Datasheet LTM8074 (Analog Devices) - 16

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APPLICATIONS INFORMATION
is disabled, and the switching frequency will slow down The LTM8074 does not operate in forced continuous mode only during overcurrent conditions. regardless of SYNC signal.
Synchronization Negative Output
To select low ripple Burst Mode operation, tie the SYNC The LTM8074 is capable of generating a negative output pin below about 0.4V (this can be ground or a logic low voltage by connecting its VOUT to system GND and the output). To synchronize the LTM8074 oscil ator to an LTM8074 GND to the negative voltage rail. An example of external frequency, connect a square wave (with about this is shown in the Typical Application section. The most 20% to 80% duty cycle) to the SYNC pin. The square versatile way to generate a negative output is to use a dedi- wave amplitude should have val eys that are below 0.4V cated regulator that was designed to generate a negative and peaks above 1.5V. voltage, but using a buck regulator like the LTM8074 to The LTM8074 will not enter Burst Mode operation at low generate a negative voltage is a simple and cost effective output loads while synchronized to an external clock, but solution, as long as certain restrictions are kept in mind. instead will pulse skip to maintain regulation. The LTM8074 Figure 1 shows a typical negative output voltage application. may be synchronized over a 200kHz to 2.2MHz range. Note that LTM8074 VOUT is tied to system GND and input The RT resistor should be chosen to set the switching power is applied from VIN to LTM8074 VOUT. As a result, frequency equal to or below the lowest synchronization the LTM8074 is not behaving as a true buck regulator, input. For example, if the synchronization signal will be and the maximum output current depends upon the input 500kHz and higher, the RT should be selected for 500kHz. voltage. In the example shown in the Typical Application For some applications it is desirable for the LTM8074 to section, there is an attending graph that shows how much operate in pulse-skipping mode, offering two major dif- current the LTM8074 can deliver for given input voltages. ferences from Burst Mode operation. The first is that the clock stays awake at all times and all switching cycles VIN are aligned to the clock. The second is that full switching VIN VOUT frequency is reached at lower output load than in Burst LTM8074 Mode operation. These two differences come at the expense of increased quiescent current. To enable pulse-skipping GND mode, the SYNC pin is floated. NEGATIVE OUTPUT VOLTAGE The LTM8074 features spread spectrum operation to 8074 F01 further reduce EMI/EMC emissions. To enable spread
Figure 1. The LTM8074 Can Be Used to Generate a Negative Voltage
spectrum operation, apply between 2.9V and 4.2V to the Note that this configuration requires that any load current SYNC pin. In this mode, triangular frequency modulation transient will directly impress the transient voltage onto is used to vary the switching frequency between the value the LTM8074 GND, as shown in Figure 2, so fast load programmed by RT to about 20% higher than that value. transients can disrupt the LTM8074’s operation or even The modulation frequency is about 3kHz. For example, when cause damage. the LTM8074 is programmed to 2MHz, the frequency will vary from 2MHz to 2.4MHz at a 3kHz rate. When spread spectrum operation is selected, Burst Mode operation is disabled, and the part will run in pulse-skipping mode. Rev. A 16 For more information www.analog.com 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 Package Photo Package Description Typical Application Related Parts