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LT3954IUHE

40VIN LED Converter with Internal PWM Generator

Linear Technology 

LT3954

APPLICATIONS INFORMATION

cycle operation. Lower frequency operation gives better

performance at the cost of larger external component

size. For an appropriate RT resistor value see Table 2.

An external resistor from the RT pin to GND is required—do

not leave this pin open.

Table 2. Switching Frequency (fSW) vs RT Value

fSW (kHz)

RT (kΩ)

100

95.3

200

48.7

300

33.2

400

25.5

500

20.5

600

16.9

700

14.3

800

12.1

900

10.7

1000

8.87

Duty Cycle Considerations

Switching duty cycle is a key variable defining converter

operation, therefore, its limits must be considered when

programming the switching frequency for a particular ap-

plication. The minimum duty cycle of the switch is limited

by the fixed minimum on-time and the switching frequency

(fSW). The maximum duty cycle of the switch is limited

by the fixed minimum off-time and fSW. The following

equations express the minimum/maximum duty cycle:

Min Duty Cycle = 220ns • fSW

Max Duty Cycle = 1 – 170ns • fSW

Besides the limitation by the minimum off-time, it is

also recommended to choose the maximum duty cycle

below 95%.

DBOOST

=

VLED − VIN

VLED

DBUCK _ MODE

=

VLED

VIN

DSEPIC, DCUK

=

VLED

VLED + VIN

300

250

SW MINIMUM ON-TIME

200

SW MINIMUM OFF-TIME

150

100

50

0

–50 –25

0 25 50 75 100 125

TEMPERATURE (°C)

3954 F08

Figure 8. Typical Switch Minimum On

and Off Pulse Width vs Temperature

Thermal Considerations

The LT3954 is rated to a maximum input voltage of 40V.

Careful attention must be paid to the internal power dis-

sipation of the IC at higher input voltages to ensure that

a junction temperature of 125°C is not exceeded. This

junction limit is especially important when operating at

high ambient temperatures. If LT3954 junction temperature

reaches 165°C, the power switch will be turned off and

the PWMOUT pin will be driven to GND and the soft-start

(DIM/SS) pin will be discharged to GND. Switching will

be enabled after device temperature is reduced 10°C. This

function is intended to protect the device during momentary

thermal overload conditions.

The major contributors to internal power dissipation are

the current in the linear regulator to drive the switch, and

the ohmic losses in the switch. The linear regulator power

is proportional to VIN and switching frequency, so at high

VIN the switching frequency should be chosen carefully

to ensure that the IC does not exceed a safe junction

temperature. The internal junction temperature of the IC

can be estimated by:

TJ = TA + [VIN • (IQ + fSW • 7nC) + ISW2 • 0.04Ω • DSW]

• θJA

where TA is the ambient temperature, IQ is the quiescent

current of the part (maximum 2.2mA) and θJA is the pack-

age thermal impedance (34°C/W for the 5mm × 6mm QFN

package). For example, an application with TA(MAX) = 85°C,

For more information www.linear.com/3954

3954fa

17

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