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  • Solving Light-Load Ripple Without an LDO: TPS54202 Eco-Mode vs TPS54308 FCCM — A Hands-On Comparison

    Solving Light-Load Ripple Without an LDO: TPS54202 Eco-Mode vs TPS54308 FCCM — A Hands-On Comparison

    When selecting a step-down converter for ripple-sensitive circuits, picking solely by rated current and efficiency is a recipe for trouble. Converters that engage Eco-mode or pulse skipping at light loads can see output ripple soar to tens of mV, severely degrading the performance of analog sensors, RF front-ends, and PLL/VCO supply rails. Texas Instruments’ TPS54202 and TPS54308 share the same 6-pin SOT-23 package and identical pinout, yet they differ decisively in light-load operating mode — a difference that completely changes the choice for ripple-sensitive designs.

    The Ripple Problem in Sensitive Circuits

    When generating a 5V or 3.3V local rail from a 24V industrial bus, the most common choice is a synchronous step-down converter. But in systems where the load is not constant — think IoT gateways where current consumption swings from a few mA during sensor polling idle periods to hundreds of mA during active cycles, or wireless modules that draw single-digit mA in standby but surge past 1A during transmission — the output ripple during light-load intervals becomes a real problem. As the load lightens, the converter enters pulse-skipping mode, the switching frequency becomes irregular, and the output capacitor repeatedly charges and discharges, dramatically increasing ripple amplitude.

    The trouble with this ripple goes beyond mere voltage variation. When the ripple frequency components overlap with an ADC’s sampling bandwidth, effective resolution (ENOB) takes a hit. It couples directly into PLL phase noise, and survives as system noise that even the CMRR of an analog signal chain cannot reject. Switching to an LDO eliminates the ripple, but in a 24V-to-5V step-down, an LDO manages only about 21% efficiency — at 1A load, that means 19W of heat. You’re trading ripple for a heatsink and extra board area.

    TPS54202 vs TPS54308 output ripple comparison by load current
    Figure 1: Output ripple (mVpp) comparison between TPS54202 (Eco-mode) and TPS54308 (FCCM) across load currents — the ripple difference becomes dramatic at light loads.

    Why Eco-Mode Ripple Grows at Light Load

    The TPS54202 is a 2A converter employing fixed 500kHz peak current-mode control with Advanced Eco-mode™. Eco-mode works by skipping switching cycles when the load drops below the level that the high-side FET’s minimum on-time can sustain — the converter only turns on the FET when the output voltage falls below the setpoint, then returns to idle once the target is reached. The impressively low 45μA no-load quiescent current (Iq) is a direct consequence of this Eco-mode operation.

    This efficiency optimization comes at a cost, however. During pulse-skipping intervals, the effective switching frequency can drop to just a few hundred Hz. A single burst of energy overcharges the output capacitor, and the load slowly bleeds it off — a pattern that repeats. As a result, output ripple amplitude climbs from 10–15mV at heavy load to 40–80mV at light load, and can exceed 100mV under certain conditions. The ripple frequency also becomes irregular, making it difficult to filter out any specific band with a notch filter.

    TPS54202 Eco-mode light-load 100mA output ripple oscilloscope waveform
    Figure 2: TPS54202 oscilloscope capture — at IOUT=100mA light load, VOUT ripple appears large on a 20mV/div scale, and the switching node (PH) waveform becomes irregular due to pulse skipping. (Source: TI TPS54202 datasheet Figure 7-8)

    How FCCM Solves the Ripple Problem

    The TPS54308 comes in the same 6-pin SOT-23 package with an identical pinout to the TPS54202, but there is one decisive difference: it operates in FCCM (Forced Continuous Conduction Mode), never stopping switching even at light load. No matter how low the load current drops, the 350kHz switching frequency remains constant, and the inductor current is allowed to flow in the negative direction to maintain continuous conduction. This design philosophy trades away some light-load efficiency (Iq = 300μA, versus 45μA for the TPS54202) in exchange for keeping output ripple predictably low across the entire load range.

    In FCCM, output ripple is fundamentally determined by \( \Delta V_{OUT} pprox \Delta I_L imes ESR + \Delta I_L / (8 imes f_{SW} imes C_{OUT}) \), where \( \Delta I_L \) itself is fixed by the constant switching frequency and duty cycle. There is no structural reason for ripple to increase at light load. In practice, with a 12V-to-5V, \( L = 10\mu H \), \( C_{OUT} = 2 imes 22\mu F \) MLCC configuration, the TPS54202 exhibits over 60mVpp ripple at light load (10mA), while the same board fitted with a TPS54308 keeps ripple within 15–20mVpp. With ripple frequency locked at 350kHz, an additional notch filter targeting that single band can further suppress it.

