TNY253/254/255
circuit is sampled at the rising edge of the oscillator Clock
signal (at the beginning of each cycle). If it is high, then the
power MOSFET is turned on (enabled) for that cycle, otherwise
the power MOSFET remains in the off state (cycle skipped).
Since the sampling is done only once at the beginning of each
cycle, any subsequent changes at the ENABLE pin during the
cycle are ignored.
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 continu-
ously 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.
Undervoltage
The undervoltage 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 has to rise back to 5.8 V to
enable (turn-on) the power MOSFET.
Hysteretic Over Temperature Protection
The thermal shutdown circuitry senses the die junction tem-
perature. The threshold is set at 135 °C with 70 °C hysteresis.
When the junction temperature rises above this threshold
(135 °C) the power MOSFET is disabled and remains disabled
until the die junction temperature falls by 70 °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.
TinySwitch Operation
TinySwitch is intended to 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 a given TinySwitch device
are 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. As long as the TinySwitch
device chosen is rated for the 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
The TinySwitch senses the ENABLE pin to determine whether
or not to proceed with the next switch cycle as described earlier.
Once a cycle is started TinySwitch always completes the cycle
(even when the ENABLE 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 ENABLE feedback.
The ENABLE signal is generated on the secondary by comparing
the power supply output voltage with a reference voltage. The
ENABLE signal is high when the power supply output voltage
is less than the reference voltage.
In a typical implementation, the ENABLE pin is driven by
an optocoupler. The collector of the optocoupler transistor is
connected to the ENABLE 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 ENABLE
pin low. The Zener could be replaced by a TL431 device for
improved accuracy.
The ENABLE pin pull-down current threshold is nominally
50 µA, but is set to 40 µA the instant the threshold is exceeded.
This is reset to 50 µA when the ENABLE pull-down current
drops below the current threshold of 40 µA.
ON/OFF Control
The internal clock of the TinySwitch runs all the time. At the
beginning of each clock cycle the TinySwitch samples the
ENABLE pin to decide whether or not to implement a switch
cycle. If the ENABLE pin is high (< 40 µA), then a switching
cycle takes place. If the ENABLE pin is low (greater than
50 µA) then no switching cycle occurs, and the ENABLE pin
status is sampled again at the start of the subsequent clock cycle.
At full load TinySwitch will conduct during the majority of
its clock cycles (Figure 4). At loads less than full load, the
TinySwitch will “skip” more cycles in order to maintain volt-
age regulation at the secondary output (Figure 5). At light
load or no load, almost all cycles will be skipped (Figure 6).
A small percentage of cycles will conduct to support the power
consumption of the power supply.
Rev E
02/12
3
POWERINT [ POWER INTEGRATIONS, INC. ]
