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STR-W6750 Dataheets PDF



Part Number STR-W6750
Manufacturers Allegro MicroSystems
Logo Allegro MicroSystems
Description Off-Line Quasi-Resonant Switching Regulators
Datasheet STR-W6750 DatasheetSTR-W6750 Datasheet (PDF)

g n i h tors c t i a Sw egul R Application Note 28103.30 PRODUCT DESCRIPTION www.DataSheet4U.net Series STR-W6750 Off-Line Quasi-Resonant Switching Regulators INTRODUCTION TERMINAL FUNCTIONS VCC (Pin 4) Start-up circuit The start-up circuit detects the VCC pin voltage (pin 4), and makes the control IC start and stop operation. The power supply of the control IC (VCC pin input) employs a circuit as shown in Figure 1. At start-up, C3 is charged through a startup resistor R2. The R2 value needs.

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g n i h tors c t i a Sw egul R Application Note 28103.30 PRODUCT DESCRIPTION www.DataSheet4U.net Series STR-W6750 Off-Line Quasi-Resonant Switching Regulators INTRODUCTION TERMINAL FUNCTIONS VCC (Pin 4) Start-up circuit The start-up circuit detects the VCC pin voltage (pin 4), and makes the control IC start and stop operation. The power supply of the control IC (VCC pin input) employs a circuit as shown in Figure 1. At start-up, C3 is charged through a startup resistor R2. The R2 value needs to be set for more than the hold current of the latch circuit (140 µA max.) and to operate at the minimum ac input. The Series STR-W6750 devices are hybrid integrated circuits (HICs) with a built-in power MOSFET and a control IC designed for quasi-resonant type switch-mode power supplies (SMPS). In normal operation, the HIC provides high efficiency and low EMI noise with bottom-skip quasi-resonant operation during light output loads. Low power consumption is also achieved by blocking (intermittent) oscillation during an auto-burst mode and reduced even further in a manually triggered (clamping an output voltage) standby mode. The HIC is supplied in a seven-pin fully-molded TO-220-style package with pin 2 deleted, which is suitable for downsizing and standardizing of an SMPS by reducing external component count and simplifying circuit design. Features Blocking (or intermittent) oscillation operation by reducing output voltage in the standby mode. In addition to the standard quasi-resonant operation, a bottom-skip function is available for increased efficiency from light to medium load. Soft-start operation at start-up. Reduced switching noise (compared to conventional PWM hard-switching solution) with a step-drive function. Built-in avalanche-energy-guaranteed power MOSFET (to simplify surge-absorption circuit; no VDSS derating is required). Overcurrent protection (OCP), overvoltage protection (OVP), overload protection (OLP), and maximum ON-time control circuits are incorporated. OVP and OLP go into a latched mode. Able to save SMPS design time with present designs and evaluation processes. All performance characteristics given are typical values for circuit or system baseline design only and, unless otherwise stated, are at the nominal operating voltage and an ambient temperature of +25°C, unless otherwise stated. Figure 1 – External start-up circuit If the value of R2 is too high, the C3 charge current will be reduced. Consequently, it will take longer to reach the operation start-up voltage. The VCC pin voltage falls immediately after the control circuit starts its operation. The voltage drop can be reduced by increasing C3’s capacitance. Therefore, to maintain the start-up operation, even if the rise of the bias winding voltage is slow, the VCC pin voltage would not fall to the operation-stop voltage. However, too large a C3 capacitance will cause an improperly long time to reach the operation start after the initial power turn on. In general, SMPS performs its start-up operation properly with a value of C3 between 4.7 µF and 47 µF, and R2 between 47 kΩ and 150 kΩ for 120 V narrow or universal ac input, and 82 kΩ to 330 kΩ for 200 V narrow ac input. Sanken Power Devices from Allegro MicroSystems Series STR-W6750 Off-Line Quasi-Resonant Switching Regulators As shown in Figure 2, the circuit current is limited to 100 µA max (VCC = 15 V, and resistor R2 with appropriate high resistance value for the circuit) until the control circuit starts its operation. Once the VCC pin voltage reaches 18.2 V, the control circuit starts its operation by the start-up circuit, and supply current is increased. Once the VCC pin voltage drops down to lower than the operation-stop voltage 9.7 V, the UVLO circuit operates to stop the control circuit, and the IC returns to its initial state prior to start-up. g n i h tors c t i a Sw egul R Figure 3 – VCC after start-up Figure 2 – ICC vs. VCC Figure 4 – VCC vs. IO (secondary load) Bias/drive winding After the control circuit starts its operation, the power supply is operated by rectifying and smoothing the voltage of the bias winding. Figure 3 shows the start-up voltage waveform of the VCC pin. The bias winding voltage does not immediately increase up to the set voltage after the control circuit starts its operation. That is why the VCC pin voltage starts dropping. The operation-stop voltage is set as low as 10.6 V (max), the bias winding voltage reaches a stabilized voltage before it drops to the operation-stop voltage, and the control circuit continues its operation. The bias winding voltage, in normal power supply operation, is set for the voltage across C3 to be higher than the operation-stop voltage [VCC(OFF) 10.6 V(max.)] and lower than the OVP-operation voltage [VCC(OVP) 25.5 V(min.)]. In an actual power supply circuit, the Vcc pin voltage might be changed by the value of secondary output current as shown in Figure 4. Because of the low circuit current of the STRW6750, C3.


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