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RT8206M Datasheet(PDF) 21 Page - Richtek Technology Corporation |
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RT8206M Datasheet(HTML) 21 Page - Richtek Technology Corporation |
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21 / 27 page  RT8206L/M 21 DS8206L/M-07 June 2012 www.richtek.com © Copyright 2012 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation. Reference and Linear Regulator (REF, LDO and 14V Charge Pump) The 2V reference (REF) is accurate within ±1% over the entire temperature range, making REF useful as a precision system reference. Bypass REF to GND with as 0.22 μF(MIN) capacitor. REF can supply up to 50 μA for external loads. Loading REF degrades FBxand output accuracy according to the REF load-regulation error. An internal regulator produces a fixed output voltage 5V. The LDO regulator can supply up to 70mA for external loads. Bypass LDO with a minimum 4.7 μF ceramic capacitor. When the output voltage of the VOUT1 is higher than the switchover threshold, an internal 1.5 Ω N-Channel MOSFET switch connects VOUT1 to LDO through BYP while simultaneously shutting down the internal linear regulator. In typical application circuit, the external 14V charge pump is driven by LGATE1. When LGATE1 is low, D1 charges C5 sourced from VOUT1. C5 voltage is equal to VOUT1 minus a diode drop. When LGATE1 transitions to high, the charge from C5 will transfer to C6 through D2 and charge it to VLGATE1 plus VC5. As LGATE1 transients low on the next cycle, C6 will charge C7 to its voltage minus a diode drop through D3. Finally, C7 charges C8 through D4 when LGATE1 transitions to high. CP output voltage is : VCP = VOUT1 +2 x VLGATE1 − 4 x VD where : VLGATE1 is the peak voltage of the LGATE1 driver VD is the forward diode dropped across the Schottkys SECFB (RT8206L) is used to monitor the charge pump via the resistive divider. In an event when SECFB drops below 2V the controller forces ultrasonic mode operation which keeps the switching frequency above 25kHz to maintain charge pump voltage. Reducing the CP decoupling capacitor and placing a small ceramic capacitor C19 (10pF to 47pF) in parallel will the upper leg of the SECFB resistor feedback network, R11, will also increase the robustness of the charge pump. Current Limit Setting (ILIMx) The RT8206L/M has a cycle-by-cycle current limiting control. The current limit circuit employs a unique “valley” current sensing algorithm. If the magnitude of the current sense signal at PHASEx is above the current limit threshold, the PWM is not allowed to initiate a new cycle (Figure 4). The actual peak current is greater than the current limit threshold by an amount equal to the inductor ripple current. Therefore, the exact current limit characteristic and maximum load capability are a function of the sense resistance, inductor value, battery voltage, and output voltage. IL t 0 IL, peak ILIM ILoad Figure 4. Valley Current Limit The RT8206L/M uses the on-resistance of the synchronous rectifier as the current-sense element. Use the worse- case maximum value for RDS(ON) from the MOSFET datasheet, and add a margin of 0.5%/ °C for the rise in RDS(ON) with temperature. The current limit threshold is adjusted with an external resistor for the RT8206L/M at ILIMx. The current limit threshold adjustment range is from 50mV to 200mV. In the adjustment mode, the current limit threshold voltage is precise to 1/10 the voltage seen at ILIMx. The threshold defaults to 100mV when ILIMx is connected to VCC. The logic threshold for switchover to the 100mV default value is higher than VCC −1V. Carefully observe the PC board layout guidelines to ensure that noise and DC errors do not corrupt the current sense signal at PHASEx and GND. Mount or place the IC close to the low side MOSFET. MOSFET Gate Driver (UGATEx, LGATEx) The high side driver is designed to drive high current, low RDS(ON) N-MOSFET(s). When configured as a floating driver the instantaneous drive current is supplied by the flying |
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