L6615 데이터 시트보기 (PDF) - STMicroelectronics

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L6615

HIGH/LOW SIDE LOAD SHARE CONTROLLER

STMicroelectronics 

L6615

– maximum share bus voltage is internally limited up to 2.2V below L6615 VCC voltage (pin#8);

– VSH(MAX) represents an upper limit but the designer should select the full scale share bus voltage

keeping in mind that every Volt on the share bus will increase the master controller's supply current

by approximately 45µA for each slave unit connected in parallel; this total current, provided by the

master share drive amplifier, must be lower than its minimum output capabilty (8mA) so

VSH(MA

X)

<

-R----i-(--M-----I-N-----)

N

⋅

8mA

This condition is not tough to meet in normal applications, as one can easily see by using sensible

values for N (number of paralleled power supplies) and VSH(MAX). For example, with VSH(MAX)=8V,

solving for N, we obtain Nmax=20;

– maximum share drive amplifier current capability (ICGA(MAX)=2mA);

– for safety reasons the following relation must be met:

RG

>

1--

2

⋅





---V----o---u---t--

10mA

–

40

in this way no fault will cause ICS+ (or ICS-) to overcome its absolute maximum ratings.

At full load, ∆VSENSE(MAX) = IOUT(MAX) · RSENSE(MAX) is the maximum voltage drop across the resistor

RSENSE (typically few hundreds of millivolt).

IOUT(MAX) is the maximum current carried by each of the paralleled power supply; in non redundant sys-

tems composed by N power supplies, each of them works at its nominal current, so:

IOUT(MAX)

=

I--L---O-----A---D--

N

This relationship is true also in N+M redundant system, even if under normal condition each power supply

provides ILOAD/(N+M).

For example in a system composed by two paralleled power supplies 100% redundant (N=M=1), each

module is sized to sustain the entire load current (in normal operation it carries only one half): for this rea-

son the sense resistor must be sized considering the whole load current.

The temperature variation of the sense resistor (hence of its resistance value) has to be taken into ac-

count, so RSENSE(MAX) is the value at maximum operating temperature to avoid saturating the share bus.

Once fixed VSENSE(MAX), the ratio RCGA/RG (gain from the sensing section to the share bus) can be cal-

culated:

-R----C----G----A-- = ------V----S----H----(--M----A----X---)------

RG

VSENSE(MAX)

where VSH(MAX) is defined by the application.

A small capacitor in parallel to RCGA is useful to reduce the noise.

The effect of current sharing feedback loop is to force the voltages of the slave's CGA pins to be equal to

VSH (that is to reduce the voltage difference at the inputs of the L6615 error amplifier). For the sake of

simplicity we consider 2 paralleled power supplies (as in fig. 11): under closed loop condition:

IOUT(1)

⋅

R-----S----N---S----(--1---)

RG(1)

⋅

RCGA(1)

=

IOUT(2)

⋅

R-----S----N----S---(--2---)

RG(2)

⋅

RCGA(2)

Ideally all the external component and α are matched so:

IOUT(1)

=

IOUT(2)

=

-I-L----O----A---D--

2

Any mismatch will have repercussion on the sharing precision: in particular the maximum difference be-

tween the output currents (sharing error) will be given by the sum of the mismatches amongst the relevant

values.

11/20

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