LM3477EVAL National Semiconductor, LM3477EVAL Datasheet
LM3477EVAL
Specifications of LM3477EVAL
Related parts for LM3477EVAL
LM3477EVAL Summary of contents
Page 1
... Features n 500kHz switching frequency n Adjustable current limit n 1.5% reference Typical Application Circuit Typical High Efficiency Step-Down (Buck) Converter © 2004 National Semiconductor Corporation n Thermal shutdown n Frequency compensation optimized with a single capacitor and resistor n Internal softstart n Current mode operation n Undervoltage lockout with hysteresis ...
Page 2
Connection Diagram Package Marking and Ordering Information Order Number Package Type LM3477MM LM3477MMX LM3477AMM LM3477AMMX Pin Description Pin Name Pin Number I 1 SEN COMP/ GND www.national.com ...
Page 3
... Absolute Maximum Ratings If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Input Voltage < Peak Driver Output Current ( 10µs) CB Pin Voltage (Note 2) I Pin Voltage SEN Power Dissipation Storage Temperature Range Junction Temperature ...
Page 4
Electrical Characteristics Specifications in Standard type face are for T Range. Unless otherwise specified, V Symbol Parameter V Current Limit Voltage at 100% CL(100) Duty Cycle V Short-Circuit Current Limit Sense SC Voltage V Internal Compensation Ramp SL Voltage Height ...
Page 5
Electrical Characteristics Specifications in Standard type face are for T Range. Unless otherwise specified, V Symbol Parameter TSD Thermal Shutdown T Thermal Shutdown Hysteresis SH θ Thermal Resistance JA Note 1: Absolute Maximum Ratings are limits beyond which damage to ...
Page 6
Typical Performance Characteristics I (Shutdown) vs Temperature & Supply Voltage Temperature & Supply Voltage (Switching) Supply V −V vs Supply Voltage CB SW www.national.com Unless otherwise specified Temperature & Supply Voltage (Non-Switching) Supply 20003313 ...
Page 7
Typical Performance Characteristics Efficiency vs Load Current (V = 24V 12V) IN OUT Efficiency vs Load Current (V = 12V 3.3V) IN OUT Error Amplifier Phase Shift Unless otherwise specified, V Efficiency vs Load Current (V ...
Page 8
Typical Performance Characteristics Short Circuit vs Temperature Shutdown Threshold Hysteresis vs Temperature www.national.com Unless otherwise specified, V Slope Compensation Ramp vs Slope Compensation 200033J8 Current Sense Voltage vs Duty Cycle 20003318 8 = 12V 25˚C. (Continued ...
Page 9
Functional Block Diagram Functional Description GENERAL DESCRIPTION The LM3477 switching regulator controller for topolo- gies incorporating a high side switch. The most common of these topologies is the step-down, or buck, converter. Other topologies such as the inverting ...
Page 10
Functional Description The important differences between the LM3477 and the LM3477A are summarized in Figure 1. The voltages in Fig- ure 1 can be referred to the switch current by dividing through The LM3477A has a lower ...
Page 11
Functional Description FIGURE 3. The Feedback Voltage Experiences an Oscillation if the Input Voltage Crosses the 7.2V Internal Bias Threshold DEFAULT/ADJUSTABLE SLOPE COMPENSATION The LM3477/A uses a current mode control scheme. There are many advantages in a current mode architecture ...
Page 12
Functional Description calculations for minimum inductance and necessary slope resistance R are carried out based on this resonant peak- SL ing. START-UP/SOFT-START The LM3477/A incorporates an internal soft-start during start-up. The soft-start forces the inductor current to rise slowly and ...
Page 13
Design Section (Continued) FIGURE 6. LM3477 Buck Converter Reference Schematic PROGRAMMING THE OUTPUT VOLTAGE The output voltage can be programmed using a resistor divider between the output and the feedback pins, as shown in Figure 6. The resistors are selected ...
Page 14
Design Section (Continued) where D is the duty cycle at the lowest V MAX To avoid current limit, < SEN(peak) CL(MIN) Therefore, Example 4.5V 2.5V, I IN(MIN) OUT www.national.com condition. IN The hysteretic threshold ...
Page 15
Design Section (Continued) Continuing with the example above, If the peak switch current decreases below this threshold, the LM3477/A will operate in hysteretic mode (see OVER VOLTAGE PROTECTION section). In some designs, it will be desired to use R so ...
Page 16
Design Section (Continued) Where, D’ = 1− the MOSFET when it is conducting OUT DS(ON). 1.8 = voltage gain of the current sense amp Voltage across the sense resistor ) ...
Page 17
Output Capacitor Selection (Continued) FIGURE 10. Output Voltage Overshoot Violation The ESR and the capacitance of the output capacitor must be carefully chosen so that the output voltage overshoot is within the design’s specification V OS(MAX) bined ESR of the ...
Page 18
Power Mosfet Selection MOSFET can be categorized into conduction losses and ac switching or transition losses. R DS(ON) the conduction losses. The conduction loss loss across the MOSFET. The maximum conduction loss is given by: where ...
Page 19
Compensation (Continued) FIGURE 13. LM3477 Compensation Components Figure 14 shows a bode plot of a typical current mode buck regulator estimate of the actual plot using the as- ymptotic approach. The three plots shown are of the ...
Page 20
Compensation (Continued) The strategy taken here for choosing R crossover frequency with R , and set the compensator zero c with C . Using the selected target crossover frequency set R to Crossover frequency in ...
Page 21
Compensation (Continued) −9 Choosing will set the power pole and insuring a −20dB/decade slope in the low frequency magnitude response. In other words, the phase margin below the crossover frequency will ...
Page 22
Compensation (Continued) www.national.com A third option is to strategically place the ESR zero f the output capacitor to cancel out the sampling pole. In this case, the capacitor C will not be used to cancel out should ...
Page 23
... BANNED SUBSTANCE COMPLIANCE National Semiconductor certifies that the products and packing materials meet the provisions of the Customer Products Stewardship Specification (CSP-9-111C2) and the Banned Substances and Materials of Interest Specification (CSP-9-111S2) and contain no ‘‘Banned Substances’’ as defined in CSP-9-111S2. ...