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LCS705HG Datasheet(PDF) 5 Page - Power Integrations, Inc. |
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LCS705HG Datasheet(HTML) 5 Page - Power Integrations, Inc. |
5 / 26 page Rev. B 062011 5 LCS700-708 www.powerint.com HiperLCS Basic Operation The HiperLCS is designed for half-bridge LLC converters, which are high-efficiency resonant, variable frequency converters. The HiperLCS is an LLC controller chip with built-in drivers and half-bridge MOSFETs. LLC converters require a fixed dead-time between switching half-cycles. The dead-time, maximum frequency at start-up, and burst threshold frequencies, are programmed with a resistor divider on the DT/BF pin from the VREF to the GROUND pins. The FEEDBACK (FB) pin is the frequency control input for the feedback loop. Frequency is proportional to FEEDBACK pin current. The FEEDBACK pin V-I characteristic resembles a diode to ground. Burst Mode If the frequency commanded by the FEEDBACK pin current exceeds the upper burst threshold frequency (f STOP, ISTOP) programmed by the resistor divider on the DT/BF pin, the output MOSFETs will turn off, and will resume switching when the current drops below the value which corresponds to the frequency equal to the lower burst threshold frequency (f START, ISTART). As a first approximation, burst mode control resembles a hysteretic controller where the frequency ramps from f START to fSTOP, stops and repeats. An external component network connected from the VREF pin to the FEEDBACK pin determines the minimum and start-up FEEDBACK pin currents, and thus the minimum and start-up switching frequencies. A soft-start capacitor in this network determines soft-start timing. The VREF pin provides a nominal 3.4 V as a reference for this FEEDBACK pin external network and other functions. Maximum current from this pin must be ≤4 mA. The Dead-Time/Burst Frequency (DT/BF) pin also has a diode-to- ground V-I characteristic. A resistor divider from VREF to GROUND programs dead-time, maximum start-up switching frequency (f MAX), and the burst threshold frequencies. The current flowing from the resistor divider to the DT/BF pin determines f MAX. The ratio of the resistors selects from 3 discrete, burst threshold frequency ratios, which are fixed fractions of f MAX. The OV/UV pin senses the high-voltage B+ input through a resistor divider. It implements brown-in, brown-out, and OV with hysteresis. The ratios of these voltages are fixed; the user must select the resistor divider ratio such that the brown-in voltage is below the minimum nominal bulk (input) voltage regulation set-point to ensure start-up, and the OV (lower) restart voltage is above the maximum nominal bulk voltage set-point, to ensure that the LCS will restart after a voltage swell event that triggers the OV upper threshold. If different brown-in to brown-out to OV ratios are required, external circuitry needs to be added to the resistor divider. VCC Pin UVLO The VCC pin has an internal UVLO function with hysteresis. The HiperLCS will not start until the voltage exceeds the VCC start threshold V UVLO(+). HiperLCS will turn off when the VCC drops to the VCC Shutdown Threshold V UVLO(-). VCCH Pin UVLO The VCCH pin is the supply pin for the high-side driver. It also has a UVLO function similar to the VCC pin, with a threshold lower than the VCC pin. This is to allow for a VCCH voltage that is slightly lower than VCC because the VCCH pin is fed by a bootstrap diode and series current-limiting resistor from the VCC supply. Start-Up and Auto-Restart Before start-up the FEEDBACK pin is internally pulled up to the VREF pin to discharge the soft-start capacitor and to keep the output MOSFETs off. When start-up commences the internal pull-up transistor turns off, the soft-start capacitor charges, the outputs begin switching at f MAX, the FEEDBACK pin current diminishes, the switching frequency drops, and the PSU output rises. When the output reaches the voltage set-point, the optocoupler will conduct, closing the loop and regulating the output. Whenever the VCC pin is powered up, the DT/BF pin goes into high impedance mode for 500 ms in order to sense the voltage divider ratio and select the Burst Threshold. This setting is stored until the next VCC recycle. The DT/BF pin then goes into normal mode, resembling a diode to ground, and the sensed current continuously sets the f MAX frequency. The burst threshold frequencies are fixed fractions of f MAX. The internal oscillator runs the internal counters at f MAX whenever the FEEDBACK pin internal pull-up is on. When a fault is detected on the IS, OV/UV, or VCC pin (UVLO), the internal FEEDBACK pin pull-up transistor turns on for 131,072 clock cycles, to discharge the soft-start capacitor completely, then a restart is attempted. The first power-up after a VCC recycle only waits 1024 cycles, including the condition where the OV/UV pin rises above the brown-in voltage for the first time, after VCC is powered up. Remote-Off Remote-off can be invoked by pulling down the OV/UV pin to ground, or by pulling up the IS pin to >0.9 V. Both will invoke a 131,072 cycle restart cycle. VCC can also be pulled down to shut the device off, but when it is pulled up, the FEEDBACK pin is pulled up to the VREF pin to discharge the soft-start capacitor for only 1024 f MAX clock cycles. If this scheme is used, the designer must ensure that the time the VCC is pulled down, plus 1024 cycles, is sufficient to discharge the soft-start capacitor, or if not, that the resulting lower starting frequency is high enough so as not to cause excessive primary currents that may cause the over-current protection to trip. Current Sense The IS pin senses the primary current. It resembles a reverse diode to the GROUND pin. It is tolerant of negative voltages provided the negative current is limited to <5 mA. Therefore it must be connected to the current sense resistor (or primary capacitive voltage divider + sense resistor) via a series current limiting resistor of >220 W. Thus it can accept an AC waveform and does not need a rectifier or peak detector circuit. If the IS pin senses a nominal positive peak voltage of 0.5 V for 7 consecutive cycles, an auto-restart will be invoked. The IS pin also has a second, higher threshold at nominally 0.9 V, which will invoke an auto-restart with a single pulse. The minimum |
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