CS5305 datasheet(22/33 Pages) ONSEMI

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CS5305

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Share Bus Components

Five external components are required to implement the

module−to−module current

function.

These

components set the current sense load line, provide the share

bus pull−down and compensate the share adjust amplifier.

The share current sense amplifier monitors the total

module current and provides a DC voltage output

proportional to that current. The share sense amplifier gain

is programmable and allows the user to set the share bus

transconductance. It is important that all modules in a

system have a share current load line that approximates that

all the other modules to ensure accurate

module−to−module current sharing. Let us arbitrarily set the

share bus maximum voltage for full load at 2 V. If a module

is designed to provide 81 A at full load, the module share

transconductance should be 81 A/2 V or 40.5 A/V. Two

resistors and a capacitor set the share current sense amplifier

gain. The resistors set the DC gain while the capacitor

provides a zero to minimize errors due to noise.

The total module current is measured as described in the

section dealing with the OCSET current limit function. That

is, each phase within a module generates a voltage between

the CSx and CSREF leads that is proportional to the current

flow in the output inductor and the inductor’s ESR:

VCSx * VCSREF + (IL)(ESRL)

This signal is amplified by a factor of 3.7 for each phase

and then summed for all three phases. This signal is provided

as input to the share current sense amplifier. If we assume

that all three phases are sharing current equally within a

single module, the input to the share current sense amplifier

can be expressed as:

VIN(SENSE) + 11.1(VCSx * VCSREF)

+ 11.1(IL)(ESRL) + 3.7(IOUT)(ESRL)

If we set IL equal to the maximum per phase current at full

load, and if we know the value of ESR for our inductors, we

can calculate the required share current sense amplifier

gain as:

AV(SHARESENSE) +

share maximimum voltage

VIN(SENSE)

As an example, let us again consider the case for a module

providing full load current of 81 A. Each output phase is

conducting 27 A. If we assume ESR = 1.5 mΩ then input to

the share current sense amplifier is (11.1)(27 A)(1.5 mΩ) =

0.45 V. The required share current sense amplifier gain is

then 2 V/0.45 V = 4.44.

−

RIOUT

CIOUT

IFB

RIFB

VIN(SENSE)

IOUT

Figure 38.

From the schematic in Figure 38, we derive the DC gain as:

AV(SHARESENSE) + 4.44 + RIOUT RIFB ) 1

This specifies that RIOUT should be 3.44 times greater than

RIFB.

Another important consideration is the type of resistor

selected for RIFB. The thermal performance of RIFB must

match that of whatever sense element is being used to monitor

module current. Inductive sensing has been shown to be

reasonably accurate, but copper’s thermal coefficient of

resistivity is approximately +4000 parts per million per °C

(0.4% per °C). In order to maintain accurate control of the

share bus over temperature, RIFB must have a similar thermal

coefficient. This requires a positive temperature coefficient

element such as the KOA−Speer LT73. If a standard sense

resistor is used in series between the inductor and the load,

there is no need to use special resistors for sensing, but

efficiency will suffer due to power dissipation in the sense

resistor.

As regards the value of CIOUT, it should be noted that the

complete transfer function for the share current sense

amplifier in Figure 38 is:

AV(SHARESENSE) +

RIOUT

RIFB(1 ) sCIOUT RIOUT)

) 1

CIOUT causes the gain for high frequency noise to

decrease, thus quieting the share bus.

The share resistor provides a passive pull−down on the

SHARE lead. This allows the share bus voltage to be pulled

all the way down to ground. The share resistor is selected to

satisfy a number of criteria. First, the resistor cannot be made

too small. The SHARE lead source current is guaranteed to

be above 1 mA and must be capable of driving the SHARE

lead voltage to 3 V. The share bus of one module serves as

master to all and drives the total resistance of all SHARE

leads. Thus, the total impedance of all share resistors should

be made greater than or equal to 3 kΩ. That is,

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