LM3429 datasheet(11/50 Pages) TI1

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)

fSW =

25 x VIN - VO

RT x CT X VIN

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)

LM3429

RCT

Start tON

VIN/25

Reset timer

RSNS

VIN

LED-

VSW

LM3429

RCT

Start tON

VIN/25

Reset timer

LM3429

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SNVS616F – APRIL 2009 – REVISED JANUARY 2010

SWITCHING FREQUENCY

An external resistor (RT) connected between the RCT pin and the switch node (where D1, Q1, and L1 connect),

in combination with a capacitor (CT) between the RCT and AGND pins, sets the off-time (tOFF) as shown in

Figure 3. For boost and buck-boost topologies, the VIN proportionality ensures a virtually constant switching

frequency (fSW).

Figure 3. Off-timer Circuitry for Boost and Buck-boost Regulators

For a buck topology, RT and CT are also used to set tOFF, however the VIN proportionality will not ensure a

constant switching frequency. Instead, constant ripple operation can be achieved. Changing the connection of RT

in Figure 3 from VSW to VIN will provide a constant ripple over varying VIN. Adding a PNP transistor as shown in

Figure 4 will provide constant ripple over varying VO.

Figure 4. Off-timer Circuitry for Buck Regulators

The switching frequency is defined:

Buck (Constant Ripple vs. VIN)

(4)

Buck (Constant Ripple vs. VO)

(5)

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