NPN (Q1 in the Block SWITCHING FREQUENCY (kHz) 100 Diagram) cannot remain on for 100% of each clock cycle. The maximum allowable duty cycle is given by: 00.1 1 10 100 1000 T LOAD CURRENT (mA) DC P –Minimum Switch Off-Time •100% 8494 F01 MAX = T
Figure 1. Switching Frequency in Burst Mode Operation
P 8494fa 10 For more information www.linear.com/LT8494 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Application Related Parts