Key Terminologies for Selecting LDO, Buck, Boost, or Buck-Boost ICs in Low Power Applications
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Quiescent Current in the context of Buck and Boost\( I_Q \)
Quiescent current is the current drawn by the IC when it is enabled, non-switching, and no-load.
- No load: No current flows from the IC to the output, meaning all of \( I_Q \) flows to ground inside the IC.
- Non-switching: The IC's power switch is off, meaning the power stage is disconnected from the output, except for some devices with integrated MOSFET body diodes that cannot be turned off.
- Enabled: The IC is active (not in UVLO or shutdown), and \( I_Q \) measures operating current, not shutdown current.
- For buck converters, \( I_Q \) typically comes from the input. For boost or buck-boost converters, \( I_Q \) can come from both the input and output.
- \( I_Q \) includes the current needed for internal functions like the precision reference voltage, oscillator, and logic gates, but excludes current for the power stage or gate drivers.
For more information, check this application note.
How to Use \( I_Q \)
Knowing the quiescent current (\( I_Q \)) helps in comparing the low-power performance of different ICs. However, \( I_Q \) is only part of the system's input current, which is influenced by:
- The internal design of each IC (\( I_Q \)).
- External components around the IC.
- The overall system configuration.
In applications that do not run at no load but in “standby” or “hibernate” modes, where some current is still drawn by the processor or other loads, the usefulness of \( I_Q \) decreases. For example, consider the TPS62120 powering TI's MSP430™ and other circuitry consuming 100 μA at 2 V. With an 8-V input and 60% efficiency, as shown below:
The resulting input current is \(\frac{2\, \text{V} \times 100\, \mu\text{A}}{0.6 \times 8\, \text{V}} = 42\, \mu\text{A}\). This includes \( I_Q \) (11 μA), which constitutes about 26% of the total input current. However, if the standby load increases to 1 mA, the input current becomes \(\frac{2\, \text{V} \times 1\, \text{mA}}{0.8 \times 8\, \text{V}} = 313\, \mu\text{A}\). Here, \( I_Q \) is only about 3.5% of the total input current. Therefore, \( I_Q \) alone does not accurately estimate battery current draw in standby mode.
NOTE: Instead of using \( I_Q \) to estimate battery current draw, measure and use the no-load input current of the system. For better accuracy, define the system's load in low-power mode and measure the actual battery current draw at that operating point.
How to Calculate the Input Current to a Boost Converter with No Load
The total current of a boost converter, when there is no load, is the sum of the current into the \( V_{\text{out}} \) pin, the current flowing through the resistor divider, and the load current (\( I_L \)). This current is transferred from the battery when the device is switching.
For more information on this formula, please visit the Texas Instruments application note.
Let's take an example of the TPS61021A Boost converter:
- Input Quiescent Current: \( I_{Q_{\text{vin}}} = 3\, \mu\text{A} \) (max)
- Output Quiescent Current: \( I_{Q_{\text{vout}}} = 30\, \mu\text{A} \)
- Output Voltage: \( V_{\text{out}} = 3.3\, \text{V} \)
- Input Voltage: \( V_{\text{in}} = 1.5\, \text{V} \)
- Feedback Current: \( I_{FB} = 20\, \text{nA} \)
- Load Current: \( I_{\text{load}} = 0\, \text{A} \)
- Efficiency: 87% (from the figure below)
We can choose the efficiency value at \( I_{\text{Load}} = 100 \times max(I_{Q_{\text{vout}}}, I_{Q_{\text{vin}}}) = 3000\, \mu\text{A} \) condition to be \(\eta_1\).
So \( I_{\text{IN}} = 3\, \mu\text{A} + \frac{3.3\, \text{V}}{1.5\, \text{V} \times 0.87} \times (30\, \mu\text{A} + 0.020\, \mu\text{A} + 0) = \mathbf{79\, \mu\text{A}} \) @ no load.
For \( V_{\text{out}} = 1.8\, \text{V} \), with all other parameters remaining the same:
- Input Voltage: \( V_{\text{in}} = 1.5\, \text{V} \)
- Output Voltage: \( V_{\text{out}} = 1.8\, \text{V} \)
- Efficiency: 95% (extracted from the same figure)
Input Current Calculation: \( I_{\text{IN}} = 3\, \mu\text{A} + \frac{1.8\, \text{V}}{1.5\, \text{V} \times 0.95} \times (30\, \mu\text{A} + 0.020\, \mu\text{A} + 0) = \mathbf{40.92\, \mu\text{A}} \) @ no load.
Let's take an example of the TLV61220 Boost converter:
- Input Quiescent Current: \( I_{Q_{\text{vin}}} = 0.9\, \mu\text{A} \) (max)
- Output Quiescent Current: \( I_{Q_{\text{vout}}} = 7.5\, \mu\text{A} \)
- Output Voltage: \( V_{\text{out}} = 1.8\, \text{V} \)
- Input Voltage: \( V_{\text{in}} = 1.5\, \text{V} \)
- Feedback Current: \( I_{FB} = 0.01\, \mu\text{A} \)
- Load Current: \( I_{\text{load}} = 0\, \text{A} \)
- Efficiency: 83% (from the figure below)
We can choose the efficiency value at \( I_{\text{Load}} = 100 \times max(I_{Q_{\text{vout}}}, I_{Q_{\text{vin}}}) = 750\, \mu\text{A} \) condition to be \(\eta_1\).
So \( I_{\text{IN}} = 0.9\, \mu\text{A} + \frac{1.8\, \text{V}}{1.5\, \text{V} \times 0.83} \times (7.5\, \mu\text{A} + 0.010\, \mu\text{A} + 0) = \mathbf{11.75\, \mu\text{A}} \) @ no load.