    TPS54308 FCCM light-load 100mA output ripple oscilloscope waveform
    Figure 3: TPS54308 oscilloscope capture — under identical light-load conditions (IOUT=100mA), VOUT ripple is suppressed to a 10mV/div scale thanks to FCCM, and the PH waveform remains steady at a fixed 350kHz. (Source: TI TPS54308 datasheet Figure 8-8)

    Why Not Just Use an LDO?

    The first workaround an engineer facing a ripple problem reaches for is an LDO. Output ripple is virtually nonexistent at the μV level, and the external BOM is just two capacitors. But this choice inflicts a severe efficiency penalty, especially when the input-to-output voltage differential is large. For example, an LDO supplying 5V at 0.5A from a 24V input dissipates \( (24V – 5V) imes 0.5A = 9.5W \) — nearly four times the power delivered to the load (2.5W). A SOT-223 package cannot handle this thermally, and even a TO-220 with a heatsink will cook nearby components.

    The TPS54308 FCCM converter, under the same conditions (24V to 5V, 0.5A), delivers roughly 88–90% efficiency, meaning the converter itself dissipates only about 0.28W. That’s a 97% reduction in loss compared to the LDO — translating directly to no heatsink required, a tiny SOT-23 6-pin package, and layout freedom on tight boards. Moreover, the FCCM keeps ripple at a sufficiently low 15–20mVpp — not quite LDO levels, but adding a single LC filter stage after the converter can bring it below 5mVpp for ultra-low-noise rails.

    ApproachOutput RippleEfficiency (24V→5V, 0.5A)Power LossHeatsink
    LDO (e.g., LM7805)~μVpp21%9.5WRequired (TO-220 + heatsink)
    TPS54202 (Eco-mode)60–80mVpp (light load)~90%0.28WNot required
    TPS54308 (FCCM)15–20mVpp~89%0.31WNot required
    Table 1: Ripple-efficiency trade-off among LDO, Eco-mode converter, and FCCM converter

    Real-World Swap: TPS54202 to TPS54308

    On a sensor interface board operating from 24V, producing 5V for a variable 10mA–500mA load, the initial design using the TPS54202 performed well at medium-to-heavy loads (200mA and above). But during the idle periods between sensor polling cycles (approximately 10–15mA), output ripple shot up to 70mVpp. This ripple jittered the LSB of a 16-bit ADC by 4–5 bits, dragging effective resolution down to roughly 11 bits — and software averaging could not fully eliminate it.

    After swapping to the TPS54308 on the same board, with the same \( L = 10\mu H \) and \( C_{OUT} = 2 imes 22\mu F \) components, light-load ripple dropped to 18mVpp — a 74% reduction. Thanks to the fixed 350kHz switching frequency, the ripple FFT spectrum also concentrated into a single peak. ADC effective resolution recovered to 14.2 bits, and with software oversampling, better than 15 bits was achievable. Power dissipation increased by roughly 30mW (from the Iq difference of 45μA to 300μA), but overall efficiency remained at 89% with no thermal issues whatsoever.

    Notably, the TPS54202 and TPS54308 share the exact same pinout (1: GND, 2: SW, 3: VIN, 4: FB, 5: EN, 6: BOOT). All these improvements were obtained by simply desoldering the TPS54202 and soldering in a TPS54308 — zero PCB changes required. One minor consideration: the TPS54308 switches at 350kHz versus the TPS54202’s 500kHz, so adjusting the inductor value can further optimize for the same ripple target if desired.

    Before and after replacing TPS54202 with TPS54308 — ripple reduction
    Figure 4: Ripple comparison before and after replacing TPS54202 with TPS54308 on the same PCB — a 74% reduction from 70mVpp to 18mVpp, restoring ADC effective resolution from 11 bits to 14.2 bits.

    Spec Comparison and Selection Criteria

    ParameterTPS54202TPS54308
    Input Voltage4.5–28V4.5–28V
    Output Current2A3A
    Integrated FETs (HS+LS)148 + 78mΩ85 + 40mΩ
    Switching Frequency500kHz (spread spectrum)350kHz (fixed)
    Light-Load ModeAdvanced Eco-mode™ (pulse skip)FCCM (forced continuous conduction)
    Quiescent Current (Iq)45μA300μA
    Soft Start5ms (internal)5ms (internal)
    ProtectionOCP, OVP, TSDOCP, OVP, TSD
    PackageSOT-23 (6)SOT-23 (6)
    PinoutGND-SW-VIN-FB-EN-BOOTGND-SW-VIN-FB-EN-BOOT (identical!)
    Table 2: TPS54202 vs TPS54308 specification comparison

    For ripple-sensitive analog circuits, precision sensor supplies, or local rails in RF blocks, the predictable low ripple that the TPS54308’s FCCM delivers is far more valuable than the TPS54202’s light-load efficiency advantage. Conversely, in battery-powered devices where light-load efficiency directly governs runtime and the load consists of ripple-tolerant digital circuits, the TPS54202’s 45μA Iq and Eco-mode make it the better choice. Since both parts share an identical pinout, the most practical approach during prototyping is to test both, measure ripple and efficiency, and make the final decision based on real data.

