Power Integrations TOP234Y, TOP234P, TOP234G, TOP233Y, TOP233P Datasheet

®

TOP232-234

TOPSwitch-FX

Family

Design Flexible,

EcoSmart

®

, Integrated

Off-line Switcher

Figure 1. Typical Flyback Application.

®

PART

ORDER

NUMBER

3 Adapter

1

Open

Frame

2

Open

Frame

2

OUTPUT POWER TABLE

Table 1. Notes: 1. Typical continuous power in a non-ventilated
enclosed adapter measured at 50 ˚C ambient. 2. Maximum practical
continuous power in an open frame design with adequate heat sinking,
measured at 50 ˚C ambient. See key applications section for detailed
conditions. 3. Packages: P: DIP-8B, G: SMD-8B, Y: TO-220-7B.

PI-2503-073099

AC

IN

DC

OUT

D

S

C

TOPSwitch-FX

CONTROL

M

+

-

F

July 2001

Product Highlights

Lower System Cost, High Design Flexibility

• Features eliminate or reduce cost of external components

• Fully integrated soft-start for minimum stress/overshoot

• Externally settable accurate current limit

• Wider duty cycle for more power, smaller input capacitor

• Line under-voltage (UV) detection: no turn off glitches

• Line overvoltage (OV) shutdown extends line surge limit

• Line feed forward with maximum duty cycle (DC

MAX

)

reduction rejects ripple and limits DC

MAX

at high line

• Single resistor sets OV/UV thresholds, DC

MAX

reduction

• Frequency jittering reduces EMI and EMI filtering costs

• Regulates to zero load without dummy loading

• 132 kHz frequency reduces transformer/power supply size

• Half frequency option for video applications

• Hysteretic thermal shutdown for automatic recovery

• Large thermal hysteresis prevents PC board overheating

• Standard packages with omitted pins for large creepage

• Active-on and active-off remote ON/OFF capability

• Synchronizable to a lower frequency

EcoSmart

®

- Energy Efficient

• Cycle skipping reduces no-load consumption

• Reduced consumption in remote off mode

• Half frequency option for high efficiency standby

• Allows shutdown/wake-up via LAN/input port

Description

TOPSwitch-FX uses the proven TOPSwitch topology and cost
effectively integrates many new functions that reduce system
cost and, at the same time, improve design flexibility,
performance and energy efficiency. Like TOPSwitch, the high
voltage power MOSFET, PWM control, fault protection and
other control circuitry are all integrated onto a single CMOS
chip, but with two added terminals. The first one is a MULTI­FUNCTION (M) pin, which implements programmable line
OV/UV shutdown and line feed forward/DC

MAX

reduction with
line voltage. The same pin can be used instead to externally set
an accurate current limit. In either case, this pin can also be used
for remote ON/OFF or to synchronize the oscillator to an
external, lower frequency signal. The second added terminal is
the FREQUENCY (F) pin and is available only in the Y
package. This pin provides the half frequency option when
connected to CONTROL (C) instead of SOURCE (S). The
features on the new pins can be disabled by shorting them to the
SOURCE, which allows the device to operate in a three terminal

TOPSwitch mode, but with the following new transparent
features: soft-start, cycle skipping, 132 kHz switching frequency,
frequency jittering, wider DC

MAX

, hysteretic thermal shutdown
and larger creepage. In addition, all critical parameters such as
frequency, current limit, PWM gain, etc. have tighter temperature
and absolute tolerances compared to the TOPSwitch-II family.
Higher current limit accuracy and larger DC

MAX

, when combined
with other features allow for a 10% to 15% higher power
capability on the TOPSwitch-FX devices compared to equivalent
TOPSwitch-II devices for the same input/output conditions.

TOP232P
TOP232G

TOP232Y
TOP233P
TOP233G
TOP233Y
TOP234P
TOP234G
TOP234Y

230 VAC ±15%

Adapter

1

85-265 VAC

9 W 15 W 6.5 W 10 W

10 W 25 W 7 W 15 W
13 W 25 W 9 W 15 W
20 W 50 W 15 W 30 W
16 W 30 W 11 W 20 W
30 W 75 W 20 W 45 W

TOP232-234

2

B
7/01

Section List

Pin Functional Description.........................................................................................................................................3

TOPSwitch-FX Family Functional Description .........................................................................................................4

CONTROL (C) Pin Operation .................................................................................................................................4

Oscillator and Switching Frequency .......................................................................................................................5

Pulse Width Modulator and Maximum Duty Cycle .................................................................................................5