For \( V_{\text{out}} = 3.3\, \text{V} \), keeping everything else the same:
- Input Voltage: \( V_{\text{in}} = 1.5\, \text{V} \)
- Output Voltage: \( V_{\text{out}} = 3.3\, \text{V} \)
- Efficiency: 87% (extracted from the above figure)
Input Current Calculation: \( I_{\text{IN}} = 0.9\, \mu\text{A} + \frac{3.3\, \text{V}}{1.5\, \text{V} \times 0.87} \times (7.5\, \mu\text{A} + 0.010\, \mu\text{A} + 0) = \mathbf{20\, \mu\text{A}} \) @ no load.
| Boost | IQ_Vin | IQ_Vout | Efficiency | IFB | Iload | Vin | Vout | IIN |
|---|---|---|---|---|---|---|---|---|
| TLV61220 | 0.9uA | 7.5uA | 83% | 0.01uA | 0A | 1.5V | 1.8V | 11.75uA |
| TLV61220 | 0.9uA | 7.5uA | 87% | 0.01uA | 0A | 1.5V | 3.3V | 20uA |
| TPS61021A | 3uA | 30uA | 95% | 0.02uA | 0A | 1.5V | 1.8V | 40.92uA |
| TPS61021A | 3uA | 30uA | 87% | 0.02uA | 0A | 1.5V | 3.3V | 79uA |
No load input current for Boost converter (Excluding feedback R-divider network)
LDO Key Parameters
-
Dropout Voltage:
Dropout voltage is the input-to-output differential voltage at which the circuit ceases to regulate against further reductions in input voltage; this point occurs when the input voltage approaches the output voltage.In the dropout region, the PMOS pass element is simply a resistor, and dropout is expressed in terms of its on-resistance (RDSON).
\[ V_{\text{dropout}} = I_{\text{OUT}} * RDS_{\text{ON}} \]
-
Quiescent Current \( I_{\text{Q}} \) and Ground Current:
Quiescent current (IQ) is the current required to power the LDO’s internal circuitry when the external load current is zero. It includes the operating currents of the band-gap reference, error amplifier, output voltage divider, and overcurrent and overtemperature sensing circuits. Ground current (IGND) is the difference between the input and output currents, and necessarily includes the quiescent current. A low ground current maximizes the LDO efficiency. -
Standby/Shutdown Current:
Standby current is the input current drawn by a regulator when the output voltage is disabled by a shutdown signal. -
Efficiency:
To have a high efficiency, dropout voltage and quiescent current must be minimized. In addition, the voltage difference between input and output must be minimized.
\[ \text{Efficiency} = \frac{I_{\text{OUT}} V_{\text{OUT}}}{(I_{\text{OUT}} + I_{\text{GND}}) V_{\text{IN}}} \times 100 \] -
Line Regulation:
Line regulation is a measure of the circuit’s ability to maintain the specified output voltage with varying input voltage. Line regulation is defined as:
\[ \text{Line regulation} = \frac{\Delta V_{\text{OUT}}}{\Delta V_{\text{IN}}} \] -
Load Regulation:
Load regulation is a measure of the circuit’s ability to maintain the specified output voltage under varying load conditions. Load regulation is defined as:
\[ \text{Load regulation} = \frac{\Delta V_{\text{OUT}}}{\Delta I_{\text{OUT}}} \] -
Power Supply Rejection Ratio (PSRR):
Power supply rejection ratio (PSRR), also known as ripple rejection, measures the LDO regulator’s ability to prevent the regulated output voltage from fluctuating due to input voltage variations. The same relation for line regulation applies to PSRR except that the whole frequency spectrum is considered. More than -40dB is the recommended value. -
Power Dissipation:
\[ P_{\text{D}} = (V_{\text{IN}} - V_{\text{OUT}}) \times I_{\text{OUT}} + V_{\text{IN}} \times I_{\text{GND}} \]- \( V_{\text{IN}} \): Input voltage (V)
- \( V_{\text{OUT}} \): Output voltage (V)
- \( I_{\text{OUT}} \): Output current (A)
- \( I_{\text{GND}} \): Ground current(includes \( I_{\text{Q}} \)) when an LDO is on (A)
The calculated power dissipation should be lower than the Absolute Maximum Rating specified in the datasheet to ensure safe operation of the device.
-
Junction Temperature:
\[ T_{\text{j}} = P_{\text{D}} \times R_{\text{th(j-a)}} + T_{\text{a}} \] \[ R_{\text{th(j-a)}} = \frac{T_{\text{j(max)}} - T_{\text{a}}}{P_{\text{D(max)}}} \]- \( P_{\text{D}} \): Power dissipation of an LDO (W)
- \( R_{\text{th(j-a)}} \): Junction-to-ambient thermal resistance (°C/W)
- \( T_{\text{j(max)}} \): Maximum rated junction temperature specified in the datasheet (150°C) (°C)
- \( P_{\text{D(max)}} \): Maximum rated power dissipation under the board conditions specified in the datasheet (W)
- \( T_{\text{a}} \): Ambient temperature at which the absolute maximum ratings are specified in the datasheet (25°C) (°C)
-
ThetaJA (\( \theta_{JA} \)):
ThetaJA is the thermal resistance from junction to ambient, expressed in degrees Celsius per Watt (°C/W). It indicates how efficiently the device can dissipate heat into the surrounding environment. This value will give you an idea about whether to use a package for a requirement or not. Suppose that ThetaJA = 170 C/W and datasheet says it is 150 C/W for SO-8 package, 200 C/W for SOT23-5. so we cannot use SOT23-5 because it will burn, and SO-8 will be the suitable option. Rule: any package with lower C/W than calculated can be okay.
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