®
TNY256
TinySwitch Plus
Energy Efficient, Low Power Off-line Switcher
Product Highlights
TinySwitch Plus
Features
• Extended power range
• Fully integrated auto-restart reduces short circuit current
• Line under-voltage sense eliminates turn-off glitches
• Frequency jittering dramatically reduces EMI (5 to 10 dB)
• TO-220 package option
Lowest Cost, Low Power Switcher Solution
• Lower cost than RCC, discrete PWM and other
integrated/hybrid solutions
• Cost effective replacement for bulky linear adapters
• Lowest component count
• Simple ON/OFF control – no loop compensation components
• No bias winding – simpler, lower cost transformer
• Designed to work with low cost external components
+
Optional
UV Resistor
®
+
DC Output
–
Wide-Range
HV DC Input
D
EN/UV
BP
TinySwitch Plus
S
–
PI-2363-022699
Figure 1. Typical Standby Application.
OUTPUT POWER CAPABILITY*
ORDER
PART
NUMBER
TNY256P
TNY256G
TNY256Y
PACKAGE
DIP-8
SMD-8
TO-220-7B
8-19 W
5-11 W
230 VAC or
115 VAC
w/Doubler
85-265
VAC
Extremely Energy Efficient
• Consumes only 30/60 mW at 115/230 VAC with no load
• Meets Blue Angel, Energy Star, Energy 2000 and
200mW European cell phone requirements for standby
• Saves $1 to $4 per year in energy costs (at $0.12/kWHr)
compared to bulky linear adapters
• Ideal for cellular phone chargers and adapters
High Performance at Low Cost
• High voltage powered – ideal for charger applications
• High bandwidth provides fast turn on with no overshoot
• Current limit operation rejects line frequency ripple
• Built-in current limit and thermal protection
8-15 W
5-10 W
Table 1. * The low end of the power ranges shown represent enclosed
adapters with minimal heat sinking whereas, the high end of the power
ranges represent open frame power supplies with adequate heat
sinking, both measured at an ambient of 50
o
C. Please refer to the Key
Application Considerations section for more details.
Description
The TNY256 extends the power range of the
TinySwitch
family
of energy efficient, low power off-line switchers.
TinySwitch
devices use a breakthrough design to provide the lowest cost,
high efficiency, off-line switching solution for low power
applications. They integrate a 700 V power MOSFET, oscillator,
high voltage switched current source, current limit and thermal
shutdown circuitry into a single, monolithic device. The
devices start-up and operate on power derived from the DRAIN
voltage, eliminating the need for a transformer bias winding and
associated circuitry.
TinySwitch's
low operating current allows
power supply no-load consumption to be kept under 100 mW,
even at 265 VAC input.
The
TinySwitch Plus
incorporates auto-restart, line under-voltage
sense, and frequency jittering features. The auto-restart circuit
safely limits output power during fault conditions such as output
short or open loop. The auto-restart circuit is fully integrated and
does not require external timing components. The line under-
voltage sense threshold can be externally programmed using a line
sense resistor. During start-up, this feature keeps the TNY256 off
until the input line voltage reaches the under-voltage threshold.
When the input line voltage is removed, the line under-voltage
circuit prevents auto-restart attempts after the output goes out of
regulation. This eliminates power down glitches caused by the
slow discharge of input storage capacitors present in applications
such as standby supplies. A single resistor is used to implement
this feature, eliminating what normally takes five to six components.
The line sense resistor is optional. The TNY256 operating frequency
of 130 kHz is jittered (frequency modulated) to reduce both quasi-
peak and average EMI, minimizing filtering costs.
July 2001
TNY256
BYPASS
(BP)
REGULATOR
5.8 V
LINE UNDER-VOLTAGE
50
µA
DRAIN
(D)
AUTO-
RESTART
COUNTER
CLOCK
RESET
5.8 V
5.1 V
BYPASS PIN
UNDER-VOLTAGE
+
-
+
-
JITTER
CLOCK
1.5 V + VTH
DCMAX
VI
LIMIT
THERMAL
SHUTDOWN
OSCILLATOR
ENABLE/
UNDER-VOLTAGE
(EN/UV)
1.5 V
S
Q
R
Q
LEADING
EDGE
BLANKING
SOURCE
(S)
PI-2367-122398
Figure 2. Functional Block Diagram.