Minimum Duty Cycle and Cycle Skipping ...............................................................................................................6

Error Amplifier......................................................................................................................................................... 6

On-chip Current Limit with External Programability................................................................................................6

Line Under-Voltage Detection (UV) ........................................................................................................................ 6

Line Overvoltage Shutdown (OV)...........................................................................................................................7

Line Feed Forward with DC

MAX

Reduction .............................................................................................................. 7

Remote ON/OFF and Synchronization...................................................................................................................7

Soft-Start ................................................................................................................................................................8

Shutdown/Auto-Restart ..........................................................................................................................................8

Hysteretic Over-Temperature Protection ................................................................................................................8

Bandgap Reference................................................................................................................................................8

High-Voltage Bias Current Source..........................................................................................................................8

Using FREQUENCY and MULTI-FUNCTION Pins ..................................................................................................... 9

FREQUENCY (F) Pin Operation.............................................................................................................................9

MULTI-FUNCTION (M) Pin Operation ....................................................................................................................9

Typical Uses of FREQUENCY (F) Pin ...................................................................................................................... 11

Typical Uses of MULTI-FUNCTION (M) Pin ............................................................................................................. 12

Application Examples............................................................................................................................................... 14

A High Efficiency, 30 W, Universal Input Power Supply........................................................................................14

35 W Multiple Output Power Supply.....................................................................................................................15

17 W PC Standby Power Supply..........................................................................................................................16

Processor Controlled Supply Turn On/Of f ............................................................................................................ 17

Key Application Considerations ..............................................................................................................................19

TOPSwitch-FX vs. TOPSwitch-ll...........................................................................................................................19

TOPSwitch-FX Design Considerations.................................................................................................................20

TOPSwitch-FX Selection................................................................................................................................20

Input Capacitor............................................................................................................................................... 20

Primary Clamp and Output Reflected Voltage VOR.........................................................................................20

Output Diode ..................................................................................................................................................21

Soft-Start ........................................................................................................................................................21

EMI................................................................................................................................................................. 21

Transformer Design........................................................................................................................................ 21

Standby Consumption.................................................................................................................................... 23

TOPSwitch-FX Layout Considerations ................................................................................................................. 23

Primary Side Connections..............................................................................................................................23

Y-Capacitor.....................................................................................................................................................23

Heat Sinking................................................................................................................................................... 23

Quick Design Checklist.........................................................................................................................................23

Design Tools ......................................................................................................................................................... 23

Product Specifications and Test Conditions ..........................................................................................................24

Typical Performance Characteristics ...................................................................................................................... 30

Package Outlines ...................................................................................................................................................... 34

TOP232-234

B

7/01

3

Figure 2. Functional Block Diagram.

Pin Functional Description

DRAIN (D) Pin:

High voltage power MOSFET drain output. The internal start­up bias current is drawn from this pin through a switched high­voltage current source. Internal current limit sense point for
drain current.

CONTROL (C) Pin:

Error amplifier and feedback current input pin for duty cycle
control. Internal shunt regulator connection to provide internal
bias current during normal operation. It is also used as the
connection point for the supply bypass and auto-restart/
compensation capacitor.

MULTI-FUNCTION (M) Pin:

Input pin for OV, UV, line feed forward with DC

MAX

reduction,
external set current limit, remote ON/OFF and synchronization.
A connection to SOURCE pin disables all functions on this pin
and makes TOPSwitch-FX operate in simple three terminal
mode (like TOPSwitch-II).

FREQUENCY (F) Pin: (Y package only)

Input pin for selecting switching frequency: 132 kHz if connected
to SOURCE pin and 66 kHz if connected to CONTROL pin.

Figure 3. Pin Configuration.

PI-2535-083099

The switching frequency is internally set for 132 kHz only
operation in P and G packages.

SOURCE (S) Pin:

Output MOSFET source connection for high voltage power
return. Primary side control circuit common and reference point.

SHUTDOWN/

AUTO-RESTART

PWM

COMPARATOR

CLOCK

SAW

HALF
FREQUENCY

OSCILLATOR WITH JITTER

CONTROLLED

TURN-ON

GATE DRIVER

CURRENT LIMIT

COMPARATOR

INTERNAL UV

COMPARATOR

INTERNAL
SUPPLY

5.8 V

4.8 V

SOURCE (S)

SRQ

Q

D

MAX

STOP

SOFT-

START

-

+

CONTROL (C)

MULTI-

FUNCTION (M)

FREQUENCY (F)

(Y Package Only)

