TC105 Datasheet, PDF (5/14 Pages) Microchip Technology – PFM/PWM Step-Down DC/DC Controller

3.8 External Switching Transistor
Selection
EXT is a complementary output with a maximum ON
resistance of 22Ω to V_DD when high and 19Ω to ground
when low. It is designed to directly drive a P-channel
MOSFET or a PNP bipolar transistor through a base
current limiting resistor (Figure 4-2). A PNP transistor is
recommended in applications where V_IN is less than
2.5V. Otherwise, a P-channel MOSFET is preferred as
it affords the highest efficiency because it does not
draw any gate drive current. However, P-channel
MOSFETs are typically more expensive than bipolar
transistors.
P-channel MOSFET selection is determined mainly by
the on-resistance, gate-source threshold, and gate
charge requirements. Also, the drain-to-source and
gate-to-source breakdown voltage ratings must be
greater than V_DDMAX. The total gate charge specifica-
tion should be less than 100nC for best efficiency. The
MOSFET must be capable of handling the required
peak inductor current, and should have a very low
on-resistance at that current. For example, an Si9430
MOSFET has a drain-to-source rating of -20V, and a
typical on-resistance r_DSON of 0.07Ω at 2A, with V_GS =
-4.5V. Table 4-1 lists suppliers of external components
recommended for use with the TC105.
TC105
3.8.1 BOARD LAYOUT GUIDELINES
As with all inductive switching regulators, the TC105
generates fast switching waveforms, which radiate
noise. Interconnecting lead lengths should be mini-
mized to keep stray capacitance, trace resistance and
radiated noise as low as possible. In addition, the GND
pin, input bypass capacitor and output filter capacitor
ground leads should be connected to a single point.
The input capacitor should be placed as close to power
and ground pins of the TC105 as possible. The length
of the EXT trace must also be kept as short as possible.
© 2002 Microchip Technology Inc.
DS21349B-page 5