Pin Functional Description
DRAIN (D) Pin:
Power MOSFET drain connection. Provides internal operating
current for both start-up and steady-state operation.
BYPASS (BP) Pin:
Connection point for a 0.1
µF
external bypass capacitor for the
internally generated 5.8 V supply.
ENABLE/UNDER-VOLTAGE (EN/UV) Pin:
This pin has dual functions, enable input and line under-voltage
sense. During normal operation, switching of the power
MOSFET is controlled by this pin. MOSFET switching is
terminated when a current greater than 50
µA
is drawn out of
this pin. This pin also senses line under-voltage conditions
through an external resistor connected to the DC line voltage.
If there is no external resistor connected to this pin, TNY256
detects this and disables the line under-voltage function.
SOURCE (S) Pin:
Power MOSFET source connection. Primary return.
Tab Internally
Connected to SOURCE Pin
7D
5 NC
4S
3 BP
1 EN/UV
Y Package (TO-220-7B)
BP
S
S
EN/UV
1
2
3
4
8
7
6
5
S
S
S
D
P Package (DIP-8)
G Package (SMD-8)
PI-2500-031501
Figure 3. Pin Configuration.
NO CONNECT (N) Pin
No connection.
2
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TNY256
TinySwitch
Functional Description
TinySwitch
combines a high voltage power MOSFET switch
with a power supply controller in one device. Unlike conventional
PWM (Pulse Width Modulator) controllers,
TinySwitch
uses a
simple ON/OFF control to regulate the output voltage.
The TNY256 controller consists of an Oscillator, Enable (Sense
and Logic) circuit, 5.8 V Regulator, Bypass pin Under-Voltage
circuit, Over Temperature Protection, Current Limit circuit,
Leading Edge Blanking and a 700 V power MOSFET. The
TNY256 incorporates additional circuitry for Line Under-Voltage
Sense, Auto-Restart and Frequency Jitter. Figure 2 shows the
functional block diagram with the most important features.
Oscillator
The typical oscillator frequency is internally set to an average of
130 kHz. Two signals are generated from the oscillator, the
Maximum Duty Cycle signal (DC
MAX
) and the Clock signal that
indicates the beginning of each cycle.
The TNY256 oscillator incorporates circuitry that introduces a
small amount of frequency jitter, typically 5 kHz peak-to-peak,
to minimize EMI emission. The modulation rate of the frequency
jitter (1 kHz) is set to optimize EMI reduction for both average
and quasi-peak emissions. The frequency jitter should be
measured with the oscilloscope triggered at the falling edge of
the DRAIN waveform. The waveform in Figure4 illustrates the
frequency jitter of the TNY256.
Enable Input Circuit
The enable input circuit at the EN/UV pin consists of a low
impedance source follower output set at 1.5 V. The current
through the source follower is limited to 50
µA
with 10
µA
of
hysteresis. When the current drawn out of the this pin exceeds
PI-2366-021299
50
µA,
a low logic level (disable) is generated at the output of
the enable circuit. This output is sampled at the beginning of
each cycle on the rising edge of the clock signal. If high, the
power MOSFET is turned on for that cycle (enabled), otherwise
the power MOSFET remains off (disabled). Since the sampling
is done only at the beginning of each cycle, subsequent
changes in the EN/UV pin voltage or current during the
remainder of the cycle are ignored.
Under most operating conditions (except when close to no-
load), the low impedance of the source follower, keeps the
voltage on the EN/UV pin from going much below 1.5 V, in the
disabled state. This improves the response time of the
optocoupler that is usually connected to this pin.
