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
8.2.1.2.7 Power MOSFET Selection
The drive pin of the LM3478 must be connected to the gate of an external MOSFET. The drive pin (DR) voltage depends on the input voltage (see Typical Characteristics). In most applications, a logic level MOSFET can be used. For very low input voltages, a sub logic level MOSFET should be used. The selected MOSFET has a great influence on the system efficiency. The critical parameters for selecting a MOSFET are:
- Minimum threshold voltage, VTH(MIN)
- On-resistance, RDS(ON)
- Total gate charge, Qg
- Reverse transfer capacitance, CRSS
- Maximum drain to source voltage, VDS(MAX)
The off-state voltage of the MOSFET is approximately equal to the output voltage. Vds(max) must be greater than the output voltage. The power losses in the MOSFET can be categorized into conduction losses and switching losses. RDS(ON) is needed to estimate the conduction losses, Pcond:
The temperature effect on the RDS(ON) usually is quite significant. Assume 30% increase at hot.
For the current I in Equation 28 the average inductor current may be used.
Especially at high switching frequencies the switching losses may be the largest portion of the total losses.
The switching losses are very difficult to calculate due to changing parasitics of a given MOSFET in operation. Often the individual MOSFET's data sheet does not give enough information to yield a useful result. Equation 29 and Equation 30 give a rough idea how the switching losses are calculated:
