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LTM8040

8040p

Ceramic capacitors are also piezoelectric. While the

LTM8040 is a ﬁxed 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 ﬁnal 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 efﬁciency 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 40.3cm2 4 layer FR4 PC board, can be used a guide.

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

Total dimming ratio (DIMRATIO) is the product of the PWM

dimming ratio and the current dimming ratio.

Example: IMAX = 1A, IMIN = 0.1A, tMAX = 1.0ms,

tMIN = 25μs

RATIO =

0.1A

=10:1

PWM

RATIO =

10ms

25µs

= 400:1

DIM

RATIO = 10 • 400 = 4000 :1

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 VIN). See Typical Performance Characteristics for

details.

Capacitor Selection Considerations

The CIN and capacitor values in tables 1 and 2 are the

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 coefﬁcients 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.

APPLICATION INFORMATION

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