5.8 V Regulator
The 5.8 V regulator charges the bypass capacitor connected to
the BYPASS pin to 5.8 V by drawing a current from the voltage
on the DRAIN, whenever the MOSFET is off. The BYPASS
pin is the internal supply voltage node for the
TinySwitch.
When the MOSFET is on, the
TinySwitch
runs off of the energy
stored in the bypass capacitor. Extremely low power
consumption of the internal circuitry allows the
TinySwitch
to
operate continuously from the current drawn from the DRAIN
pin. A bypass capacitor value of 0.1
µF
is sufficient for both
high frequency de-coupling and energy storage.
BYPASS Pin Under-Voltage
The BYPASS pin under-voltage circuitry disables the power
MOSFET when the BYPASS pin voltage drops below 5.1 V.
Once the BYPASS pin voltage drops below 5.1 V, it must rise
back to 5.8 V to enable (turn-on) the power MOSFET.
Over Temperature Protection
The thermal shutdown circuitry senses the die temperature.
The threshold is set at 135
o
C with 70
o
C hysteresis. When the
die temperature rises above this threshold (135
o
C) the power
MOSFET is disabled and remains disabled until the die
temperature falls by 70
o
C, at which point it is re-enabled.
Current Limit
The current limit circuit senses the current in the power
MOSFET. When this current exceeds the internal threshold
(I
LIMIT
), the power MOSFET is turned off for the remainder of
that cycle.
The leading edge blanking circuit inhibits the current limit
comparator for a short time (t
LEB
) after the power MOSFET is
turned on. This leading edge blanking time has been set so that
current spikes caused by primary-side capacitance and
secondary-side rectifier reverse recovery time will not cause
premature termination of the switching pulse.
Auto-Restart
In the event of a fault condition such as output overload, output
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600
500
400
300
200
100
0
132.5 kHz
127.5 kHz
V
DRAIN
0
.5
1
Time (µs)
Figure 4. Frequency Jitter.
3
TNY256
short, or an open loop condition, TNY256 enters into auto-
restart operation. An internal counter clocked by the oscillator
gets reset every time the EN/UV pin is pulled low. If the EN/
UV pin is not pulled low for 32 ms, the power MOSFET
switching is disabled for 128 ms (except in the case of line
under-voltage condition). The auto-restart alternately enables
and disables the switching of the power MOSFET until the fault
condition is removed. Figure 5 illustrates auto-restart circuit
operation in the presence of a temporary output short.
In the event of line under-voltage condition, the switching of the
power MOSFET is disabled beyond its normal 128 ms time
until the line under-voltage condition goes away.
Line Under-Voltage (UVLO) Sense Circuit
The DC line voltage can be monitored by connecting an
external resistor from the DC line to the EN/UV pin. During
power-up or when the switching of the power MOSFET is
disabled in auto-restart, the current into the EN/UV pin must
exceed 50
µA
to initiate switching of the power MOSFET.
During power-up, this is implemented by holding the BYPASS
pin to 5.1 V while the line under-voltage condition exists. The
BYPASS pin then rises from 5.1 V to 5.8V when the line under-
voltage condition goes away. When the switching of the power
MOSFET is disabled in auto-restart mode and the line under-
voltage condition exists, the counter is stopped. This stretches
the disable time beyond its normal 128ms until the line under-
voltage condition goes away.
The line under-voltage circuit also detects when there is no
external resistor connected to the EN/UV pin. In this case the
line under-voltage function is disabled.
PI-2342-121198
300
200
100
0
20
10
0
DRAIN
VOLTAGE
OUTPUT
VOLTAGE
0
250
500
Time (ms)
Figure 5. TNY256 Auto-Restart Operation.
TinySwitch
Operation
TinySwitch
devices operate in the current limit mode. When
enabled, the oscillator turns the power MOSFET on at the
beginning of each cycle. The MOSFET is turned off when the
current ramps up to the current limit. The maximum on-time of
the MOSFET is limited to DC
MAX
by the oscillator. Since the
current limit and frequency of the TNY256 is constant, the
power delivered is proportional to the primary inductance of the
transformer and is relatively independent of the input voltage.
