ZXLD1370Q

Document number: DS37117 Rev. 4 - 2

21 of 39

www.diodes.com

February 2018

© Diodes Incorporated

ZXLD1370Q

MOSFET Selection

The ZXLD1370Q requires an external NMOSFET 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.

P

P

P

SWITCHING



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

LED

MAX

MAX

MOSFET

I

x

D

I





This is of most importance in Buck mode where a Bootstrap cannot be implemented.

Boost and Buck-Boost Mode

i

D

1

D

I

LED

MAX

MAX

MAX

MOSFET









enhanced.

The approximate RMS current in the MOSFET will be:

Buck Mode

D

I

I

LED

RMS

MOSFET





Boost and Buck-Boost Mode

LED

RMS

MOSFET

I

x

D

1

D

I







The resistive power dissipation of the MOSFET is:

ON

DS

2

RMS

MOSFET

RESISTIVE

R

x

I

P







Switching Power Losses

Calculating the switching MOSFET's switching loss depends on many factors that influence both turn-on and turn-off. Using a first order rough

approximation, the switching power dissipation of the MOSFET is:

GATE

LOAD

sw

IN

2

RSS

SWITCHING

I

I

x

f

x

V

x

C

P



Where: