© 2002 Microchip Technology Inc.

DS21349B-page 5

TC105

3.8

External Switching Transistor

Selection

EXT is a complementary output with a maximum ON

resistance of 22

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

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

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

-4.5V. Table 4-1 lists suppliers of external components

recommended for use with the 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.