Design Goals

Design Description

This design accurately steps down a voltage level and holds it stable at a fixed output voltage (low dropout regulator). The regulator takes a –4.5V to –5.5V input voltage and steps it down to a –3V rail that supplies current up to 600mA.

Design Notes

1. Use the op amp in a linear operating region. Verify that the inputs of the op amp do not exceed the common-mode range of the device. Linear output swing is usually specified under the A_OL test conditions.
2. The common-mode voltage is equal to the inverting input voltage, set by the TLV431 reference of –1.24V.
3. Using a high-gain BJT reduces the output current requirement for the op amp.
4. The majority of the power loss is |V_ee – V_out| × I_out and is dissipated in transistor T₁. A larger V_ee increases power loss and the temperature of T₁.
5. Other op amps can be used in place of the TLV9002, but can require adjustment of the feedback stabilization.
6. Positive feedback to the amplifier is used, because an inversion is performed by T₁.

Design Steps

The transfer function of the circuit is:

1. Based on the desired output voltage, in this case -3V, select a ratio of R₁ and R₂ that satisfies the above equation.
   $$\begin{gathered} -\mathrm{3V}=-1.24\mathrm{V}\times \frac{R_1+R_2}{R_2} \\ 1.419\times R_2=R_1 \end{gathered}$$

   Selecting standard resistors, select 1.02kΩ and 715Ω.

   
   
2. For sizing the output capacitance, the product of C₂ and R_esr must generate a zero below 10kHz to verify stability. The ESR zero is located at:
   $F_{z\left(ESR\right)}=\frac{1}{2{\mathrm{\pi R}}_{\mathrm{esr}}{C}_2}=\frac{1}{2\mathrm{\pi }\left(\mathrm{1\Omega }\right)(47\times {10}^{-6}F)}=\mathrm{3.38kHz}$

Design Results

DC Analysis Results

Vee Sweep

Output Current Sweep

Dropout Voltage

AC Analysis Results

Bode 60mA

Bode 600mA

Transient Analysis Results

Transient Response (100mA Step)

Design References

See the Analog Engineer’s Circuit Cookbooks for TI’s comprehensive circuit library.

AN-1482 LDO Regulator Stability Using Ceramic Output Capacitors

Space-Grade, 100-krad, –2.5V, Discrete Negative LDO Linear Regulator Circuit

For more information on many op amp topics including common-mode range, output swing, and bandwidth, please visit TI Precision Labs.

Spice Simulation File

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