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ISL6754 Datasheet(PDF) 11 Page - Intersil Corporation |
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ISL6754 Datasheet(HTML) 11 Page - Intersil Corporation |
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11 / 19 page  11 FN6754.1 September 29, 2008 The average current signal on IOUT remains accurate provided the output inductor current remains continuous (CCM operation). Once the inductor current becomes discontinuous (DCM operation), IOUT represents 1/2 the peak inductor current rather than the average current. This occurs because the sample and hold circuitry is active only during the on time of the switching cycle. It is unable to detect when the inductor current reaches zero during the off time. If average overcurrent limit is desired, IOUT may be used with the error amplifier of the ISL6754. Typically IOUT is divided down and filtered as required to achieve the desired amplitude. The resulting signal is input to the current error amplifier (IEA). The IEA is similar to the voltage EA found in most PWM controllers, except it cannot source current. Instead, VERR has a separate internal 1mA pull-up current source. Configure the IEA as an integrating (Type I) amplifier using the internal 0.6V reference. The voltage applied at FB is integrated against the 0.6V reference. The resulting signal, VERR, is applied to the PWM comparator where it is compared to the sawtooth voltage on RAMP. If FB is less than 0.6V, the IEA will be open loop (can’t source current), VERR will be at a level determined by the voltage loop, and the duty cycle is unaffected. As the output load increases, IOUT will increase, and the voltage applied to FB will increase until it reaches 0.6V. At this point the IEA will reduce VERR as required to maintain the output current at the level that corresponds to the 0.6V reference. When the output current again drops below the average current limit threshold, the IEA returns to an open loop condition, and the duty cycle is again controlled by the voltage loop. The average current control loop behaves much the same as the voltage control loop found in typical power supplies except it regulates current rather than voltage. The EA available on the ISL6754 may also be used as the voltage EA for the voltage feedback control loop rather than the current EA as described above. An external op-amp may be used as either the current or voltage EA providing the circuit is not allowed to source current into VERR. The external EA must only sink current, which may be accomplished by adding a diode in series with its output. The 4x gain of the sample and hold buffer allows a range of 150 - 1000mV peak on the CS signal, depending on the resistor divider placed on IOUT. The overall bandwidth of the average current loop is determined by the integrating current EA compensation and the divider on IOUT. The current EA cross-over frequency, assuming R6 >> (R4||R5), is: where fCO is the cross-over frequency. A capacitor in parallel with R4 may be used to provide a double-pole roll-off. The average current loop bandwidth is normally set to be much less than the switching frequency, typically less than 5kHz and often as slow as a few hundred hertz or less. This is especially useful if the application experiences large surges. The average current loop can be set to the steady state overcurrent threshold and have a time response that is longer than the required transient. The peak current limit can be set higher than the expected transient so that it does not interfere with the transient, but still protects for short-term larger faults. In essence a 2-stage overcurrent response is possible. The peak overcurrent behavior is similar to most other PWM controllers. If the peak current exceeds 1.0V, the active output pulse is terminated immediately. FIGURE 6. DYNAMIC BEHAVIOR OF CS vs IOUT CHANNEL 1 (YELLOW): OUTLL CHANNEL 3 (BLUE): CS CHANNEL 2 (RED): OUTLR CHANNEL 4 (GREEN): IOUT FIGURE 7. AVERAGE OVERCURRENT IMPLEMENTATION SS OUTUR OUTUL OUTLR OUTLL VDD VREF GND 1 2 4 3 5 6 7 8 19 20 11 12 13 14 15 16 N/C GND 9 10 17 18 150 - 1000 mV + - 0.6V S&H 4x R6 R5 R4 C10 CS FB IOUT VERR ISL6754 f CO 1 2 π R6 C10 ⋅⋅ ----------------------------------- = Hz (EQ. 6) ISL6754 |
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