LT3474 datasheet(15/28 Pages) AD

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LT3755/LT3755-1/LT3755-2

Rev. E

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APPLICATIONS INFORMATION

zero and full current to achieve a precisely programmed

average current. To make PWM dimming more accurate,

the switch demand current is stored on the VC node dur-

ing the quiescent phase when PWM is low. This feature

minimizes recovery time when the PWM signal goes 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 is affected by choice

of operating frequency and external component selection.

The data sheet application titled “Buck Mode 500mA LED

Driver for 20kHz PWM Dimming” demonstrates regulated

current pulses as short as 1µs are achievable. The best

overall combination of PWM and analog dimming capabil-

ity 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,

respectively. The following equations express the mini-

mum/maximum duty cycle:

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

frequency

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.

Figure 4. Typical Minimum On and Off

Pulse Width vs Temperature

100

200

300

150

250

37551 F04

TEMPERATURE (°C)

–50

–25

100

150

125

MINIMUM ON-TIME

MINIMUM OFF-TIME

CGATE = 3300pF

Thermal Considerations

The LT3755 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 (150°C for H-Grade and

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

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