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LT3475/LT3475-1
APPLICATIONS INFORMATION
LT3475 to maintain diode current regulation with PWM
pulse widths as short as 7.5 switching cycles (12.5μs for
f SW = 600kHz). Maximum PWM period is determined by
the system and is unlikely to be longer than 12ms. Using
PWM periods shorter than 100μs is not recommended.
The maximum PWM dimming ratio (PWM RATIO ) can be
calculated from the maximum PWM period (t MAX ) and
minimum PWM pulse width (t MIN ) as follows:
t MAX /t MIN = PWM RATIO
Total dimming ratio (DIM RATIO ) is the product of the PWM
dimming ratio and the current dimming ratio.
Example:
I MAX = 1A, I MIN = 0.1A, t MAX = 9.9ms
t MIN = 3.3μs (f SW = 1.4MHz)
I RATIO = 1A/0.1A =10:1
Layout Hints
As with all switching regulators, careful attention must
be paid to the PCB layout and component placement. To
maximize ef?ciency, switch rise and fall times are made
as short as possible. To prevent electromagnetic interfer-
ence (EMI) problems, proper layout of the high frequency
switching path is essential. The voltage signal of the SW
and BOOST pins have sharp rise and fall edges. Minimize
the area of all traces connected to the BOOST and SW
pins and always use a ground plane under the switching
regulator to minimize interplane coupling. In addition, the
ground connection for frequency setting resistor R T and
capacitors at V C1 , V C2 pins (refer to the Block Diagram)
should be tied directly to the GND pin and not shared
with the power ground path, ensuring a clean, noise-free
connection.
PWM RATIO = 9.9ms/3.3μs = 3000:1
PWM1
SHDN
PWM2
DIM RATIO = 10 ? 3000 = 30000:1
To achieve the maximum PWM dimming ratio, use the
circuit shown in Figure 9. This allows PWM pulse widths
as short as 4.5 switching cycles (7.5μs for f SW = 600kHz).
Note that if you use the circuit in Figure 9, the rising edge
of the two PWM signals must align within 100ns.
V IN
220pF
R T
V C
1M
LT3475
3.3nF
10k
0.1μF
PWM1
R T
GND
3475 F09
3475 F10
VIA TO LOCAL GND PLANE
Figure 9. Extending the PWM Dimming Range
Figure 10. Recommended Component Placement
3475fb
15
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