0

5

10

15

20

25

30

35

INPUT VOLTAGE (V)

0

250

500

750

1000

1250

1500

= calculated value

68 PH

22 PH

L =

D

D

+ (2 x delay)

50

110

175

255

340

425

505

0

300

600

900

1200

1500

1800

0

300

600

900

1200

1500

1800

22 PH freq

68 PH freq

68 PH rip

22 PH rip

LM3401

www.ti.com

SNVS516C – AUGUST 2007 – REVISED MAY 2013

Figure 16. Switching Frequency and Ripple Current vs Hysteresis

SWITCHING FREQUENCY

Although hysteretic control is a simple control method, the switching frequency depends on application conditions

and components. If the inductance, input voltage, or LED forward voltage is changed, there will be a change in

the switching frequency. Therefore, care must be taken to select components which will provide the desired

frequency range. Usually, the best approach is to determine a nominal switching frequency for the application

frequency range more precisely. This design process usually involves a few iterations to select appropriate

standard values that will result in the desired frequency and ripple current.

Switching frequency can be approximately calculated using the formula:

(8)

voltages, and delay is the sum of the LM3401 propagation delay time and the PFET delay time. The LM3401

propagation delay is 46 ns typically (See the Propagation Delay curve). Alternately, the inductor value can be

calculated from a known frequency by re-arranging the same equation:

(9)

Switching frequency for a typical application is shown in Figure 17 along with the calculated frequency.

Figure 17. Frequency vs Input Voltage

Maximum switching frequency will typically occur around the input voltage which corresponds to 25% duty cycle.

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