2002 Microchip Technology Inc. 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 43Ω
to V
DD
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 (r
DSON
) 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 @ V
GS
= 5V. This equates to a gate
current of:
I
GATEMAX
=f
MAX
x Qg = 115kHz x 17nC = 2mA
The two most significant losses in the N-channel
MOSFET are switching loss and I
2
R loss. To minimize
these, a transistor with low r
DSON
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 R
B
as large
as possible, but still guaranteeing the switching transis-
tor is completely saturated when the minimum value of
h
FE
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.
