Every buck regulator will have a specified maximum output current rating capability that the device can handle before an over-current condition is triggered. In this over-current condition the device will shut-down, stop switching, and check if the fault is removed before switching is resumed. Normally if a design calls for a higher output current requirement above the device’s maximum rated output current limit, a new device must be selected. However this would require a new schematic and PCB layout design if the device is not pin-to-pin compatible with the original layout.

With a simple external differential op-amp circuit two single-output buck regulators can be configured in a two-phase design to allow twice the load current capability of a single buck regulator. This provides customers the following benefits:

1. Tying two regulator outputs together to share current allows the spread of power loss between two regulators leading to lower tempt rise and overall thermally robust design. This removes the potential need for designing with an external heatsink which increases system design complexity, size, and overall cost.
2. Implementing 180 degrees phase shift circuit provides the following benefits:
   1. Each path will have less switching losses. Doing this creates a design that leads to better EMI and thermal performance.
   2. Phase shift circuits provide input and output ripple current cancellation. For more details refer to Section 8.1.2 from the LM5119-Q1 data sheet.
3. Easier to design than a dual phase controller since power MOSFETS are already integrated into the regulator/converter.

Two Types of Current Sensing

Inductor DCR Current Sensing

When using the inductor DCR current sensing method, a carefully selected RC network is used to mimic or recreate the voltage that would be seen across the DCR of the inductor. It is as if the DCR of the inductor is the current sense resistor. In order to accurately represent the current flowing through the DCR, the RC time constant (τ), needs to be the same as that of the inductor and DCR

Equation 1.

• L = power inductor
• DCR = inductor direct-current-resistance

One of the ways to find the values for the RC components is by first calculating an approximate resistor value for R (Rtemporary), from which we will find the capacitor value. Once an actual capacitor is selected (i.e from a component vendor), we can then re-calculate a more accurate resistor value. The reason for performing this slightly iterative process is due to the fact that it is usually more difficult to find a capacitor with the desired value, temperature coefficient, voltage rating, and availability, compared to the resistor.

1. Calculate a temporary resistor value: A good-sized resistor for most applications is either an 0402 or 0603 (Imperial) package size resistor. For most applications 0402 resistors will work fine. The characteristics of these (0402) resistors vary between manufacturers and product-series, however they usually have voltage ratings of about 50V and power ratings of about 1/16W, or 0.063W. Worst-case power dissipation estimation, along with buffers, let us say is 50mW for this 0402-package resistor
   Equation 2.
2. Calculate a capacitor value:
   Equation 3.
3. Find an actual capacitor that is closest to that calculated in Step 2
4. Calculate the actual resistor value to be selected/used:
   Equation 4.

Current Sense Resistor for Current Sensing

As a rule-of-thumb a good full-scale current sense voltage range (VIsns) is approximately between 75mV and 100mV at the maximum peak inductor current.

Equation 5.

The power loss through the current sense resistor involves the maximum continuous load current, which for a buck converter is the average inductor current at this condition.

Equation 6.

For example, in the case of PMP22993, the maximum peak load current (per phase) is 6A, giving us a peak inductor current of 7AIL_pk. Figure 2-1 is a GUI from the Power Stage Designer Tool to help illustrate and simulate the expected inductor current given a typical design value (VIN = 16V, VOUT = 3.3V, IOUT = 6A, Fsw = 2.1MHz).

Figure 2-1 Inductor Current Simulation from Power State Designer Tool

If we were to use 100mV as the full-scale current sense voltage we get,

Equation 7.

Now let us see how much power will be dissipated in this resistor when operating at the maximum continuous load current:

Equation 8.

Let us check how much this amount of power loss will decrease our total system efficiency:

Equation 9.

Equation 10.

For this application, a loss of 2% of efficiency at maximum continuous load is deemed acceptable. Designing for a reduced system efficiency of 3% or lower is generally acceptable but will depend on the application requirements. As a rule-of-thumb, it is always good to select a current sense resistor that has a power dissipation rating that is twice that of the calculated power loss. This is so that the resistor temperature does not get too high.

To assist in calculating, as well as visualizing, many of the signals associated with switch-mode power supplies, such as the peak inductor current, please download Power Stage Designer (link).

Current sharing between two buck regulators can be achieved using a simple current sharing op-amp circuit using a OPA991. This current sharing amplifier design is configured as a differential amplifier which compares the primary and secondary phase inductor currents. The op amp circuit servos the secondary converter's load current, by controlling the output voltage, to keep the difference between the primary and secondary output currents zero. For example, if the primary phase were to source more current than the secondary phase, the output of the differential amplifier will decrease causing the secondary phase voltage to slightly increase. This results in the secondary phase sourcing more current until it achieves balanced current sharing.

R₂ and R_F should be calculated such that, when the primary and secondary output voltage is matched, the output of the difference amplifier is equal to the reference voltage (Vref) of the buck regulator (Refer to schematic in Figure 3-1 for more details). When both phases are evenly matched no current will be sourced or sinked from the feedback node. Note that the op-amp current sense design requires a compensation capacitor that is placed in parallel to R_F. This capacitor value may need to be adjusted per application parameters.

Equation 11.

• V_{C_SECONDARY} is the voltage before the current sense resistor of the secondary converter

Equation 12.

Equation 13.

• Note: This equation assumes the ideal op-amp voltage on both inverting and non-inverting inputs

Equation 14.

• V_{C_MAIN} is the voltage before the current sense resistor of the main converter

Equation 15.

• V_REF is the internal reference voltage of the converter

Equation 16.

Figure 3-1 Current Sharing Differential Op-Amp Circuit Design