Datasheet LTM8022 (Analog Devices) - 10

LTM8022
APPLICATIONS INFORMATION
of operating frequencies, a haphazardly chosen one may maximum voltage at the BIAS pin is less than 16V and result in undesirable operation under certain operating or the sum of V + IN BIAS is less than 56V. If BIAS power is fault conditions. A frequency that is too high can reduce applied from a remote or noisy voltage source, it may effi ciency, generate excessive heat or even damage the be necessary to apply a decoupling capacitor locally to LTM8022 if the output is overloaded or short-circuited. A the LTM8022. frequency that is too low can result in a fi nal design that has too much output ripple or too large of an output cap.
Minimum Input Voltage
The maximum frequency and corresponding R The LTM8022 is a step-down converter, so a minimum T value at which the LTM8022 should be allowed to switch is given amount of headroom is required to keep the output in in Table 1 in the R regulation. For most applications at full load, the input T(MIN) and fMAX columns, while the recommended frequency and R needs to be at least 1.5V above the desired output. In T value over the given input range is given in the R addition, the input voltage required to turn on depends T(OPTIMAL) and fOPTIMAL columns. upon how the RUN/SS pin is tied. As shown in Figure 1, There are additional conditions that must be satisfi ed if it takes more input voltage to turn on if RUN/SS is tied the synchronization function is used. Please refer to the to VIN than if the turn-on is controlled by raising RUN/SS Synchronization section for details. when VIN is in the required operating range. This is shown in Figure 1.
Burst Mode Operation
6.0 To enhance effi ciency at light loads, the LTM8022 auto- VOUT = 3.3V T matically switches to Burst Mode operation which keeps A = 25°C 5.5 f = 650kHz the output capacitor charged to the proper voltage while minimizing the input quiescent current. During Burst Mode 5.0 TO START operation, the LTM8022 delivers single cycle bursts of 4.5 current to the output capacitor followed by sleep periods RUN/SS ENABLED where the output power is delivered to the load by the output 4.0 INPUT VOLTAGE (V) capacitor. In addition, VIN and BIAS quiescent currents are TO RUN 3.5 reduced to typically 20μA and 50μA, respectively, during the sleep time. As the load current decreases towards a no 3.0 0 200 400 600 800 1000 load condition, the percentage of time that the LTM8022 LOAD CURRENT (mA) operates in sleep mode increases and the average input 8022 F01a current is greatly reduced, resulting in higher effi ciency. 7.5 VOUT = 5V Burst Mode operation is enabled by tying SYNC to GND. To TA = 25°C f = 650kHz disable Burst Mode operation, tie SYNC to a stable voltage 7.0 source above 0.7V.
Do not leave this pin fl oating.
6.5 TO START
BIAS Pin Considerations
RUN/SS 6.0 ENABLE The BIAS pin is used to provide drive power for the internal INPUT VOLTAGE (V) power switching stage and operate internal circuitry. For 5.5 proper operation, it must be powered by at least 2.4V. If TO RUN the output voltage is programmed to be 2.4V or higher, 5.0 0 200 400 600 800 1000 simply tie BIAS to AUX, which is internally tied to VOUT . LOAD CURRENT (mA) If VOUT is less than 2.4V, BIAS can be tied to VIN or 8022 F01b some other voltage source. In all cases, ensure that the
Figure 1. The LTM8022 Needs More Voltage to Start Than to Run
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