LTM8040EV-PBF LINER [Linear Technology], LTM8040EV-PBF Datasheet - Page 9

• Manufacturer Part Number: LTM8040EV-PBF
• Description: 36V, 1A ?Module LED Driver
• Manufacturer: LINER [Linear Technology]

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
4V TO 36V
Figure 2a. Tie BIAS to LPWR if it is Greater Than 2.6V
V
IN
R
137.0
175.0
215.0
487.0
T
open
13.0
16.0
18.7
24.9
29.4
35.8
54.9
75.0
88.7
(kΩ)
C1
2.2μF
LTM8040
V
SHDN
V
PWM
RT
IN
ADJ
GND
Figure 2c. Tie BIAS to an External Power Source if Neither V
LPWR
4V TO 36V
LEDA
BIAS
Frequency (MHz)
V
IN
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
3.3V
8040 F02a
C1
2.2μF
WHITE
LED
LPWR are Suitable
LTM8040
V
SHDN
V
PWM
RT
IN
ADJ
GND
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 effi ciency 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 effi ciency. 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 V
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-
LPWR
Figure 2b. BIAS May be Tied to XV
LEDA
BIAS
5.5V
V
IN
2.7V
C1
2.2μF
8040 F02c
RED
LED
3.3V
IN
Nor
LTM8040
V
SHDN
V
PWM
RT
IN
ADJ
GND
LPWR
LEDA
IN
BIAS
if LPWR is Below 2.6V
LTM8040
IN
2.7V
and BIAS is
8040 F02b
RED
LED
9
8040p