ZXLD1370

Document number: DS32165 Rev. 6 - 2

20 of 40

www.diodes.com

December 2016

© Diodes Incorporated

ZXLD1370

































196

.

0

305

.

0

x

35

.

0

225

.

0

V

V

ADJx

_

xGI

I

225

.

0

R

REF

ADJ

LED

SBOOSTBB

0.2Ω will give the desired LED current with an error of 2% due to the preferred value selection.

Table 1 shows typical resistor values used to determine the GI_ADJ ratio with E24 series resistors.

Table 1

GI Ratio

0.2

30k

Ω

120k

Ω

0.25

33k

Ω

100k

Ω

0.3

39k

Ω

91k

Ω

0.35

30k

Ω

56k

Ω

0.4

100k

Ω

150k

Ω

0.45

51k

Ω

62k

Ω

0.5

30k

Ω

30k

Ω

This completes the LED current setting.

Inductor Selection and Frequency Control

The selection of the inductor coil, L1, requires knowledge of the switching frequency and current ripple, and 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 ZXLD1370 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 330kHz in Buck configuration, and 300kHz in Boost and Buck-

Boost configurations. This is helpful in terms of EMC and other system requirements.

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-









I

t

R

R

R

I

V

V

L

ON

S

COIL

DSON

OUT

LED

IN











for Buck

L1 =









I

t

R

R

R

I

V

L

ON

S

COIL

DSON

IN

IN









for Boost

Equation 18











I

t

R

R

R

I

I

V

L

ON

S

COIL

DSON

OUT

IN

IN











for Buck-Boost

f

D

Equation 19