ltm8040 Linear Technology Corporation, ltm8040 Datasheet - Page 8

• Manufacturer Part Number: ltm8040
• Description: 36v, 1a ?module Led Driver
• Manufacturer: Linear Technology Corporation

For most applications, the design process is straight
forward, summarized as follows:
1. Look at Table 1 and fi nd the row that has the desired
2. Apply the recommended C
3. Connect BIAS as indicated.
4. Connect LEDA to the anode of the LED string.
5. Connect the remaining pins as needed by the system
While these component combinations have been tested
for proper operation, it is incumbent upon the user to
verify proper operation over the intended system’s line,
load and environmental conditions.
Open LED Protection
The LTM8040 has internal open LED circuit protection. If
the LED is absent or fails open, the LTM8040 clamps the
voltage on the LEDA pin to 14V. The switching regulator
then skips cycles to limit the input current. The input current
and output voltage during an open LED condition is shown
in the Typical Performance Characteristics section.
Undervoltage Lockout
Undervoltage lockout (UVLO) is typically used in situations
where the input supply is current limited, or has high source
resistance. A switching regulator draws constant power
from the source, so the source current increases as the
source voltage drops. This looks like a negative resistance
load to the source and can cause the source to current
limit or latch low under low source voltage conditions.
UVLO prevents the regulator from operating at source
voltages where this might occur. An internal comparator
will force the part into shutdown when V
An adjustable UVLO threshold is also realized through
the SHDN pin, as the internal comparator that performs
this function has a 2.65V threshold. An internal resistor
pulls 10.3μA to ground from the SHDN pin at the UVLO
threshold in order to provide hysteresis.
LTM8040
APPLICATION INFORMATION
8
input voltage range LED string voltage range and output
current.
IN
, R
T
and R
IN
falls below 3.5V.
ADJ
requirements.
values.
Choose resistors according to the following formula:
where V
Suppose that the output needs to stay off until the input
is above 8V.
V
Let R1 = 100k
Keep the connections from the resistors to the SHDN pin
short. If high resistance values are used, the SHDN pin
should be bypassed with a 1nF capacitor to prevent cou-
pling problems from switching nodes.
Setting the Switching Frequency
The LTM8040 uses a constant frequency architecture that
can be programmed over a 500kHz to 2MHz range with a
single external timing resistor from the RT pin to ground.
The current that fl ows into the timing resistor is used to
charge an internal oscillator capacitor. The LTM8040 is
confi gured such that the default frequency is 500KHz
without adding any resistor. Many applications use this
value. If another frequency is desired, a graph for selecting
TH
R2 =
R2 =
= 8V
UVLO
V
8V − 2.65V
UVLO
100k
V
is the desired UVLO Threshold
IN
R1
Figure 1. Undervoltage Lockout
− 2.65V
2.65V
2.65V
C1
– 10.3µA
– 10.3µA
R1
R2
V
SHDN
IN
LTM8040
= 61.9
GND
8040 F01
8040p