LT3669 View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LT3669

IO-Link Transceiver with Integrated Step-Down Regulator and LDO

Linear Technology 

LT3669/LT3669-2

APPLICATIONS INFORMATION

The last parameter to take into account is the quiescent

current required to keep all circuits working properly which

is about 6mA. As an example, assume an L+ voltage of

24V, a programmed VOUT and VLDO voltages of 5V and

3.3V respectively and load currents for both line drivers

of 250mA. The LDO input is connected to the switching

regulator output and both switching regulator and LDO

outputs are driving full load (300mA and 150mA, respec-

tively). The total power dissipation can be estimated as:

PD = 24V • 0.006A + (5V – 3.3V) • 0.15A +

5V • 0.3A • 25% + 2 • 1.5V • 0.25A = 1.524W

With a θJA of 44°C/W, the increase in junction temperature

compared to ambient will be 67°C.

The LT3669 protects itself against internal overheating with

the help of two independent thermal shutdown circuits.

One of them, with a hysteresis of 12°C, shuts only the line

drivers off if the junction temperature exceeds 140°C, and

pulls both SC1 and SC2 outputs low during the thermal

shutdown event. The LDO and switching regulator outputs

keep in regulation, allowing a µC to process the event. This

thermal shutdown circuit keeps the junction temperature

under control in those cases where only the line drivers

are under heavy load or short-circuit conditions. In case of

fault conditions on the LDO or switching regulator outputs,

a second thermal shutdown circuit shuts them off if the

junction temperature exceeds 168°C. Figure 21 depicts

waveforms during a thermal shutdown event.

Reverse-Polarity Protection

The LT3669 is designed to withstand ±60V between any

combination of the line driver ports (L+, CQ1, Q2 and GND).

The switching regulator’s power devices are powered from

L+ through an integrated reverse-polarity protection diode

whose cathode is also wired to the DIO pin. In order to

avoid damaging this diode due to surge currents during

hot plugging, do not place any bypass capacitors at the

DIO pin (leave it unconnected) unless an external diode

is connected to bolster the integrated one.

Surge and ESD Protection Considerations

The LT3669 contains internal protection against ESD pulses

(HBM 100pF/1.5kΩ) of ±4kV for the interface ports (L+,

CQ1, Q2 and GND) and ±2kV for all other pins.

In order to protect the LT3669 interface ports against

surge and contact/air discharge events based on the

IEC 61000-4-5 and IEC 61000-4-2 standards, additional

external protection is required. TVS diodes with break-

down voltages above the maximum operating voltage

of the application and clamp voltages below 60V (for

the maximum expected short-circuit current during

the surge/ESD event) are required.

SM6T39A or equivalent TVS clamps are recommended

for IO-Link and most other applications with L+ operating

voltages as high as 36V and will protect the part against

VLDO

5V/DIV

VOUT

5V/DIV

VSC1/SC2

5V/DIV

LINE DRIVERS ON

(PULLING LOAD HIGH)

VCQ1/Q2

10V/DIV

3.3V

5V

THERMAL

SHUTDOWN

EVENT

0V

THERMAL

SHUTDOWN

SHUTS LINE

DRIVERS OFF

0V

100ms/DIV

36692 F21

Figure 21. Thermal Shutdown Waveforms

For more information www.linear.com/LT3669

3669fa

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