Datasheet LTC1541, LTC1542 (Analog Devices) - 9

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