Datasheet

LTC3824
9
3824fg
applicaTions inForMaTion
Inductor Selection
The maximum inductor current is determined by :
I
L(MAX)
= I
OUT(MAX)
+
I
RIPPLE
2
where I
RIPPLE
=
(V
IN
– V
OUT
) • D
f • L
and Duty Cycle D =
V
OUT
+ V
D
V
IN
+ V
D
V
D
is the catch diode D1 forward voltage and f is the
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.
L =
(V
IN–
V
OUT
) • D
f • 0.4 •I
OUT(MAX)
The saturation current level of the inductor should be
sufficiently larger than I
L(MAX)
.
Power MOSFET Selection
Important parameters for the power MOSFET include the
drain-to-source breakdown voltage (BV
DSS
), the threshold
voltage (V
GS(TH)
), the on-resistance (R
DS(ON)
) versus gate-
to-source voltage, the gate-to-source and gate-to-drain
charges (Q
GS
and Q
GD
, respectively), the maximum drain
current (I
D(MAX)
) and the MOSFET’s thermal resistance
(R
TH(JC)
) and R
TH(JA)
.
The gate drive voltage is set by the 8V internal regulator.
Consequently, at least 10V V
GS
rated MOSFETs are required
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 coefficient of R
DS(ON)
)
.
The power
dissipation calculation should be based on the worst-cast
specifications for V
SENSE(MAX)
, the required load current at
maximum duty cycle, the voltage and temperature ranges,
and the R
DS(ON)
of the MOSFET listed in the data sheet.
The power dissipated by the MOSFET when the LTC3824
is in continuous mode is given by :
P
MOSFET
=
V
OUT+
V
D
V
IN
+ V
D
(I
OUT
)
2
(1+ δ)R
DS(ON)
+ K(V
IN
)
2
(I
OUT
)(C
RSS
)(f)
The first term in the equation represents the I
2
R losses in
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 coefficient of the R
DS(ON)
of the MOSFET, which is typically 0.4%/°C. C
RSS
is the
MOSFET reverse transfer capacitance. Figure 1 illustrates
the variation of normalized R
DS(ON)
over temperature for
a typical power MOSFET.
Figure 1. Normalized R
DS(ON)
vs Temperature
From a known power dissipated in the power MOSFET, its
junction temperature can be obtained using the following
formula:
T
J
= T
A
+ P
MOSFET
• R
TH(JA)
The R
TH(JA)
to be used in this equation normally includes
the R
TH(JC)
for the device plus the thermal resistance from
the case to the ambient temperature (R
TH(CA)
). This value
of T
J
can then be compared to the original assumed value
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
JUNCTION TEMPERATURE (°C)
–50
δ NORMALIZED ON-RESISTANCE
1.0
1.5
150
3824 F01
0.5
0
0
50
100
2.0