Diodes ZXLD1370 - Manual (Page 22)

ZXLD1370
Document number: DS32165 Rev. 5 - 2
22 of 39
www.diodes.com
September 2012
© Diodes Incorporated
ZXLD1370
A Product Line of
Diodes Incorporated
Application Information
(cont.)
MOSFET Selection
The ZXLD1370 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.
P P P
SWITCHING RESISTIVE
+ =
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 =
−
Boost / Buck-Boost Mode
i
D 1
D
I
LED
MAX
MAX
MAX MOSFET
×
−
=
−
The approximate RMS current in the MOSFET will be:
Buck Mode
D I I
LED RMS MOSFET
=
−
Boost / 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
C
RSS
is the MOSFET's reverse-transfer capacitance (a data sheet parameter),
f
SW
is the switching frequency,
I
GATE
is the MOSFET gate-driver's sink/source current at the MOSFET's turn-on threshold.