Datasheet LT1676 (Analog Devices) - 7

LT1676
U OPERATIO
Please refer to the High dV/dt Mode Timing Diagram. A threshold reference, VTH. (Remember that in a current typical oscillator cycle is as follows: The logic section first mode switching topology, the VC voltage determines the generates an SWDR signal that powers up the current peak switch current.) When the VC signal is above VTH, the comparator and allows it time to settle. About 1µs later, the previously described “high dV/dt” action is performed. SWON signal is asserted and the BOOST signal is pulsed When the VC signal is below VTH, the boost pulses are for a few hundred nanoseconds. After a short delay, the absent, as can be seen in the Low dV/dt Mode Timing VSW pin slews rapidly to VIN. Later, after the peak switch Diagram. Now the DC current, activated by the SWON current indicated by the control voltage VC has been signal alone, drives Q4 and this transistor drives Q1 by reached (current mode control), the SWON and SWDR itself. The absence of a boost pulse, plus the lack of a signals are turned off, and SWOFF is pulsed for several second NPN driver, result in a much lower slew rate which hundred nanoseconds. The use of an explicit turn-off aids light load controllability. device, i.e., Q5, improves turn-off response time and thus A further aid to overall efficiency is provided by the aids both controllability and efficiency. specialized bias regulator circuit, which has a pair of The system as previously described handles heavy loads inputs, VIN and VCC. The VCC pin is normally connected to (continuous mode) at good efficiency, but it is actually the switching supply output. During start-up conditions, counterproductive for light loads. The method of jam- the LT1676 powers itself directly from VIN. However, after ming charge into the PNP bases makes it difficult to turn the switching supply output voltage reaches about 2.9V, them off rapidly and achieve the very short switch ON the bias regulator uses this supply as its input. Previous times required by light loads in discontinuous mode. generation Buck controller ICs without this provision Furthermore, the high leading edge dV/dt rate similarly typically required hundreds of milliwatts of quiescent adversely affects light load controllability. power when operating at high input voltage. This both degraded efficiency and limited available output current The solution is to employ a “boost comparator” whose due to internal heating. inputs are the VC control voltage and a fixed internal
U U W U APPLICATIONS INFORMATION Selecting a Power Inductor
 V  V – V  L OUT IN OUT = There are several parameters to consider when selecting  f I  V PK IN  • a power inductor. These include inductance value, peak current rating (to avoid core saturation), DC resistance, For example, substituting 48V, 5V, 200mA and 100kHz construction type, physical size, and of course, cost. respectively for VIN, VOUT, IPK and f yields a value of about 220µH. Note that the left half of this expression is indepen- In a typical application, proper inductance value is dictated dent of input voltage while the right half is only a weak by matching the discontinuous/continuous crossover point function of VIN when VIN is much greater than VOUT. This with the LT1676 internal low-to-high dV/dt threshold. This means that a single inductor value will work well over a is the best compromise between maintaining control with range of “high” input voltage. And although a progres- light loads while maintaining good efficiency with heavy sively smaller inductor is suggested as VIN begins to loads. The fixed internal dV/dt threshold has a nominal approach VOUT, note that the much higher ON duty cycles value of 1.4V, which referred to the VC pin threshold and under these conditions are much more forgiving with control voltage to switch transconductance, corresponds respect to controllability and efficiency issues. Therefore to a peak current of about 200mA. Standard Buck con- when a wide input voltage range must be accommodated, verter theory yields the following expression for induc- say 10V to 50V for 5VOUT, the user should choose an tance at the discontinuous/continuous crossover: inductance value based on the maximum input voltage. 7