Datasheet LT1776 (Analog Devices) - 8

LT1776
U OPERATIO
The system as previously described handles heavy loads signal alone, drives Q4 and this transistor drives Q1 by (continuous mode) at good efficiency, but it is actually itself. The absence of a boost pulse, plus the lack of a counterproductive for light loads. The method of jam- second NPN driver, result in a much lower slew rate which ming charge into the PNP bases makes it difficult to turn aids light load controllability. them off rapidly and achieve the very short switch ON A further aid to overall efficiency is provided by the times required by light loads in discontinuous mode. specialized bias regulator circuit, which has a pair of Furthermore, the high leading edge dV/dt rate similarly inputs, V adversely affects light load controllability. IN and VCC. The VCC pin is normally connected to the switching supply output. During start-up conditions, The solution is to employ a “boost comparator” whose the LT1776 powers itself directly from VIN. However, after inputs are the VC control voltage and a fixed internal the switching supply output voltage reaches about 2.9V, threshold reference, VTH. (Remember that in a current the bias regulator uses this supply as its input. Previous mode switching topology, the VC voltage determines the generation buck controller ICs without this provision peak switch current.) When the VC signal is above VTH, the typically required hundreds of milliwatts of quiescent previously described “high dV/dt” action is performed. power when operating at high input voltage. This both When the VC signal is below VTH, the boost pulses are degraded efficiency and limited available output current absent, as can be seen in the Low dV/dt Mode Timing due to internal heating. Diagram. Now the DC current, activated by the SWON
U U W U APPLICATIONS INFORMATION Selecting a Power Inductor
For example, substituting 40V, 5V, 200mA and 200kHz respectively for V There are several parameters to consider when selecting IN, VOUT, IPK and f yields a value of about 100µH. Note that the left half of this expression is indepen- a power inductor. These include inductance value, peak dent of input voltage while the right half is only a weak current rating (to avoid core saturation), DC resistance, function of V construction type, physical size, and of course, cost. IN when VIN is much greater than VOUT. This means that a single inductor value will work well over a In a typical application, proper inductance value is dictated range of “high” input voltage. And although a progres- by matching the discontinuous/continuous crossover point sively smaller inductor is suggested as VIN begins to with the LT1776 internal low-to-high dV/dt threshold. This approach VOUT, note that the much higher ON duty cycles is the best compromise between maintaining control with under these conditions are much more forgiving with light loads while maintaining good efficiency with heavy respect to controllability and efficiency issues. Therefore loads. The fixed internal dV/dt threshold has a nominal when a wide input voltage range must be accommodated, value of 1.4V, which referred to the VC pin threshold and say 10V to 40V for 5VOUT, the user should choose an control voltage to switch transconductance, corresponds inductance value based on the maximum input voltage. to a peak current of about 200mA. Standard buck con- Once the inductance value is decided, inductor peak verter theory yields the following expression for induc- current rating and resistance need to be considered. Here, tance at the discontinuous/continuous crossover: the inductor peak current rating refers to the onset of  saturation in the core material, although manufacturers V  V – V  L OUT IN OUT = sometimes specify a “peak current rating” which is de-  f I  V PK IN  • rived from a worst-case combination of core saturation and self-heating effects. Inductor winding resistance alone 8