LTC3824EMSE-PBF LINER [Linear Technology], LTC3824EMSE-PBF Datasheet - Page 8

• Manufacturer Part Number: LTC3824EMSE-PBF
• Description: High Voltage Step-Down Controller With 40?A Quiescent Current
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LTC3824
Inductor Selection
The maximum inductor current is determined by :
V
switching frequency.
A small inductance will result in larger ripple current,
output ripple voltage and also larger inductor core loss.
An empirical starting point for the inductor ripple current
is about 40% of maximum DC current.
The saturation current level of the inductor should be
suffi ciently larger than I
Power MOSFET Selection
Important parameters for the power MOSFET include the
drain-to-source breakdown voltage (BV
voltage (V
to-source voltage, the gate-to-source and gate-to-drain
charges (Q
current (I
(R
The gate drive voltage is set by the 8V internal regulator.
Consequently, at least 10V V
in high voltage applications.
In order to calculate the junction temperature of the power
MOSFET, the power dissipated by the device must be known.
This power dissipation is a function of the duty cycle, the
load current and the junction temperature itself (due to the
positive temperature coeffi cient of R
dissipation calculation should be based on the worst-cast
specifi cations for V
maximum duty cycle, the voltage and temperature ranges,
and the R
8
D
TH(JC)
I
L =
where I
and Duty Cycle D =
L(MAX)
is the catch diode D1 forward voltage and f is the
f • 0.4 •I
) and R
(V
D(MAX)
DS(ON)
GS(TH)
RIPPLE
IN–
=I
GS
OUT(MAX)
V
and Q
OUT
OUT(MAX)
TH(JA)
), the on-resistance (R
) and the MOSFET’s thermal resistance
of the MOSFET listed in the data sheet.
=
SENSE(MAX)
) • D
(V
GD
.
IN
, respectively), the maximum drain
+
V
L(MAX)
V
– V
OUT
I
IN
RIPPLE
f • L
GS
OUT
+ V
2
+ V
, the required load current at
rated MOSFETs are required
.
D
) • D
D
DS(ON)
DS(ON)
DSS
), the threshold
) versus gate-
)
.
The power
The power dissipated by the MOSFET when the LTC3824
is in continuous mode is given by :
The fi rst term in the equation represents the I
the device and the second term is the switching losses. K
(estimated as 1.7) is an empirical factor inversely related
to the gate drive current and has the unit of 1/Amps. The δ
term accounts for the temperature coeffi cient of the R
of the MOSFET, which is typically 0.4%/°C. C
MOSFET reverse transfer capacitance. Figure 1 illustrates
the variation of normalized R
a typical power MOSFET.
From a known power dissipated in the power MOSFET, its
junction temperature can be obtained using the following
formula:
The R
the R
the case to the ambient temperature (R
of T
used in the calculation.
Output Diode Selection
The catch diode carries load current during the switch
off-time. The average diode current is therefore dependent
T
P
J
J
MOSFET
TH(JC)
can then be compared to the original assumed value
= T
TH(JA)
Figure 1. Normalized R
A
+ P
0.5
for the device plus the thermal resistance from
2.0
1.5
1.0
=
to be used in this equation normally includes
0
–50
+ K(V
MOSFET
V
V
OUT+
IN
+ V
JUNCTION TEMPERATURE (°C)
IN
V
0
D
• R
)
D
2
(I
(I
TH(JA)
OUT
OUT
50
)
)(C
DS(ON)
DS(ON)
2
(1+ δ)R
RSS
vs Temperature
100
)(f)
over temperature for
DS(ON)
TH(CA)
3824 F01
150
). This value
2
R losses in
RSS
DS(ON)
is the
3824fc