Zxld1370, Application information – Diodes ZXLD1370 User Manual
Page 21
ZXLD1370
Document number: DS32165 Rev. 5 - 2
21 of 39
September 2012
© Diodes Incorporated
ZXLD1370
A Product Line of
Diodes Incorporated
Application Information
(cont.)
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 ZXLD1370 sets the ripple current, ΔI
L
is monitored by the ZXLD1370 which sets this to be 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 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
12
.
0
03
.
0
I
COIL
REF
ADJ
LMAX
−
⎪⎭
⎪
⎬
⎫
⎪⎩
⎪
⎨
⎧
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
+
=
Δ
I
ADJ
_
GI
D
1
V
V
04
.
0
01
.
0
I
COIL
REF
ADJ
LMIN
−
⎪⎭
⎪
⎬
⎫
⎪⎩
⎪
⎨
⎧
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
+
=
Δ
Equation 20
I
ADJ
_
GI
D
1
V
V
08
.
0
02
.
0
I
COIL
REF
ADJ
LMID
−
⎪⎭
⎪
⎬
⎫
⎪⎩
⎪
⎨
⎧
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
+
=
Δ
If ADJ is connected to REF, this simplifies to
I
ADJ
_
GI
D
1
15
.
0
I
COIL
LMAX
−
=
Δ
I
ADJ
_
GI
D
1
05
.
0
I
COIL
LMIN
−
=
Δ
Equation 20a
I
ADJ
_
GI
D
1
1
.
0
I
COIL
LMID
−
=
Δ
where ΔI
LMID
is the value we must use in Equation 18. We have now established the inductance value.
The chosen coil should have a saturation current higher than the peak sensed current. This saturation current is the DC current for which the
inductance has decreased by 10% compared to the low current value.
Assuming ±10% ripple current, we can find this peak current from Equation 8, adjusted for ripple current:
1.1
I
LED
for
Buck
I
COILPEAK
=
1.1 I
INMAX
for Boost
Equation 21
1.1
I
INMAX
+ I
LED
for
Buck-boost
where I
INMAX
is the value of I
IN
at minimum V
IN
.
The mean current rating is also a factor, but normally the saturation current is the limiting factor.
The following websites may be useful in finding suitable components