# LTC4269CDKD-1#PBF Linear Technology, LTC4269CDKD-1#PBF Datasheet - Page 33

#### 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

#### Available stocks

Company

Part Number

Manufacturer

Quantity

Price

Company:

Bonase Electronics (HK) Co., Limited

Part Number:

LTC4269CDKD-1#PBF

Manufacturer:

VOLTRONIC

Quantity:

2 140

Price:

APPLICATIONS INFORMATION

Short-Circuit Conditions

Loss of current limit is possible under certain conditions

such as an output short-circuit. If the duty cycle exhibited

by the minimum on-time is greater than the ratio of

secondary winding voltage (referred-to-primary) divided

by input voltage, then peak current is not controlled at

the nominal value. It ratchets up cycle-by-cycle to some

higher level. Expressed mathematically, the requirement

to maintain short-circuit control is

where:

Trouble is typically encountered only in applications with

a relatively high product of input voltage times secondary

to primary turns ratio and/or a relatively long minimum

switch on time. Additionally, several real world effects such

as transformer leakage inductance, AC winding losses and

output switch voltage drop combine to make this simple

theoretical calculation a conservative estimate. Prudent

design evaluates the switcher for short-circuit protection

and adds any additional circuitry to prevent destruction.

Output Voltage Error Sources

The LTC4269-1’s feedback sensing introduces additional

minor sources of errors. The following is a summary list:

• The internal bandgap voltage reference sets the reference

• The external feedback resistive divider ratio directly

• Leakage inductance on the transformer secondary

t

I

N

(other variables as previously deﬁ ned)

voltage for the feedback ampliﬁ er. The speciﬁ cations

detail its variation.

affects regulated voltage. Use 1% components.

reduces the effective secondary-to-feedback winding

turns ratio (NS/NF) from its ideal value. This increases

the output voltage target by a similar percentage. Since

secondary leakage inductance is constant from part to

part (within a tolerance) adjust the feedback resistor

ratio to compensate.

DC

ON(MIN)

SC

SP

MIN

is the short-circuit output current

is the secondary-to-primary turns ratio (N

= t

is the primary-side switch minimum on-time

ON(MIN)

• f

OSC

<

I

SC

• R

(

V

SEC

IN

• N

+ R

SP

DS(ON)

SEC

)

/N

PRI

)

• The transformer secondary current ﬂ ows through the

If the output load current is relatively constant, the feedback

resistive divider is used to compensate for these losses.

Otherwise, use the LTC4269-1 load compensation circuitry

(see Load Compensation). If multiple output windings are

used, the ﬂ yback winding will have a signal that represents

an amalgamation of all these windings impedances. Take

care that you examine worst-case loading conditions when

tweaking the voltages.

Power MOSFET Selection

The power MOSFETs are selected primarily on the criteria of

on-resistance R

breakdown voltage (BV

and maximum drain current (ID

For the primary-side power MOSFET, the peak current is:

where XMIN is peak-to-peak current ratio as deﬁ ned

earlier.

For each secondary-side power MOSFET, the peak cur-

rent is:

Select a primary-side power MOSFET with a BVDSS

greater than:

where NSP reﬂ ects the turns ratio of that secondary-to

primary winding. LLKG is the primary-side leakage induc-

tance and CP is the primary-side capacitance (mostly from

the drain capacitance (COSS) of the primary-side power

MOSFET). A clamp may be added to reduce the leakage

inductance as discussed.

impedances of the winding resistance, synchronous

MOSFET R

equivalent current for these errors is higher than the

load current because conduction occurs only during

the converter’s off-time. So, divide the load current by

(1 – DC).

I

I

BV

PK(PRI)

PK(SEC)

DSS

≥I

=

=

PK

V

1− DC

DS(ON)

IN(MIN)

DS(ON)

I

L

OUT

C

LKG

P

MAX

P

and output capacitor ESR. The DC

IN

• DC

, input capacitance, drain-to-source

DSS

+ V

• 1+

MAX

⎛

⎝ ⎜

IN(MAX)

), maximum gate voltage (V

• 1+

X

(MAX)

⎛

⎝ ⎜

MIN

2

+

LTC4269-1

V

⎞

⎠ ⎟

X

).

OUT(MAX)

MIN

2

N

SP

⎞

⎠ ⎟

33

42691fb

GS

)