ZXLD1371Q

ZXLD1371Q

Document number: DS37116 Rev. 1 - 2

28 of 42

www.diodes.com

September 2015

© Diodes Incorporated

ZXLD1371Q

Buck Mode

Boost / Buck-Boost Mode

The resistive power dissipation of the MOSFET is:

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:

where

Matching the MOSFET with the controller is primarily based on the rise and fall time of the gate voltage. The best rise/fall time in the

application is based on many requirements, such as EMI (conducted and radiated), switching losses, lead/circuit inductance, switching

frequency, etc. How fast a MOSFET can be turned on and off is related to how fast the gate capacitance of the MOSFET can be charged

and discharged. The relationship between C (and the relative total gate charge Qg), turn-on/turn-off time and the MOSFET driver current

rating can be written as:

where

dt = turn-on/turn-off time

dV = gate voltage

C = gate capacitance = Qg/V

I = drive current

– constant current source (for the given voltage value)

Here the constant current source” I ” usually is approximated with the peak drive current at a given driver input voltage.

Example 1:

Assuming that cumulatively the rise time and fall time can account for a maximum of 10% of the period, the maximum frequency allowed in

this condition is:



This frequency is well above the max frequency the device can handle, therefore the DNM6068 can be used with the ZXLD1371Q in the

whole spectrum of frequencies recommended for the device (from 300kHz to 1MHz).

D

I

I

LED

LED

MOSFETRMS

I

x

D

1

D

I





)

ON

(

DS

R

x

2

MOSFETRMS

I

resistiv e

P



GATE

I

LOAD

I

x

sw

f

x

IN

2

V

x

RSS

C

switching

P



I

Qg

I

C

dV

dt







ns

35

mA

300

nC

3

.

10

PEAK

I

g

Q

dt





