LTM8040 Ver la hoja de datos (PDF) - Linear Technology

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Fabricante

LTM8040

36V, 1A µModule LED Driver

Linear Technology 

LTM8040

APPLICATION INFORMATION

the value of RT for a given operating frequency is shown

in the Typical Performance Characteristics section. Table

2 shows suggested RT selections for a variety of switch-

ing frequencies.

Table 2. RT vs Frequency

RT (kΩ)

13.0

16.0

18.7

24.9

29.4

35.8

54.9

75.0

88.7

137.0

175.0

215.0

487.0

open

Frequency (MHz)

2.00

1.84

1.70

1.50

1.37

1.25

1.00

0.90

0.85

0.75

0.68

0.65

0.57

0.50

BIAS Pin Considerations

For proper operation, The BIAS pin must be powered by

at least 3.2V. Figure 2 shows three ways to arrange the

circuit. For outputs of 3.2V or higher, the standard circuit

(Figure 2a) is best, because the circuit’s efficiency is better

for lower voltages above 3.2V. For output voltages below

3.2V, the BIAS pin can be tied to the input (Figure 2b) at

the cost of some efficiency. Finally, the BIAS pin can be

tied to another source that is at least 3.2V (Figure 2c).

For example, if a 3.3V source is on whenever the LED is

on, the BIAS pin can be connected to the 3.3V output. In

all cases, be sure that the maximum voltage at the BIAS

pin is both less than 25V and the sum of VIN and BIAS is

less than 51V. If BIAS is powered by a source other than

LPWR, a local decoupling capacitor may be necessary.

The value of the decoupling cap is dependent upon the

source and PCB layout.

Programming LED Current

The LED current can be set by adjusting the voltage on

the ADJ pin. The ADJ pin is internally pulled up to a pre-

VIN

4V TO 36V

C1

2.2μF

LTM8040

VIN

SHDN

LEDA

LPWR

VADJ

PWM

BIAS

RT

GND

3.3V

WHITE

LED

VIN

5.5V

C1

LTM8040

VIN

SHDN

LEDA

LPWR

2.2μF

VADJ

BIAS

PWM

RT

GND

2.7V

RED

LED

8040 F02a

Figure 2a. Tie BIAS to LPWR if it is Greater Than 2.6V

8040 F02b

Figure 2b. BIAS May be Tied to XVIN if LPWR is Below 2.6V

VIN

4V TO 36V

C1

2.2μF

LTM8040

VIN

SHDN

LEDA

LPWR

VADJ

PWM

BIAS

RT

GND

2.7V

3.3V

RED

LED

8040 F02c

Figure 2c. Tie BIAS to an External Power Source if Neither VIN Nor

LPWR are Suitable

8040p

9

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