­+

5.8 V

I

FB

R

E

Z

C

V

C

+

-

LEADING

EDGE

BLANKING

÷ 8

1

HYSTERETIC

THERMAL

SHUTDOWN

SHUNT REGULATOR/

ERROR AMPLIFIER

+

-

DRAIN (D)

ON/OFF

DC

MAX

V

BG

DC

MAX

V

BG

+ V

T

0

OV/UV

V

I (LIMIT)

CURRENT

LIMIT

ADJUST

LINE

SENSE

SOFT START

PI-2501-031901

Tab Internally

Connected to SOURCE Pin

Y Package (TO-220-7B)

P Package (DIP-8B)

G Package (SMD-8B)

C

D

S
S

S

S

1 C

3 M

5 F
4 S

7 D

M

8

5

7

1

4

2

3

TOP232-234

4

B
7/01

TOPSwitch-FX

Family Functional Description

Figure 4. Relationship of Duty Cycle to CONTROL Pin Current.

PI-2504-072799

Duty Cycle (%)

IC (mA)

IM = 140 µA

I

M

= 190 µA

I

M

< I

M(DC)

1.91.5 5.5 5.9

Slope = PWM Gain

I

CD1

I

B

Auto-restart

78

1.5

47

Like TOPSwitch, TOPSwitch-FX is an integrated switched
mode power supply chip that converts a current at the control
input to a duty cycle at the open drain output of a high voltage
power MOSFET. During normal operation the duty cycle of the
power MOSFET decreases linearly with increasing CONTROL
pin current as shown in Figure 4.

In addition to the three terminal TOPSwitch features, such as the
high voltage start-up, the cycle-by-cycle current limiting, loop
compensation circuitry, auto-restart, thermal shutdown, etc.,
the TOPSwitch-FX incorporates many additional functions that
reduce system cost, increase power supply performance and
design flexibility. A patented high voltage CMOS technology
allows both the high voltage power MOSFET and all the low
voltage control circuitry to be cost effectively integrated onto a
single monolithic chip.

Two terminals, FREQUENCY (available only in Y package)
and MULTI-FUNCTION, have been added to implement some
of the new functions. These terminals can be connected to the
SOURCE pin to operate the TOPSwitch-FX in a TOPSwitch-
like three terminal mode. However, even in this three terminal
mode, the TOPSwitch-FX offers many new transparent features
that do not require any external components:

1. A fully integrated 10 ms soft-start reduces peak currents and

voltages during start-up and practically eliminates output
overshoot in most applications.

2. DC

MAX

of 78% allows smaller input storage capacitor, lower

input voltage requirement and/or higher power capability.

3. Cycle skipping at minimum pulse width achieves regulation

and very low power consumption at no load.

4. Higher switching frequency of 132 kHz reduces the

transformer size with no noticeable impact on EMI or on
high line efficiency.

5. Frequency jittering reduces EMI.

6. Hysteretic over-temperature shutdown ensures automatic

recovery from thermal fault. Large hysteresis prevents circuit
board overheating.

7. Packages with omitted pins and lead forming provide large

DRAIN creepage distance.

8. Tighter absolute tolerances and smaller temperature vari-

ations on switching frequency, current limit and PWM gain.

The MULTI-FUNCTION pin is usually used for line sensing by
connecting a resistor from this pin to the rectified DC high
voltage bus to implement line over-voltage (OV)/under-voltage
(UV) and line feed forward with DC

MAX

reduction. In this
mode, the value of the resistor determines the OV/UV thresholds
and the DC

MAX

is reduced linearly starting from a line voltage
above the under-voltage threshold. In high efficiency
applications, this pin can be used in the external current limit
mode instead, to reduce the current limit externally (to a value

close to the operating peak current), by connecting the pin to
SOURCE through a resistor. The same pin can also be used as
a remote ON/OFF and a synchronization input in both modes.
The FREQUENCY pin in the TO-220 package sets the switching
frequency to the default value of 132 kHz when connected to
SOURCE pin. A half frequency option can be chosen by
connecting this pin to CONTROL pin instead. Leaving this pin
open is not recommended.

CONTROL (C) Pin Operation

The CONTROL pin is a low impedance node that is capable of
receiving a combined supply and feedback current. During
normal operation, a shunt regulator is used to separate the
feedback signal from the supply current. CONTROL pin voltage
VC is the supply voltage for the control circuitry including the
MOSFET gate driver. An external bypass capacitor closely
connected between the CONTROL and SOURCE pins is
required to supply the instantaneous gate drive current. The
total amount of capacitance connected to this pin also sets the
auto-restart timing as well as control loop compensation.

