LT3756 LINER [Linear Technology], LT3756 Datasheet - Page 12

• Manufacturer Part Number: LT3756
• Description: 100VIN, 100VOUT LED Controller
• Manufacturer: LINER [Linear Technology]

LT3756/LT3756-1
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 f
by the system and is unlikely to be longer than 12ms.
The maximum PWM dimming ratio (PWM
calculated from the maximum PWM period (t
minimum PWM pulse width (t
t
PWM
Programming the Switching Frequency
The RT frequency adjust pin allows the user to program
the switching frequency from 100kHz to 1MHz to optimize
effi ciency/performance or external component size. Higher
frequency operation yields smaller component size but
increases switching losses and gate driving current, and
may not allow suffi ciently high or low duty cycle operation.
Lower frequency operation gives better performance at the
cost of larger external component size. For an appropri-
ate R
APPLICATIONS INFORMATION
12
MAX
PWM
T
= 9ms, t
RATIO
resistor value see Table 1 or Figure 4. An external
SW
RATIO
10000
1000
= 9ms/6μs = 1500:1
100
= 1MHz). Maximum PWM period is determined
10
Figure 4. Switching Frequency vs R
MIN
10
=
t
t
= 6μs (f
MAX
MIN
SW
R
T
= 1MHz)
(k)
MIN
) as follows:
37561 F04
(RATIO)
100
T
MAX
) and the
) can be
resistor from the RT pin to GND is required—do not leave
this pin open.
Table 1. Switching Frequency vs R
Duty Cycle Considerations
Switching duty cycle is a key variable defi ning converter
operation, therefore, its limits must be considered when
programming the switching frequency for a particular
application. The fi xed minimum on-time and minimum
off-time (see Figure 5) and the switching frequency defi ne
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 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 100ns to allow margin for PWM control latitude,
GATE rise/fall times and SW node rise/fall times.
f
OSC
300
250
200
150
100
50
1000
0
400
200
100
–50
Figure 5. Typical Minimum On and
Off Pulse Width vs Temperature
(kHz)
–25
MINIMUM OFF-TIME
MINIMUM ON-TIME
0
TEMPERATURE (°C)
25
T
50
Value (1% Resistors)
75
100
R
T
28.7
53.6
100
37561 F05
10
(kΩ)
125
37561f