FunctionDescription
The B628 uses a fixed frequency, peakcurrent mode boost regulator architecture to regulate voltage atthe feedback pin. The operation of the B628 can be understood byreferring to the block diagram of (Functional Block Diagram). Atthe start of each oscillator cycle the MOSFET is turned on throughthe control circuitry. To prevent sub-harmonic oscillations at dutycycles greater than 50 percent, a stabilizing ramp is added to theoutput of the current sense amplifier and the result is fed intothe negative input of the PWM comparator. When this voltage e output voltage of the error amplifier the power MOSFET isturned off. The voltage at the output of the error amplifier is anamplified version of the difference between the 0.6V bandgapreference voltage and the feedback voltage. In this way the peakcurrent level keeps the output in regulation. If the feedbackvoltage starts to drop, the output of the error amplifierincreases. These results in more current to flow through the powerMOSFET, thus increasing the power delivered to the output. The B628has internal soft start to limit the amount of input current atstartup and to also limit the amount of overshoot on theoutput.
ApplicationInformation
Setting theOutput Voltage
The internal reference VREF is 0.6V(Typical). The output voltage is divided by a resistor divider,R1and R2 to the FB pin. The output voltage is given by
InductorSelection
The recommended values of inductor are4.7 to 22μH. Small size and better efficiency are the majorconcerns for portable device, such as B628 used for mobile phone.The inductor should have low core loss at 1.2MHz and low DCR forbetter efficiency. To avoid inductor saturation current ratingshould be considered.