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SI91871 Datasheet(PDF) 9 Page - Vishay Siliconix |
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SI91871 Datasheet(HTML) 9 Page - Vishay Siliconix |
9 / 10 page Si91871 Vishay Siliconix Document Number: 72012 S-51147—Rev. F, 20-Jun-05 www.vishay.com 9 DETAILED DESCRIPTION The Si91871 is a low-noise, low drop-out and low quiescent current linear voltage regulator, packaged in a small footprint MLP33-5 package. The Si91871 can supply loads up to 300 mA. As shown in the block diagram, the circuit consists of a bandgap reference error, amplifier, p-channel pass transistor and feedback resistor string. An external bypass capacitor connected to the BP pin reduces noise at the output. Additional blocks, not shown in the block diagram, include a precise current limiter, reverse battery and current protection and thermal sensor. Thermal Overload Protection The thermal overload protection limits the total power dissipation and protects the device from being damaged. When the junction temperature exceeds 150_C, the device turns the p-channel pass transistor off. Reverse Battery Protection The Si91871 has a battery reverse protection circuitry that disconnects the internal circuitry when VIN drops below the GND voltage. There is no current drawn in such an event. When the SD pin is hardwired to VIN, the user must connect the SD pin to VIN via a 100-kW resistor if reverse battery protection is desired. Hardwiring the SD pin directly to the VIN pin is allowed when reverse battery protection is not desired. Noise Reduction An external 10-nF bypass capacitor at BP is used to create a low pass filter for noise reduction. The start-up time is fast, since a power-on circuit pre-charges the bypass capacitor. After the power-up sequence the pre-charge circuit is switched to standby mode in order to save current. It is therefore not recommended to use larger bypass capacitor values than 50 nF. When the circuit is used without a capacitor, stable operation is guaranteed. Auto-Discharge The Si91871 VOUT has an internal 100-W (typ.) discharge path to ground when the SD pin is low. Stability The circuit is stable with only a small output capacitor equal to 6 nF/mA (= 2 mF @ 300 mA). Since the bandwidth of the error amplifier is around 1−3 MHz and the dominant pole is at the output node, the capacitor should be capacitive in this range, i.e., for 150-mA load current, an ESR <0.2 W is necessary. Parasitic inductance of about 10 nH can be tolerated. Safe Operating Area The ability of the Si91871 to supply current is ultimately dependent on the junction temperature of the pass device. Junction temperature is in turn dependent on power dissipation in the pass device, the thermal resistance of the package and the circuit board, and the ambient temperature. The power dissipation is defined as PD = (VIN – VOUT) * IOUT . Junction temperature is defined as TJ = TA + ((PD * (RθJC + RθCA)). To calculate the limits of performance, these equations must be rewritten. Allowable power dissipation is calculated using the equation PD = (TJ − TA )/ (RθJC + RθCA) While allowable output current is calculated using the equation IOUT = (TJ − TA )/ (RθJC + RθCA) * (VIN – VOUT). Ratings of the Si91871 that must be observed are TJmax = 125 _C, TAmax = 85 _C, (VIN – VOUT)max = 5.3 V, RθJC = 8 _C/W. The value of RθCA is dependent on the PC board used. The value of RθCA for the board used in device characterization is approximately 46 _C/W. Figure 1 shows the performance limits graphically for the Si91871 mounted on the circuit board used for thermal characterization. 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0 123 456 Figure 1. Safe Operating Area VIN − VOUT (V) (VIN − VOUT)MAX = 5.3 V TA = 50_C TA = 85_C TA = 70_C Vishay Siliconix maintains worldwide manufacturing capability. Products may be manufactured at one of several qualified locations. Reliability data for Silicon Technology and Package Reliability represent a composite of all qualified locations. For related documents such as package/tape drawings, part marking, and reliability data, see http://www.vishay.com/ppg?72012. |
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