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LTC4278

4278fc

APPLICATIONS INFORMATION

when the peak design current is exceeded. This results

in an abrupt increase in inductor ripple current and,

consequently, output voltage ripple. Do not allow the core

to saturate! The maximum peak primary current occurs

at minimum VIN:

IPK =

VIN(MIN) •DCMAX

• 1+ XMIN













now :

DCMAX =

N • VIN MIN

(

)

VOUT

1+ 1

• 41

49.4%

XMIN =

VIN(MIN) •DCMAX

(

)

fOSC •LP •PIN

41• 49.4%

(

)

200kHz • 260µH • 29.5W

0.267

Using the example numbers leads to:

IPK =

29.5W

41• 0.494

• 1+ 0.267











=1.65A

Multiple Outputs

One advantage that the flyback topology offers is that

additionaloutputvoltagescanbeobtainedsimplybyadding

windings. Designing a transformer for such a situation is

beyondthescopeofthisdocument.Formultiplewindings,

realize that the flyback winding signal is a combination of

activityonallthesecondarywindings.Thusloadregulation

is affected by each winding’s load. Take care to minimize

cross regulation effects.

Setting Feedback Resistive Divider

TheexpressionforVOUTdevelopedintheOperationsection

is rearranged to yield the following expression for the

feedback resistors:

R1=R2

VOUT +ISEC • ESR+RDS(ON)

(

)









VFB •NSF

−1













Continuing the example, if ESR + RDS(ON) = 8mW, R2 =

3.32k, then:

R1=3.32k 5

5.3 • 0.008

1.237 • 1/ 3

−











= 37.28k

choose 37.4k.

It is recommended that the Thevenin impedance of the

resistive divider (R1||R2) is roughly 3k for bias current

cancellation and other reasons.

Current Sense Resistor Considerations

The external current sense resistor is used to control peak

primary switch current, which controls a number of key

converter characteristics including maximum power and

external component ratings. Use a noninductive current

sense resistor (no wire-wound resistors). Mounting the

resistordirectlyaboveanunbrokengroundplaneconnected

with wide and short traces keeps stray resistance and

inductance low.

ThedualsensepinsallowforafullKelvinconnection.Make

sure that SENSE+ and SENSE– are isolated and connect

close to the sense resistor.

Peak current occurs at 100mV of sense voltage VSENSE.So

the nominal sense resistor is VSENSE/IPK. For example, a

peakswitchcurrentof10Arequiresanominalsenseresistor

of 0.010

W Note that the instantaneous peak power in the

sense resistor is 1W, and that it is rated accordingly. The

use of parallel resistors can help achieve low resistance,

low parasitic inductance and increased power capability.

Size RSENSE using worst-case conditions, minimum LP,

VSENSE and maximum VIN. Continuing the example, let us

assume that our worst-case conditions yield an IPKof40%

above nominal, so IPK = 2.3A. If there is a 10% tolerance

on RSENSE and minimum VSENSE = 88mV, then RSENSE •

110% = 88mV/2.3A and nominal RSENSE = 35mW. Round

to the nearest available lower value, 33m

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