LT3669EUFD データシートの表示（PDF） - Linear Technology

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LT3669EUFD

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

Linear Technology 

LT3669/LT3669-2

APPLICATIONS INFORMATION

until the residual voltages approach within 2.95V of the

intended power rail, in which case the loads are driven

continuously. The on-time depends on the sum of the

residual voltages for the active switches (provided that

the residual voltage is higher than 2.95V) and since their

current limit is fixed, it is inversely proportional to their

power dissipation. The lower the power dissipated by the

switches, the longer the on-time, thus optimizing the time

to drive these heavy loads fully. In order to account for the

case of normal load slew rate, the internal on-time timer

only increases after a blanking period dependent on the

SR setting. A thermal shutdown circuit with a trigger tem-

perature of 140°C (typical) provides additional protection.

Short-Circuit and Thermal Shutdown Flags SC1 and SC2

A short-circuit is defined as the condition where the driver’s

output is within 2.95V from the opposite targeted rail,

for instance if the CQ1 output is programmed to be high

level (close to VL+) but stays within 2.95V from GND. If

either CQ1 or Q2 is short-circuited, the internal pulsing

mechanism and thermal shutdown circuitry will protect

the drivers. Open-collector outputs SC1 and SC2 will pull

low during short-circuit events on CQ1 an Q2, respectively.

A heavy output load can be interpreted as a short-circuit

condition during the first pulses, and SC1 and SC2 outputs

will flag it accordingly. This information can be used by an

external microcontroller to decide whether there is a real

short-circuit or a heavy load is attached to the outputs. A

heavy load requires a minimum amount of time to bring

the driver’s output outside of the short-circuit range. By

setting timers using SC1 and SC2, a short-circuit condition

can be found and the microcontroller will react accordingly

(by disabling the affected driver, for example).

Figures 10, 11 and 12 show the behavior of the pulsing

scheme when driving a light bulb, a 470µF capacitor and

a short-circuit.

12V/5W BULB

VL+ = 12V

VCQ1/Q2 RILIM = 42.2kΩ

5V/DIV

VSC1

5V/DIV

VSC2

5V/DIV

50ms/DIV

36692 F10

Figure 10. SC1 and SC2 Outputs While Driving a Light Bulb

RILIM = 42.2kΩ

VCQ1

1V/DIV

0V

VSC1

5V/DIV

5ms/DIV

36692 F11

Figure 11. SC1 Output While Driving 470µF

RILIM = 42.2kΩ

VTXEN1

5V/DIV

VCQ1

0.5V/DIV

VSC1

5V/DIV

2ms/DIV

36692 F12

Figure 12. SC1 Output While Driving a Short-Circuit

3669fa

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For more information www.linear.com/LT3669

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