ZXLD1371

ZXLD1371

Document number: DS35436 Rev. 1 - 2

28 of 42

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February 2012

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The ZXLD1371 sets the ripple current,

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

range of ripple current control is also modulated by other circuit parameters as follows.

∆I

0.06 +0.24

1-D

GI_ADJ

∆I

0.02 +0.08

1-D

GI_ADJ

Equation 20

∆I

0.04 +0.16

1-D

GI_ADJ

If ADJ is connected to REF, this simplifies to

∆I

1-D

GI_ADJ

∆I

1-D

GI_ADJ

Equation 20a

∆I

1-D

GI_ADJ

where

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:

for Buck

for Boost

Equation 21

for Buck-boost

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

www.coilcraft.com

www.niccomp.com

www.wuerth-elektronik.de

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

−

=