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LTC1541 Datasheet(PDF) 9 Page - Linear Technology |
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LTC1541 Datasheet(HTML) 9 Page - Linear Technology |
9 / 12 page 9 LTC1541/LTC1542 15412fd The LTC1541/LTC1542 are a combination of a micropower op amp, comparator and 1.2V ±1% reference (LTC1541) in an 8-pin package. The LTC1541 has the negative input of the comparator internally connected to the reference output. The supply voltage range is from 2.5V to 12.6V for single supply and ±1.25V to ±6.3V for dual supplies. The supply current is a mere 5 µA (typical) with a 5V single supply. Op Amp The op amp is internally compensated to be unity-gain stable, with typical GBW at 12kHz and slew rate of 8V/ms. The output can drive a capacitive load of up to 1000pF and swings from rail-to-rail. The input range is from the negative rail to within 1.3V of the positive rail. The input bias current is less than 1nA maximum at the extended temperature range. The output can source up to 2mA and sink up to 20 µA with a 5V supply. The output can drive a bypass capacitor of up to 0.01 µF without oscillation and by inserting a series resistor, capacitance values up to 100 µF can be used (Figure 1). Figure 2 shows the resistor value required for different capacitor values to achieve critical damping. Bypassing the reference can help prevent false tripping of the comparator by preventing glitches on VCC or reference load transients from disturbing the reference output volt- age. Figures 3a and 3b show the bypass reference output with a square wave applied to the VCC pin while resistor R1 damps the reference response. Note that the comparator output doesn’t trip. Op Amp Stability Unlike other industry standard micropower CMOS op amps, the op amp in the LTC1541/LTC1542 maintain stability in unity-gain configuration while driving heavy capacitive loads of up to 1000pF. Although this family is primarily designed for low frequency applications,goodlayoutisextremelyimportant.Lowpower, high impedance circuits may increase the effects of board leakage and stray capacitance. For example, the combina- tion of a 10M resistance (from leakage between traces on a contaminated, poorly designed PC board) and a 1pF stray capacitance provides a pole at approximately 16kHz, which is near the amplifier’s bandwidth. Board routing and layout should minimize leakage and stray capacitance. In some cases, stray capacitance may be unavoidable and it may be necessary to add a small capacitor across the feedback resistor to compensate (Figure 4); select the smallest capacitor value that ensures stability. Inputs The input common mode range for both the op amp and comparator is from the negative supply to within 1.3V of the positive supply. The inputs can be taken more than 300mV below the negative supply without damaging the device if the current out of the pin is limited to less than 1mA. Unlike bipolar input op amps and comparators, the outputs of the CMOS LTC1541/LTC1542 will not reverse phase when the inputs are taken above the common mode input range. Figure 1. Damping the Reference Output 8 4 LTC1541 1541/42 F01 6 REF VCC VSS R1 C1 ×1 Comparator The comparator has a high impedance differential input stage with a common mode input range from the negative rail to within 1.3V of the positive rail. The CMOS output stage can swing from rail-to-rail and source up to 20mA continuously. The output stage has been designed to eliminate the power supply glitches that normally occur when the output changes logic state. In addition, internal hysteresis ( ±2.25mV) ensures clean output switching even with slow moving input signals. The negative input is internally connected to the reference for the LTC1541. Reference The internal bandgap reference has an output voltage of 1.2V ±1% over the industrial grade temperature range. APPLICATIO S I FOR ATIO |
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