Zxld1371, Applications information – Diodes ZXLD1371 User Manual
Page 27
ZXLD1371
ZXLD1371
Document number: DS35436 Rev. 1 - 2
27 of 42
February 2012
© Diodes Incorporated
A Product Line of
Diodes Incorporated
Applications Information
(cont.)
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 ZXLD1371 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.
Figure 7. Frequency vs. V
IN
for Boost LED Driver with
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-peak ripple current in the coil,
ΔI
L
and the MOSFET on time, t
ON
.
L1 = V
IN
- N V
LED
- I
OUT
R
DSON
+ R
COIL
+ R
S
t
ON
∆I
L
for Buck
L1 = V
IN
- I
IN
R
DSON
+ R
COIL
+ R
S
t
ON
∆I
L
for Boost
L1 = V
IN
- (I
IN
+ I
OUT
) R
DSON
+ R
COIL
+ R
S
t
ON
∆I
L
for Buck-boost
Equation 18
Therefore In order to calculate L1, we need to find I
IN
, t
ON
, and
ΔI
L
. The effects of the resistances are small and will be
estimated.
I
IN
is estimated from Equation 9.
t
ON
is related to switching frequency, f, and duty cycle, D, as follows:
t
ON
=
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 calculation of t
ON
.
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
°
Ω
Ω
Ω
L = 33µH
0
50
100
150
200
250
300
350
400
450
500
SW
IT
CHING F
REQU
E
N
CY
(
k
Hz
)
L = 100µH
L = 68µH