  • How to select a bridge rectifier diode

    How to select a bridge rectifier diode

    Characteristics of Diodes

    As shown in the figure, a diode is a component that can allow current to flow only in the direction of ▶. This is a nonlinear relationship in which the forward current \( V_{F} \) rapidly increases when a voltage is applied across the diode above the forward voltage \( V_{F} \). In addition, when the forward current \( I_{F} \) flows, a forward voltage drop \( V_{F} \) must occur.

    Since power loss occurs at this time, attention should be paid to component heating in a high-current rectification circuit. When the diode is heated, leakage current increases, and there is a risk of fire due to thermal runaway. Therefore, most diode plastic molds meet the flame retardant standard of UL94V-0 or higher.

    Diode Forward Voltage(\( V_{F} \)) – Forward Current(\( I_{F} \))Characteristic

    Select the Reverse Maximum Voltage \(V_{RM}\) of the bridge diode

    In the bridge rectification circuit, while the diode is conducting, the applied voltage \( e\) of each diode terminal voltage \(V_{D}\) becomes \(\sqrt{2}\times e_{rms}\). In reality, if the input voltage of the AC fluctuates, \( e_{rms}\) also changes in proportion to it, so that \(V_{D}\) should not exceed the reverse maximum voltage \(V_{RM}\) of the diode even at the maximum input voltage.

    In addition, since the actual rectification circuit has the influence of external noise such as surges, \(V_{RM}\) should be selected sufficiently. In general, a Reverse voltage of twice the rectified voltage is selected, and if the applied voltage is unstable, a higher Reverse voltage selection is required.

    Selection of forward current \(I_{F}\) for bridge diode

    In a typical rectifying circuit, the current \(i_{c}\) flowing through the diode flows in a pulse waveform rather than a sine wave. This pulse current has a maximum value that changes under various conditions.

    First, since the average value \(I_{ave}\) of \(i_{c}\) flowing through the diode must be equal to the DC current \(I_{O}\) after rectification, if the period of half-cycle is \(T\), and the period of current flowing is \(t_{1}\), it becomes \(\frac{1}{T}\int_{0}^{t_{1}}i_{c}dt=I_{0}\).

    Relation of RMS and Peak to Average Diode Current in Capacitor-input Circuits
    From O.H Schade, Proc. IRE, Vol. 31, 1943, p. 356

    In general, the forward current \(I_{F}\) of a rectifying diode is maximum rated from the average value of this \(i_{c}\). However, this is the value when the current flows in direct current, and the pulse current should be considered low in the rated value. The maximum value \(i_{CP}\) of this pulsed current can be obtained from O.H. Schade’s graph. (n\) of \(n\omega CR_{L}\) on the horizontal axis has a coefficient of 0.5 at double voltage rectification, 1 at half-wave rectification, and 2 at full-wave rectification. \(C\) is the capacitor capacity, and \(R_{L}\) is the load resistance value.

    Next, \(R_{S}/\left ( nR_{L} \right )\) of the vertical axis means a ratio of the load resistance and the line impedance. The line impedance \(R_{S}\) should be considered to include not only the resistance value of the wiring but also the winding resistance of the power transformer.

    Under these conditions, the values of the left vertical axis are read from the graph below. This value multiplied by the output current \(I_{O}\) is the maximum current value \(i_{CP}\).

    Selection of diodes considering surge current

    Another current condition of the diode is surge current \(I_{FSM}\). In the rectifying circuit, when the power switch is initially operated, the charging voltage of the capacitor is set to 0V. Therefore, at the moment when the switch is operated, a large charging current flows to the capacitor. This is called the inrush current, and the terminal voltage of the capacitor is increased by this large charging current, and accordingly, the current value of the charging gradually becomes a normal state.

    In general, the surge current \(I_{FSM}\) of a rectifying diode has a value of about 10 times that of forward current \(I_{F}\). However, this is a guarantee value of one cycle and the value decreases when the temperature of the diode is high.

