TC110301ECT datasheet(7/16 Pages) MICROCHIP

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DS21355B-page 7

TC110

3.7

Output Diode

For best results, use a Schottky diode such as the

MA735, 1N5817, MBR0520L or equivalent. Connect

the diode between the FB (or SENSE) input as close to

the IC as possible. Do not use ordinary rectifier diodes

since the higher threshold voltages reduce efficiency.

3.8

External Switching Transistor

Selection

The EXT output is designed to directly drive an

N-channel MOSFET or NPN bipolar transistor. N-

channel

MOSFETs

afford

the

highest

efficiency

because they do not draw continuous gate drive

current, but are typically more expensive than bipolar

transistors. If using an N-channel MOSFET, the gate

should be connected directly to the EXT output as

shown in Figure 3-1 and Figure 3-1. EXT is a compli-

mentary output with a maximum ON resistances of 43

Ω

to VDD when high and 27Ω to ground when low. Peak

currents should be kept below 10mA.

When selecting an N-channel MOSFET, there are three

important parameters to consider: total gate charge

(Qg); ON resistance (rDSON) and reverse transfer

capacitance (CRSS). Qg is a measure of the total gate

capacitance that will ultimately load the EXT output.

Too high a Qg can reduce the slew rate of the EXT

output sufficiently to grossly lower operating efficiency.

Transistors with typical Qg data sheet values of 50nC

or less can be used. For example, the Si9410DY has a

Qg (typ) of 17nC @ VGS = 5V. This equates to a gate

current of:

IGATEMAX =fMAX x Qg = 115kHz x 17nC = 2mA

The two most significant losses in the N-channel

MOSFET are switching loss and I2R loss. To minimize

these, a transistor with low rDSON and low CRSS should

be used.

Bipolar NPN transistors can be used, but care must be

taken when determining base current drive. Too little

current will not fully turn the transistor on, and result in

unstable regulator operation and low efficiency. Too

high a base drive causes excessive power dissipation

in the transistor and increase switching time due to

over-saturation. For peak efficiency, make RB as large

as possible, but still guaranteeing the switching transis-

tor is completely saturated when the minimum value of

hFE is used.

3.9

Board Layout Guidelines

As with all inductive switching regulators, the TC110

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 TC110 as possible.

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