ZXLD1370

ZXLD1370

Document number: DS32165 Rev. 4 - 2

20 of 36

www.diodes.com

March 2011

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Boost / Buck-boost mode

LED

I

x

D

1

D

RMS

MOSFET

I

−

=

−

The resistive power dissipation of the MOSFET is:

ON

DS

R

x

2

RMS

MOSFET

I

resistive

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

I

LOAD

I

x

sw

f

x

IN

2

V

x

RSS

C

switching

P

=

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:

I

Qg

I

C

dV

dt

=

⋅

=

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)

ns

35

mA

300

nC

3

.

10

PEAK

I

g

Q

dt

=

=

=

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

ZXLD1370 in the whole spectrum of frequencies recommended for the device (from 300kHz to 1MHz).

Example 2)

ns

97

mA

300

nC

29

PEAK

I

g

Q

dt

=

=

=