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LTC4278

4278fc

APPLICATIONS INFORMATION

Selecting the Load Compensation Resistor

The expression for RCMP was derived in the Operation

section as:

RCMP =K1•

RSENSE • 1–DC

(

)

ESR

+RDS(ON)

•R1•NSF

Continuing the example:

K1=

VOUT

VIN •Eff











=

48 • 90%

0.116

DC=

N•VIN(NOM)

VOUT

1+ 1

• 48

45.5%

If ESR+RDS(ON) =8mΩ

RCMP =0.116 •

33mΩ • 1−0.455

(

)

8mΩ

• 37.4kΩ • 1

3.25k

This value for RCMP is a good starting point, but empirical

methods are required for producing the best results.

This is because several of the required input variables

are difficult to estimate precisely. For instance, the ESR

term above includes that of the transformer secondary,

but its effective ESR value depends on high frequency

behavior, not simply DC winding resistance. Similarly, K1

appears as a simple ratio of VIN to VOUT times efficiency,

but theoretically estimating efficiency is not a simple

calculation.

The suggested empirical method is as follows:

1. Build a prototype of the desired supply including the

actual secondary components.

2. Temporarily ground the CCMP pin to disable the load

compensation function. Measure output voltage while

sweeping output current over the expected range.

Approximate the voltage variation as a straight line.

ΔVOUT/ΔIOUT = RS(OUT) .

3. Calculate a value for the K1 constant based on VIN, VOUT

and the measured efficiency.

4. Compute:

RCMP =K1•

RSENSE

RS(OUT)

•R1•NSF

5. Verify this result by connecting a resistor of this value

from the RCMP pin to ground.

6.DisconnectthegroundshorttoCCMPandconnecta0.1µF

filter capacitor to ground. Measure the output imped-

anceRS(OUT)=ΔVOUT/ΔIOUTwiththenewcompensation

in place. RS(OUT) should have decreased significantly.

Fine tuning is accomplished experimentally by slightly

altering RCMP. A revised estimate for RCMP is:

′

RCMP =RCMP • 1+

RS(OUT)CMP

RS(OUT)













where R

′CMP is the new value for the load compensation

resistor. RS(OUT)CMP is the output impedance with RCMP

in place and RS(OUT) is the output impedance with no

load compensation (from step 2).

Setting Frequency

The switching frequency of the LTC4278 is set by an

external capacitor connected between the OSC pin and

ground. Recommended values are between 200pF and

33pF, yielding switching frequencies between 50kHz and

250kHz.Figure12showsthenominalrelationshipbetween

external capacitance and switching frequency. Place the

capacitor as close as possible to the IC and minimize OSC

COSC (pF)

100

200

300

100

200

4278 F12

Figure 12. fOSC vs OSC Capacitor Values

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