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SC2616 Datasheet(PDF) 10 Page - Semtech Corporation |
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SC2616 Datasheet(HTML) 10 Page - Semtech Corporation |
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10 / 16 page  10 © 2003 Semtech Corp. www.semtech.com POWER MANAGEMENT SC2616 Compensation Components Applications Information (Cont.) The control model of SC2616 can be depicted in Fig. 1. This model can also be used in Spice kind of simulator to generate loop gain Bode plots. The bandgap reference is 1.25 V and trimmed to +/-1% accuracy. The desired output voltage can be achieved by setting the resistive divider network, R1 and R2. The error amplifier is transconductance type with fixed gain of: The compensation network includes a resistor and a ca- pacitor in series, which terminates from the output of the error amplifier to the ground. This device uses voltage mode control with input voltage feed forward. The peak-to-peak ramp voltage is propor- tional to the input voltage, which results in an excellent performance to reject input voltage variation. The PWM gain is inversion of the ramp amplitude, and this gain is given by: where the ramp amplitude (peak-to-peak) is 0.55 volts when input voltage is 5 volts. The total control loop-gain can then be derived as follows: Ts () T o 1s R . C . sR . C . . 1s R c . C o . 1s R c C o . L R o . s 2 L . C o . 1 R c R o . . where The task here is to properly choose the compensation network for a nicely shaped loop-gain Bode plot. The following design procedures are recommended to accom- plish the goal: (1) Calculate the corner frequency of the output filter: (2) Calculate the ESR zero frequency of the output filter capacitor: (3) Check that the ESR zero frequency is not too high. If this condition is not met, the compensation structure may not provide loop stability. The solution is to add some electrolytic capacitors to the output capacitor bank to correct the output filter corner frequency and the ESR zero frequency. In some cases, the filter inductance may also need to be adjusted to shift the filter corner frequency. It is not recommended to use only high fre- quency multi-layer ceramic capacitors for output filter. (4) Choose the loop gain cross over frequency (0 dB fre- quency). It is recommended that the crossover frequency is always less than one fifth of the switching frequency : If the transient specification is not stringent, it is better to choose a crossover frequency that is less than one tenth of the switching frequency for good noise immunity. The resistor in the compensation network can then be calculated as: when T o Gm Gpwm ⋅ V in ⋅ R ⋅ V bg V o ⎛ ⎜ ⎝ ⎞ ⎠ ⋅ := G m 0.0008A ⋅ V := G pwm 1 V ramp F o 1 2 π ⋅ LC o ⋅ ⋅ := F esr 1 2 π ⋅ R c ⋅ C o ⋅ := F x_over F sw 5 ≤ Fig. 1. SC2616 control model. R 1 G pwm Vin ⋅ G m ⋅ F esr F o ⎛ ⎜ ⎝ ⎞ ⎠ 2 ⋅ F x_over F esr ⎛ ⎜ ⎝ ⎞ ⎠ ⋅ V o V bg ⎛ ⎜ ⎝ ⎞ ⎠ ⋅ := C Vbg 1.25Vdc L Rc R Co Gpwm EA Ro R2 Vin R1 F esr F sw 5 < F o F esr < F x_over < |
Similar Part No. - SC2616_03 |
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Similar Description - SC2616_03 |
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