Datasheet

LT3755/LT3755-1/LT3755-2
13
37551fd
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/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 appropriate
R
T
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 R
T
Value
f
OSC
(kHz) R
T
(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
100 100
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
0
100
200
300
50
150
250
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TIME (ns)
TEMPERATURE (°C)
–50 0
50
75
–25
25
100
150125
MINIMUM ON-TIME
MINIMUM OFF-TIME
C
GATE
= 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) is
not exceeded. This junction limit is especially important