TNY174-180 데이터 시트보기 (PDF) - Power Integrations, Inc

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TNY174-180

TinySwitch-LT Family Energy Efficient, Offline Switcher with Enhanced Flexibility and Extended Power Range®

Power Integrations, Inc 

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 75

μA and 115 μA.

SOURCE (S) Pin:

This pin is internally connected to the output MOSFET source

for high voltage power return and control circuit common.

TinySwitch-LT Functional

Description

TinySwitch-LT 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-LT incorporates additional circuitry for

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 (DCMAX) 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

600

500

VDRAIN

400

300

200

100

0

136 kHz

128 kHz

0

5

10

Time (μs)

Figure 4. Frequency Jitter.

TNY174-180

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 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 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-LT 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 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-LT 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-LT externally through a bias winding to decrease

the no-load consumption to well below 50 mW.

A

08/07

3

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