Therefore, the design of the power supply involves calculating
the primary inductance of the transformer for the maximum
power required. If the TNY256 is appropriately chosen for the
V
EN
CLOCK
V
EN
CLOCK
D
MAX
D
MAX
I DRAIN
I DRAIN
V DRAIN
V DRAIN
PI-2373-120998
PI-2377-120998
Figure 6. TNY256 Operation at Heavy Load.
Figure 7. TNY256 Operation at Light Load.
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TNY256
PI-2383-122398
PI-2381-122398
200
100
0
10
5
0
400
200
0
0
1
200
100
0
10
V
DC-BUS
V
DC-BUS
V
BYPASS
5
0
400
V
BYPASS
V
DRAIN
2
200
0
0
V
DRAIN
1
2
Time (ms)
Figure 8. TNY256 Power-up With External Resistor (2 M
Ω
)
Connected to EN/UV Pin.
Time (ms)
Figure 9. TNY256 Power-up Without External Resistor Connected
to EN/UV Pin.
power level at the lowest input voltage, the calculated inductance
will ramp up the current to the current limit before the DC
MAX
limit is reached.
Enable Function
TNY256 senses the EN/UV pin to determine whether or not to
proceed with the next switch cycle as described earlier. Once
a cycle is started, it always completes the cycle (even when the
EN/UV pin changes state half way through the cycle). This
operation results in a power supply whose output voltage ripple
is determined by the output capacitor, amount of energy per
switch cycle and the delay of the feedback.
The EN/UV pin signal is generated on the secondary by
comparing the power supply output voltage with a reference
voltage. The EN/UV pin signal is high when the power supply
output voltage is less than the reference voltage.
In a typical implementation, the EN/UV pin is driven by an
optocoupler. The collector of the optocoupler transistor is
connected to the EN/UV pin and the emitter is connected to the
SOURCE pin. The optocoupler LED is connected in series with
a Zener across the DC output voltage to be regulated. When the
output voltage exceeds the target regulation voltage level
(optocoupler diode voltage drop plus Zener voltage), the
optocoupler diode will start to conduct, pulling the EN/UV pin
low. The Zener can be replaced by a TL431 device for
improved accuracy.
The EN/UV pin pull-down current threshold is nominally
50
µA,
but is set to 40
µA
the instant the threshold is exceeded.
This is reset back to 50
µA
when the EN/UV pull-down current
drops below the current threshold of 40
µA.
ON/OFF Control
The internal clock of the TNY256 runs all the time. At the
beginning of each clock cycle, it samples the EN/UV pin to
decide whether or not to implement a switch cycle. If the
EN/UV pin is high (< 40
µA),
then a switching cycle takes
place. If the EN/UV pin is low (greater than 50
µA)
then no
switching cycle occurs, and the EN/UV pin status is sampled
again at the start of the subsequent clock cycle.
At full load, TNY256 will conduct during the majority of its
clock cycles (Figure6). At loads less than full load, it will
“skip” more cycles in order to maintain voltage regulation at the
secondary output. At light load or no load, almost all cycles will
be skipped (Figure7). A small percentage of cycles will
conduct to support the power consumption of the power supply.
The response time of the TNY256 ON/OFF control scheme is
very fast compared to normal PWM control. This provides tight
regulation and excellent transient response.
Power Up/Down
The TNY256 requires only a 0.1
µF
capacitor on the BYPASS
pin. Because of the small size of this capacitor, the power-up
delay is kept to an absolute minimum, typically 0.3 ms. Due to
the fast nature of the ON/OFF feedback, there is no overshoot
at the power supply output. When an external resistor (2 MΩ)
is connected to the EN/UV pin, the power MOSFET switching
will be delayed during power-up until the DC line voltage
exceeds the threshold (100 V). Figures 8 and 9 illustrate the
power-up timing waveform of TNY256 in applications with
and without an external resistor (2 MΩ) connected to the
EN/UV pin.
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