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ZXLD1371
Applications Information (cont.)
During the on-time, the MOSFET switch current is equal to the coil current. The rms MOSFET current is I COIL √ D where
I COIL is the mean coil current. Therefore the approximate RMS current in the MOSFET during t ON is:
Buck mode
I MOSFETRMS = I LED D
Boost / Buck-boost mode
I MOSFETRMS =
D
1 ? D
x I LED
The resistive power dissipation of the MOSFET is:
P resistive = I MOSFETRMS 2 x R DS ( ON )
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:
P switching = RSS
C
x V 2 IN x f sw x I LOAD
I GATE
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.
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:
dt =
dV ? C
I
=
Qg
I
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)
Using the DMN6068 MOSFET (V DS(MAX) = 60V, I D(MAX) = 8.5A):
Q G = 10.3nC at V GS = 10V
ZXLD1371 I PEAK = I GATE = 300mA
dt =
Q g
I PEAK
=
10 . 3nC
300 mA
= 35 ns
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:
t PERIOD = 20*dt
f = 1/ t PERIOD = 1.43MHz
This frequency is well above the max frequency the device can handle, therefore the DNM6068 can be used with the
ZXLD1371 in the whole spectrum of frequencies recommended for the device (from 300kHz to 1MHz).
ZXLD1371
Document number: DS35436 Rev. 1 - 2
29 of 42
www.diodes.com
February 2012
? Diodes Incorporated
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