When rectified DC high voltage is applied to the DRAIN pin
during start-up, the MOSFET is initially off, and the CONTROL
pin capacitor is charged through a switched high voltage current
source connected internally between the DRAIN and CONTROL
pins. When the CONTROL pin voltage VC reaches
approximately 5.8 V, the control circuitry is activated and the
soft-start begins. The soft-start circuit gradually increases the
duty cycle of the MOSFET from zero to the maximum value
over approximately 10 ms. If no external feedback/supply
current is fed into the CONTROL pin by the end of the soft-start,
the high voltage current source is turned off and the CONTROL
pin will start discharging in response to the supply current
drawn by the control circuitry. If the power supply is designed
properly, and no fault condition such as open loop or shorted
output exists, the feedback loop will close, providing external

TOP232-234

B

7/01

5

PI-2545-082299

S1 S2 S6 S7 S1 S2 S6 S7S0

S1 S7

S0

S0

5.8 V

4.8 V

S7

0 V

0 V

0 V

V

LINE

V

C

V

DRAIN

V

OUT

Note: S0 through S7 are the output states of the auto-restart counter

2

1

2

3

4

0 V

~

~

~

~

~

~

~

~

~

~

S6 S7

~

~

~

~

~

~

~

~

V

UV

~

~

~

~

~

~

~

~

~

~

S2

~

~

CONTROL pin current, before the CONTROL pin voltage has
had a chance to discharge to the lower threshold voltage of
approximately 4.8 V (internal supply under-voltage lockout
threshold). When the externally fed current charges the
CONTROL pin to the shunt regulator voltage of 5.8 V, current
in excess of the consumption of the chip is shunted to SOURCE
through resistor R

E

as shown in Figure 2. This current flowing
through RE controls the duty cycle of the power MOSFET to
provide closed loop regulation. The shunt regulator has a finite
low output impedance ZC that sets the gain of the error amplifier
when used in a primary feedback configuration. The dynamic
impedance ZC of the CONTROL pin together with the external
CONTROL pin capacitance sets the dominant pole for the
control loop.

When a fault condition such as an open loop or shorted output
prevents the flow of an external current into the CONTROL pin,
the capacitor on the CONTROL pin discharges towards 4.8 V.
At 4.8 V auto-restart is activated which turns the output MOSFET
off and puts the control circuitry in a low current standby mode.
The high-voltage current source turns on and charges the
external capacitance again. A hysteretic internal supply under­voltage comparator keeps VC within a window of typically 4.8
to 5.8 V by turning the high-voltage current source on and off
as shown in Figure 5. The auto-restart circuit has a divide-by­8 counter which prevents the output MOSFET from turning on
again until eight discharge/charge cycles have elapsed. This is
accomplished by enabling the output MOSFET only when the
divide-by-8 counter reaches full count (S7). The counter
effectively limits TOPSwitch-FX power dissipation by reducing
the auto-restart duty cycle to typically 4%. Auto-restart mode
continues until output voltage regulation is again achieved
through closure of the feedback loop.

Oscillator and Switching Frequency

The internal oscillator linearly charges and discharges an internal
capacitance between two voltage levels to create a sawtooth
waveform for the pulse width modulator. The oscillator sets the
pulse width modulator/current limit latch at the beginning of
each cycle.

The nominal switching frequency of 132 kHz was chosen to
minimize transformer size while keeping the fundamental EMI
frequency below 150 kHz. The FREQUENCY pin (available
only in TO-220 package), when shorted to the CONTROL pin,
lowers the switching frequency to 66 kHz (half frequency)
which may be preferable in some cases such as noise sensitive
video applications or a high efficiency standby mode. Otherwise,
the FREQUENCY pin should be connected to the SOURCE pin
for the default 132 kHz. Trimming of the current reference
improves oscillator frequency accuracy.

To further reduce the EMI level, the switching frequency is
jittered (frequency modulated) by approximately ±4 kHz at
250 Hz (typical) rate as shown in Figure 6. Figure 28 shows the
typical improvement of EMI measurements with frequency
jitter.

Pulse Width Modulator and Maximum Duty Cycle

The pulse width modulator implements voltage mode control
by driving the output MOSFET with a duty cycle inversely
proportional to the current into the CONTROL pin that is in
excess of the internal supply current of the chip (see Figure 4).
The excess current is the feedback error signal that appears
across RE (see Figure 2). This signal is filtered by an RC
network with a typical corner frequency of 7 kHz to reduce the
effect of switching noise in the chip supply current generated by

Figure 5. Typical Waveforms for (1) Power Up (2) Normal Operation (3) Auto-restart (4) Power Down .

