Unit-3-LDO- Notes IoT Design- IS3202.pptx

Electronics

1. Unit-3 System design for low power: LDO, DC-DC converters, power software.

2. Low Dropout Regulators (LDO)

3. Real life scenerios Powering image sensors : A DSLR camera manufacturer uses LDOs to power their image sensors because of the low noise required to process sensor signals. Powering infrared sensors : A thermal camera manufacturer uses LDOs to power their infrared sensors. as a lithium-ion battery drops from 4.2 V (fully charged) to 2.7 V (almost discharged), an LDO can maintain a constant 2.5 V at the load.

4. Low Dropout Regulators (LDO) Low dropout regulators (LDOs) are inexpensive way to regulate an output voltage that is powered from a higher voltage input. There are two types of linear regulators: standard linear regulators and low dropout linear regulators (LDOs). The difference between the two is in the pass element and the amount of dropout voltage, required to maintain a regulated output voltage. The dropout voltage is the minimum voltage required across the regulator to maintain regulation. Pass element : component that controls output voltage by reducing the input voltage .  It's a transistor that's in series with the load.   A 3.3 V regulator that has 1 V of dropout requires the input voltage to be at least 4.3 V.

5. Darlington transistor This circuit is a two-stage transistor amplifier , specifically a Darlington pair amplifier or cascade amplifier . It is designed to provide high gain and is commonly used in signal amplification applications

6. Darlington transistor Applications: Used in audio amplifiers for signal boosting. Common in sensor circuits to amplify weak signals. Found in RF and communication systems .

7. Understanding LDO This figure illustrates two different pass element configurations used in Low Dropout (LDO) voltage regulators : NPN Darlington Pass Element (Top Diagram) N-Channel FET Pass Element (Bottom Diagram)

8. Understanding LDO

9. Standard linear regulators have voltage drops as high as 2 V which are acceptable for applications with large input-to-output voltage difference such as generating 2.5 V from a 5 V input. A typical application such as generating 3.3 V from a 3.6 V Li-Ion battery requires a much lower dropout voltage (less than 0.3V/300 mV). These applications require the use of an LDO to achieve the lower dropout voltage. Most LDOs use an N-channel or P-channel FET pass element and can have dropout voltages less than 100 mV.

10. Fig. 2 shows an LDO block diagram. The input voltage is applied to a pass element, which is an N-channel or P-channel FET, or NPN or PNP transistor. 1. Input Voltage ( ​) The LDO takes an input voltage of 5V . This is supplied through a power source (shown as a voltage source and capacitor). 2. Pass Element The Pass Element (typically a MOSFET or bipolar transistor) regulates the voltage drop between VINV_{IN}VIN​ and VOUTV_{OUT}VOUT​. It controls how much current passes through to the output. 3. Error Amplifier The Error Amplifier compares the feedback voltage (taken from the output) with a stable reference voltage (VREFV_{REF}VREF​). If there’s a deviation from the expected voltage (3.3V), it adjusts the Gate Drive signal to correct it.  

11. 4. Gate Drive The Gate Drive controls the Pass Element by adjusting its conductivity. This ensures that the output remains stable despite changes in input voltage or load current. 5. Feedback Resistors A resistor divider network is used to scale down VOUTV_{OUT}VOUT​ for comparison with VREFV_{REF}VREF​. The Error Amplifier uses this feedback to maintain the desired output voltage. 6. Output Voltage (VOUTV_{OUT}VOUT​) The output voltage is regulated at 3.3V . A capacitor is placed at the output to filter any noise and improve stability. The regulated voltage is supplied to the load .

12. The error amplifier drives the pass element’s gate to the appropriate operating point to ensure that the output is at the correct voltage. As the input voltage changes, the error amplifier modulates the pass element to maintain a constant output voltage.

13. Why LDOs are Used in IoT? Stable Power Supply : Ensures constant voltage for IoT devices like microcontrollers (MCUs), sensors, and wireless modules . Low Noise : Unlike switching regulators, LDOs do not introduce high-frequency noise , making them ideal for RF circuits, sensors, and analog components . Small Size & Simplicity : Requires few external components (just capacitors), making them perfect for compact IoT devices . Battery Efficiency : Helps regulate voltage from Li-ion, NiMH, or other battery sources efficiently

14. Dropout Voltage Condition V in(min)= V out +V dropout If V in falls below this, V out will not be regulated properly = 0.3

15. LDO Equations (1) Power Loss in LDO P loss =(V in − V out )× I load power loss, causing heat dissipation. (2) LDO Efficiency η= V out ×I load / V in ×I load ×100 If V in is much greater than V out , efficiency drops significantly. (3) Thermal Power Dissipation T junction = T ambient +( P loss ×R θJA ) R θJA = Junction-to-ambient thermal resistance (°C/W).

16. LDO Numerical Problems Problem 1: Power Loss in an IoT Sensor An IoT temperature sensor requires 3.3V @ 50mA and is powered by a 5V battery using an LDO. Find: Power loss in the LDO Efficiency of the LDO Solution: (1) Power Loss Calculation P loss =(V in − V out )× I load P loss = (5V - 3.3V) x50mA = 0.085W   (2) Efficiency Calculation η=3.3V×50mA/5V×50mA×100 = 66%

17. Problem 2: Thermal Power Dissipation in a Smartwatch A smartwatch processor operates at 1.8V @ 100mA , powered by a 3.7V Li-ion battery using an LDO . LDO has a thermal resistance of 100°C/W . Find: The junction temperature if the ambient temperature is 25°C . Solution: P loss =(3.7V−1.8V)×0.1A = 0.19W T junction =25°C+(0.19W×100°C/W) = 44°C Junction Temperature is the highest operating temperature of a semiconductor device at its p-n junction (inside the transistor, diode, or IC).

18. If T junction ​ is too high, thermal management is needed: Use a heatsink (reduces R θJA ​) Improve airflow (cooling fans, ventilation) Use a Switching Regulator instead of LDO (higher efficiency) Reduce input voltage (minimizing power dissipation)