EL7571C 데이터 시트보기 (PDF) - Elantec -> Intersil

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EL7571C

Programmable PWM Controller

Elantec -> Intersil 

EL7571C

Programmable PWM Controller

tion. The following table lists some of the differences

between the various configurations:

Converter Topologies

Topology

5V only Non-synchronous

5V only Synchronous

5V &12V Non-synchronous

5V & 12V Synchronous

12V only Synchronous

Diagram

figure 1

figure 2

figure 3

figure 4

Connection

Diagram

Efficiency

92%

95%

92%

95%

92%

Circuit schematics and Bills of Material (BOMs) for the

various topologies are provided at the end of this data

sheet. If your application requirements differ from the

included samples, the following design guide lines

should be used to select the key component values.

Refer to the front page connection diagram for compo-

nent locations.

Output Inductor, L1

Two key converter requirements are used to determine

inductor value:

• IMIN- minimum output current; the current level at

which the converter enters the discontinuous mode of

operation (refer to Elantec application note #18 for a

detailed discussion of discontinuous mode)

• IMAX- maximum output current

Although many factors influence the choice of the

inductor value, including efficiency, transient response

and ripple current, one practical way of sizing the induc-

tor is to select a value which maintains continuous mode

operation, i.e. inductor current positive for all condi-

tions. This is desirable to optimize load regulation and

light load transient response. When the minimum induc-

tor ripple current just reaches zero and with the mean

ripple current set to IMIN, peak inductor ripple current is

twice IMAX, independent of duty cycle. The minimum

inductor value is given by:

L1MIN = (---V----I--N-----–-----1V---P-O--E--U--A--T-K--)----×-----T---O----N-- = -(-V-V---I-I-N-N----×-–---F--V--S--O-W---U---×-T---)-2---×-×----V-I--M-O----UI--N--T-

Cost

low

higher

lowest

high

highest

Complexity

low

higher

lowest

high

highest

Input Noise

high

high

high

high

high

Transient

Response

good

good

good

good

best

where:

IPEAK = peak ripple current

TON = top switch on time

VIN = input voltage

FSW = switching frequency

VOUT = output voltage

IMIN = minimum load

Since inductance value tends to decrease with current,

ripple current will generally be greater than 21MIN at

higher output current.

Once the minimum output inductance is determined, an

off the shelf inductor with current rating greater than the

maximum DC output required can be selected. Pulse

Engineering and Coil Craft are two manufactures of

high current inductors. For converter designers who

want to design their own high current inductors, for

experimental purposes or to further reduce costs, we rec-

ommend the Micrometals Powered Iron Cores data

sheet and applications note as a good reference and start-

ing point.

Current Sense Resistor, R1

Inductor current is monitored indirectly via a low value

resistor R1. The voltage developed across the current

sense resistor is used to set the maximum operating cur-

rent, the current reversal threshold and the system load

regulation. To ensure reliable system operation it is

important to sense the actual voltage drop across the

resistor. Accordingly a four wire Kelvin connection

should be made to the controller current sense inputs.

9

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