Modern Step-Up Regulator ICs
In contemporary electronics, a range of switching regulator integrated circuits (ICs) has emerged, operating on the step-up principle. These ICs efficiently convert input voltage into a higher output voltage. This conversion is facilitated through the periodic grounding of coil L via the LX connection of the IC. This action results in the creation of a magnetic field within the coil, storing energy. When the step-up regulator IC deactivates, the collapsing magnetic field within L compels the current to persist. But now it must pass through diode D towards the output capacitor and the external load linked to Vout. This process generates a voltage exceeding the initial input voltage. The values of resistors R1 and R2 constitute a voltage divider, adjusting the output voltage according to the presented formula. Vref’s typical value hovers around 1.2 V.
Problem with the Step-Up Regulator
When the step-up regulator’s IC is inactive, a continuous current path exists from the input to the output through coil L and diode D. As a result, the output voltage remains nonzero, instead of being zero like the input voltage. To resolve this issue, a straightforward solution involves using a transistor and a series base resistor. In this case, a PNP transistor, such as the BCP69, is introduced into the output circuit to periodically transfer the DC output voltage from the switching regulator to output capacitor C2. The base of transistor T is connected to the switch pin LX of the step-up regulator IC through the series resistor R.
Voltage Waveforms in the Diagram
The diagram illustrates voltage waveforms. Pin LX is cyclically switched to ground. When the switch opens, a voltage pulse that supplements the input voltage appears at LX. Diode D briefly conducts and conveys this voltage to C1, causing it to charge to a voltage level determined by the voltage divider R1/R2, which is 0.3 V higher than the output voltage. It’s worth noting that the small charging peaks shown in curve 2 are not drawn to scale.
Transistor Operation and Voltage Sags
If VLX is more than 0.7 V lower than VC1, transistor T becomes conductive and transfers the voltage from C1 to C2. The small voltage sags depicted in curve 3 are also not drawn to scale for clarity. In cases where the step-up regulator IC is disabled, the voltage across C1 matches the input voltage. This voltage is also present at LX, which means there’s insufficient base bias voltage to activate the transistor, causing it to be in the off state.