    Power loss of diode

    The diode suffers power loss due to the forward voltage drop \(V_{F}\) and the forward current \(I_{F}\). And as a result, it generates heat and increases the temperature. Silicon diodes currently in general must not exceed the maximum junction temperature \(T_{j(max)}\) 150℃. Since the current rating of the diode is determined under the condition of reaching the junction temperature, it is necessary to install a heatsink to lower the temperature when the temperature is high.

    It is not easy to calculate the power loss of a diode strictly. The simple calculation method is calculated by multiplying the output current \(I_{O}\) after rectification at the forward voltage drop \(V_{F}\) as the loss. In addition, in a bridge diode, the total loss should be doubled because current always flows through two diodes.

  • Type of rectifier according to the number of diodes

    Type of rectifier according to the number of diodes

    A half-wave rectifier with one diode

    Commercial power is a sinusoidal wave of 50/60 Hz, which is a symmetrical waveform of positive and negative voltages at every half of the frequency. Rectifying only a positive voltage with one diode is called a half-wave rectifier.

    At this time, the diode charges the capacitor with a positive voltage and prevents the charge in the opposite direction by reverse voltage to the diode in a negative voltage cycle. In this case, the direct current output current \( I_{O} \) is an average value of the capacitor charging current \( i_{C} \) and \( I_{O}=\frac{1}{T}\int_{0}^{t}i_{C}dt \).

    As such, in a half-wave rectifier, the charging current \(i_{C} \) of the capacitor is charged only once per cycle of the power supply frequency, so the current maximum \(i_{C,peak} \) gets that big. Therefore, if the output current is large, the half-wave rectifier has a large capacitor to reduce the output ripple. Therefore, it should only be used in circuits with small output currents.

    Half-wave rectifier

    A full-wave rectifier with two diodes

    The full-wave rectifier uses two diodes to rectify both positive and negative voltages of sine waves. A full-wave rectifier using two diodes requires two windings around the center tap of the transformer on the secondary side. In each transformer winding, diode \(D_{1}\) turn-on in the positive half cycle, and diode \(D_{2}\) turn-on in the negative half cycle. Therefore, the rectified waveform is a pulse waveform in which the negative half-period of the sine wave is inverted.

    Full-wave rectifier

    A full-wave rectifier with four diodes(Bridge Diode)

    The most commonly used is a full-wave rectifier using four diodes, also called a bridge rectifier. Four diodes should be used instead of one trans-winding, but it is not a big drawback because many bridge diodes with four diodes packaged are on the market.

    The current in the positive and negative half cycles alternately charges the capacitor, so it is fully rectified, but two diodes are inserted in series in the current path, which doubles the forward voltage drop \(V_{F} \) of the diode and increases the loss.

    For example, let calculate the efficiency of a 12W circuit with a rectified voltage of 12V and an output current of 1A. If the forward voltage drop \(V_{F} \) of the diode is 1V, for full-wave rectification using the center tap,

    $$\eta = \frac{12W}{\left ( 12V + 1V_{F} \right )\times 1A}=92\%$$

    For full-wave rectifier using a bridge diode,

    $$\eta = \frac{12W}{\left ( 12V + 2V_{F} \right )\times 1A}=86\%$$

    There is a big difference in efficiency.

    Despite these shortcomings, bridge rectifier are widely used because the center tap of the transformer can be removed and the circuit can be simplified using commercially available bridge diodes.

    Bridge rectifier

    Use a rectifier suitable for the purpose of the circuit

    As above, each rectifier has clear disadvantages and advantages. If one diode is used, the circuit is simple, but because the charging current of the capacitor is large, it must be used for small output, and if two diodes are used, the efficiency is high, but the center tab of the transformer has to be designed. Bridge diodes are simple to design, but efficiency should be consider the forward voltage drop of the diodes.

    Recently, synchronous rectifier, which are methods of rectifying using FET instead of diodes, have been used for high efficiency, but this will be explained later.

  • What is the difference between a linear regulator and a switching regulator?

    What is the difference between a linear regulator and a switching regulator?

    Use a linear regulator if power stability is required

    Linear regulator, called series regulator or shunt regulator, are mainly used when precise voltages are needed or when small power is needed, and when the unit price of the product has to be lowered. Linear regulator have very small electrical noise generation in a simple circuit configuration and have a small output ripple voltage, allowing them to configure high-stability power sources.

    However, a linear regulator uses a transistor to create a difference between an input voltage and an output voltage, resulting in a large power loss when the output current is large. Since all power losses are generated by heat, heat dissipation measures such as heat sinks are needed not to exceed the rated operating temperature. Therefore, when high output is required, power loss increases, making it difficult to use.

