1、Buck Regulator Architectures,4.3 Hysteretic Buck Regulators,Hysteretic Buck-Regulator Architecture,2,Ripple is needed to properly switch the comparator!,Simplest and fastest topology but variable frequency!,Hysteretic Regulator Waveforms,3,DI = DV Dt L,Output Ripple Voltage Detail,4,LM3485 Architect
2、ure,5,LM3485 Hysteretic Controller,6,Calculating Switching Frequency,In most cases, switching frequency is determined by the output ripple voltage (VOUT) resulting from the output capacitors ESR. The amplitude of VOUT is described by the following two equations: Combining these two equations yields
3、an expression for the switching frequency. Note: If a speed up capacitor is used the circled term in the denominator of this equation becomes 1 which means the switching frequency value will increase.,7,Pros & Cons of Hysteretic Control,8,LM3475/85 Using Electrolytic COUT,The graphic shows the outpu
4、t ripple and switch node voltage The operating frequency is 1.43MHz The feedback network does not use a CFF speed up capacitor,9,LM3475/85 Using Ceramic COUT,An example of using a ceramic output capacitor with almost no ESR Operating frequency has dropped and can not be calculated using the equation
5、 mentioned previously Reason: The output ripple is 90phase shifted from the switching action,10,LM3475/85 Working with Ceramic Capacitors,Desire Use ceramic capacitors Challenge Ceramic capacitors have very low ESR Results in 90phase shift of output voltage ripple, resulting in low operating frequen
6、cy and increased output ripple Solution Add a low value resistor in series with the ceramic output capacitor to provide an ESR value Although counter intuitive, this combination provides highly accurate control over output voltage ripple,11,Working with Ceramic Capacitors Another Technique,By adding the three components circled in the diagram, we provide AC feedback in phase with the switching action. The 100pF capacitor provides bypassing of any high frequency edge noise which may cause improper triggering of the FB comparator. This method has a number of advantages.,12,Thank you!,13,
