LT3755 datasheet(13/24 Pages) LINER

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LT3755/LT3755-1

37551fa

APPLICATIONS INFORMATION

between zero and full current to achieve a precisely pro-

grammed average current. To make this method of current

control more accurate, the switch demand current is stored

on the VC node during 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 will

depend on the choice of operating frequency through the

RT input. For best current accuracy, the minimum PWM

low or high time should be at least six switching cycles

(6μs for fSW=1MHz).MaximumPWMperiodisdetermined

by the system and is unlikely to be longer than 12ms.

The maximum PWM dimming ratio (PWM(RATIO)) can be

calculated from the maximum PWM period (tMAX) and the

minimum PWM pulse width (tMIN) as follows:

PWM

RATIO

MAX

MIN

tMAX = 9ms, tMIN = 6μs (fSW = 1MHz)

PWMRATIO = 9ms/6μs = 1500:1

Programming the Switching Frequency

The RT frequency adjust pin allows the user to program

the switching frequency from 100kHz to 1MHz to optimize

efﬁciency/performance or external component size. 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.

Lower frequency operation gives better performance at the

cost of larger external component size. For an appropri-

ate RT resistor value see Table 1 or Figure 4. An external

resistor from the RT pin to GND is required—do not leave

this pin open.

Table 1. Switching Frequency vs RT Value (1% Resistors)

fOSC (kHz)

RT (kΩ)

1000

400

28.7

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 5) 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 datasheet should be increased by

Figure 4. Switching Frequency vs RT

RT (k)

37551 F04

10000

1000

100

Figure 5. Typical Minimum On and Off

Pulse Width vs Temperature

100

200

300

150

250

37551 F05

TEMPERATURE (°C)

–50

–25

100

125

MINIMUM ON-TIME

MINIMUM OFF-TIME

CGATE = 3300pF

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