TNY274-280
®
TinySwitch-III
Family
Product Highlights
Lowest System Cost with Enhanced Flexibility
• Simple ON/OFF control, no loop compensation needed
• Selectable current limit through BP/M capacitor value
- Higher current limit extends peak power or, in open
frame applications, maximum continuous power
- Lower current limit improves efficiency in enclosed
adapters/chargers
- Allows optimum
TinySwitch-III
choice by swapping
devices with no other circuit redesign
• Tight I
2
f parameter tolerance reduces system cost
- Maximizes MOSFET and magnetics power delivery
- Minimizes max overload power, reducing cost of
transformer, primary clamp & secondary components
• ON-time extension – extends low line regulation range/
hold-up time to reduce input bulk capacitance
• Self-biased: no bias winding or bias components
• Frequency jittering reduces EMI filter costs
• Pin-out simplifies heatsinking to the PCB
• SOURCE pins are electrically quiet for low EMI
Enhanced Safety and Reliability Features
• Accurate hysteretic thermal shutdown protection with
automatic recovery eliminates need for manual reset
• Improved auto-restart delivers <3% of maximum power
in short circuit and open loop fault conditions
• Output overvoltage shutdown with optional Zener
• Line under-voltage detect threshold set using a single
optional resistor
• Very low component count enhances reliability and
enables single-sided printed circuit board layout
• High bandwidth provides fast turn on with no overshoot
and excellent transient load response
• Extended creepage between DRAIN and all other pins
improves field reliability
EcoSmart
– Extremely Energy Efficient
• Easily meets all global energy efficiency regulations
• No-load <150 mW at 265 VAC without bias winding,
<50 mW with bias winding
• ON/OFF control provides constant efficiency down to
very light loads – ideal for mandatory CEC regulations
and 1 W PC standby requirements
Applications
• Chargers/adapters for cell/cordless phones, PDAs, digital
cameras, MP3/portable audio, shavers, etc.
®
Energy Efficient, Off-Line Switcher with
Enhanced Flexibility and Extended Power Range
+
+
DC
Output
-
Wide-Range
HV DC Input
TinySwitch-III
S
D
EN/UV
BP/M
-
PI-4095-082205
Figure 1. Typical Standby Application.
OUTPUT POWER TABLE
230 VAC ±15%
PRODUCT
3
TNY274 P or G
TNY275 P or G
TNY276 P or G
TNY277 P or G
TNY278 P or G
TNY279 P or G
TNY280 P or G
85-265 VAC
Peak or
Peak or
Adapter
1
Open Adapter
1
Open
Frame
2
Frame
2
6W
8.5 W
10 W
13 W
16 W
18 W
20 W
11 W
15 W
19 W
23.5 W
28 W
32 W
36.5 W
5W
6W
7W
8W
10 W
12 W
14 W
8.5 W
11.5 W
15 W
18 W
21.5 W
25 W
28.5 W
Table 1.
Notes:
1.
Minimum continuous power in a typical non-
ventilated enclosed adapter measured at 50
°C
ambient. Use of an
external heatsink will increase power capability
2.
Minimum peak
power capability in any design or minimum continuous power in an
open frame design (see Key Application Considerations).
3.
Packages:
P: DIP-8C, G: SMD-8C. See Part Ordering Information.
•
•
•
PC Standby and other auxiliary supplies
DVD/PVR and other low power set top decoders
Supplies for appliances, industrial systems, metering, etc.
Description
TinySwitch-III
incorporates a 700 V power MOSFET, oscillator,
high voltage switched current source, current limit (user
selectable) and thermal shutdown circuitry. The IC family uses
an ON/OFF control scheme and offers a design flexible solution
with a low system cost and extended power capability.
February 2006
TNY274-280
BYPASS/
MULTI-FUNCTION
(BP/M)
REGULATOR
5.85 V
LINE UNDER-VOLTAGE
DRAIN
(D)
115
µA
25
µA
AUTO-
RESTART
COUNTER
6.4 V
FAULT
PRESENT
+
-
BYPASS PIN
UNDER-VOLTAGE
RESET
CURRENT
LIMIT STATE
MACHINE
5.85 V
4.9 V
V
I
LIMIT
CURRENT LIMIT
COMPARATOR
ENABLE
-
+
JITTER
CLOCK
1.0 V + V
T
DCMAX
THERMAL
SHUTDOWN
OSCILLATOR
ENABLE/
UNDER-
VOLTAGE
(EN/UV)
1.0 V
S
Q
R
Q
LEADING
EDGE
BLANKING
SOURCE
(S)
PI-4077-013106
Figure 2. Functional Block Diagram.
Pin Functional Description
DRAIN (D) Pin:
This pin is the power MOSFET drain connection. It provides
internal operating current for both start-up and steady-state
operation.
