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L6569 Datasheet(PDF) 5 Page - STMicroelectronics |
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L6569 Datasheet(HTML) 5 Page - STMicroelectronics |
5 / 13 page 5/13 L6569 L6569A Bootstrap Function The L6569 has an internal Bootstrap structure that enables the user to avoid the external diode needed, in sim- ilar devices, to perform the charge of the bootstrap capacitor that, in turns, provide an appropriate driving to the Upper External Mosfet. The operation is achieved with an unique structure (patented) that uses a High Voltage Lateral DMOS driven by an internal charge pump (see Block Diagram) and synchronized, with a 50 nsec delay, with the Low Side Gate driver (LVG pin), actually working as a synchronous rectifier . The charging path for the Bootstrap capacitor is closed via the Lower External Mosfet that is driven ON (i.e. LVG High) for a time interval: TC = RF · CF · In2 → 1.1 · RF · CF starting from the time the Supply Voltage VS has reached the Turn On Voltage (VSUVP = 9 V typical value). After time T1 (see waveform Diagram) the LDMOS that charges the Bootstrap Capacitor, is on with a RON=120Ω (typical value). In the L6569A a different start up procedure is followed (see waveform Diagram). The Lower External Mosfet is drive OFF until VS has reached the Turn On Threshold (VSUVPp), then again the TC time interval starts as above. Being the LDMOS used to implement the bootstrap operation a "bi-directional" switch the current flowing into the BOOT pin (pin 8) can lead an undue stress to the LDMOS itself if a ZERO VOLTAGE SWITCHING opera- tions is not ensured, and then an high voltage is applied to the BOOT pin. This condition can occur, for example, when the load is removed and an high resistive value is placed in series with the gate of the external Power Mos. To help the user to secure his design a SAFE OPERATING AREA for the Bootstrap LDMOS is provided (fig. 7). Let's consider the steps that should be taken. 1) Calculate the Turn on delay ( td ) of your Lower Power MOS: 2) Calculate the Fall time ( tf ) of your Lower Power MOS: where: Rg = External gate resistor Rid = 50Ω, typical equivalent output resistance of the driving buffer (when sourcing current) VTH, Ciss and Qgd are Power MOS parameters VS = Low Voltage Supply. 3) Sketch the VBOOT waveform (using log-log scales) starting from the Drain Voltage of the Lower Power MOS (remember to add the Vs, your Low Voltage Supply, value) on the Bootstrap LDMOS SOA . On fig. 8 an example is given where: VS = Low Voltage Supply VHV = High Voltage Supply Rail The VBOOT voltage swing must fall below the curve identified by the actual operating frequency of your applica- tion. t d R g R id + () C iss 1 1 V TH V S ----------- – -------------------- ln ⋅⋅ = t f R g R id + V S V TH – ------------------------ Q gd ⋅ = |
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