TMC603 DATA SHEET (V. 1.06 / 26. Mar. 2009)
26
transistor manufacturer gate charge max. frequency max. voltage max. load
type
current
(typ.)
BC857
div.
- (bipolar)
100 kHz
300 kHz
230 kHz
175 kHz
300 kHz
40V
50V
60V
60V
30V
80 mA
BSS84
Fairchild, NXP 0.9 nC
120 mA
150 mA
150 mA
350 mA
TP0610K
NDS0605
TP0202K
Vishay
Fairchild
Vishay
1.3 nC
1.8 nC
1 nC
For the catching diode, use a Schottky type with sufficient voltage and current rating.
The choice of a high switching frequency allows the use of a smaller and less expensive inductor as
well as a lower capacitance for the Villard circuit and the switching regulator output capacitor.
However, the combination of inductor, transistor and switching frequency should be carefully selected
and should be adapted to the load current, especially if a high load current is desired.
Choice of capacitor for the switching frequency (examples):
COSC
frequency fOSC
inductivity Remark
example
470 pF 100 kHz
220 pF 175 kHz
100 pF 300 kHz
220 µH
150 µH
100 µH
Not recommended
for VVM < 14V
The switcher inductivity shall be chosen in a way, that it can sustain part of the load current between
each two switching events. If the inductivity is too low, the current will drop to zero and higher
frequency oscillations for the last part of each cycle will result (discontinuous mode). The required
transistor peak current will rise and thus efficiency falls.
For a low load current, operation in discontinuous mode is possible. If a high output current is required,
a good design value for continuous mode is to target a current ripple in the coil of +/-40%.
To give a coarse hint on the required inductor you can use the following formula for calculating the
minimum inductivity required for continuous operation, based on a ripple current which is 100% of the
load current:
VVM is the supply voltage. For low voltage operation (15V or less), the output voltage sinks from 12V to
0.85*VVM. The formula can be adapted accordingly.
IOUT is the current draw at 12V.
For 40% current ripple, you can use roughly the double inductivity.
If ripple is not critical, you can use a much smaller inductivity, e.g. only 5% to 50% of the calculated
value. But at the same time switching losses will rise and efficiency and current capability sink due to
higher losses in the switching transistor. If the TMC603 does not supply additional external circuitry,
current draw is very low, about 20mA in normal operation. This would lead to large inductivity values.
In this case we recommend going for the values given in the table above in order to optimize coil cost.
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