LM3429_14 Datasheet, PDF (11/50 Pages) Texas Instruments – LM3429Q1 N-Channel Controller for Constant Current LED Drivers

LM3429
www.ti.com
SNVS616F – APRIL 2009 – REVISED JANUARY 2010
SWITCHING FREQUENCY
An external resistor (R_T) connected between the RCT pin and the switch node (where D1, Q1, and L1 connect),
in combination with a capacitor (C_T) between the RCT and AGND pins, sets the off-time (t_OFF) as shown in
Figure 3. For boost and buck-boost topologies, the V_IN proportionality ensures a virtually constant switching
frequency (f_SW).
V_SW
LM3429
V_IN/25
R_T
RCT
Start tON
C_T
Reset timer
Figure 3. Off-timer Circuitry for Boost and Buck-boost Regulators
For a buck topology, R_T and C_T are also used to set t_OFF, however the V_IN proportionality will not ensure a
constant switching frequency. Instead, constant ripple operation can be achieved. Changing the connection of R_T
in Figure 3 from V_SW to V_IN will provide a constant ripple over varying V_IN. Adding a PNP transistor as shown in
Figure 4 will provide constant ripple over varying V_O.
V_IN
R_SNS
LED-
R_T
LM3429
V_IN/25
RCT
Start tON
C_T
Reset timer
Figure 4. Off-timer Circuitry for Buck Regulators
The switching frequency is defined:
Buck (Constant Ripple vs. V_IN)
25 x (V_IN - V_O )
f_SW = _{RT x CT X VIN}
(4)
Buck (Constant Ripple vs. V_O)
f_SW =
25 x ⁽V_IN x V_O - V_O2⁾
R_T x C_T x V_IN2
(5)
Copyright © 2009–2010, Texas Instruments Incorporated
Product Folder Links: LM3429
Submit Documentation Feedback
11