TSM108(2001) View Datasheet(PDF) - STMicroelectronics
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TSM108 Datasheet PDF : 13 Pages
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TSM108
This component value is valid if the above
described characteristics are fixed... but in the
automotive field, the input voltage of the converter
is dependant of the car battery conditions. Also,
the frequency may vary depending on the
temperature, due to the fact that the frequency is
fixed by an external capacitor. Therefore, we must
calculate the inductor value considering the worst
case condition in order to avoid the saturation of
the inductor, which is when the battery voltage is
at it’s highest, and the switching frequency at it’s
lowest. Thanks to the OVLO function integrated in
TSM108, the operation of the DC/DC converter
will be stopped as soon as the voltage exceeds
the OVLO level. Let’s suppose the OVLO pin has
been left open, therefore, the maximum input
voltage of the DC/DC converter will be Vin max. =
32V. Frequency min stands in the range of 75kHz
In this case, D = 6 / 32 = 0.1875, therefore Lmin =
52µH.
If we allow a 25% security margin
Lmin = 68µH
9.2. Capacitor
The capacitor choice will depend mainly on the
accepted voltage ripple on the output
Ripple = DVout / Vout = (1-D) / 8LCF²
Therefore, C = (1-D) / 8LRippleF². If C = 22µF,
then Ripple = 0.4% which should be far
acceptable.
Here again, the worst conditions for the ripple are
set when the input voltage is at the highest (32V)
and the frequency at it's lowest (75kHz).
with C = 22µF, Ripple = 1.2%
9.3. Ratings for the Inductor, Capacitor,
Transistor and Diode
The inductor wire must be rated at the rms current,
and the core should not saturate for peak inductor
current. The capacitor must be selected to limit the
output ripple to the design specifications, to
withstand peak output voltage, and to carry the
required rms current.
The transistor and the diode should be rated for
the maximum input voltage (up to 60V in
automotive applications). The diode recovery time
must be in accordance with the time period and
the maximum authorized switching time of the
power transistor.
A compromise between the switching and
conducting performances of the transistor must be
found, because choosing a very low ohmic Mosfet
aiming at the benefit of low conduction losses may
bring much higher switching losses than the
expected benefit.
Losses in the switch are:
Pswitch = Prise + Pfall + Pon
where Prise + Pfall represent the switching losses
and where Pon represents the conduction losses.
Prise + Pfall = Iout x Vin x (Trise + Tfall) x F / 2
Pon = Ron x Iout² x d
where Trise is the switching on time, and Tfall is
the switching off time, and where d is the duty
cycle of the switching profile, which can be
approximated to 1 under full load conditions.
With the two last equations, we can see easily that
what we may gain by choosing a performing low
Rdson P-channel MOSFET (for example) may be
jeopardized by the long on and off switching times
required when using a large input gate
capacitance.
10. Electromagnetic Compatibility
The small schematic hereafter shows how to
reduce the EMC noise when used in an EMC
sensitive environment:
EMC Improvement
MOSFET P
Q1
L1
D1
GD
TSM108
The RC components should realize a time
constant corresponding to one tenth of the
switching time constant of the TSM108 (i.e. in our
example, the oscillator frequency is set to 10µs
corresponding to 100kHz, therefore, the RC
couple should realise a time constant close to
1µs).
Choosing the components must privilege a rather
small resistivity (between 10 to 100W). A guess
couple of values for RC in our example would be:
R= 22W, C= 47nF
11. Efficiency Calculations (rough estimation)
The following gives a rough estimation of the
efficiency of a car phone charger, knowing that the
exact calculations depend on a lot of parameters,
as well as on a wide choice of external
components.
Let’s consider the following characteristics of a
classical car phone charger application:
9/13
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