Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.
The LM4132 family of precision voltage references can deliver up to 20 mA without an output capacitor or buffer amplifier. The LM4132 is ideal for battery-powered solutions, with a low quiescent current of 60 µA, and a low dropout voltage of 400 mV. The LM4132 enters the shutdown mode (3 µA, typical) when EN is 0 V.
For this design example, use the parameters listed as the input parameters.
The foundation of any voltage reference is the band-gap circuit. While the reference in the LM4132 is developed from the gate-source voltage of transistors in the device, principles of the band-gap circuit are easily understood using a bipolar example. For a detailed analysis of the bipolar band-gap circuit, refer to AN-56 LM113 1.2V Reference (SNVA514).
To ensure proper operation, VEN and VIN must be within a specified range. An acceptable range of input voltages is calculated by Equation 2:
The EN pin uses an internal pullup current source (IPULLUP ≊ 2 µA) that may be left floating or triggered by an external source. If the device is not enabled by an external source, it may be connected to VIN. An acceptable range of enable voltages is given by Figure 4. See Electrical Characteristics LM4132-1.8 (VOUT = 1.8 V) and Figure 3 for more detail. The device does not operate correctly for VEN > VIN.
A small ceramic (X5R or X7R) capacitor on the input must be used to ensure stable operation. The value of CIN must be sized according to the output capacitor value. The value of CIN must satisfy the relationship CIN ≥ COUT. When no output capacitor is used, CIN must have a minimum value of 0.1 µF. Noise on the power-supply input may affect the output noise. Larger input capacitor values (typically 4.7 µF to 22 µF) may help reduce noise on the output and significantly reduce overshoot during start-up. Use of an additional optional bypass capacitor from the input and ground may help further reduce noise on the output. With an input capacitor, the LM4132 drives any combination of resistance and capacitance up to VREF / 20 mA and 10 µF, respectively.
The LM4132 is designed to operate with or without an output capacitor and is stable with capacitive loads up to 10 µF. Connecting a capacitor from the output and ground significantly improves the load transient response when switching from a light load to a heavy load. The output capacitor must not be made arbitrarily large because capacitor selection affects the turnon time as well as line and load transients.
While a variety of capacitor chemistry types may be used, it is typically advisable to use low equivalent series resistance (ESR) ceramic capacitors. Such capacitors provide a low impedance to high frequency signals, effectively bypassing them to ground. Bypass capacitors must be mounted close to the device. Mounting bypass capacitors close to the device helps reduce the parasitic trace components thereby improving performance.
Temperature drift is defined as the maximum deviation in output voltage over the operating temperature range. This deviation over temperature may be shown in Figure 48:
Temperature coefficient may be expressed analytically as Equation 3:
Long-term stability refers to the fluctuation in output voltage over a long period of time (1000 hours). The LM4132 features a typical long-term stability of 50 ppm over 1000 hours. The measurements are made using 5 units of each voltage option, at a nominal input voltage (5 V), with no load, at room temperature.
Electrical characteristics are typically expressed in mV, ppm, or a percentage of the nominal value. Depending on the application, one expression may be more useful than the other. To convert one quantity to the other one may apply the following:
ppm to mV error in output voltage:
Bit error (1 bit) to voltage error (mV):
mV to ppm error in output voltage:
Voltage error (mV) to percentage error (percent):