LDO(LT3080)

Datasheet

Goal of Application: Drop from 4.2 V to a consistent 3.3 V. Also known as a DC-to-DC step-down converter.

Specific Part: LT3080EDD#TRPBF

https://www.digikey.com/short/hdhdfn83

Notes: DD Package, 8-Lead DFN (3mm x 3mm)

Cost: $6.40

Typical Application

3.3V Application

$$V_{out} = R_{set} \cdot (10 \times 10^{-6})$$

$$R_{set} = V_{out} \cdot 10 \times 10^{6}$$

$$R_{set} = (3.3) \cdot 10^5 = 330 \text{ k} \Omega$$

Application of LDO in LTSpice.

What is an LDO?

  • A Low Dropout Regulator(LDO) is a voltage regulator that can get a regulated low output voltage from a higher input voltage.
    • The output is steady, low noise, DC.
    • This is compared to a Linear Voltage Regulator, which requires a higher voltage drop to function as designed.

LDO Parameters

Quiescent Current (IQ) - Current drawn by system when light or no load is connected

Power Supply Rejection Ratio (PSRR) - Ability to reject AC elements of source(e.g. ripple voltage). The datasheet of the LT3080 simply calls it "Ripple Rejection".

Defined by: \(20\log(\frac{V_{ripple, in}}{V_{ripple,out}})\)

Ex: Let \(V_{ripple, in}=2 \text V\) & \(V_{ripple, out}=0.2 \text V\)

$$\text{PSRR} \vert_{dB} = 20\log \left(\frac{V_{ripple, in}}{V_{ripple,out}} \right) = 20 \log \left(\frac{2}{0.2} \right) = 20 \text{ dB}$$

Ex: Let \(V_{ripple, in}=2 \text V\) & \(V_{ripple, out}= 2 \text {mV}\)

$$\text{PSRR} \vert_{dB} = 20\log \left(\frac{V_{ripple, in}}{V_{ripple,out}} \right) = 20 \log \left(\frac{2}{2\times10^{-3}} \right) = 60 \text{ dB}$$

Note: For the PSRR relating to our current application of the part, refer to the "Ripple Rejection, Single Supply" graph. If the \(V_{IN}\) and \(V_{CONTROL}\) pins are powered by different sources, then refer to the "Ripple Rejection, Dual Supply" graphs.

Load Regulation - Ability to maintain specified output voltage under varying load conditions

\(\text{Load Regulation} = \frac{\Delta V_{out}}{\Delta I_{out}}\)

Applications Information

  • For minimizing board leakage: recommends encircling the SET pin and circuitry with a guard ring operated at a potential close to itself, tied to the OUT pin.

  • \(V_{IN}\) must be more than 0.1V to 0.5V greater than the output voltage. This depends on the current that needs to flow through the load. For more information, refer to pages 4 and 8 of the datasheet.

  • There must be a minimum load current of 1 mA

OUTPUT CAPACITOR 2.2u

  • Capacitor must have an ESR(Equivalent Series Resistance) of 0.5 Ohms or less
    • How to find the ESR of a capacitor is explained here
  • Materials with a low ESR include: Ceramic, Tantalum, or low ESR electrolytic

A Note on Ceramic Capacitors

  • Ceramic capacitors are manufactured with a variety of dielectrics, each with different behavior across temperature and applied voltage
  • Most common dielectrics are specified with EIA temperature characteristic codes of: Z5U, Y5V, X5R, X7R
  • Z5U, Y5V dielectrics are good for providing high capacitances in a small package, but tend to have strong voltage and temp coefficients
  • X7R or X5R are better to use, but be aware of their temperature characteristics
  • Try not to find a ceramic capacitor with a piezoelectric response. (It's no requirement, but MIL-STD-202 testing is vibration testing.)

Additional Purchased Components

330K Resistor

ERA-3AEB334V

https://www.digikey.com/short/nzfdzw40

Notes: 0.1% Tolerance, 0.1 W Power Rating(This is fine)

Cost: $0.35

1u Capacitor

C0603C105K8PAC7867

https://www.digikey.com/short/z8pd0t38

Notes: Tolerance: 10%, Temperature Coefficient: X5R, Rated Voltage: 10V, Package: 0603, ESR: 0.008 Ohms < 0.5 (Source: LTSpice). Normally stocking, RoHS compliant, has datasheet. Has gone through MIL–STD–202 testing. Optionally, you could buy another as a bypass capacitor of the 330k resistor. The LDO’s datasheet claims that would lead to “improvement in transient performance” and reduction of system noise.

Cost: $0.10

2.2u Capacitor

C0603C225K8PAC7867

https://www.digikey.com/short/4v45dvz4

*Notes: Tolerance: 10%, Temperature Coefficient: X5R, Rated Voltage: 10V, Package: 0603, ESR: 0.017 Ohms < 0.5 (Source: LTSpice). Normally stocking, RoHS compliant, has datasheet. Has gone through MIL–STD–202 testing.

Cost: $0.23