Datasheet

LTM8040
10
8040fa
If another frequency is desired, Table 1 shows suggested
R
T
selections for a variety of switching frequencies.
Table 1. R
T
vs Frequency
R
T
(kΩ) FREQUENCY (MHz)
13 2.00
16 1.84
18.7 1.70
24.9 1.50
29.4 1.37
35.7 1.25
54.9 1.00
75 0.90
88.7 0.85
137 0.75
174 0.68
215 0.65
487 0.57
OPEN 0.50
GND
3.3V
WHITE
LED
LTM8040
8040 F02a
V
IN
SHDN
ADJ
PWM
RT
V
IN
4V TO 36V
C1
2.2μF
LEDA
LPWR
BIAS
Figure 2a. Tie BIAS to LPWR if LPWR is Greater Than 2.6V
GND
2.5V
RED
LED
LTM8040
8040 F02b
V
IN
SHDN
ADJ
PWM
RT
V
IN
5.5V
C1
2.2μF
LEDA
LPWR
BIAS
GND
3.3V
2.5V
1μF
RED
LED
LTM8040
8040 F02c
V
IN
SHDN
ADJ
PWM
RT
V
IN
4V TO 36V
C1
2.2μF
LEDA
LPWR
BIAS
Figure 2b. BIAS May be Tied to V
IN
if LPWR is Below 2.6V
Figure 2c. Tie BIAS to an External Power Source if Neither V
IN
Nor LPWR are Suitable
APPLICATION INFORMATION
BIAS Pin Considerations
For proper operation, the BIAS pin must be powered by
at least 2.6V. Figure 2 shows three ways to arrange the
circuit. For outputs of 2.6V or higher, the standard circuit
(Figure 2a) is recommended. For output voltages below
2.6V, 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 2.6V (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
IN
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 capacitor is dependent upon
the source and PCB layout.