SNVS523G September   2007  – January 2018 LM3103

PRODUCTION DATA.  

  1. Features
  2. Applications
  3. Description
    1.     Typical Application Schematic
  4. Revision History
  5. Pin Configuration and Functions
    1.     Pin 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 Functional Block Diagram
    2. 7.2 Feature Description
      1. 7.2.1  COT Control Circuit Overview
      2. 7.2.2  Startup Regulator (VCC)
      3. 7.2.3  Regulation Comparator
      4. 7.2.4  Zero Coil Current Detect
      5. 7.2.5  Over-Voltage Comparator
      6. 7.2.6  ON-Time Timer, Shutdown
      7. 7.2.7  Current Limit
      8. 7.2.8  N-Channel MOSFET and Driver
      9. 7.2.9  Soft-Start
      10. 7.2.10 Thermal Protection
  8. Applications and Implementation
    1. 8.1 Application Information
      1. 8.1.1 External Components
  9. Device and Documentation Support
    1. 9.1 Receiving Notification of Documentation Updates
    2. 9.2 Community Resources
    3. 9.3 Trademarks
    4. 9.4 Electrostatic Discharge Caution
    5. 9.5 Glossary
  10. 10Mechanical, Packaging, and Orderable Information

Package Options

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

External Components

The following guidelines can be used to select external components.

RFB1 and RFB2 : These resistors should be chosen from standard values in the range of 1.0 kΩ to 10 kΩ, satisfying the following ratio:

Equation 7. RFB1/RFB2 = (VOUT/0.6 V) - 1

For VOUT = 0.6 V, the FB pin can be connected to the output directly with a pre-load resistor drawing more than 20 µA. This is because the converter operation needs a minimum inductor current ripple to maintain good regulation when no load is connected.

RON:Equation 2 can be used to select RON if a desired operating frequency is selected. But the minimum value of RON is determined by the minimum on-time. It can be calculated as follows:

Equation 8. LM3103 30029730.gif

If RON calculated from Equation 2 is smaller than the minimum value determined in Equation 8, a lower frequency should be selected to re-calculate RON by Equation 2. Alternatively, VIN(MAX) can also be limited in order to keep the frequency unchanged. The relationship of VIN(MAX) and RON is shown in Figure 22.

On the other hand, the minimum off-time of 240 ns can limit the maximum duty ratio. This may be significant at low VIN. A larger RON should be selected in any application requiring a large duty ratio.

LM3103 30029738.pngFigure 22. Maximum VIN for selected RON

L: The main parameter affected by the inductor is the amplitude of the inductor current ripple (ILR), which is recommended to be greater than 0.3 A. Once ILR is selected, L can be determined by:

Equation 9. LM3103 30029731.gif

where VIN is the input voltage and fSW is determined from Equation 2.

If the output current IOUT is known, by assuming that IOUT = IL, the peak and valley of ILR can be determined. Beware that the peak of ILR should not be larger than the saturation current of the inductor and the current rating of the main and synchronous MOSFETs. Also, the valley of ILR must be positive if CCM operation is required.

LM3103 30029732.pngFigure 23. Inductor selection for VOUT = 3.3 V
LM3103 30029733.pngFigure 24. Inductor selection for VOUT = 0.6 V

Figure 23 and Figure 24 show curves on inductor selection for various VOUT and RON. According to Equation 8, VIN is limited for small RON. Some curves are therefore limited as shown in the figures.

CVCC: The capacitor on the VCC output provides not only noise filtering and stability, but also prevents false triggering of the VCC UVLO at the main MOSFET on/off transitions. CVCC should be no smaller than 1 µF for stability, and should be a good quality, low ESR, ceramic capacitor.

COUT and COUT3: COUT should generally be no smaller than 10 µF. Experimentation is usually necessary to determine the minimum value for COUT, as the nature of the load may require a larger value. A load which creates significant transients requires a larger COUT than a fixed load.

COUT3 is a small value ceramic capacitor located close to the LM3103 to further suppress high frequency noise at VOUT. A 47 nF capacitor is recommended.

CIN and CIN3: The function of CIN is to supply most of the main MOSFET current during the on-time, and limit the voltage ripple at the VIN pin, assuming that the voltage source connecting to the VIN pin has finite output impedance. If the voltage source’s dynamic impedance is high (effectively a current source), CIN supplies the difference between the instantaneous input current and the average input current.

At the maximum load current, when the main MOSFET turns on, the current to the VIN pin suddenly increases from zero to the valley of the inductor’s ripple current and ramps up to the peak value. It then drops to zero at turn-off. The average current during the on-time is the load current. For a worst case calculation, CIN must be capable of supplying this average load current during the maximum on-time. CIN is calculated from:

Equation 10. LM3103 30029734.gif

where IOUT is the load current, ton is the maximum on-time, and ΔVIN is the allowable ripple voltage at VIN.

CIN3’s purpose is to help avoid transients and ringing due to long lead inductance at the VIN pin. A low ESR 0.1 µF ceramic chip capacitor located close to the LM3103 is recommended.

CBST: A 33 nF high quality ceramic capacitor with low ESR is recommended for CBST since it supplies a surge current to charge the main MOSFET gate driver at each turn-on. Low ESR also helps ensure a complete recharge during each off-time.

CSS: The capacitor at the SS pin determines the soft-start time, i.e. the time for the reference voltage at the regulation comparator and therefore, the output voltage to reach their final value. The time is determined from the following equation:

Equation 11. LM3103 30029735.gif

CFB: If the output voltage is higher than 1.6 V, CFB is needed in the Discontinuous Conduction Mode to reduce the output ripple. The recommended value for CFB is 10 nF.