ZXLD1371Q

ZXLD1371Q

Document number: DS37116 Rev. 1 - 2

26 of 42

www.diodes.com

September 2015

© Diodes Incorporated

ZXLD1371Q

Inductor Selection and frequency Control

The selection of the inductor coil, L1, requires knowledge of the switching frequency and current ripple, and also depends on the duty cycle

to some extent. In the hysteretic converter, the frequency depends upon the input and output voltages and the switching thresholds of the

current monitor. The peak-to-peak coil current is adjusted by the ZXLD1371Q to control the frequency to a fixed value. This is done by

controlling the switching thresholds within particular limits. This effectively much reduces the overall frequency range for a given input

voltage range. Where the input voltage range is not excessive, the frequency is regulated to approximately 390kHz. This is helpful in terms

of EMC and other system requirements. Figure 7 shows practical results of switching frequency driving 8 LEDs at 350mA.

350mA LED Current and Various Inductor Values

For larger input voltage variation, or when the choice of coil inductance is not optimum, the switching frequency may depart from the

regulated value, but the regulation of LED current remains successful. If desired, the frequency can to some extent be increased by using a

smaller inductor, or decreased using a larger inductor. The web Calculator will evaluate the frequency across the input voltage range and

the effect of this upon power efficiency and junction temperatures.

Determination of the input voltage range for which the frequency is regulated may be required. This calculation is very involved, and is not

given here. However the performance in this respect can be evaluated within the web Calculator for the chosen inductance.

The inductance is given as follows in terms of peak-to-

L1

DSO

COIL

S

for Buck

L1

DSO

COIL

S

for Boost

L1

DSO

COIL

S

for Buck boost

Equation 18

D

f

Equation 19

As the regulated frequency is known, and we have already found D from Equation 7 or the approximation Equation 7b, this allows

6

5

7

8

10

9

11

12

13

14

15

16

17

18

19

20

V

(V)

IN

T = 25 C, V

= V

8 LEDs, R = 150m ,

R9 = 120k , R10 = 36k ,

C

= 100µF

A

AUX

IN

S

IN







L = 33µH

0

50

100

150

200

250

300

350

400

450

500

L = 100µH

L = 68µH