LT3669IUFD-2TRPBF View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LT3669IUFD-2TRPBF

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

Linear Technology 

LT3669/LT3669-2

APPLICATIONS INFORMATION

LINE DRIVERS

Setting the Current Limit

The LT3669 line drivers have an accurate current limit

that is programmed by a resistor tied from the ILIM pin

to ground. Table 1 lists the necessary RILIM values for

desired current limits. Tying the ILIM pin to ground sets

the default current limit of 140mA.

Table 1. Current Limit vs RILIM Value

CURRENT LIMIT (mA)

70

90

130

170

210

250

290

330

RILIM VALUE (kΩ)

221

169

113

84.5

68.1

56.2

48.7

42.2

The accurate current-limit circuit loop has a time constant of

approximately 10µs. Additionally, high speed current-limit

clamps protect the part in case of heavy loads or short-

circuits. Figure 8 depicts the high and low side driver’s

output current waveforms in a short-circuit condition.

Slew Rate Control

The LT3669 line drivers feature a controlled programmable

slew rate for optimum EMC performance. CQ1 and Q2

rising and falling times can be programmed using the SR

input pin and are independent of the L+ supply voltage.

Forcing SR below 0.4V sets the rising/falling times to a

typical value of 1.6µs/2.1µs. Forcing SR above 0.9V sets

these times to a typical value of 260ns/340ns.

The LT3669 output drivers achieve a well controlled

slew rate for a wide variety of output loads while of-

fering a low residual voltage (< 2.1V) for output load

currents of up to 250mA. In order to do so, the output

drivers switch to a low residual voltage mode after a

defined time once the TXD signal has toggled. This time

is dependent on the SR pin input level. For SR low, the

drivers will enter this mode after 8.5µs; for SR high,

after 1.8µs. This gives enough time for the controlled slew

rate mechanism to bring the outputs to within 2.95V from

the supply rails, therefore minimizing EMI during the main

part of the level transition. Once the timer is expired the

outputs will further approach the supply rails to within 2.1V.

Figure 9 depicts the output waveforms during transitions.

Driving Heavy Loads

The LT3669 is equipped with a pulsing mechanism to drive

heavy output loads like big capacitors and incandescent

bulbs, and also protect it against output short-circuit

conditions.

Under heavy load or output short-circuit conditions, the

power dissipated in the switches may increase its local

junction temperature to excessive levels if the loads were

driven continuously. In order to maintain robust operation,

the LT3669 output drivers use pulses of variable on-time

and fixed off-time (2.2ms typical) to cool the drivers down

VTXEN1

5V/DIV

ICQ1

0.5A/DIV

0A

ICQ1

0.5A/DIV

0A

LOW SIDE

(SHORTED TO L+)

HIGH SIDE

(SHORTED TO GND)

RILIM = 42.2kΩ

VL+ = 24V

100µs/DIV

36692 F08

Figure 8. Current Limit Waveform in Short-Circuit

VCQ1

10V/DIV

0V

VCQ1

10V/DIV

0V

100Ω PULL-UP

100Ω PULL-DOWN

2µs/DIV

RILIM = 42.2kΩ

VL+ = 24V

SR = 0V

36692 F09

Figure 9. Output Waveforms During a Transition

For more information www.linear.com/LT3669

3669fa

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