LTM8005 datasheet(13/26 Pages) LINER

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LTM8005

8005f

For more information www.linear.com/LTM8005

APPLICATIONS INFORMATION

Example: Calculate the resistor values required to increase

the VOUT range of a boost LED driver to 7.5:1 and have

OPENLED occur when VOUT is 38V:

Step 1: Choose R10 = 374k

Step 2: VOUTMIN = 38/7.5 = 5.1

Step 3:

R11=

1.7 • 374kΩ

1.65 • 38

(

) – 0.8 • 5.1

(

) – 1.7

= 11.2kΩ

Use R11 = 11.3k.

R12 =

1.7 • 374kΩ

0.35 • 38

(

) – 1.2 • 5.1

(

)

= 88.6kΩ

Use R12 = 88.7K.

The resistor values for R14 and R15 in Figure 6 can be

calculated as shown below. See the example that follows

for a suggested R13 value.

R14 =

1.7 • R13

1.65 • VOUTMAX

(

) – 0.8 • VOUTMIN

(

) – 0.85 • VBE(Q1)

(

)

R15 =

1.7 • R13

0.35 • VOUTMAX

(

) – 1.2 • VOUTMIN

(

) – 0.85 • VBE(Q1)

(

)

Example: Calculate the resistor values required to increase

the VOUT range of a buck-boost mode LED driver to

5:1 and have OPENLED occur when VOUT is 17V. Use

VBE(Q1) =0.7V:

Step 1: Choose R13 = 187k

Step 2: VOUTMIN = 17/5 = 3.4

Step 3:

R14 =

1.7 • 187kΩ

1.65 • 17

(

) – 0.8 • 3.4

(

) – 0.85 • 0.7

(

)

= 12.9kΩ

Use R14 = 12.7k

R15 =

1.7 • 187kΩ

0.35 • 17

(

) – 1.2 • 3.4

(

) – 0.85 • 0.7

(

)

= 249kΩ

Use R15 = 249k

LED Overcurrent Protection Feature

The LTM8005 has an overload protection feature inde-

pendent of the output LED current regulation. This feature

prevents the development of excessive switching currents

and protects the power components. The overload protec-

tion threshold (2.4A typical) is designed to be 50% higher

than the default LED current sense threshold. Once the

LED overcurrent is detected, the internal power switch is

turned off to stop switching, the PWM MOSFET is turned

off to disconnect the LED array from the power path, and

fault protection is initiated via the SS pin.

An anti-parallel Schottky or ultrafast diode D2 should be

connected as shown in Figure 7 to protect the LED node

from swinging well below ground when being shorted to

ground through a long cable. The internal protection loop

takes a finite amount of time to respond to the overload,

so the diode is recommended if the system must survive

an overload on the LED string.

LTM8005

LED

GND

8005 F07

LED

STRING

PARASITIC

INDUCTANCE OF

LONG CABLE

Figure7. Connect an Anti-Parallel Diode D2 from

LED to GND to Protect the LTM8005 from Negative

Voltage Swings when the Connecting to a LED

String through a Long Cable

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