SNVS085X July 2000 – December 2017 LM3478
PRODUCTION DATA.
- 1 Features
- 2 Applications
- 3 Description
- 4 Revision History
- 5 Pin Configuration and Functions
- 6 Specifications
- 7 Detailed Description
-
8 Application and Implementation
- 8.1 Application Information
- 8.2
Typical Applications
- 8.2.1
Typical High Efficiency Step-Up (Boost) Converter
- 8.2.1.1 Design Requirements
- 8.2.1.2
Detailed Design Procedure
- 8.2.1.2.1 Custom Design with WEBENCH Tools
- 8.2.1.2.2 Power Inductor Selection
- 8.2.1.2.3 Programming the Output Voltage
- 8.2.1.2.4 Setting the Current Limit
- 8.2.1.2.5 Current Limit with External Slope Compensation
- 8.2.1.2.6 Power Diode Selection
- 8.2.1.2.7 Power MOSFET Selection
- 8.2.1.2.8 Input Capacitor Selection
- 8.2.1.2.9 Output Capacitor Selection
- 8.2.1.2.10 Compensation
- 8.2.1.3 Application Curves
- 8.2.2 Typical SEPIC Converter
- 8.2.1
Typical High Efficiency Step-Up (Boost) Converter
- 9 Power Supply Recommendations
- 10Layout
- 11Device and Documentation Support
- 12Mechanical, Packaging, and Orderable Information
7.1 Overview
The LM3478 device uses a fixed frequency, Pulse Width Modulated (PWM) current mode control architecture. The Functional Block Diagram shows the basic functionality. In a typical application circuit, the peak current through the external MOSFET is sensed through an external sense resistor. The voltage across this resistor is fed into the ISEN pin. This voltage is fed into the positive input of the PWM comparator. The output voltage is also sensed through an external feedback resistor divider network and fed into the error amplifier negative input (feedback pin, FB). The output of the error amplifier (COMP pin) is added to the slope compensation ramp and fed into the negative input of the PWM comparator. At the start of any switching cycle, the oscillator sets the RS latch using the switch logic block. This forces a high signal on the DR pin (gate of the external MOSFET) and the external MOSFET turns on. When the voltage on the positive input of the PWM comparator exceeds the negative input, the RS latch is reset and the external MOSFET turns off.
The voltage sensed across the sense resistor generally contains spurious noise spikes, as shown in Figure 20. These spikes can force the PWM comparator to reset the RS latch prematurely. To prevent these spikes from resetting the latch, a blank-out circuit inside the IC prevents the PWM comparator from resetting the latch for a short duration after the latch is set. This duration is about 325 ns and is called the blanking interval and is specified as minimum on-time in the Electrical Characteristics section. Under extremely light-load or no-load conditions, the energy delivered to the output capacitor when the external MOSFET in on during the blanking interval is more than what is delivered to the load. An over-voltage comparator inside the LM3478 prevents the output voltage from rising under these conditions. The over-voltage comparator senses the feedback (FB pin) voltage and resets the RS latch. The latch remains in reset state until the output decays to the nominal value.
Figure 20. Basic Operation of the PWM Comparator