TOP232-234

6

B
7/01

Figure 6. Switching Frequency Jitter.

the MOSFET gate driver. The filtered error signal is compared
with the internal oscillator sawtooth waveform to generate the
duty cycle waveform. As the control current increases, the duty
cycle decreases. A clock signal from the oscillator sets a latch
which turns on the output MOSFET. The pulse width modulator
resets the latch, turning off the output MOSFET. Note that a
minimum current must be driven into the CONTROL pin
before the duty cycle begins to change.

The maximum duty cycle, DC

MAX,

is set at a default maximum
value of 78% (typical). However, by connecting the MULTI­FUNCTION pin to the rectified DC high voltage bus through a
resistor with appropriate value, the maximum duty cycle can be
made to decrease from 78% to 38% (typical) as shown in
Figure 8 when input line voltage increases (see line feed
forward with DC

MAX

reduction).

Minimum Duty Cycle and Cycle Skipping

To maintain power supply output regulation, the pulse width
modulator reduces duty cycle as the load at the power supply
output decreases. This reduction in duty cycle is proportional
to the current flowing into the CONTROL pin. As the
CONTROL pin current increases, the duty cycle reduces linearly
towards a minimum value specified as minimum duty cycle,
DC

MIN

. After reaching DC

MIN

, if CONTROL pin current is
increased further by approximately 0.4 mA, the pulse width
modulator will force the duty cycle from DC

MIN

to zero in a
discrete step (refer to Figure 4). This feature allows a power
supply to operate in a cycle skipping mode when the load at its
output consumes less power than the power that TOPSwitch-FX
delivers at minimum duty cycle, DC

MIN

. No additional control
is needed for the transition between normal operation and cycle
skipping. As the load increases or decreases, the power supply
automatically switches between normal operation and cycle
skipping mode as necessary.

Cycle skipping may be avoided, if so desired, by connecting a
minimum load at the power supply output such that the duty
cycle remains at a level higher than DC

MIN

at all times.

Error Amplifier

The shunt regulator can also perform the function of an error
amplifier in primary feedback applications. The shunt regulator
voltage is accurately derived from a temperature-compensated
bandgap reference. The gain of the error amplifier is set by the
CONTROL pin dynamic impedance. The CONTROL pin
clamps external circuit signals to the VC voltage level. The
CONTROL pin current in excess of the supply current is
separated by the shunt regulator and flows through RE as a
voltage error signal.

On-chip Current Limit with External Programmability

The cycle-by-cycle peak drain current limit circuit uses the
output MOSFET ON-resistance as a sense resistor. A current

limit comparator compares the output MOSFET on-state drain
to source voltage, V

DS(ON)

with a threshold voltage. High drain

current causes V

DS(ON)

to exceed the threshold voltage and turns
the output MOSFET off until the start of the next clock cycle.
The default current limit of TOPSwitch-FX is preset internally.
However, with a resistor connected between MULTI­FUNCTION pin and SOURCE pin, current limit can be
programmed externally to a lower level between 40% and
100% of the default current limit. Please refer to the graphs in
the typical performance characteristics section for the selection
of the resistor value. By setting current limit low, a
TOPSwitch-FX that is bigger than necessary for the power
required can be used to take advantage of the lower R

DS(ON)

for
higher efficiency. With a second resistor connected between
the MULTI-FUNCTION pin and the rectified DC high voltage
bus providing a small amount of feed forward current, a true
power limiting operation against line variation can be
implemented. When using an RCD clamp, this feed forward
technique reduces maximum clamp voltage at high line allowing
for higher reflected voltage designs. The current limit
comparator threshold voltage is temperature compensated to
minimize the variation of the current limit due to temperature
related changes in R

DS(ON)

of the output MOSFET.

The leading edge blanking circuit inhibits the current limit
comparator for a short time after the output MOSFET is turned
on. The leading edge blanking time has been set so that, if a
power supply is designed properly, current spikes caused by
primary-side capacitances and secondary-side rectifier reverse
recovery time will not cause premature termination of the
switching pulse.

The current limit can be lower for a short period after the
leading edge blanking time as shown in Figure 33. This is due
to dynamic characteristics of the MOSFET. To avoid triggering
the current limit in normal operation, the drain current waveform
should stay within the envelope shown.

