Zxld1371, Applications information – Diodes ZXLD1371 User Manual
Page 28
ZXLD1371
ZXLD1371
Document number: DS35436 Rev. 1 - 2
28 of 42
February 2012
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Applications Information
(cont.)
The ZXLD1371 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
LMAX
= 0.06 +0.24
V
ADJ
V
REF
1-D
GI_ADJ
I
COIL
∆I
LMIN
= 0.02 +0.08
V
ADJ
V
REF
1-D
GI_ADJ
I
COIL
Equation 20
∆I
LMID
= 0.04 +0.16
V
ADJ
V
REF
1-D
GI_ADJ
I
COIL
If ADJ is connected to REF, this simplifies to
∆I
LMAX
=
0.3
1-D
GI_ADJ
I
COIL
∆I
LMIN
= 0.1
1-D
GI_ADJ
I
COIL
Equation 20a
∆I
LMID
= 0.2
1-D
GI_ADJ
I
COIL
where
ΔI
LMID
is the value we must use in Equation 18. We have now established the inductance value.
The chosen coil should saturate at a current greater 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:
I
COILPEAK
= 1.1 I
LED
for Buck
I
COILPEAK
= 1.1 I
INMAX
for Boost
Equation 21
I
COILPEAK
= 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
MOSFET Selection
The ZXLD1371 requires an external NMOS FET as the main power switch with a voltage rating at least 15% higher than
the maximum circuit voltage to ensure safe operation during the overshoot and ringing of the switch node. The current
rating is recommended to be at least 10% higher than the average transistor current. The power rating is then verified by
calculating the resistive and switching power losses.
Resistive power losses
The resistive power losses are calculated using the RMS transistor current and the MOSFET on-resistance.
Calculate the current for the different topologies as follows:
Buck mode
Boost / Buck-boost mode
switching
P
resistive
P
P
+
=
LED
I
MOSFETMAX
I
=
MAX
D
1
LED
I
MOSFETMAX
I
−
=