EDN Access--01.02.97 Regulator generates sub-bandgap voltage
|Design IdeasJanuary 2, 1997|
Regulator generates sub-bandgap voltages
Ronald Young and Luong Huynh, Maxim Integrated Products, Sunnyvale, CA
Figure 1 shows a 100-mA regulator that uses a dual linear-regulator IC to produce output voltages below 1.25V. For example, it can provide the 1V output required by pager receivers and other single-cell equipment. The input-voltage range is 2.9 to 5.5V. One side of the dual regulator provides a preset output of 2.84V; the other side is configured as an adjustable regulator whose output depends on RB and RF. The preset output injects current via RB into the adjustable regulator's SET2 terminal. The voltage at SET2 is 1.25V. SET2 acts like the summing junction of an op amp, whose output (OUT2) assumes the voltage necessary to balance the currents at the summing junction.
Unlike an op amp, though, the OUT2 terminal cannot sink current. To ensure proper operation, therefore, RL must draw a current greater than the one injected in SET2. First, choose RB to inject 5 m A: RB=(2.84–1.25)V/5 m A;316 k[ohms]. Then RF=(1.25–1)V/5 m A=50 k[ohms]. RL must be lower than 1V/5 m A=200 k[ohms], so choose 180 k[ohms]. The input and output capacitances are as specified in the data sheet. Choose CF=100 pF to aid stability. The resulting circuit generates a regulated 1V, with load currents to 100 mA and load regulation similar to that of standard configurations described in the data sheet. (DI #1973)
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