Datasheet

I
O
1-D
2
P
C
= D x
x R
DSON
x 1.3
D =
V
O
- V
IN
+ V
D
V
O
+ V
D
LM5022
www.ti.com SNVS480G –JANUARY 2007–REVISED DECEMBER 2013
where
• V
D
is the forward voltage drop of the output diode (2)
The following is a design procedure for selecting all the components for the boost converter circuit shown in
Figure 12. The application is "in-cabin" automotive, meaning that the operating ambient temperature ranges from
-20°C to 85°C. This circuit operates in continuous conduction mode (CCM), where inductor current stays above
0A at all times, and delivers an output voltage of 40.0V ±2% at a maximum output current of 0.5A. Additionally,
the regulator must be able to handle a load transient of up to 0.5A while keeping V
O
within ±4%. The voltage
input comes from the battery/alternator system of an automobile, where the standard range 9V to 16V and
transients of up to 32V must not cause any malfunction.
SWITCHING FREQUENCY
The selection of switching frequency is based on the tradeoffs between size, cost, and efficiency. In general, a
lower frequency means larger, more expensive inductors and capacitors will be needed. A higher switching
frequency generally results in a smaller but less efficient solution, as the power MOSFET gate capacitances must
be charged and discharged more often in a given amount of time. For this application, a frequency of 500 kHz
was selected as a good compromise between the size of the inductor and efficiency. PCB area and component
height are restricted in this application. Following the equation given for R
T
in Applications Information, a 33.2 kΩ
1% resistor should be used to switch at 500 kHz.
MOSFET
Selection of the power MOSFET is governed by tradeoffs between cost, size, and efficiency. Breaking down the
losses in the MOSFET is one way to determine relative efficiencies between different devices. For this example,
the SO-8 package provides a balance of a small footprint with good efficiency.
Losses in the MOSFET can be broken down into conduction loss, gate charging loss, and switching loss.
Conduction, or I
2
R loss, P
C
, is approximately:
(3)
The factor 1.3 accounts for the increase in MOSFET on resistance due to heating. Alternatively, the factor of 1.3
can be ignored and the maximum on resistance of the MOSFET can be used.
Gate charging loss, P
G
, results from the current required to charge and discharge the gate capacitance of the
power MOSFET and is approximated as:
P
G
= VCC x Q
G
x f
SW
(4)
Q
G
is the total gate charge of the MOSFET. Gate charge loss differs from conduction and switching losses
because the actual dissipation occurs in the LM5022 and not in the MOSFET itself. If no external bias is applied
to the VCC pin, additional loss in the LM5022 IC occurs as the MOSFET driving current flows through the VCC
regulator. This loss, P
VCC
, is estimated as:
P
VCC
= (V
IN
– VCC) x Q
G
x f
SW
(5)
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