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
powerMOSFETisturnedon(enabled)forthatcycle,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.
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 DCMAX limit is reached.
5.8 V Regulator
The 5.8 V regulator charges the bypass capacitor connected to
theBYPASSpinto5.8Vbydrawingacurrentfromthevoltage
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.
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,
amountofenergyperswitchcycleandthedelayoftheENABLE
feedback.
TheENABLEsignalisgeneratedonthesecondarybycomparing
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.
Under Voltage
Theunder-voltagecircuitrydisablesthepowerMOSFETwhen
theBYPASSpinvoltagedropsbelow5.1V. OncetheBYPASS
pin voltage drops below 5.1 V, it has to rise back to 5.8V to
enable (turn-on) the power MOSFET.
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.
Hysteretic Over Temperature Protection
The thermal shutdown circuitry senses the die junction
temperature. Thethresholdissetat135 °Cwith70 °Chysteresis.
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
(ILIMIT), the power MOSFET is turned off for the remainder of
that cycle.
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.
The leading edge blanking circuit inhibits the current limit
comparator for a short time (tLEB) 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.
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
statusissampledagainatthestartofthesubsequentclockcycle.
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 DCMAX by the oscillator.
Since the current limit and frequency of a given TinySwitch
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 voltage
regulation at the secondary output (Figure 5). At light load or
no load, almost all cycles will be skipped (Figure 6). A small
C
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