ltc3824 Linear Technology Corporation, ltc3824 Datasheet - Page 8

• Manufacturer Part Number: ltc3824
• Description: High Voltage Step-down Controller With 40?a Quiescent Current
• Manufacturer: Linear Technology Corporation

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
threshold voltage(V
versus gate-to-source voltage, the gate-to-source and
gate-to-drain charges(Q
maximum drain current(I
resistance(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
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
(V
DS(ON)
RIPPLE
IN–
=I
TH(JC)
OUT(MAX)
V
OUT
OUT(MAX)
of the MOSFET listed in the data sheet.
=
) and R
SENSE(MAX)
) • D
(V
GS(TH)
IN
+
V
L(MAX)
V
D(MAX)
– V
OUT
GS
I
IN
RIPPLE
TH(JA)
f • L
), the on-resistance(R
GS
OUT
+ V
2
and Q
+ V
, the required load current at
rated MOSFETs are required
.
) and the MOSFET’s thermal
D
.
) • D
D
GD
, respectively), the
DS(ON)
)
.
The power
DSS
DS(ON)
), the
)
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%/
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
P
MOSFET
T
J
J
TH(JC)
can then be compared to the original assumed value
= T
TH(JA)
Figure 1. Normalized R
A
=
+ P
+K(V
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
MOSFET
IN
)
V
2
JUNCTION TEMPERATURE (°C)
V
OUT+
(I
IN
0
• R
OUT
+ V
TH(JA)
V
)(C
D
D
RSS
50
DS(ON)
DS(ON)
)(f)
vs Temperature
100
(I
over temperature for
OUT
TH(CA)
3824 F01
)
°
2
C. C
(1+ )R
150
). This value
2
R losses in
RSS
DS(ON)
DS(ON)
is the
3824fb