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CS5651 Fiches technique(PDF) 6 Page - Cherry Semiconductor Corporation |
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CS5651 Fiches technique(HTML) 6 Page - Cherry Semiconductor Corporation |
6 / 8 page 6 outputs are disabled when the error amplifier output is at its lowest state (VOUT(LOW)). This occurs when the power supply is operating at light or no-load conditions, or at the beginning of a soft-start interval. The minimum allowable error amplifier feedback resis- tance is limited by the amplifier’s source current capability (0.5 mA) and the output voltage (VOUT(High)) required to reach the current sense comparator 1.0V clamp level with the error amplifier inverting input at ground. This condi- tion happens during initial system start up or when the sensed output is shorted: RF(min) ≈ = 8.8kΩ The CS5651 operates as a current mode controller. Output switch conduction is initiated by the oscillator and termi- nated when the peak inductor current reaches the thresh- old level established by the error amplifier output. The error signal controls the peak inductor current on a cycle- by-cycle basis. The current sense comparator-PWM Latch combination ensures that only a single pulse appears at the output during any given oscillator cycle. The current is converted to a voltage by connecting sense resistor RSense in series with the source of output switch Q1 and ground. This voltage is monitored via the Sense1,2 pins and com- pared to a voltage derived from the error amp output. The peak current under normal operating conditions is con- trolled by the voltage at COMP where: Ipk = Abnormal operating conditions occur when the power supply output is overloaded or if output voltage is too high. Under these conditions, the current sense comparator threshold will be internally clamped to 1.0V. Therefore the maximum peak switch current is: Ipk(max) = Erratic operation due to noise pickup can result if there is an excessive reduction of the Ipk(max) clamp voltage. A narrow spike on the leading edge of the current wave- form can usually be observed and may cause the power supply to exhibit an instability when the output is lightly loaded. The addition of an RC filter on the current sense input reduces this spike to an acceptable level. Two undervoltage lockout comparators have been incor- porated to guarantee that the IC is fully functional before the output stages are enabled. VCC and the reference out- put VREF are monitored by separate comparators. Each comparator has built-in hysteresis to prevent erratic output behavior as their respective thresholds are crossed. The VCC comparator upper and lower thresholds are 14V and 10V for the CS5651. The VREF comparator disables the out- puts until the internal circuitry is functional. This compara- tor has upper and lower thresholds of 3.6V and 3.4V. The guaranteed minimum operating voltage after turn-on is 11V for CS5651. Each channel contains a single totem-pole output stage specifically designed for driving a power MOSFET. The outputs have up to ±1.0A peak current capability and have a typical rise and fall time of 28ns with a 1.0nF load. Internal circuitry has been added to keep the outputs in active pull-down mode whenever undervoltage lockout is active. An external pull-down resistor is not needed. Cross-conduction current in the totem-pole output stage has been minimized for high speed operation. The average added power due to cross-conduction with VCC = 15V is only 60mW at 500kHz. Although the outputs were optimized for MOSFET’s, they can easily supply the negative base current required by bipolar NPN transistors for enhanced turn-off. Because the outputs do not contain internal current limiting circuitry, an external series resistor may be required to prevent the peak output current from exceeding the ±1.0A maximum rating. The sink saturation voltage (VOL) is less than 0.4V at 100mA. A separate Power Ground pin is provided and will signifi- cantly reduce the level of switching transient noise imposed on the control circuitry. This becomes particularly important when the Ipk(max) clamp level is reduced. This input is used to switch VOUT 2. VOUT1 can be used to control circuitry that runs continuously; e.g. volatile mem- ory, the system clock, or a remote controlled receiver. The VOUT 2 output can control the high power circuitry that can be turned off when not needed. The 5.0V bandgap reference is trimmed to ±2.0% tolerance. The reference has short circuit protection and is capable of sourcing 30mA for powering any additional external cir- cuitry. High frequency circuit layout techniques are imperative to prevent pulse-width jitter. This is usually caused by exces- sive noise pick-up imposed on the current sense or voltage feed-back inputs. Noise immunity can be improved by lowering circuit impedances at these points. The printed circuit board layout should contain a ground plane with low current signal and high current switch and output grounds returning on separate paths back to the input fil- 1.0V RSense VCOMP – 1.4V 3RSense (3 x 1.0V) + 1.4V 0.5mA Operating Description: continued Outputs and Power Ground ENABLE2 Voltage Reference Undervoltage Lockout Current Sense Comparator and PWM Latch Design Considerations |
Numéro de pièce similaire - CS5651 |
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Description similaire - CS5651 |
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