Zxld1374, Application information – Diodes ZXLD1374 User Manual
Page 22
ZXLD1374
Document number: DS35032 Rev. 3 - 2
22 of 39
September 2012
© Diodes Incorporated
ZXLD1374
A Product Line of
Diodes Incorporated
Application 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 ZXLD1374 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.
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
N
V
1
L
L
ON
S
COIL
DSON
OUT
LED
IN
Δ
+
+
−
−
=
for Buck
(
)
{
}
I
t
R
R
R
I
V
1
L
L
ON
S
COIL
DSON
IN
IN
Δ
+
+
−
=
for Boost
(
)(
)
{
}
I
t
R
R
R
I
I
V
1
L
L
ON
S
COIL
DSON
OUT
IN
IN
Δ
+
+
+
−
=
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:
f
D
t
ON
=
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
.
The ZXLD1374 sets the ripple current, ΔI
L
, to between nominally 10% and 30% of the mean coil current, I
COIL
, which is found from Equation 8.
The device adjusts the ripple current within this range in order to regulate the switching frequency. We therefore need to use a ΔI
L
value of 20%
of I
COIL
to find an inductance which is optimized for the input voltage range. The range of ripple current control is also modulated by other circuit
parameters as follows.
I
ADJ
_
GI
D
1
V
V
24
.
0
06
.
0
I
COIL
REF
ADJ
LMAX
−
⎪⎭
⎪
⎬
⎫
⎪⎩
⎪
⎨
⎧
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
+
=
Δ
I
ADJ
_
GI
D
1
V
V
08
.
0
02
.
0
I
COIL
REF
ADJ
LMIN
−
⎪⎭
⎪
⎬
⎫
⎪⎩
⎪
⎨
⎧
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
+
=
Δ
Equation 20
I
ADJ
_
GI
D
1
V
V
16
.
0
04
.
0
I
COIL
REF
ADJ
LMID
−
⎪⎭
⎪
⎬
⎫
⎪⎩
⎪
⎨
⎧
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
+
=
Δ
If ADJ is connected to REF, this simplifies to
I
ADJ
_
GI
D
1
3
.
0
I
COIL
LMAX
−
=
Δ
I
ADJ
_
GI
D
1
1
.
0
I
COIL
LMIN
−
=
Δ
Equation 20a
I
ADJ
_
GI
D
1
2
.
0
I
COIL
LMID
−
=
Δ
where ΔI
LMID
is the value we must use in Equation 18. We have now established the inductance value.