Datasheet LT3971A, LT3971A-5 (Analog Devices) - 10

LT3971A/LT3971A-5
APPLICATIONS INFORMATION Achieving Ultralow Quiescent Current
leakage at room temperature. These two considerations are reiterated in the FB Resistor Network and Catch Diode To enhance efficiency at light loads, the LT3971A operates Selection sections. in low ripple Burst Mode, which keeps the output capacitor charged to the desired output voltage while minimizing It is important to note that another way to decrease the the input quiescent current. In Burst Mode operation the pulse frequency is to increase the magnitude of each LT3971A delivers single pulses of current to the output single current pulse. However, this increases the output capacitor followed by sleep periods where the output power voltage ripple because each cycle delivers more power to is supplied by the output capacitor. When in sleep mode the output capacitor. The magnitude of the current pulses the LT3971A consumes 1.7μA, but when it turns on all the was selected to ensure less than 15mV of output ripple in circuitry to deliver a current pulse, the LT3971A consumes a typical application. See Figure 2. 1.5mA of input current in addition to the switch current. Therefore, the total quiescent current will be greater than VSW 1.7μA when regulating. 5V/DIV As the output load decreases, the frequency of single cur- rent pulses decreases (see Figure 1) and the percentage IL 500mA/DIV of time the LT3971A is in sleep mode increases, resulting in much higher light load efficiency. By maximizing the VOUT time between pulses, the converter quiescent current 20mV/DIV gets closer to the 1.7μA ideal. Therefore, to optimize the 5μs/DIV 3971A F02 quiescent current performance at light loads, the current VIN = 12V in the feedback resistor divider and the reverse current VOUT = 3.3V ILOAD = 10mA in the catch diode must be minimized, as these appear to the output as load currents. Use the largest possible
Figure 2. Burst Mode Operation
feedback resistors and a low leakage Schottky catch diode in applications utilizing the ultralow quiescent current While in Burst Mode operation, the burst frequency and performance of the LT3971A. The feedback resistors the charge delivered with each pulse will not change with should preferably be on the order of MΩ and the Schottky output capacitance. Therefore, the output voltage ripple catch diode should have less than 1μA of typical reverse will be inversely proportional to the output capacitance. In a typical application with a 22μF output capacitor, the output ripple is about 10mV, and with a 47μF output ca- 1000 pacitor the output ripple is about 5mV. The output voltage VIN = 12V VOUT = 3.3V ripple can continue to be decreased by increasing the 800 output capacitance. At higher output loads (above 92mA for the front page 600 application) the LT3971A will be running at the frequency programmed by the R 400 T resistor, and will be operating in standard PWM mode. The transition between PWM and low ripple Burst Mode operation will exhibit slight frequency SWITCHING FREQUENCY (kHz) 200 jitter, but will not disturb the output voltage. 0 0 20 40 60 80 100 120 LOAD CURRENT (mA) 3971A F01
Figure 1. Switching Frequency in Burst Mode Operation
3971af 10 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 TYPICAL APPLICATION RELATED PARTS