SNVS178H January   2002  – December 2015 LM3485

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 Hysteretic Control Circuit
      2. 7.3.2 Current Limit Operation
    4. 7.4 Device Functional Modes
      1. 7.4.1 Start Up
      2. 7.4.2 External Sense Resistor
      3. 7.4.3 PGATE
  8. Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
        1. 8.2.2.1 Step by Step Design Procedure
        2. 8.2.2.2 Inductor Selection (L1)
        3. 8.2.2.3 Output Voltage Set Point
        4. 8.2.2.4 Output Capacitor Selection (COUT)
        5. 8.2.2.5 Input Capacitor Selection (CIN)
        6. 8.2.2.6 Programming the Current Limit (RADJ)
        7. 8.2.2.7 Catch Diode Selection (D1)
        8. 8.2.2.8 P-Channel MOSFET Selection (Q1)
      3. 8.2.3 Application Curves
  9. Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
  11. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Third-Party Products Disclaimer
    2. 11.2 Community Resources
    3. 11.3 Trademarks
    4. 11.4 Electrostatic Discharge Caution
    5. 11.5 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

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

10 Layout

10.1 Layout Guidelines

The PC board layout is very important in all switching regulator designs. Poor layout can cause switching noise into the feedback signal and general EMI problems. For minimal inductance, the wires indicated by heavy lines should be as wide and short as possible. Keep the ground pin of the input capacitor as close as possible to the anode of the diode. This path carries a large AC current. The switching node, the node with the diode cathode, inductor, and FET drain, should be kept short. This node is one of the main sources for radiated EMI because it is an AC voltage at the switching frequency. It is always good practice to use a ground plane in the design, particularly at high currents.

The two ground pins, PWR GND and GND, should be connected by as short a trace as possible; they can be connected underneath the device. These pins are resistively connected internally by approximately 50 Ω. The ground pins should be tied to the ground plane, or to a large ground trace in close proximity to both the FB divider and COUT grounds.

The gate pin of the external PFET should be located close to the PGATE pin. However, if a very small FET is used, a resistor may be required between PGATE and the gate of the FET to reduce high frequency ringing. Because this resistor will slow the rise time of the PFET, the current limit blanking time should be taken into consideration (see Current Limit Operation).

The feedback voltage signal line can be sensitive to noise. Avoid inductive coupling to the inductor or the switching node, by keeping the FB trace away from these areas.

Layout Example

LM3485 20034642.gif Figure 30. Top Layer,
Typical PCB Layout (3.3-V Output)
LM3485 20034641.gif Figure 32. Silk Screen,
Typical PCB Layout (3.3-V Output)
LM3485 20034644.gif Figure 31. Bottom Layer,
Typical PCB Layout (3.3-V Output)
LM3485 20034628.png Figure 33. Typical PCB Layout Schematic (3.3-V Output)

Table 2. Typical Application BOM

DESIGNATOR DESCRIPTION PART NUMBER DISTRIBUTOR
C1 COUT 22-µF to 35-V EEJL1VD226R Panasonic
C2 CIN 100-µF to 6.3-V 6TPC100M
C3 CADJ 1-nF ceramic chip capacitor
C4 CFF 100-pF ceramic chip capacitor
D1 1 A to 40 V MBRS140T3 On Semiconductor
L1 22 µH QH66SN220M01L Murata
Q1 FDC5614P Fairchild
R1 33k-Ω chip resistor
R2 20-kΩ chip resistor
R3 RADJ 240-kΩ chip resistor