SNVS288K September   2004  – September 2018 LM2734

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      Typical Application Circuit
      2.      Efficiency vs Load Current VIN = 5 V, VOUT = 3.3 V
  4. 4 Revision History
  5. 5 Pin Configuration and Functions
    1.     Pin 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 Typical Characteristics
  7. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 Output Overvoltage Protection
      2. 7.3.2 Undervoltage Lockout
      3. 7.3.3 Current Limit
      4. 7.3.4 Thermal Shutdown
    4. 7.4 Device Functional Modes
      1. 7.4.1 Enable Pin / Shutdown Mode
      2. 7.4.2 Soft Start
  8. 8 Application and Implementation
    1. 8.1 Application Information
      1. 8.1.1 Boost Function
    2. 8.2 Typical Applications
      1. 8.2.1  LM2734X (1.6 MHz) VBOOST Derived from VIN 5V to 1.5 V/1 A
        1. 8.2.1.1 Design Requirements
        2. 8.2.1.2 Detailed Design Procedure
          1. 8.2.1.2.1 Custom Design With WEBENCH® Tools
          2. 8.2.1.2.2 Inductor Selection
          3. 8.2.1.2.3 Input Capacitor
          4. 8.2.1.2.4 Output Capacitor
          5. 8.2.1.2.5 Catch Diode
          6. 8.2.1.2.6 Boost Diode
          7. 8.2.1.2.7 Boost Capacitor
          8. 8.2.1.2.8 Output Voltage
        3. 8.2.1.3 Application Curves
      2. 8.2.2  LM2734X (1.6 MHz) VBOOST Derived from VOUT 12 V to 3.3 V /1 A
        1. 8.2.2.1 Design Requirements
        2. 8.2.2.2 Detailed Design Procedure
        3. 8.2.2.3 Application Curves
      3. 8.2.3  LM2734X (1.6 MHz) VBOOST Derived from VSHUNT 18 V to 1.5 V /1 A
        1. 8.2.3.1 Design Requirements
        2. 8.2.3.2 Detailed Design Procedure
        3. 8.2.3.3 Application Curves
      4. 8.2.4  LM2734X (1.6 MHz) VBOOST Derived from Series Zener Diode (VIN) 15 V to 1.5 V / 1 A
        1. 8.2.4.1 Design Requirements
        2. 8.2.4.2 Detailed Design Procedure
        3. 8.2.4.3 Application Curves
      5. 8.2.5  LM2734X (1.6 MHz) VBOOST Derived from Series Zener Diode (VOUT) 15 V to 9 V /1 A
        1. 8.2.5.1 Design Requirements
        2. 8.2.5.2 Detailed Design Procedure
        3. 8.2.5.3 Application Curves
      6. 8.2.6  LM2734Y (550 kHz) VBOOST Derived from VIN 5 V to 1.5 V / 1 A
        1. 8.2.6.1 Design Requirements
        2. 8.2.6.2 Detailed Design Procedure
        3. 8.2.6.3 Application Curves
      7. 8.2.7  LM2734Y (550 kHz) VBOOST Derived from VOUT 12 V to 3.3 V / 1 A
        1. 8.2.7.1 Design Requirements
        2. 8.2.7.2 Detailed Design Procedure
        3. 8.2.7.3 Application Curves
      8. 8.2.8  LM2734Y (550 kHz) VBOOST Derived from VSHUNT 18 V to 1.5 V / 1 A
        1. 8.2.8.1 Design Requirements
        2. 8.2.8.2 Detailed Design Procedure
        3. 8.2.8.3 Application Curves
      9. 8.2.9  LM2734Y (550 kHz) VBOOST Derived from Series Zener Diode (VIN) 15 V to 1.5 V / 1 A
        1. 8.2.9.1 Design Requirements
        2. 8.2.9.2 Detailed Design Procedure
        3. 8.2.9.3 Application Curves
      10. 8.2.10 LM2734Y (550 kHz) VBOOST Derived from Series Zener Diode (VOUT) 15 V to 9 V / 1 A
        1. 8.2.10.1 Design Requirements
        2. 8.2.10.2 Detailed Design Procedure
        3. 8.2.10.3 Application Curves
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Development Support
      1. 11.1.1 Custom Design With WEBENCH® Tools
    2. 11.2 Receiving Notification of Documentation Updates
    3. 11.3 Community Resources
    4. 11.4 Third-Party Products Disclaimer
    5. 11.5 Trademarks
    6. 11.6 Electrostatic Discharge Caution
    7. 11.7 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

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

8.1.1 Boost Function

Capacitor CBOOST and diode D2 in Figure 13 are used to generate a voltage VBOOST. VBOOST - VSW is the gate drive voltage to the internal NMOS control switch. To properly drive the internal NMOS switch during its on-time, VBOOST needs to be at least 1.6 V greater than VSW. Although the LM2734 device will operate with this minimum voltage, it may not have sufficient gate drive to supply large values of output current. Therefore, it is recommended that VBOOST be greater than 2.5 V above VSW for best efficiency. VBOOST – VSW should not exceed the maximum operating limit of 5.5 V.

