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GM3255S8T Datasheet(PDF) 11 Page - Gamma Microelectronics Inc. |
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GM3255S8T Datasheet(HTML) 11 Page - Gamma Microelectronics Inc. |
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11 / 19 page  Another solution to the current limiting problem is to externally measure the current through the switch us- ing a sense resistor. Such a circuit is illustrated in Figure 13. The switch current is limited to Where: V = the base - emitter voltage drop of Q1, typi- BE(Q1) cally 0.65V. The improved circuit does not require a require a regulated voltage to operate properly. Unfortunately, a price must be paid for this convenience in the overall efficiency of the circuit. The designer should note that the input and output grounds are no longer common. Also, the addition of the current sense resistor, R , results in a considerable power loss which in- SENSE crease with the duty cycle. Resistor R2 and capacitor C3 form a low - pass filter to remove noise. Subharmonic Oscillation Subharmonic oscillation (SHM) is a problem found in Current-mode control systems, where instability re- sults when duty cycle exceeds 50%. SHM only occurs in switching regulators with a continuous inductor cur- rent. This instability is not harmful to the converter and usually does not affect the output voltage regula- tion. SHM will increase the radiated EM noise from the converter and can cause, under certain circum- stances, the inductor to emit high - frequency audile noise. SHM is an easily remedied problem. The rising slope of the inductor current is supplemented with internal “slope compensation” to prevent any duty cycle insta- bility from carrying through to the next switching cycle. In the GM3255, slope compensation is added during the entire switch on-time, typically in the amount of 180 mA/µs. Figure 13.Current Limiting using a Current Sense Resistor VC RSENSE Q1 VCC R1 VIN C2 C1 R2 C3 Output Ground PGND AGND + - I = SWITCH(PEAK) V BE(Q1) R ESENSE In some cases, SHM can rear its ugly head despite the presence of the onboard slope compensation. The simple cure to this problem is more slope compensa- tion avoid the unwanted oscillation. In that case, an ex- ternal circuit, shown in Figure 14, can be added to in- crease the amount of slope compensation used. This circuit requires only a few components and is "tacked on" to the compensation network. The dashed box contains the normal compensation circuitry to limit the bandwidth of the error amplifier. Resistors R2 and R3 form a voltage divider off of the V pin. In normal operation V looks similar to a SW SW square wave, and is dependent on the converter topol- ogy. Formulas for calculating V in the boost and flyback SW topologies are given in the section "V Voltage Limit." SW The voltage on V charges capacitor C3 when the SW switch is off, causing the voltage at the V pin to shift C upwards. When the switch turns on, C3 discharges through R3, producing a negative slope at the V pin. C The negative slope provides the slope compensation. The amount of slope compensation added by this cir- cuit is VSW Figure 14. Technique for Increasing Slope Compensation VC R1 C2 C1 R2 R3 VSW C3 DI DT = V ( ) (1 - e ) ( ) SW R3 R2 + R3 -(1 - D) RCf 33 SW f SW (1 - D) R A EV 11 |
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