Almost all of high speed digital PC boards wouldn’t be come without using Buck switching regulator. Because, in common, digital ICs need wide range of low voltage levels like 1.0, 1.2, 1.8 and etc for operation and buck regulator will perfectly suit to generate these voltage from higher input voltage (Usually 5V or 12V). But in real, using buck regulator we can’t generate any voltage for a given input voltage. In simple words, high to very low voltage conversion will not be possible if you use buck regulator (Ex: 12V to 1V). In this post we see how this scenario comes into picture.
To understand this, we first see some basic functions of a Buck regulator. It uses a PWM controller which runs at switching frequency of buck regulator (can be found in device datasheet). PWM controller’s duty cycle will vary based on our output voltage. PWM controller switches an inductor using MOSFET. Average current of inductor will generate a constant voltage with ripple (These ripple will be smoothen by using a capacitor). So we come to know that if a user wants to change output voltage, he has to change duty cycle (This can be done usually using resistor divider in many buck regulators). We can relate input voltage, output voltage and duty cycle by the following mathematical equation.
Vout = D X Vin (D is duty cycle)
(We have to do lot of mathematical jobs to arrive the above equation. For simplicity and to lack competency I / we avoid it)!!!!!
D = Ton * Fsw (Ton is on period of PWM controller and Fsw is switching frequency)
So Vout = Vin X Ton X Fsw -------------à (A)
In the above formula, Ton is the variable and by changing it Vout can be varied for a given Vin. Now by rearranging the above formula,
Ton = Vout / (Vin X Fsw)
Now we come to know that Ton is directly proportional to Vout (For fixed Fsw and Vin). If we want to have lower Vout, obviously Ton also will be reduced. Now the question arises? How far the Ton can be reduced (means how much less Vout we can generate). This parameter will be defined in device datasheet as “Minimum Controllable On Time”. This limitation comes because, gated pulsing mechanism of PWM controller. If we have an on time (Ton, to generate required Vout) which is lesser than specified value, output signal of PWM controller will more likely a DC voltage and consequently switching in MOSFET will not function properly. Finally we don’t get output voltage what we really want.
In datasheet of all buck regulators, they specify the minimum controllable on time parameter. For example, let take minimum controllable on time is 200ns and Fsw = 600KHz. In our board let take we use 12V as input voltage. By using above formula A, we can find minimum output voltage.
Vout_min = 200 X10-9 X 12 X 600000 = 1.44V
So any voltage below than 1.44V, it is impossible to generate using that buck regulator. Bu this applies in ideal condition and in real we have to take the loss in upper and lower MOSFETs of buck regulator. If we consider them also, Vout_min would be little bit higher. So for safer side, we can say that Vout_min = 1.5V. But in datasheet, you can’t find this 1.44V or 1.5V. They say Vout_min is Vref (usually 0.8V or 1.2 or 0.6). So as a designer, we have to find what the real minimum voltage we can generate by using the formula A.
Summary: Never try to generate below than Vin X Ton X Fsw in buck regulator.
Best Regards,
Arun Alagarsamy...
Best Regards,
Arun Alagarsamy...
