SLLSFM0A March   2022  – June 2022 SN6507

PRODUCTION DATA  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Pin Configuration and Functions
  6. 6 Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 ESD Ratings
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics
    6. 6.6 Switching Characteristics
    7. 6.7 Typical Characteristics, SN6507
  7. 7 Parameter Measurement Information
  8. 8 Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 Push-Pull Converter
      2. 8.3.2 Core Magnetization
      3. 8.3.3 Duty Cycle Control
      4.      Programmable Switching Frequency
      5. 8.3.4 Spread Spectrum Clocking
      6. 8.3.5 Slew Rate Control
      7. 8.3.6 Protection Features
        1. 8.3.6.1 Over Voltage Protection (OVP)
        2. 8.3.6.2 Over Current and Short Circuit Protection (OCP)
        3. 8.3.6.3 Under Voltage Lock-Out (UVLO)
        4. 8.3.6.4 Thermal Shut Down (TSD)
    4. 8.4 Device Functional Modes
      1. 8.4.1 Start-Up Mode
        1. 8.4.1.1 Soft-Start
      2. 8.4.2 Operation Mode
      3. 8.4.3 Shutdown Mode
      4. 8.4.4 SYNC Mode
  9. 9 Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Application
      1. 9.2.1 Design Requirements
      2. 9.2.2 Detailed Design Procedure
        1. 9.2.2.1 Pin Configuration
        2. 9.2.2.2 LDO Selection
        3. 9.2.2.3 Diode Selection
        4. 9.2.2.4 Capacitor and Inductor Selection
        5. 9.2.2.5 Transformer Selection
          1. 9.2.2.5.1 V-t Product Calculation
          2. 9.2.2.5.2 Turns Ratio Estimate
        6. 9.2.2.6 Low-Emissions Designs
      3. 9.2.3 Application Curves
      4. 9.2.4 System Examples
        1. 9.2.4.1 Higher Output Voltage Designs
        2. 9.2.4.2 Commercially-Available Transformers
    3. 9.3 Power Supply Recommendations
    4. 9.4 Layout
      1. 9.4.1 Layout Guidelines
      2. 9.4.2 Layout Example
  10. 10Device and Documentation Support
    1. 10.1 Documentation Support
      1. 10.1.1 Related Documentation
    2. 10.2 Receiving Notification of Documentation Updates
    3. 10.3 Community Resources
    4. 10.4 Trademarks
  11. 11Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

8.3.3 Duty Cycle Control

The SN6507 implements a duty cycle control feature to provide line regulation to a certain degree through a resistor on DC pin. By making the DC pin voltage a function of the input, the duty cycle will adjust with VIN, so that VOUT can be kept constant. Compared to fixed duty cycle transformer drivers, this dynamic duty cycle control feature reduces LDO power loss for wide VIN variations by pseudo-regulating the output. For applications where input variation is within a certain range, this feature can eliminate the post-regulation LDO. Another benefit of duty cycle control is to reduce the transformer cost and size because of the limited input range to primary side of the transformer.

Figure 8-3 Schematic with duty cycle control

The calculation of DC pin resistor is shown in Equation 1, where both RDC and RCLK are in kΩ.

Equation 1. RDC=0.816×D×VCC×RCLK+1-1

For fixed oscillator cases where RCLK is shorted to GND, a value of RCLK = 9.6kΩ should be used in the equation above to calculate RDC.

The duty cycle control can compensate for input variation up to ±35%, where line regulation within ±5% can be achieved. To achieve this range, it is recommended that duty cycle at nominal VIN is centered at 25% (D = 0.25). The transformer turns ratio needs take this duty cycle into calculation to ensure the expected output voltage level at all VIN voltages, as discussed in Section 9.2.2.5.

The duty cycle control features supports up to a certain duty cycle and VIN range. The minimum duty cycle is determined by the charge and discharge time of the gate capacitance of Power FETs, while the maximum duty cycle is limtied by the dead time (70 ns typical). For example, at 1 MHz, the adjustable duty cycle is between 10% and 43%. Exceeding above duty cycle range, the line regulation may saturate and input compensation does not work anymore. Meanwhile, if the duty cycle is lower than the minimum spec, the part may hit current limit at heavy loads. The VIN range that duty cycle feature is applicable is from 6 V to 36 V.

To enable the duty cycle control feature, an inductor is required on the output side. The selection of the output inductor should make sure the inductor current will not go into discountinous conduction mode (DCM), meaning the inductor current ramp should not drop to zero at any time. The minimum inductance LMIN is therefore calculated by the conditions that the part stays in continuous conduction mode (CCM) where the load DC current is smaller than half the current ramp amplitude seen on the inductor. Therefore LMIN is a function of the load current and switching frequency as shown by below equation where Iload is in A, fSW is in Hz, D is the duty cycle as a decimal (for 25% duty cycle, 0.25 would be used), and Lmin is in H.

Equation 2. LMIN=VOUT×1-2×D×(VIN TYP/VIN MAX)4×ILOAD MIN×fSW
Figure 8-4 Waveforms in Continuous Conduction Mode (CCM)
Figure 8-5 Waveforms in Discontinuous conduction Mode (DCM)