Datasheet LTM8071 (Analog Devices) - 15

LTM8071
APPLICATIONS INFORMATION
reduce current surges on the input supply. During the target voltage. This in turn limits the amount of energy soft-start ramp the output voltage will proportionally that can be delivered to the load under fault. During the track the TR/SS pin voltage. For output tracking applica- start-up time, frequency foldback is also active to limit tions, TR/SS can be externally driven by another voltage the energy delivered to the potentially large output capaci- source. From 0V to 1.6V, the TR/SS voltage will override tance of the load. When a clock is applied to the SYNC the internal 0.97V reference input to the error amplifier, pin, the SYNC pin is floated or held high, the frequency thus regulating the FB pin voltage to that of the TR/SS foldback is disabled and the switching frequency will slow pin. When TR/SS is above 1.6V, tracking is disabled and down only during overcurrent conditions. the feedback voltage will regulate to the internal reference voltage. The TR/SS pin may be left floating if the function
Synchronization
is not needed. To select low ripple Burst Mode operation, tie the SYNC During start-up, if a relatively low capacitor is used on pin below about 0.4V (this can be ground or a logic low TR/SS, the output voltage may take longer to reach output). To synchronize the LTM8071 oscillator to an regulation than expected. If accurate start-up timing is external frequency connect a square wave (with about required, please refer to the LTM8071 simulation model 20% to 80% duty cycle) to the SYNC pin. The square in LTspice for help selecting an appropriate soft-start wave amplitude should have valleys that are below 0.4V capacitor. and peaks above 1.5V. An active pull-down circuit is connected to the TR/SS pin The LTM8071 will not enter Burst Mode operation at which will discharge the external soft-start capacitor in low output loads while synchronized to an external the case of fault conditions and restart the ramp when the clock, but instead will pulse skip to maintain regulation. faults are cleared. Fault conditions that clear the soft-start The LTM8071 may be synchronized over a 200kHz to capacitor are the RUN pin transitioning low, VIN voltage 2.2MHz range. The RT resistor should be chosen to set falling too low, or thermal shutdown. the LTM8071 switching frequency equal to or below the lowest synchronization input. For example, if the synchro-
Prebiased Output
nization signal will be 500kHz and higher, the RT should As discussed in the Output Voltage Tracking and Soft-Start be selected for 500kHz. section, the LTM8071 regulates the output to the FB volt- For some applications it is desirable for the LTM8071 to age determined by the TR/SS pin whenever TR/SS is less operate in pulse-skipping mode, offering two major dif- than 1.6V. If the LTM8071 output is higher than the target ferences from Burst Mode operation. The first is that the output voltage, the LTM8071 will attempt to regulate the clock stays awake at all times and all switching cycles output to the target voltage by returning a small amount are aligned to the clock. Second is that full switching fre- of energy back to the input supply. If there is nothing load- quency is reached at lower output load than in Burst Mode ing the input supply, its voltage may rise. Take care that operation. These two differences come at the expense it does not rise so high that the input voltage exceeds the of increased quiescent current. To enable pulse-skipping absolute maximum rating of the LTM8071. mode, the SYNC pin is floated.
Frequency Foldback
The LTM8071 features spread spectrum operation to fur- ther reduce EMI emissions. To enable spread spectrum The LTM8071 is equipped with frequency foldback which operation, apply between 2.9V and 4.2V to the SYNC pin. acts to reduce the thermal and energy stress on the inter- In this mode, triangular frequency modulation is used nal power elements during a short-circuit or output over- to vary the switching frequency between the value pro- load condition. If the LTM8071 detects that the output grammed by RT to about 20% higher than that value. The has fal en out of regulation, the switching frequency is modulation frequency is about 3kHz. For example, when reduced as a function of how far the output is below the the LTM8071 is programmed to 1MHz, the frequency will Rev 0 For more information www.analog.com 15 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Package Photos Package Description Typical Application Related Parts