LT3474 datasheet(14/26 Pages) AD

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LT3756/LT3756-1/LT3756-2

Rev. C

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APPLICATIONS INFORMATION

method uses the PWM pin to modulate the current source

between zero and full current to achieve a precisely pro-

grammed average current. To make PWM dimming more

accurate, the switch demand current is stored on the VC

node during the quiescent phase when PWM is low. This

featureminimizesrecoverytimewhenthePWMsignalgoes

high. To further improve the recovery time, a disconnect

switch may be used in the LED current path to prevent the

ISP node from discharging during the PWM signal low

phase. The minimum PWM on or off time will depend on

the choice of operating frequency and external component

selection. With operation in discontinuous conduction

mode (DCM), regulated current pulses as short as 1µs are

achievable. But, the best overall combination of PWM and

analog dimming (with CTRL) is available if the minimum

PWM pulse is at least six switching cycles.

Programming the Switching Frequency

The RT frequency adjust pin allows the user to program

the switching frequency from 100kHz to 1MHz to optimize

efficiency/performanceorexternalcomponentsize.Higher

frequency operation yields smaller component size but

increases switching losses and gate driving current, and

may not allow sufficiently high or low duty cycle operation.

Lowerfrequencyoperationgivesbetterperformanceatthe

cost of larger external component size. For an appropriate

RT resistor value see Table 1. An external resistor from the

RT pin to GND is required—do not leave this pin open.

Table 1. Switching Frequency vs RT Value

fOSC (kHz)

RT (kΩ)

1000

10.0

900

11.8

800

13.0

700

15.4

600

17.8

500

21.0

400

26.7

300

35.7

200

53.6

100

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

application. The fixed minimum on-time and minimum

off-time (see Figure 4) and the switching frequency define

the minimum and maximum duty cycle of the switch, re-

spectively. The following equations express the minimum/

maximum duty cycle:

Min Duty Cycle = (minimum on-time) • switching fre-

quency

Max Duty Cycle = 1 – (minimum off-time) • switching

frequency

When calculating the operating limits, the typical values

for on/off-time in the data sheet should be increased by

at least 60ns to allow margin for PWM control latitude,

GATE rise/fall times and SW node rise/fall times.

100

200

300

150

250

375612 F04

TEMPERATURE (°C)

–50

–25

100

150

125

MINIMUM ON-TIME

MINIMUM OFF-TIME

CGATE = 3300pF

Figure 4. Typical Minimum On and Off

Pulse Width vs Temperature

Thermal Considerations

The LT3756 series is rated to a maximum input voltage of

100V. Careful attention must be paid to the internal power

dissipation of the IC at higher input voltages to ensure that

a junction temperature of 125°C (150°C for H-grade and

J-Grade) is not exceeded. This junction limit is especially

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