    Disadvantages of Linear Regulator
    Disadvantages of Linear Regulator

    Use switching regulators when high efficiency power is required

    Switching regulators are mainly used when high-efficiency power is required or when circuits need to be miniaturized. For example, since heat loss in linear regulators can be solved by switching loss in switching regulators, the power conversion efficiency is high and the area required for heat dissipation is small.

    In addition, the lower the operating frequency, the larger the size of the power transformer, so the linear regulator that converts 50/60Hz, which is a commercial power source, has a big and heavy power transformer. On the other hand, switching regulators can make the operating frequency several tens of kHz or more, making the transformer used for power conversion smaller and lighter.

    In addition, the linear regulator must make a DC voltage by dropping and rectifying the voltage by a transformer of commercial power. Therefore, the output current flows through the rectifying circuit as it is, and the loss of the rectifier diode is large, and the smoothing capacitor must also be large. However, the switching regulator uses a direct current voltage that directly rectifies commercial power, so the loss of the rectifier diode is small due to the small current, and the smoothing capacitor can be used small with an operating frequency of several tens of kHz or more.

    However, switching regulators are complicated in circuit configuration and operation. In addition, measures to reduce noise caused by switching are needed.

    Linear RegulatorSwitching egulator
    Step Down(Buck)
    Step Up(Boost)
    Buck-Boost
    Invert    		O
    X
    X
    X    		X
    X
    X
    X
    EfficiencyLowHigh
    Output CurrentLowHigh
    NoiseLowHigh
    DesignSimpleComplicated
    CostLowMiddle

    Recently, switching regulators are mainly used

    Recently, circuit integration technology has developed, and circuits that require complex functions are implemented as one IC. Switching regulators are also able to configure high-efficiency switching regulators with only a few peripheral circuit configurations. Of course, the types of parts depending on the use are also subdivided.

    However, if the method of using such an IC is not accurate, it may cause accidents such as a decrease in reliability or damage to parts. Therefore, the design of switching regulators is very important.

    Examples of switching regulators by TI (link)
  • Why do electronic circuits need regulated power supply?

    Why do electronic circuits need regulated power supply?

    The electronic circuit operates on DC power

    All electronic devices require power supply through an AC 110V/220V voltage which is a commercial power system(or a battery) for the operation of the device. In addition, these electronic devices require stable power sources such as 3.3V, 5V, and 12V.

    Electronic devices supplied with power through commercial power supply convert and rectified voltage to the required value by the power transformer to create a DC voltage and use it in a circuit. However, in a rectified DC power source, the performance of the device cannot be fully demonstrated because the stability and precision of the voltage are not good due to changes in the input voltage or a voltage drop of a transformer or rectifier diode.

    Causes of voltage fluctuations

    Quality of commercial power supply voltage

    Commercial power fluctuations exist even in countries with very good power systems using sufficient costs for power plants, etc. Most of them have small fluctuations of around ±5%, but some countries under development have a very large voltage drop of more than 10-20V.

    Power Transformer Voltage Drop

    Although it depends on the size of the transformer, a voltage drop occurs depending on the resistance of the wire because the copper wire is wound more than hundreds of times. In addition, since the leakage inductance between the primary and secondary of the transformer is inserted in series, a voltage drop occurs.

    Voltage drop in rectifier diode

    Bridge diodes, which are widely used for rectification, have forward voltage drops depending on the current

    Ripple Voltage

    Since the AC voltage of the commercial power source is sinusoidal, ripple voltage occurs due to charging and discharging even if it is smooth with a rectifier capacitor. This is represented by voltage fluctuations of twice the frequency in the case of full-wave rectification. In addition, when a load fluctuation occurs, a larger ripple voltage fluctuation occurs due to the imbalance of charging and discharging of the rectifier capacitor.

    full_wave_rectifier
    Full-wave rectifier

    Electronic circuits require a rated voltage

    All electronic components, such as motors and relays, as well as semiconductors such as ICs, have a rated voltage that is recommended to be used and a maximum voltage that guarantees operation. Therefore, if the voltage value is exceeded, the electronic component may not operate as designed, have a shorter lifespan, or may be damaged.

    For example, the rated voltage of most TTL ICs is 5V, the voltage that guarantees the operation is 4.5 to 5.5V, and the maximum voltage is 6 to 7V. In addition, in signal amplification circuits such as OPAMP, supply voltage fluctuations become signal fluctuations or noise. As a result, the designed precision or stability cannot be obtained.

    As such, the fluctuation of the power voltage is a problem to be solved in terms of the performance and reliability of the device. Therefore, power stabilization and regulated power supply are required through circuit design