ZXLD1371Q

ZXLD1371Q

Document number: DS37116 Rev. 1 - 2

27 of 42

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September 2015

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ZXLD1371Q

The ZXLD1371Q

Equation 8. The device adjusts the ripple current within this range in order to regulate the switching frequency. We therefore need to use a

modulated by other circuit parameters as follows.

0.0

0.24

1 D

GI ADJ

0.02 0.0

1 D

GI ADJ

Equation 20

0.04 0.1

1 D

GI ADJ

If ADJ is connected to REF, this simplifies to

0.

1 D

GI ADJ

0.1

1 D

GI ADJ

Equation 20a

0.2

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 ZXLD1371Q 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

resistiv e

P

P





LED

I

MOSFETMAX

I



MAX

D

1

LED

I

MOSFETMAX

I



