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SC2516MLTRT Datasheet(PDF) 10 Page - Semtech Corporation |
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SC2516MLTRT Datasheet(HTML) 10 Page - Semtech Corporation |
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10 / 15 page  10 © 2005 Semtech Corp. www.semtech.com POWER MANAGEMENT SC2516 Applications Information (Cont.) Step 1. Output filter corner frequency F o = 1.6 KHz Step 2. ESR zero frequency: F esr = 3.537 KHz Step 3. Check the following condition: Which is satisfied in this case. Step 4. Choose crossover frequency and calculate compensator R: F x_over = 50 KHz R = 15 K Ω Step 5. Calculate the compensator C: C = 33 nF Step 6. Generate Bode plot and check the phase margin. In this case, the phase margin is about 85oC that en- sures the loop stability. Fig. 2 shows the Bode plot of the loop. or R 1 G pwm V in ⋅ G m ⋅ F o F esr ⎛ ⎜ ⎝ ⎞ ⎠ 2 ⋅ F x_over F o ⎛ ⎜ ⎝ ⎞ ⎠ ⋅ V o V bg ⎛ ⎜ ⎝ ⎞ ⎠ ⋅ := when (5) The compensation capacitor is determined by choos- ing the compensator zero to be about one fifth of the output filter corner frequency: (6) The final step is to generate the Bode plot, either by using the simulation model in Fig. 1 or using the equa- tions provided here with Mathcad. The phase margin can then be checked using the Bode plot. Usually, this design procedure ensures a healthy phase margin. (7) An additional capacitor should be reserved at the compensation pin to ground to have another high fre- quency pole. An example is given below to demonstrate the proce- dure introduced above. The parameters of the power supply (typical for VDDQ section) are given as : F esr F sw 5 < F esr F o < F x_over < F zero F o 5 C 1 2 π . R . F zero . Vin 5V ⋅ := Vo 2.5 V ⋅ := Io 20 A ⋅ := Fsw 250 KHz ⋅ := L 2.2 µH ⋅ := Co 4500 µF ⋅ := Rc 0.01 Ω ⋅ := Vbg 1.25 V ⋅ := Vramp 0.55 V ⋅ := Gm 0.001 A ⋅ V := Compensation design of the GMCH Channel Compensation design of the GMCH Channel Compensation design of the GMCH Channel Compensation design of the GMCH Channel Compensation design of the GMCH Channel The configuration of the PWM comparator of GMCH channel is such that its inverter input is connected to Comp_GMCH and the non-inverter input is connected to the internal ramp. The peak voltage of the internal ramp is 1.1V and the valley voltage is 0.55V. When COMP_GMCH voltage is below 0.55V, the maximum duty cycle will be generated by PWM comparator. If COMP_GMCH voltage is over 1.1V then the minimum duty cycle will be generated. To ensure proper soft start function of the GMCH channel, COMP_GMCH voltage must rise above 1.1V at the beginning of soft start period quickly. So a higher compensation gain is required. The following example shows that by choosing the compensation parameters as 15kOhm and 27nF for a typical output filter with 1~2uH inductor and 2000uF capacitor (ESR of 8~12 mOhm), the circuit will yield smooth soft start, stable control loop, and satisfactory transient response. The measured Bode plot of the loop gain is shown in Figure 3. |
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