TNY375-380
pulled from the pin drops to less than a threshold current. A
frequency jitter is set to 1 kHz to optimize EMI reduction for both
modulation of the threshold current reduces group pulsing. The average and quasi-peak emissions. The frequency jitter should
threshold current is between 7ꢀ mA and 11ꢀ mA.
be measured with the oscilloscope triggered at the falling edge of
the DRAIN waveform. The waveform in Figure 4 illustrates the
The EN/UV pin also senses line undervoltage conditions through frequency jitter with an oscillator frequency of 264 kHz.
an external resistor connected to the DC line voltage. If there is
Enable Input and Current Limit State Machine
no external resistor connected to this pin,
TinySwitch-PK detects its absence and disables the line under-
voltage function.
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 11ꢀ mA. When the
current out of this pin exceeds the threshold current, a low logic
SOURCE (S) Pin:
This pin is internally connected to the output MOSFET source for level (disable) is generated at the output of the enable circuit
high voltage power return and control circuit common.
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. When a cycle is
disabled, the EN/UV pin is sampled at 264 kHz. This faster
sampling enables the power supply to respond faster without
being required to wait for completion of the full period.
TinySwitch-PK Functional Description
TinySwitch-PK 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, ꢀ.8ꢀ 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-PK 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.
The current limit state machine reduces the current limit by
discrete amounts at light loads when TinySwitch-PK 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.
Oscillator
The typical oscillator frequency is internally set to an average of
264 kHz (at the highest current limit level). Two signals are generated
from the oscillator: the maximum duty cycle signal (DCMAX) and the
clock signal that indicates the beginning of each cycle.
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.
The oscillator incorporates circuitry that introduces a small
amount of frequency jitter, typically ±35 of the oscillator
frequency, to minimize EMI emission. The modulation rate of the
5.85 V Regulator and 6.4 V Shunt Voltage Clamp
The ꢀ.8ꢀ V regulator charges the bypass capacitor connected
to the BYPASS pin to ꢀ.8ꢀ 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 the TNY37ꢀ and
TNY376 to operate continuously from current taken from the
DRAIN pin. A bypass capacitor value of 0.1 mF is sufficient for
both high frequency decoupling and energy storage.
600
500
VDRAIN
400
300
200
100
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-PK externally
through a bias winding as required for TNY377-380. Powering
the TinySwitch-PK externally in this way also decreases the
no-load consumption to below 60 mW.
0
280 kHz
248 kHz
0
2.5
5
Time (µs)
Figure 4. Frequency Jitter.
3
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Rev. C 09/12
POWERINT [ Power Integrations ]
