Many applications require a simple method for getting multiple outputs from the power supply. One way to achieve this goal is by using charge pumps.
The schematic shown in Figure 1 is an LCD bias supply using an LT1372 high-efficiency switching regulator to produce 10 V at 200 mA, −10 V at 5 mA, and 22 V at 5 mA from a 5-V input. Both the −10-V and 22-V outputs were created using charge pumps from the switch node of the LT1372.
Because a charge pump only produces a multiple of its input voltage, post regulation is needed to step the output down to the desired regulated voltage. A Zener diode offers a simple way to accomplish this task. However, one of the drawbacks of using a Zener diode as a shunt regulator is the temperature drift of the resulting output. This is due to the relatively high positive temperature coefficient of Zener diodes.
The temperature drift of the 0.5-W 18-V Zener diode (1N5248A) measured from room temperature to 120°C is about 1.3 Volts. The output voltage rises with increasing temperature. This represents a 6% error in output due to temperature changes.
The circuit shown uses a bipolar transistor’s negative temperature coefficient to compensate this unwanted characteristic of the Zener diode. The voltage across a transistor emitter-base junction changes by −2.2 mV for each 1°C rise. A VBE multiplier is used to scale up the transistor’s temperature coefficient (TC) to match that of the Zener. The temperature coefficient of VCE scales per Equation 1. Adding the two temperature coefficients together results in a near zero TC if the correct components are selected.
The circuit in Figure 1 (22-V output) shows the Zener shunt regulator with a VBE multiplier. The values of the two resistors are calculated by using the following formulas:
DVC = DVBE(R5 + R6)/R6 (1)
where (R5 + R6)/R6 = TCVZ/TCVBE; TCVZ = temperature coefficient of the Zener diode (16 mV/°C for an 18-Volt Zener); and TCVBE = temperature coefficient for VBE (−2.2 mV/°C).
In addition, the desired output voltage of the Zener regulator is calculated by using:
VOUT(desired) = VC + VZENER
A suitable combination can usually be found within a couple of iterations. Figure 2 shows the actual temperature test plot comparing an 18-Volt Zener versus a 15-Volt Zener plus the VBE multiplier.
As can Figure 2 demonstrates, the total output drift from room temperature to 120°C is about 230 mV for the temperature-compensated circuit, so the VBE multiplier created almost a factor of six improvement in the output voltage versus temperature performance when compared to the uncompensated circuit.