Zxld1370, Application information – Diodes ZXLD1370 User Manual
Page 20
ZXLD1370
Document number: DS32165 Rev. 5 - 2
20 of 39
September 2012
© Diodes Incorporated
ZXLD1370
A Product Line of
Diodes Incorporated
Application Information
(cont.)
Now we calculate R
S
from Equation 11. Assume ADJ is connected to REF.
Ω
=
=
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
=
196
.
0
305
.
0
x
35
.
0
225
.
0
V
V
ADJx
_
xGI
I
225
.
0
R
REF
ADJ
LED
SBOOSTBB
A preferred value of R
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
RGI1
RG2
0.2
30k
Ω 120kΩ
0.25
33k
Ω 100kΩ
0.3
39k
Ω 91kΩ
0.35
30k
Ω 56kΩ
0.4
100k
Ω 150kΩ
0.45
51k
Ω 62kΩ
0.5
30k
Ω 30kΩ
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 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 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-peak ripple current in the coil, ΔI
L
and the MOSFET on time, t
ON
.
(
)
{
}
I
t
R
R
R
I
V
V
L
ON
S
COIL
DSON
OUT
LED
IN
Δ
+
+
−
−
for Buck
L1 =
(
)
{
}
I
t
R
R
R
I
V
L
ON
S
COIL
DSON
IN
IN
Δ
+
+
−
for Boost Equation 18
(
)(
)
{
}
I
t
R
R
R
I
I
V
L
ON
S
COIL
DSON
OUT
IN
IN
Δ
+
+
+
−
for Buck-boost
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.