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LTM8040
APPLICATION INFORMATION
For most applications, the design process is straight
Choose resistors according to the following formula:
forward, summarized as follows:
1. Look at Table 2 and ?nd the row that has the desired
input voltage range LED string voltage range and output
current.
R2 =
2.65V
V UVLO ? 2.65V
R1
– 10.3μA
2. Apply the recommended C IN , R T and R ADJ values.
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
requirements.
where V UVLO is the desired UVLO threshold
Suppose that the output needs to stay off until the input
is above 8V.
V UVLO = 8V
Let R1 = 100k
8V ? 2.65V
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.
R2 =
2.65V
– 10.3μA
100k
= 61.9k
Open LED Protection
The LTM8040 has internal open LED circuit protection. If
the LED is absent or fails open, the LTM8040 clamps the
V IN
R1
V IN
LTM8040
SHDN
voltage on the LEDA pin to 14V. The switching regulator
C1
R2
GND
then skips cycles to limit the input current.
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 volt-
ages where this might occur. An internal comparator will
force the part to stop switching when V IN falls below 3.5V.
An adjustable UVLO threshold is also realized through the
SHDN pin, as the internal comparator that performs this
function has a 2.65V threshold. If SHDN is below 0.3V all
internal circuitry is off. An internal resistor pulls 10.3μA
to ground from the SHDN pin at the UVLO threshold in
8040 F01
Figure 1. Undervoltage Lockout
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 ?ows into the timing resistor is used to
charge an internal oscillator capacitor. The LTM8040 is
con?gured such that the default frequency is 500kHz
without adding a resistor. Many applications use this value.
order to provide hysteresis.
8040fa
9
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