LM3424 Datasheet, PDF (28/69 Pages) National Semiconductor (TI) – Constant Current N-Channel Controller with Thermal Foldback for Driving LEDs

LM3424, LM3424-Q1
SNVS603C – AUGUST 2009 – REVISED AUGUST 2015
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Application Information (continued)
8.1.7 Switching Frequency
The switching frequency of the LM3424 is programmed using an external resistor (R_T) connected from the RT pin
to GND as shown in Figure 35.
Alternatively, an external PWM signal can be applied to the RT pin through a filter (R_FLT and C_FLT) and an AC
coupling capacitor (C_AC) to synchronize the part to an external clock as shown in Figure 35. If the external PWM
signal is applied at a frequency higher than the base frequency set by the R_T resistor, the internal oscillator is
bypassed and the switching frequency becomes the synchronized frequency. The external synchronization signal
should have a pulse width of 100ns, an amplitude between 3V and 6V, and be AC coupled to the RT pin with a
ceramic capacitor (C_AC = 100pF). A 10MHz RC filter (R_FLT = 150Ω and C_FLT = 100 pF) should be placed between
the PWM signal and C_AC to eliminate unwanted high frequency noise from coupling into the RT pin.
The switching frequency is defined:
1
fSW = 1.4 x 10-10x R - 1.95 x 10-8
T
(35)
See the Typical Characteristics section for a plot of R_T vs. f_SW.
LM3424
External Synchronization
R_FLT
C_AC
PWM
C_FLT
RT
Oscillator
Start tON
R_T
Figure 35. Timing Circuitry
28
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