# LTC4269CDKD-1#PBF Linear Technology, LTC4269CDKD-1#PBF Datasheet - Page 28

#### LTC4269CDKD-1#PBF

Manufacturer Part Number

LTC4269CDKD-1#PBF

Description

IC PD/OPTO FLYBACK CTRLR 32-DFN

Manufacturer

Linear Technology

Type

Power Over Ethernet (PoE)r

Datasheet

1.LTC4269IDKD-1PBF.pdf
(44 pages)

#### Specifications of LTC4269CDKD-1#PBF

Applications

Power Interface Switch for Power Over Ethernet (PoE) Devices

Voltage - Supply

14 V ~ 16 V

Operating Temperature

0°C ~ 70°C

Mounting Type

Surface Mount

Package / Case

32-DFN

Current - Supply

1.35mA

Interface

IEEE 802.3af

Controller Type

Powered Device Interface Controller (PD)

Input Voltage

60V

Supply Current

6.4mA

Digital Ic Case Style

DFN

No. Of Pins

32

Duty Cycle (%)

88%

Frequency

100kHz

Operating Temperature Range

0Â°C To +70Â°C

Msl

MSL 1 - Unlimited

Rohs Compliant

Yes

Operating Temperature (max)

70C

Operating Temperature (min)

0C

Pin Count

32

Mounting

Surface Mount

Package Type

DFN EP

Case Length

7mm

Screening Level

Commercial

Lead Free Status / RoHS Status

Lead free / RoHS Compliant

APPLICATIONS INFORMATION

LTC4269-1

Selecting the Load Compensation Resistor

The expression for R

section as:

Continuing the example:

This value for R

methods are required for producing the best results.

This is because several of the required input variables

are difﬁ cult 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 V

but theoretically estimating efﬁ ciency is not a simple

calculation.

The suggested empirical method is as follows:

1. Build a prototype of the desired supply including the

2. Temporarily ground the C

3. Calculate a value for the K1 constant based on V

28

actual secondary components.

compensation function. Measure output voltage while

sweeping output current over the expected range.

Approximate the voltage variation as a straight line.

and the measured efﬁ ciency.

R

If ESR + R

CMP

K1=

DC=

R

ΔV

CMP

OUT

= K1•

⎛

⎝ ⎜

1+

V

= 0.116 •

= 3.25k

/ΔI

IN

V

N•V

DS(ON)

OUT

OUT

• Eff

R

CMP

V

ESR + R

1

IN(NOM)

SENSE

OUT

= R

⎞

⎠ ⎟

is a good starting point, but empirical

= 8mΩ

=

33mΩ • 1− 0.455

CMP

S(OUT)

48 • 90%

• 1− DC

(

DS(ON)

=

was derived in the Operation

5

1+

8mΩ

CMP

.

IN

(

8

1

1

to V

)

•

= 0.116

• R1• N

pin to disable the load

48

5

OUT

= 45.5%

times efﬁ ciency,

)

SF

• 37.4kΩ •

IN

, V

3

1

OUT

4. Compute:

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

6. Disconnect the ground short to C

Setting Frequency

The switching frequency of the LTC4269-1 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. Figure 12 shows the nominal relationship between

external capacitance and switching frequency. Place the

capacitor as close as possible to the IC and minimize OSC

from the R

ﬁ lter capacitor to ground. Measure the output imped-

ance R

in place. R

Fine tuning is accomplished experimentally by slightly

altering R

where R′

resistor. R

in place and R

load compensation (from step 2).

R

′ R

CMP

CMP

S(OUT)

C MP

Figure 12. f

300

200

100

= K1•

= R

CMP

S(OUT)CMP

50

CMP

S(OUT)

is the new value for the load compensation

= ΔV

CMP

. A revised estimate for R

S(OUT)

30

pin to ground.

R

R

S(OUT)

OUT

SENSE

should have decreased signiﬁ cantly.

• 1+

OSC

⎛

⎜

⎝

is the output impedance with R

/ΔI

is the output impedance with no

vs OSC Capacitor Values

R

OUT

C

• R1• N

OSC

S(OUT)CMP

R

S(OUT)

(pF)

with the new compensation

100

CMP

SF

and connect a 0.1μF

⎞

⎟

⎠

CMP

42691 F12

200

is:

42691fb

CMP