Line Under-Voltage Detection (UV)

At power up, UV keeps TOPSwitch-FX off until the input line

PI-2550-092499

128 kHz

4 ms

Time

Switching

Frequency

V

DRAIN

136 kHz

TOP232-234

B

7/01

7

Figure 7. Synchronization Timing Diagram.

voltage reaches the under-voltage threshold. At power down,
UV prevents auto-restart attempts after the output goes out of
regulation. This eliminates power down glitches caused by the
slow discharge of input storage capacitor present in applications
such as standby supplies. A single resistor connected from the
MULTI-FUNCTION pin to the rectified DC high voltage bus
sets UV threshold during power up. Once the power supply is
successfully turned on, UV is disabled to allow extended input
voltage operating range. Input voltage is not checked again
until the power supply loses regulation and attempts another
turn-on. This is accomplished by enabling the UV comparator
only when the divide-by-8 counter used in auto-restart reaches
full count (S7) which is also the state that the counter is reset to
at power up (see Figure 5). The UV feature can be disabled
independent of OV feature as shown in Figure 16.

Line Overvoltage Shutdown (OV)

The same resistor used for UV also sets an overvoltage threshold
which, once exceeded, will force TOPSwitch-FX output into
off-state. The ratio of OV and UV thresholds is preset at 4.5
as can be seen in Figure 8. This feature turns off the
TOPSwitch-FX power MOSFET when the rectified DC high
voltage exceeds the OV threshold. When the MOSFET is off,
the rectified DC high voltage surge capability is increased to the
voltage rating of the MOSFET (700 V), due to the absence of
the reflected voltage and leakage spikes on the drain. Small
amount of hysteresis is provided on the OV threshold to prevent
noise triggering. The OV feature can be disabled independent
of UV feature as shown in Figure 15.

Line Feed Forward with DC

MAX

Reduction

The same resistor used for UV and OV also implements line
voltage feed forward which minimizes output line ripple and
reduces power supply output sensitivity to line transients. This

feed forward operation is illustrated in Figure 4 by the different
values of I

M

. Note that for the same CONTROL pin current,
higher line voltage results in smaller operating duty cycle. As
an added safety measure, the maximum duty cycle DC

MAX

is
also reduced from 78% (typical) at a voltage slightly higher than
the UV threshold to 38% (typical) at the OV threshold (see
Figures 4, 8). DC

MAX

of 38% at the OV threshold was chosen
to ensure that the power capability of the TOPSwitch-FX is not
restricted by this feature under normal operation.

Remote ON/OFF and Synchronization

TOPSwitch-FX can be turned on or off by controlling the
current into or out from the MULTI-FUNCTION pin (see
Figure 8). This allows easy implementation of remote ON/OFF
control of TOPSwitch-FX in several different ways. A transistor
or an optocoupler output connected between the MULTI­FUNCTION pin and the SOURCE pin implements this function
with “active-on” (Figure 19) while a transistor or an optocoupler
output connected between the MULTI-FUNCTION pin and the
CONTROL pin implements the function with “active-off”
(Figure 20).

When a signal is received at the MULTI-FUNCTION pin to
disable the output through any of the MULTI-FUNCTION pin
functions such as OV, UV and remote ON/OFF,
TOPSwitch-FX always completes its current switching cycle as
illustrated in Figure 7 before the output is forced off. The
internal oscillator is stopped slightly before the end of the
current cycle and stays there as long as the disable signal exists.
When the signal at the MULTI-FUNCTION pin changes state
from disable to enable, the internal oscillator starts the next
switching cycle. This approach allows the use of this pin to
synchronize TOPSwitch-FX to any external signal with a
frequency lower than its internal switching frequency.

PI-2558-092999

Oscillator

(SAW)

D

MAX

Enable from

M Pin (STOP)

Time

TOP232-234

8

B
7/01

As seen above, the remote ON/OFF feature allows the
TOPSwitch-FX to be turned on and off instantly, on a cycle-by­cycle basis, with very little delay. However, remote ON/OFF
can also be used as a standby or power switch to turn off the
TOPSwitch-FX and keep it in a very low power consumption
state for indefinitely long periods. If the TOPSwitch-FX is held
in remote off state for long enough time to allow the CONTROL
pin to dishcharge to the internal supply under-voltage threshold
of 4.8 V (approximately 32 ms for a 47 µF CONTROL pin
capacitance), the CONTROL pin goes into the hysteretic mode
of regulation. In this mode, the CONTROL pin goes through
alternate charge and discharge cycles between 4.8 V and 5.8 V
(see CONTROL pin operation section above) and runs entirely
off the high voltage DC input, but with very low power
consumption (160 mW typical at 230 VAC with M pin open).
When the TOPSwitch-FX is remotely turned on after entering
this mode, it will initiate a normal start-up sequence with soft­start the next time the CONTROL pin reaches 5.8 V. In the
worst case, the delay from remote on to start-up can be equal to
the full discharge/charge cycle time of the CONTROL pin,
which is approximately 125 ms for a 47 µF CONTROL pin
capacitor. This reduced consumption remote off mode can
eliminate expensive and unreliable in-line mechanical switches.
It also allows for microprocessor controlled turn-on and turn­off sequences that may be required in certain applications such
as inkjet and laser printers. See Figure 27 under application
examples for more information.

