ZXLD1371EST16TC Diodes Inc. / Zetex, ZXLD1371EST16TC Datasheet - Page 28

• Manufacturer Part Number: ZXLD1371EST16TC
• Description: MOSFET LED Controller TSSOP TSSOP-16EP T&R 2.5K
• Manufacturer: Diodes Inc. / Zetex

Specifications of ZXLD1371EST16TC

• Rohs: yes

Applications Information
The ZXLD1371 sets the ripple current, ΔI
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
range of ripple current control is also modulated by other circuit parameters as follows.
∆I
∆I
∆I
If ADJ is connected to REF, this simplifies to
∆I
∆I
∆I
ZXLD1371
Document number: DS35436 Rev. 1 - 2
where ΔI
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:
where I
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
LMAX
LMIN
LMID
LMAX
LMIN
LMID
=
=
= 0.1
= 0.2
=
= 0.3
INMAX
I
I
I
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COILPEAK
COILPEAK
COILPEAK
P
I
LMID
I
MOSFETMAX
0.02 +0.08
0.04 +0.16
MOSFETMAX
0.06 +0.24
=
GI_ADJ
GI_ADJ
GI_ADJ
P
is the value of I
is the value we must use in Equation 18 . We have now established the inductance value.
resistive
1-D
1-D
1-D
= 1.1 I
= 1.1 I
= 1.1 I
I
I
I
V
V
V
COIL
COIL
V
V
COIL
V
LED
INMAX
INMAX
REF
REF
ADJ
ADJ
=
REF
ADJ
+
=
1
P
L
I
LED
−
switching
value of 20% of I
IN
I
+ I
LED
D
at minimum V
MAX
GI_ADJ
LED
GI_ADJ
GI_ADJ
1-D
1-D
1-D
(cont.)
L
I
I
I
, to between nominally 10% and 30% of the mean coil current, I
COIL
COIL
COIL
IN
COIL
.
to find an inductance which is optimized for the input voltage range. The
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for Buck
for Boost
for Buck-boost
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A Product Line of
COIL
Equation 20
Equation 20a
Equation 21
© Diodes Incorporated
, which is found
February 2012
ZXLD1371