BYPASS/MULTI-FUNCTION (BP/M) Pin:
This pin has multiple functions:
1. It is the connection point for an external bypass capacitor
for the internally generated 5.85 V supply.
2. It is a mode selector for the current limit value, depending
on the value of the capacitance added. Use of a 0.1
µF
capacitor results in the standard current limit value. Use of
a 1
µF
capacitor results in the current limit being reduced to
that of the next smaller device size. Use of a 10
µF
capacitor
results in the current limit being increased to that of the next
larger device size for TNY275-280.
3. It provides a shutdown function. When the current into the
bypass pin exceeds 5.5 mA, the device latches off until the
BP/M voltage drops below 4.9 V, during a power down.
This can be used to provide an output overvoltage function
P Package (DIP-8C)
G Package (SMD-8C)
EN/UV
BP/M
1
2
8
7
S
S
6
D
S
S
4
5
PI-4078-080905
Figure 3. Pin Configuration.
with a Zener connected from the BP/M pin to a bias winding
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
2
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2/06
TNY274-280
MOSFET is controlled by this pin. MOSFET switching is
terminated when a current greater than a threshold current is
drawn from this pin. Switching resumes when the current being
pulled from the pin drops to less than a threshold current. A
modulation of the threshold current reduces group pulsing. The
threshold current is between 60
µA
and 115
µA.
The EN/UV 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,
TinySwitch-III
detects
its absence and disables the line under-voltage function.
SOURCE (S) Pin:
This pin is internally connected to the output MOSFET source
for high voltage power return and control circuit common.
maximum duty cycle signal (DC
MAX
) and the clock signal that
indicates the beginning of each cycle.
The oscillator incorporates circuitry that introduces a small
amount of frequency jitter, typically 8 kHz peak-to-peak, to
minimize EMI emission. The modulation rate of the frequency
jitter is set to 1 kHz 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 Figure 4 illustrates
the frequency jitter.
Enable Input and Current Limit State Machine
The enable input circuit at the EN/UV pin consists of a low
impedance source follower output set at 1.2 V. The current
through the source follower is limited to 115
µA.
When the
current out of this pin exceeds the threshold current, a low
logic level (disable) is generated at the output of the enable
circuit, until the current out of this pin is reduced to less than
the threshold current. This enable circuit 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). If low, 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.
The current limit state machine reduces the current limit by
discrete amounts at light loads when
TinySwitch-III
is likely to
switch in the audible frequency range. The lower current limit
raises the effective switching frequency above the audio range
and reduces the transformer flux density, including the associated
audible noise. The state machine monitors the sequence of
enable events to determine the load condition and adjusts the
current limit level accordingly in discrete amounts.
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 1.2 V in the disabled
state. This improves the response time of the optocoupler that
is usually connected to this pin.
5.85 V Regulator and 6.4 V Shunt Voltage Clamp
The 5.85 V regulator charges the bypass capacitor connected
to the BYPASS pin to 5.85 V by drawing a current from the
voltage on the DRAIN pin whenever the MOSFET is off. The
BYPASS/MULTI-FUNCTION pin is the internal supply voltage
node. When the MOSFET is on, the device operates from the
energy stored in the bypass capacitor. Extremely low power
consumption of the internal circuitry allows
TinySwitch-III
to
operate continuously from current it takes from the DRAIN
pin. A bypass capacitor value of 0.1
µF
is sufficient for both
high frequency decoupling and energy storage.
TinySwitch-III
Functional
Description
TinySwitch-III
combines a high voltage power MOSFET switch
with a power supply controller in one device. Unlike conventional
PWM (pulse width modulator) controllers, it uses a simple
ON/OFF control to regulate the output voltage.
The controller consists of an oscillator, enable circuit (sense and
logic), current limit state machine, 5.85 V regulator, BYPASS/
MULTI-FUNCTION pin under-voltage, overvoltage circuit, and
current limit selection circuitry, over- temperature protection,
current limit circuit, leading edge blanking, and a 700 V power
MOSFET.
TinySwitch-III
incorporates additional circuitry for
line under-voltage sense, auto-restart, adaptive switching cycle
on-time extension, 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 132 kHz. Two signals are generated from the oscillator: the
PI-2741-041901
600
500
V
DRAIN
400
300
200
100
0
136 kHz
128 kHz
0
5
Time (µs)
10
Figure 4. Frequency Jitter.
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3
TNY274-280
In addition, there is a 6.4 V shunt regulator clamping the
BYPASS/MULTI-FUNCTION pin at 6.4 V when current
is provided to the BYPASS/MULTI-FUNCTION pin
through an external resistor. This facilitates powering of
TinySwitch-III
externally through a bias winding to decrease
the no-load consumption to well below 50 mW.