Soft-Start

An on-chip soft-start function is activated at start-up with a
duration of 10 ms (typical). Maximum duty cycle starts from
zero and linearly increases to the default maximum of 78% at
the end of the 10 ms duration. In addition to start-up, soft-start
is also activated at each restart attempt during auto-restart and
when restarting after being in hysteretic regulation of CONTROL
pin voltage (VC), due to remote off or thermal shutdown
conditions. This effectively minimizes current and voltage
stresses on the output MOSFET, the clamp circuit and the
output rectifier, during start-up. This feature also helps minimize
output overshoot and prevents saturation of the transformer
during start-up.

Shutdown/Auto-Restart

To minimize TOPSwitch-FX power dissipation under fault
conditions, the shutdown/auto-restart circuit turns the power

supply on and off at an auto-restart duty cycle of typically 4%
if an out of regulation condition persists. Loss of regulation
interrupts the external current into the CONTROL pin. V

C

regulation changes from shunt mode to the hysteretic auto­restart mode described above. When the fault condition is
removed, the power supply output becomes regulated, V

C

regulation returns to shunt mode, and normal operation of the
power supply resumes.

Hysteretic Over-Temperature Protection

Temperature protection is provided by a precision analog
circuit that turns the output MOSFET off when the junction
temperature exceeds the thermal shutdown temperature (135 ˚C
typical). When the junction temperature cools to below the
hysteretic temperature, normal operation resumes. A large
hysteresis of 70 ˚C (typical) is provided to prevent overheating
of the PC board due to a repeating fault condition. VC is
regulated in hysteretic mode and a 4.8 V to 5.8 V (typical)
sawtooth waveform is present on the CONTROL pin when the
power supply is turned off.

Bandgap Reference

All critical TOPSwitch-FX internal voltages are derived from a
temperature-compensated bandgap reference. This reference is
also used to generate a temperature-compensated current
reference which is trimmed to accurately set the switching
frequency, MOSFET gate drive current, current limit, and the
line OV/UV thresholds. TOPSwitch-FX has improved circuitry
to maintain all of the above critical parameters within very tight
absolute and temperature tolerances.

High-Voltage Bias Current Source

This current source biases TOPSwitch-FX from the DRAIN pin
and charges the CONTROL pin external capacitance during
start-up or hysteretic operation. Hysteretic operation occurs
during auto-restart, remote off and over-temperature shutdown.
In this mode of operation, the current source is switched on and
off with an effective duty cycle of approximately 35%. This
duty cycle is determined by the ratio of CONTROL pin charge
(IC) and discharge currents (I

CD1

and I

CD2

). This current source
is turned off during normal operation when the output MOSFET
is switching.

TOP232-234

B

7/01

9

for line sensing by connecting a resistor from this pin to the
rectified DC high voltage bus to implement OV, UV and DC

MAX

reduction with line voltage functions. In this mode, the value
of the resistor determines the line OV/UV thresholds, and the
DC

MAX

is reduced linearly with rectified DC high voltage
starting from just above the UV threshold. In high efficiency
applications this pin can be used in the external current limit
mode instead, to reduce the current limit externally to a value
close to the operating peak current, by connecting the pin to the
SOURCE pin through a resistor. The same pin can also be used
as a remote on/off and a synchronization input in both modes.
Please refer to Table 2 for possible combinations of the functions
with example circuits shown in Figure 13 through Figure 23. A
description of specific functions in terms of the MULTI­FUNCTION pin I/V characteristic is shown in Figure 8. The
horizontal axis represents MULTI-FUNCTION pin current
with positive polarity indicating currents flowing into the pin.
The meaning of the vertical axes varies with functions. For
those that control the on/off states of the output such as UV, OV
and remote ON/OFF, the vertical axis represents the enable/
disable states of the output. UV triggers at I

UV

(+50 µA typical)
and OV triggers at IOV (+225 µA typical). Between +50 µA and
+225 µA, the output is enabled. For external current limit and
line feed forward with DC

MAX

reduction, the vertical axis

represents the magnitude of the I

LIMIT

and DC

MAX

. Line feed

forward with DC

MAX

reduction lowers maximum duty cycle from

78% at I

M(DC)

(+90 µA typical) to 38% at IOV (+225 µA). External

current limit is available only with negative MULTI-FUNCTION
pin current. Please see graphs in the typical performance
characteristics section for the current limit programming range
and the selection of appropriate resistor value.

