1. Technical Field
The present disclosure relates to current sensing, and more particularly to a current sensing device for a multi-phase switched voltage regulator capable of generating sense currents representing the current unbalance of the phases in respect to the average current delivered by each phase, and the total current delivered by the voltage regulator.
2. Description of the Related Art
Multiphase voltage regulators are used in numerous applications, for example as power supplies in microprocessors for personal computers, workstations, servers, printers and other similar electronic equipment.
Voltage regulators allow a desired output voltage to be provided proportionally to the current required by the supplied load. This feature is commonly called “droop function” or “voltage positioning”.
The droop function allows current sense devices to read or estimate the current delivered by the regulator. In general, current sense devices read the current as a voltage drop across a sense resistance, that may be a parasitic element of the voltage regulator. For example, in power switches, the sense resistance may be the on resistance of the switch or the parasitic resistance of the phase inductor (phase winding). Alternatively, the resistance may be a series of components purposely inserted in the circuit.
If a discrete sense resistance is used, a very precise current reading may be provided. Moreover, using constantan-made resistances, the current reading is almost independent from temperature variations.
Nevertheless, a drawback of this approach consists in being expensive and in lowering current conversion efficiency.
Using parasitic resistances of the switches of the regulator for sensing the delivered current is economical because no separate element is added to the regulator. Unfortunately, the current sensing may be less precise because of the spread of the value of the conduction resistance of each integrated switch and to the variations produced by fluctuations of the working temperature of the regulator.
Sensing the phase currents by exploiting the parasitic resistance of the phase winding is preferred because the reading tolerance may be contained within about 5%.
In general, a multiphase voltage regulator with N switches outputs a saw-tooth current with a period corresponding to T/N, wherein T is the switching period of the phases. Nevertheless, switched voltage regulators use a control circuit for controlling the phase differences between the input currents of the N switches, in order to balance the current delivered by each phase and attain an effective current sharing mode of operation.
In currently available voltage regulators, the presence of particularly precise components to meet stringent specifications, poses further problems. In particular, the ability of using ever smaller inductors makes the values of parasitic series resistances RL to be exploited as sense resistances comparable to the values of parasitic resistances on PC application boards, because of the unavoidable resistance of the metal vias therein.
As emphasized in FIG. 1, for a dual-phase regulator, the presence of the parasitic resistances Rp1 in series with the resistances RL implies that a current sensing circuit capable of discriminating the current sense information on the phase winding is used for correct sensing. The solution is to implement a so-called fully differential reading.
In a known device described in U.S. Pat. No. 5,982,160 an R-C current sense series connection is connected in parallel to each output inductor of the voltage regulator, as shown in FIG. 2. The values of the shunt resistor R and of the filter capacitor C of the R-C series connection are determined such to match the time constant of the circuit RL-L (the phase winding and its parasitic series resistance) with the time constant of the R-C series connection. In this matching condition, it may be assumed that the DC component of the voltage drop on the phase winding be equal to the voltage on the filter capacitor of the R-C series connection.
Moreover, the current sense circuit current signal of each R-C network is analyzed together with the output signal through a resistance RG by a controller. In the case represented in FIG. 2b, a circuit is shown for estimating the output current of the dual-phase voltage regulator emphasizing that the controller has two dedicated pins for each R-C series connection.
The voltage regulator according to the prior approach, although advantageous under several points of view, requires a controller with 2*N pins for the total current reading, wherein N is the number of phases of the voltage regulator. This increases complexity of the regulator and silicon area requirement.
A known two-phase voltage regulator is described in U.S. Pat. No. 6,683,441 B2 and is shown in FIG. 3. The voltages of the two nodes, PHASE1 and PHASE2, are added by two resistances Rp connected in common and sent to an input of an operational amplifier, and the other input receives the output voltage of the regulator. In this case, a fully differential current reading cannot be provided because the output voltage VOUT is used as a reference voltage by the operational amplifier. Therefore, different parasitic resistances between the output of the regulator and each phase cause an error in sensing the delivered output current. Moreover, this prior architecture does not allow to sense each phase current.
Another known current sense circuit of a multi-phase voltage regulator is disclosed in the published U.S. patent application No. 2008/0169797 in the name of the same applicant, and is shown in FIG. 4. The current sense circuit composed by the operational amplifier and the controlled MOSFET uses only two pins for reading the mean current flowing through the inductances of the N phases of the voltage regulator.
Unfortunately, also this prior architecture does not allow to sense each phase current, neither a refined control of the current delivered by the voltage regulator by correcting the current unbalancing in each phase.