TOP232-234
Function
TOPSwitch-II
TOPSwitch-FX Figures Advantages
Remote ON/OFF
N/A*
Single transistor
or optocoupler
8, 19,
• Fast on/off (cycle by cycle)
20, 21, • Active-on or active-off control
interface or manual 22, 23, • Low consumption in remote off state
switch
27
• Active-on control for fail-safe
• Eliminates expensive in-line on/off
switch
• Allows processor controlled turn on/
off
• Permits shutdown/wake-up of
peripherals via LAN or parallel port
Synchronization
N/A*
Single transistor
or optocoupler
interface
• Synchronization to external lower
frequency signal
• Starts new switching cycle on
demand
Thermal Shutdown
Latched
Hysteretic (with
70 °C hysteresis)
• Automatic recovery from thermal
fault
• Large hysteresis prevents circuit
board overheating
Current Limit Tolerance 10% (@25 °C)
7% (@25 °C)
• 10% higher power capability due to
tighter tolerance
-8% (0 °C to100 °C) -4% (0 °C to 100 °C)
DRAIN
Creepage at
Package
DIP
SMD
0.037" / 0.94 mm
0.037" / 0.94 mm
0.137" / 3.48 mm
0.137" / 3.48 mm
0.068" / 1.73 mm
• Greater immunity to arcing as a
result of build-up of dust, debris and
other contaminants
TO-220 0.046" / 1.17 mm
DRAIN Creepage at
PCB for TO-220
0.045" / 1.14 mm
0.113" / 2.87 mm
(preformed leads)
• Preformed leads accommodate
large creepage for PCB layout
• Easier to meet Safety (UL/VDE)
*Not available
Table 3 (cont). Comparison Between TOPSwitch-II and TOPSwitch-FX.
Primary Clamp and Output Reflected Voltage VOR
A primary clamp is necessary to limit the peak TOPSwitch-FX
drain to source voltage. A Zener clamp (see Figure 26, VR1)
requires few parts and takes up little board space. For good
efficiency, the clamp Zener should be selected to be at least 1.5
times the output reflected voltage VOR as this keeps the leakage
spike conduction time short. When using a Zener clamp in a
universal input application, a VOR of less than 135 V is
recommended to allow for the absolute tolerances and
temperature variations of the Zener. This will ensure efficient
operation of the clamp circuit and will also keep the maximum
drain voltage below the rated breakdown voltage of the
TOPSwitch-FX MOSFET.
TOPSwitch-FX Design Considerations
TOPSwitch-FX Selection
Selecting the optimum TOPSwitch-FX depends upon required
maximum output power, efficiency, heat sinking constraints
and cost goals. With the option to externally reduce current
limit, a larger TOPSwitch-FX may be used for lower power
applications where higher efficiency is needed or minimal heat
sinking is available.
Input Capacitor
The input capacitor must be chosen to provide the minimum
DCvoltagerequiredfortheTOPSwitch-FXconvertertomaintain
regulation at the lowest specified input voltage and maximum
output power. Since TOPSwitch-FX has a higher DCMAX than
TOPSwitch-II,itispossibletouseasmallerinputcapacitor. For
TOPSwitch-FX, a capacitance of 2 µF per watt is usually
sufficient for universal input with an appropriately designed
transformer.
AhighVOR isrequiredtotakefulladvantageofthewiderDCMAX
of TOPSwitch-FX. An RCD clamp provides tighter clamp
voltage tolerance than a Zener clamp and allows a VOR as high
as165V. TheVOR canbe furtherincreasedincontinuousmode
designs up to 185 V by reducing the external current limit as a
function of input line voltage (see Figure 18). The RCD clamp
B
7/01
20
PAM [ POWER ANALOG MICOELECTRONICS ]