BYPASS/MULTI-FUNCTION Pin Under-Voltage
The BYPASS/MULTI-FUNCTION pin under-voltage circuitry
disables the power MOSFET when the BYPASS/MULTI-
FUNCTION pin voltage drops below 4.9 V in steady state
operation. Once the BYPASS/MULTI-FUNCTION pin voltage
drops below 4.9 V in steady state operation, it must rise back
to 5.85 V to enable (turn-on) the power MOSFET.
Over Temperature Protection
The thermal shutdown circuitry senses the die temperature. The
threshold is typically set at 142 °C with 75 °C hysteresis. When
the die temperature rises above this threshold the power MOSFET
is disabled and remains disabled until the die temperature falls
by 75 °C, at which point it is re-enabled. A large hysteresis of
75 °C (typical) is provided to prevent overheating of the PC
board due to a continuous fault condition.
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
current limit state machine reduces the current limit threshold
by discrete amounts under medium and light loads.
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 capacitance and secondary-side rectifier
reverse recovery time will not cause premature termination of
the switching pulse.
300
200
100
0
10
5
0
V
DC-OUTPUT
V
PI-4098-082305
Auto-Restart
In the event of a fault condition such as output overload, output
short circuit, or an open loop condition,
TinySwitch-III
enters
into auto-restart operation. An internal counter clocked by the
oscillator is reset every time the EN/UV pin is pulled low. If the
EN/UV pin is not pulled low for 64 ms, the power MOSFET
switching is normally disabled for 2.5 seconds (except in the
case of line under-voltage condition, in which case it is disabled
until the condition is removed). 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 an output short circuit.
In the event of a line under-voltage condition, the switching of
the power MOSFET is disabled beyond its normal 2.5 seconds
until the line under-voltage condition ends.
Adaptive Switching Cycle On-Time Extension
Adaptive switching cycle on-time extension keeps the cycle
on until current limit is reached, instead of prematurely
terminating after the DC
MAX
signal goes low. This feature
reduces the minimum input voltage required to maintain
regulation, extending hold-up time and minimizing the size
of bulk capacitor required. The on-time extension is disabled
during the startup of the power supply, until the power supply
output reaches regulation.
Line Under-Voltage 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 25
µA
to
initiate switching of the power MOSFET. During power-up, this
is accomplished by holding the BYPASS/MULTI-FUNCTION
pin to 4.9 V while the line under-voltage condition exists. The
BYPASS/MULTI-FUNCTION pin then rises from 4.9 V to
5.85 V when the line under-voltage condition goes away. When the
switching of the power MOSFET is disabled in auto-restart mode
and a line under-voltage condition exists, the auto-restart counter
is stopped. This stretches the disable time beyond its normal
2.5 seconds until the line under-voltage condition ends.
The line under-voltage circuit also detects when there is
no external resistor connected to the EN/UV pin (less than
~1
µA
into the pin). In this case the line under-voltage function
is disabled.
TinySwitch-III
Operation
TinySwitch-III
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 or when the DC
MAX
limit is
reached. Since the highest current limit level and frequency of
a
TinySwitch-III
design are constant, the power delivered to the
DRAIN
0
2500
5000
Time (ms)
Figure 5. Auto-Restart Operation.
4
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2/06
TNY274-280
load is proportional to the primary inductance of the transformer
and peak primary current squared. Hence, designing the supply
involves calculating the primary inductance of the transformer
for the maximum output power required. If the
TinySwitch-III
is appropriately chosen for the power level, the current in the
calculated inductance will ramp up to current limit before the
DC
MAX
limit is reached.
Enable Function
TinySwitch-III
senses the EN/UV pin to determine whether or
not to proceed with the next switching cycle. The sequence of
cycles is used to determine the current limit. 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 in which the 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 diode across the DC output voltage to be regulated.
When the output voltage exceeds the target regulation voltage
level (optocoupler LED voltage drop plus Zener voltage), the
optocoupler LED will start to conduct, pulling the EN/UV pin
low. The Zener diode can be replaced by a TL431 reference
circuit for improved accuracy.
ON/OFF Operation with Current Limit State Machine
The internal clock of the
TinySwitch-III
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, and based
on the sequence of samples over multiple cycles, it determines
the appropriate current limit. At high loads, the state machine
sets the current limit to its highest value. At lighter loads, the
state machine sets the current limit to reduced values.
V
EN
CLOCK
DC
MAX
I DRAIN
V DRAIN
PI-2749-082305
Figure 6. Operation at Near Maximum Loading.
V
EN
V
EN
CLOCK
DC
CLOCK
DC
MAX
MAX
I DRAIN
I DRAIN
V DRAIN
V DRAIN
PI-2667-082305
PI-2377-082305
Figure 7. Operation at Moderately Heavy Loading.
Figure 8. Operation at Medium Loading.
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5
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