ZXLD1370_11 DIODES [Diodes Incorporated], ZXLD1370_11 Datasheet - Page 20

• Manufacturer Part Number: ZXLD1370_11
• Description: 60V HIGH ACCURACY BUCK/BOOST/BUCK-BOOST LED DRIVER CONTROLLER
• Manufacturer: DIODES [Diodes Incorporated]

ZXLD1370
Document number: DS32165 Rev. 3 - 2
Applications Information
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:
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:
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
ZXLD1370 I
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)
ZXLD1370 I
Using the ZXMN6A09K (V
Q
Q
G
G
t
I
P
P
dt
dt
PERIOD
MOSFET
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)
= 10.3nC at V
= 29nC at V
where
C
f
I
dt
resistive
switching
SW
GATE
RSS
=
=
PEAK
PEAK
=
is the switching frequency,
I
I
dV
PEAK
PEAK
is the MOSFET's reverse-transfer capacitance (a data sheet parameter),
Q
Q
−
is the MOSFET gate-driver's sink/source current at the MOSFET's turn-on threshold.
= 20*dt
RMS
=
=
I
= I
g
= 300mA
g
I
⋅
MOSFET
C
C
=
GATE
=
=
RSS
GS
=
1
10
300
300
GS
29
−
Qg
= 10V
D
−
x
I
3 .
D
RMS
= 300mA
nC
= 10V
mA
DS(MAX)
mA
V
nC
x
2
2
I
I
IN
GATE
LED
x
=
=
R
x
DS(MAX)
35
97
DS
f
f = 1/ t
= 60V, I
sw
−
(Continued)
ns
ns
ON
I x
LOAD
= 60V, I
PERIOD
D(MAX)
= 1.43MHz
D(MAX)
= 12.2A):
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= 8.5A):
20 of 35
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ZXLD1370
February 2011