5.5 V > VBOOST – VSW > 2.5 V for best performance.

LM2734 20102308.gifFigure 13. VOUT Charges CBOOST

When the LM2734 device starts up, internal circuitry from the BOOST pin supplies a maximum of 20 mA to CBOOST. This current charges CBOOST to a voltage sufficient to turn the switch on. The BOOST pin continues to source current to CBOOST until the voltage at the feedback pin is greater than 0.76 V.

There are various methods to derive VBOOST:

  1. From the input voltage (VIN)
  2. From the output voltage (VOUT)
  3. From an external distributed voltage rail (VEXT)
  4. From a shunt or series Zener diode

In the simplified block diagram of Functional Block Diagram, capacitor CBOOST and diode D2 supply the gate-drive current for the NMOS switch. Capacitor CBOOST is charged via diode D2 by VIN. During a normal switching cycle, when the internal NMOS control switch is off (TOFF) (refer to Figure 12), VBOOST equals VIN minus the forward voltage of D2 (VFD2), during which the current in the inductor (L) forward biases the Schottky diode D1 (VFD1). Therefore, the voltage stored across CBOOST is:

Equation 1. VBOOST - VSW = VIN - VFD2 + VFD1

When the NMOS switch turns on (TON), the switch pin rises to:

Equation 2. VSW = VIN – (RDSON × IL),

forcing VBOOST to rise thus reverse biasing D2. The voltage at VBOOST is then:

Equation 3. VBOOST = 2 VIN – (RDSON × IL) – VFD2 + VFD1

which is approximately:

Equation 4. 2VIN – 0.4 V

for many applications. Thus the gate-drive voltage of the NMOS switch is approximately:

Equation 5. VIN – 0.2 V

An alternate method for charging CBOOST is to connect D2 to the output as shown in Figure 13. The output voltage should be from 2.5 V and 5.5 V, so that proper gate voltage will be applied to the internal switch. In this circuit, CBOOST provides a gate drive voltage that is slightly less than VOUT.

In applications where both VIN and VOUT are greater than 5.5 V, or less than 3 V, CBOOST cannot be charged directly from these voltages. If VIN to VOUT are greater than 5.5 V, CBOOST can be charged from VIN or VOUT minus a Zener voltage by placing a Zener diode D3 in series with D2, as shown in Figure 14. When using a series Zener diode from the input, ensure that the regulation of the input supply does not create a voltage that falls outside the recommended VBOOST voltage.

Equation 6. (VINMAX – VD3) < 5.5 V
Equation 7. (VINMIN – VD3) > 1.6 V
LM2734 20102309.gifFigure 14. Zener Reduces Boost Voltage from VIN

An alternative method is to place the Zener diode D3 in a shunt configuration as shown in Figure 15. A small 350 mW to 500 mW 5.1-V Zener diode in a SOT or SOD package can be used for this purpose. A small ceramic capacitor such as a 6.3 V, 0.1-µF capacitor (C4) should be placed in parallel with the Zener diode. When the internal NMOS switch turns on, a pulse of current is drawn to charge the internal NMOS gate capacitance. The 0.1-µF parallel shunt capacitor ensures that the VBOOST voltage is maintained during this time.

Resistor R3 should be chosen to provide enough RMS current to the Zener diode (D3) and to the BOOST pin. A recommended choice for the Zener current (IZENER) is 1 mA. The current IBOOST into the BOOST pin supplies the gate current of the NMOS control switch and varies typically according to the following formula for the X version:

Equation 8. IBOOST = 0.56 × (D + 0.54) × (VZENER – VD2) mA

IBOOST can be calculated for the Y version using the following:

Equation 9. IBOOST = 0.22 × (D + 0.54) × (VZENER – VD2) µA

where D is the duty cycle, VZENER and VD2 are in volts, and IBOOST is in milliamps. VZENER is the voltage applied to the anode of the boost diode (D2), and VD2 is the average forward voltage across D2. Note that this formula for IBOOST gives typical current. For the worst case IBOOST, increase the current by 40%. In that case, the worst case boost current will be:

Equation 10. IBOOST-MAX = 1.4 × IBOOST

R3 will then be given by:

Equation 11. R3 = (VIN – VZENER) / (1.4 × IBOOST + IZENER)

For example, using the X-version let VIN = 10 V, VZENER = 5 V, VD2 = 0.7 V, IZENER = 1 mA, and duty cycle D = 50%. Then:

Equation 12. IBOOST = 0.56 × (0.5 + 0.54) × (5 - 0.7) mA = 2.5 mA
Equation 13. R3 = (10 V – 5 V) / (1.4 × 2.5 mA + 1 mA) = 1.11 kΩ
LM2734 20102348.gifFigure 15. Boost Voltage Supplied from the Shunt Zener on VIN