TNY274-280
TinySwitch-III
Family
®
Energy-Efficient, Off-Line Switcher With
Enhanced Flexibility and Extended Power Range
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 undervoltage 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.
•
PC Standby and other auxiliary supplies
•
DVD/PVR and other low power set top decoders
•
Supplies for appliances, industrial systems, metering, etc.
®
+
+
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
Adapter
1
6W
8.5 W
10 W
13 W
16 W
18 W
20 W
Peak or
Open
Frame
2
11 W
15 W
19 W
23.5 W
28 W
32 W
36.5 W
85-265 VAC
Adapter
1
5W
6W
7W
8W
10 W
12 W
14 W
Peak or
Open
Frame
2
8.5 W
11.5 W
15 W
18 W
21.5 W
25 W
28.5 W
TNY274P/G
TNY275P/G
TNY276P/G
TNY277P/G
TNY278P/G
TNY279P/G
TNY280P/G
Table 1. Output Power Table.
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 Applications Considerations).
3. Packages: P: DIP-8C, G: SMD-8C. See Part Ordering Information.
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.
www.powerint.com
January 2009
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
RESET
FAULT
PRESENT
+
BYPASS PIN
UNDER-VOLTAGE
BYPASS
-
CAPACITOR
5.85 V
SELECT AND
4.9 V
CURRENT
LIMIT STATE
MACHINE
V
I
LIMIT
CURRENT LIMIT
COMPARATOR
ENABLE
-
+
JITTER
CLOCK
1.0 V + V
T
DCMAX
OSCILLATOR
ENABLE/
UNDER-
VOLTAGE
(EN/UV)
1.0 V
S
Q
THERMAL
SHUTDOWN
R
Q
OVP
LATCH
LEADING
EDGE
BLANKING
SOURCE
(S)
PI-4077-062306
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 startup 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 I
SD
, 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 with a
Zener connected from the BP/M pin to a bias winding supply.
P Package (DIP-8C)
G Package (SMD-8C)
EN/UV
BP/M
1
2
8
7
6
S
S
S
S
D
4
5
PI-4078-080905
Figure 3.
Pin Configuration.
ENABLE/UNDERVOLTAGE (EN/UV) Pin:
This pin has dual functions: enable input and line undervoltage
sense. During normal operation, switching of the power
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
2
Rev. I 01/09
www.powerint.com
TNY274-280
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 75
μA
and 115
μA.
The EN/UV pin also senses line undervoltage 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 undervoltage function.
SOURCE (S) Pin:
This pin is internally connected to the output MOSFET source for
high voltage power return and control circuit common.
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.
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.
136 kHz
128 kHz
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 undervoltage, 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
undervoltage 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
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
PI-2741-041901
600
500
V
DRAIN
400
300
200
100
0
0
Figure 4.
Frequency Jitter.
5
Time (μs)
10
BYPASS/MULTI-FUNCTION Pin Undervoltage
The BYPASS/MULTI-FUNCTION pin undervoltage 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.
3
www.powerint.com
Rev. I 01/09
TNY274-280
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 over-
heating 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.
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 undervoltage 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 undervoltage condition, the switching of
the power MOSFET is disabled beyond its normal 2.5 seconds
until the line undervoltage condition ends.
PI-4098-082305
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 Undervoltage 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 undervoltage condition
exists. The BYPASS/MULTI-FUNCTION pin then rises from
4.9 V to 5.85 V when the line undervoltage condition goes
away. When the switching of the power MOSFET is disabled in
auto-restart mode and a line undervoltage condition exists, the
auto-restart counter is stopped. This stretches the disable time
beyond its normal 2.5 seconds until the line undervoltage
condition ends.
The line undervoltage circuit also detects when there is no
external resistor connected to the EN/UV pin (less than ~2
μA
into the pin). In this case the line undervoltage 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
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.
300
200
100
0
10
V
V
DRAIN
5
0
DC-OUTPUT
0
2500
5000
Time (ms)
Figure 5.
Auto-Restart Operation.
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.
4
Rev. I 01/09
www.powerint.com
TNY274-280
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
V
EN
CLOCK
DC
MAX
DC
MAX
I DRAIN
I DRAIN
V DRAIN
V DRAIN
PI-2749-082305
PI-2667-082305
Figure 6.
Operation at Near Maximum Loading.
Figure 7.
Operation at Moderately Heavy Loading.
V
EN
CLOCK
V
EN
CLOCK
DC
MAX
DC
MAX
I DRAIN
I DRAIN
V DRAIN
V DRAIN
PI-2377-082305
PI-2661-082305
Figure 8.
Operation at Medium Loading.
Figure 9.
Operation at Very Light Load.
5
www.powerint.com
Rev. I 01/09
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