FREQUENCY (F) Pin Operation

The FREQUENCY pin is a digital input pin available in
TO-220 package only. Shorting the FREQUENCY pin to
SOURCE pin selects the nominal switching frequency of
132 kHz (Figure 10) which is suited for most applications. For
other cases that may benefit from lower switching frequency
such as noise sensitive video applications, a 66 kHz switching
frequency (half frequency) can be selected by shorting the
FREQUENCY pin to the CONTROL pin (Figure 11). In
addition, an example circuit shown in Figure 12 may be used to
lower the switching frequency from 132 kHz in normal operation
to 66 kHz in standby mode for very low standby power
consumption.

MULTI-FUNCTION (M) Pin Operation

When current is fed into the MULTI-FUNCTION pin, it works
as a voltage source of approximately 2.6 V up to a maximum
current of +400 µA (typical). At +400 µA, this pin turns into
a constant current sink. When current is drawn out of the
MULTI-FUNCTION pin, it works as a voltage source of
approximately 1.32 V up to a maximum current of –240 µA
(typical). At –240 µA, it turns into a constant current source.
Refer to Figure 9.

There are a total of five functions available through the use of
the MULTI-FUNCTION pin: OV, UV, line feed forward with
DC

MAX

reduction, external current limit and remote ON/OFF.
A short circuit between the MULTI-FUNCTION pin and
SOURCE pin disables all five functions and forces

TOPSwitch-FX to operate in a simple three terminal mode like
TOPSwitch-II. The MULTI-FUNCTION pin is typically used

✔

✔✔ ✔

✔✔ ✔

✔✔

✔

✔✔ ✔✔

✔✔✔✔✔

Table 2. Typical MULTI-FUNCTION Pin Configurations.

MULTI-FUNCTION PIN TABLE*

13 14 15 16 17 18 19 20 21 22 23

*This table is only a partial list of many MULTI-FUNCTION pin configurations that are possible.

Using FREQUENCY and MULTI­FUNCTION Pins

▲

Figure Number
Three Terminal Operation

Under-Voltage
Overvoltage
Line Feed Forward (DC

MAX

)

Line Feed Forward (I

LIMIT

)
External Current Limit
Remote ON/OFF

TOP232-234

10

B
7/01

-250 -200 -150 -100 -50 0 50 100 150 200 250 300 350 400

PI-2524-081999

Output
MOSFET
Switching

(Enabled)

(Disabled)

I

LIMIT

(Default)

DC

MAX

(78.5%)

Current

Limit

Maximum

Duty Cycle

V

BG

V

BG

+ V

TP

I

M

I

M

I

M

I

M

I

UV

I

REM(N)

I

OV

MULTI­FUNCTION
Pin Voltage

Note: This figure provides idealized functional characteristics of the MULTI-FUNCTION pin with typical performance values.
Please refer to the parametric table and typical performance characteristics sections of the data sheet for measured data.

MULTI-FUNCTION Pin Current (µA)

Disabled when supply
output goes out of
regulation

Figure 8. MULTI-FUNCTION Pin Characteristics.

Figure 9. MULTI-FUNCTION Pin Input Simplified Schematic.

VBG + V

T

V

BG

240 µA

400 µA

CONTROL Pin

MULTI-FUNCTION Pin

(Positive Current Sense - Under-Voltage,

Over-Voltage, Maximum Duty

Cycle Reduction)

(Negative Current Sense - ON/OFF,

Current Limit Adjustment)

PI-2548-092399

TOPSwitch-FX

TOP232-234

B

7/01

11

Figure 12. Half Frequency Standby Mode (For High Standby

Efficiency).

Figure 10. Full Frequency Operation (132 kHz).

Figure 11. Half Frequency Operation (66 kHz).

Typical Uses of FREQUENCY (F) Pin

PI-2505-081199

DC

Input

Voltage

+

-

D

S

C

CONTROL

F

PI-2506-081199

DC

Input

Voltage

+

-

D

S

C

CONTROL

F

PI-2507-040401

DC

Input

Voltage

+

-

D

S

C

STANDBY

QS can be an optocoupler output.

CONTROL

F

20 kΩ

R

HF

1 nF

Q

S

47 kΩ