Datasheet LT1959 (Analog Devices) - 17

LT1959
U U W U APPLICATIONS INFORMATION
The term inside the radical has a maximum value of 0.5 normally a problem, but at very low input voltage they may when input voltage is twice output, and stays near 0.5 for cause erratic operation because the input voltage drops a relatively wide range of input voltages. It is common below the minimum specification. Problems can also practice therefore to simply use the worst-case value and occur if the input-to-output voltage differential is near assume that RMS ripple current is one half of load current. minimum. The amplitude of these dips is normally a At maximum output current of 4.5A for the LT1959, the function of capacitor ESR and ESL because the capacitive input bypass capacitor should be rated at 2.25A ripple reactance is small compared to these terms. ESR tends to current. Note however, that there are many secondary be the dominate term and is inversely related to physical considerations in choosing the final ripple current rating. capacitor size within a given capacitor type. These include ambient temperature, average versus peak load current, equipment operating schedule, and required
SYNCHRONIZING
product lifetime. For more details, see Application Notes The SYNC pin, is used to synchronize the internal oscilla- 19 and 46, and Design Note 95. tor to an external signal. The SYNC input must pass from a logic level low, through the maximum synchronization
Input Capacitor Type
threshold with a duty cycle between 10% and 90%. The Some caution must be used when selecting the type of input can be driven directly from a logic level output. The capacitor used at the input to regulators. Aluminum synchronizing range is equal to initial operating frequency electrolytics are lowest cost, but are physically large to up to 1MHz. This means that minimum practical sync achieve adequate ripple current rating, and size con- frequency is equal to the worst-case high self-oscillating straints (especially height), may preclude their use. frequency (560kHz), not the typical operating frequency of Ceramic capacitors are now available in larger values, and 500kHz. Caution should be used when synchronizing their high ripple current and voltage rating make them above 700kHz because at higher sync frequencies the ideal for input bypassing. Cost is fairly high and footprint amplitude of the internal slope compensation used to may also be somewhat large. Solid tantalum capacitors prevent subharmonic switching is reduced. This type of would be a good choice, except that they have a history of subharmonic switching only occurs at input voltages less occasional spectacular failures when they are subjected to than twice output voltage. Higher inductor values will tend large current surges during power-up. The capacitors can to eliminate this problem. See Frequency Compensation short and then burn with a brilliant white light and lots of section for a discussion of an entirely different cause of nasty smoke. This phenomenon occurs in only a small subharmonic switching before assuming that the cause is percentage of units, but it has led some OEM companies insufficient slope compensation. Application Note 19 has to forbid their use in high surge applications. The input more details on the theory of slope compensation. bypass capacitor of regulators can see these high surges At power-up, when V when a battery or high capacitance source is connected. C is being clamped by the FB pin (see Figure 2, Q2), the sync function is disabled. This allows the Several manufacturers have developed a line of solid frequency foldback to operate in the shorted output con- tantalum capacitors specially tested for surge capability dition. During normal operation, switching frequency is (AVX TPS series for instance, see Table 3), but even these controlled by the internal oscillator until the FB pin reaches units may fail if the input voltage surge approaches the 0.7V, after which the SYNC pin becomes operational. maximum voltage rating of the capacitor. AVX recom- mends derating capacitor voltage by 2:1 for high surge applications.
THERMAL CALCULATIONS
Larger capacitors may be necessary when the input volt- Power dissipation in the LT1959 chip comes from four age is very close to the minimum specified on the data sources: switch DC loss, switch AC loss, boost circuit sheet. Small voltage dips during switch on time are not current, and input quiescent current. The following 17