LTM8040EV-PBF LINER [Linear Technology], LTM8040EV-PBF Datasheet - Page 11

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

APPLICATION INFORMATION
Total dimming ratio (DIM
dimming ratio and the current dimming ratio.
Example: I
t
Minimum Input Voltage
The LTM8040 is a step down converter, so a minimum
amount of headroom is required to keep the output in
regulation. For most applications at full load, the input
needs to be at least 1.5V above the desired output. In
addition, it takes more input voltage to initially start than
is required for continuous operation. This start voltage is
also dependent on whether turn-on is controlled by the
LTM8040’s SHDN pin or UVLO (that is, the SHDN pin is
tied to V
details.
Capacitor Selection Considerations
The C
minimum recommended values for the associated oper-
ating conditions. Applying capacitor values below those
indicated in table 1 is not recommended, and may result
in undesirable operation. Using larger values is generally
acceptable, and can yield improved performance, if it is
necessary. Again, it is incumbent upon the user to verify
proper operation over the intended system’s line, load and
environmental conditions.
Ceramic capacitors are small, robust and have very low
ESR. However, not all ceramic capacitors are suitable.
X5R and X7R types are stable over temperature and ap-
plied voltage and give dependable service. Other types,
including Y5V and Z5U have very large temperature and
voltage coeffi cients of capacitance. In an application cir-
cuit they may have only a small fraction of their nominal
capacitance resulting in much higher output voltage ripple
than expected.
MIN
I
PWM
DIM
RATIO
= 25μs
IN
RATIO
IN
RATIO
and capacitor values in tables 1 and 2 are the
=
). See Typical Performance Characteristics for
0.1A
MAX
1A
=10 • 400 = 4000:1
=
10ms
=10:1
25µs
= 1A, I
= 400:1
RATIO
MIN
) is the product of the PWM
= 0.1A, t
MAX
= 1.0ms,
Ceramic capacitors are also piezoelectric. While the
LTM8040 is a fi xed frequency device, the internal regulators
may skip cycles at light loads and extend the switching
cycle on time as the input voltage falls towards the to
output. Under either of these conditions, the LTM8040 can
excite a ceramic capacitor at audio frequencies, generating
audible noise.
If this audible noise is unacceptable, use a high performance
electrolytic capacitor at the output. The input capacitor
can be a parallel combination of a 4.7μF ceramic capacitor
and a low cost electrolytic capacitor.
A fi nal precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LTM8040. A
ceramic input capacitor combined with trace or cable
inductance forms a high Q (under damped) tank circuit.
If the LTM8040 circuit is plugged into a live supply, the
input voltage can ring to twice its nominal value, possi-
bly exceeding the device’s rating. This situation is easily
avoided by introducing a small series damping resistance
into the circuit. This is most often taken care of by the
presence of an electrolytic bulk capacitor in the board.
High Temperature Considerations
The internal operating temperature of the LTM8040 must
be lower than 125°C rating, so care should be taken in
the layout of the circuit to ensure good heat sinking of
the LTM8040. To estimate the junction temperature, ap-
proximate the power dissipation within the LTM8040 by
applying the typical effi ciency stated in this datasheet to
the desired output power, or, if you have and actual module,
by taking a power measurement. Then calculate the tem-
perature rise of the LTM8040 junction above the surface
of the printed circuit board by multiplying the module’s
power dissipation by the thermal resistance. The actual
thermal resistance of the LTM8040 to the printed circuit
board depends on the layout of the circuit board, but the
thermal resistance given on page 2, which is based upon
a 40.3cm
The LTM8040 is equipped with thermal protection that
reduces the output LED current if the internal operating
temperature is too high. This thermal protection is active
above the 125°C temperature rating of the LTM8040, so
2
4 layer FR4 PC board, can be used a guide.
LTM8040
11
8040p