SLUS191D February   1997  – July 2017 UC1525A , UC1527A , UC2525A , UC2527A , UC3525A , UC3527A

PRODUCTION DATA.  

  1. Features
  2. Applications
  3. Description
  4. Revision History
  5. Pin Configuration and Functions
  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 Typical Characteristics
  7. Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 Adjustable Dead-Time Control
      2. 7.3.2 Soft Start
      3. 7.3.3 Input Undervoltage Lockout With Hysteresis
      4. 7.3.4 Shutdown and Pulse-by-Pulse Current Limiting
    4. 7.4 Device Functional Modes
      1. 7.4.1 Shutdown Options (See )
  8. Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application
      1. 8.2.1 Theory of Operation
      2. 8.2.2 Design Requirements
      3. 8.2.3 Detailed Design Procedure
        1. 8.2.3.1 Timing Resistor and Capacitor Selection
        2. 8.2.3.2 Turns Ratio Selection
        3. 8.2.3.3 Inductor Selection
        4. 8.2.3.4 Rectification Diode Selection
        5. 8.2.3.5 VC Capacitor Selection
        6. 8.2.3.6 Output Capacitor Selection
        7. 8.2.3.7 Input Capacitor Selection
      4. 8.2.4 Application Curves
  9. Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Documentation Support
      1. 11.1.1 Related Documentation
    2. 11.2 Related Links
    3. 11.3 Community Resources
    4. 11.4 Trademarks
    5. 11.5 Electrostatic Discharge Caution
    6. 11.6 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

Refer to the PDF data sheet for device specific package drawings

Mechanical Data (Package|Pins)
  • N|16
Thermal pad, mechanical data (Package|Pins)
Orderable Information

Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.

Application Information

The UC1525A/1527A series of pulse width modulator integrated circuits are designed to offer improved performance and lowered external parts count when used in designing all types of switching power supplies. The UC1525A output stage features NOR logic, giving a LOW output for an OFF state. The UC1527A utilizes OR logic which results in a HIGH output level when OFF.

Typical Application

UC1525A UC1527A UC2525A UC2527A UC3525A UC3527A Lab_Test_Fixture_SLUS191.gif Figure 5. Lab Test Fixture

Theory of Operation

UC1525A UC1527A UC2525A UC2527A UC3525A UC3527A Low_Power_Transformers_SLUS191.gif Figure 6. Low Power Transformers

Low power transformers can be driven by the UC1525A. Automatic reset occurs during dead time, when both ends of the primary winding are switched to ground.

UC1525A UC1527A UC2525A UC2527A UC3525A UC3527A UC1525A_Oscillator_Schematic_SLUS191.gif Figure 7. UC1525A Oscillator Schematic
UC1525A UC1527A UC2525A UC2527A UC3525A UC3527A UC1525A_Error_Amplifier_SLUS191.gif Figure 8. UC1525A Error Amplifier
UC1525A UC1527A UC2525A UC2527A UC3525A UC3527A UC1525A_Output_Circuit_Half_Circuit_Shown_SLUS191.gif Figure 9. UC1525A Output Circuit (1/2 circuit shown)
UC1525A UC1527A UC2525A UC2527A UC3525A UC3527A Grounded_Driver_Outputs_For_Single-Ended_Supplies_SLUS191.gif Figure 10. Grounded Driver Outputs For Single-Ended Supplies

For single-ended supplies, the driver outputs are grounded. The VC terminal is switched to ground by the totem-pole source transistors on alternate oscillator cycles.

UC1525A UC1527A UC2525A UC2527A UC3525A UC3527A Output_Drivers_with_Low_Source_Impedance_SLUS191.gif Figure 11. Output Drivers With Low Source Impedance

The low source impedance of the output drivers provides rapid charging of power FET input capacitance while minimizing external components.

UC1525A UC1527A UC2525A UC2527A UC3525A UC3527A Conventional_Push-Pull_Bipolar_Design_SLUS191.gif Figure 12. Conventional Push-Pull Bipolar Design

In conventional push-pull bipolar designs, forward base drive is controlled by R1–R3. Rapid turn-off times for the power devices are achieved with speed-up capacitors C1 and C2.

Design Requirements

This example illustrates the design process and component selection for a push-pull DC-DC converter utilizing the UC1525A. The converter regulates a 30-V input to a 5-V output with 10-A maximum load.

Table 1. Design Parameters

PARAMETER MIN TYP MAX UNIT
VIN Input voltage range 25 30 35 V
VOUT Output voltage 5 V
iOUT Output current 1 10 A
fO Oscillator frequency 100 kHz
fS Switching frequency 50 kHz

Detailed Design Procedure

Timing Resistor and Capacitor Selection

Generally, higher switching frequency gives smaller size but have higher switching loss. Operation at 100 kHz was selected in this example as a reasonable compromise between size and efficiency. The value of RT = 10 kΩ, CT = 1.37 nF and RD = 100 Ω were chosen for 100-kHz oscillator frequency based on equation:

Equation 1. UC1525A UC1527A UC2525A UC2527A UC3525A UC3527A qu1new_lus191.gif

Turns Ratio Selection

The maximum primary-to-secondary turns ratio NMAX can be determined by the target output voltage, minimum input voltage, and the estimated maximum duty cycle. DLIM = 0.35 was selected for this example. NMAX can be calculated using Equation 1.

Equation 2. UC1525A UC1527A UC2525A UC2527A UC3525A UC3527A qu2new_lus191.gif

Rounding NMAX down to the next lowest integer results in a turns ratio of N = 3.

Inductor Selection

The maximum inductor ripple current occurs at the maximum input voltage. Typically, 20% to 40% of the full load current ripple is a good compromise between core loss and copper loss of the inductor. Higher ripple current allows for a smaller inductor size, but places more burden on the output capacitor to smooth the ripple voltage on the output. In this example, a ripple current of 25% of 10 A was chosen. The inductor value can be calculated as:

Equation 3. UC1525A UC1527A UC2525A UC2527A UC3525A UC3527A qu3new_lus191.gif

Rectification Diode Selection

A rectification diode should always possess low-forward voltage drop. When used in high-frequency switching applications, the diode must also possess a short recovery time. Schottky diodes meet both requirements and are therefore strongly recommended in push-pull converter designs.

VC Capacitor Selection

The primary purpose of the VC capacitor is to supply the peak transient currents of the drivers as well as provide stability for the VC regulator. These peak currents can be several amperes. The recommended value of VC capacitor should be no smaller than 0.1 μF, and should be a good quality, low ESR, ceramic capacitor. VC capacitor should be placed as close as possible to the VC pin to minimize potentially damaging voltage transients caused by trace inductance.

Output Capacitor Selection

The output capacitors smooth the output voltage ripple caused by inductor ripple current and provide a source of charge during load transient conditions.

Input Capacitor Selection

The input supply voltage typically has high source impedance at the switching frequency. Good quality input capacitors are necessary to limit the ripple voltage at the VIN pin while supplying most of the switch current during the on-time. The input capacitor should be selected for RMS current rating and minimum ripple voltage.

Application Curves

UC1525A UC1527A UC2525A UC2527A UC3525A UC3527A D001_SLUS191.gif
Figure 13. UC1525A Output Saturation Characteristics