A switched capacitor circuit is an active topology that uses passive capacitors and internal MOSFET switches to transfer discrete packets of charge. By rapidly toggling a "flying" capacitor between an input and an output, these circuits can double, invert, or regulate voltage without bulky inductors, or create precision active filters without resistors. But here is the bench reality: the active IC is only half the battle. The performance, efficiency, and stability of any switched capacitor network are entirely dictated by the passive capacitors you choose.
If you are building a charge pump (like a MAX232 or TPS60400) or a switched-capacitor filter (like the LTC1068), using the wrong dielectric will result in massive voltage drops, audible noise, or completely shifted filter cutoff frequencies. The direct answer for 90% of low-voltage DC-DC charge pump designs is to use X7R or X5R multilayer ceramic capacitors (MLCCs) with a voltage rating at least 2x your maximum rail. For precision switched-capacitor filters, you must strictly use C0G/NP0 dielectrics. Never use Y5V, Z5U, or high-ESR aluminum electrolytics in high-frequency switched topologies.
The Core Topology and Component Specifications
In a standard voltage-inverting or doubling switched capacitor charge pump, the circuit relies on two distinct passive roles:
- The Flying Capacitor ($C_{FLY}$): This capacitor is physically switched between the input rail and ground (or the output) by the IC's internal MOSFETs. It must have ultra-low Equivalent Series Resistance (ESR) to minimize $I^2R$ losses during the rapid charge/discharge cycles.
- The Reservoir Capacitor ($C_{RES}$): This sits at the output to smooth the pulsed DC into a steady voltage. It requires high bulk capacitance to hold the rail up between switching cycles.
The required values and dielectric types vary wildly depending on the IC's internal switching frequency. A 10 kHz legacy inverter needs vastly different passive physics than a 1 MHz modern step-down converter. Below is a data-dense reference for common switched capacitor ICs you will encounter on the bench.
| IC Part Number | Primary Function | Switching Freq | $C_{FLY}$ Value | $C_{RES}$ Value | Required Dielectric / Type |
|---|---|---|---|---|---|
| MAX232 | RS-232 Transceiver (Dual Charge Pump) | ~150 kHz | 1.0 µF (x2) | 1.0 µF (x2) | X7R MLCC or Low-ESR Tantalum |
| ICL7660 / LMC7660 | CMOS Voltage Inverter (-5V from +5V) | 10 kHz | 10 µF | 10 µF | Low-ESR Electrolytic or X7R MLCC |
| TPS60400 | Unregulated 60mA Charge Pump Inverter | 50 - 250 kHz | 1.0 µF | 1.0 µF (x2) | X5R or X7R MLCC (0402/0603) |
| LTC1068 | Clock-Tunable Switched-Capacitor Filter | Up to 50 kHz (Clock) | Internal / External | N/A | C0G/NP0 Mandatory for external integrators |
| LTC3261 | High Voltage (16V) Step-Down Charge Pump | 300 kHz - 3 MHz | 1.0 µF | 4.7 µF | X7R MLCC (Rated ≥25V to avoid VCC drop) |
Dielectric Type Comparison: Which Capacitor for Which Job?
Not all ceramics are created equal. The EIA (Electronic Industries Alliance) classifies ceramic dielectrics into Class I (stable, low capacitance density) and Class II (high density, but suffer from temperature and voltage coefficients). Choosing the wrong class for a switched capacitor application is the most common reason a prototype fails to meet its datasheet specs.
| Dielectric Code | Class | Tempco / Stability | Voltage Coefficient (VCC) | Typical Use in SC Circuits |
|---|---|---|---|---|
| C0G / NP0 | Class I | ±30 ppm/°C (Ultra-stable) | Negligible | Switched-capacitor filters, precision timing oscillators, high-Q resonant tanks. |
| X7R | Class II | ±15% over -55°C to +125°C | Moderate (drops 20-40% near rated V) | General purpose charge pumps ($C_{FLY}$ and $C_{RES}$), RS-232 level shifters, 5V/12V rails. |
| X5R | Class II | ±15% over -55°C to +85°C | High (can drop 50%+ near rated V) | Space-constrained portable device charge pumps where physical size dictates 0402 packages. |
| Y5V / Z5U | Class II | -82% to +22% (Extreme drift) | Severe | Avoid. Never use in SC circuits. Charge transfer will collapse at temperature extremes. |
| Tantalum | Polarized Electrolytic | Stable, but high ESR | None | Legacy low-frequency inverters (ICL7660). Avoid in modern >100kHz SC circuits due to ESR heating. |
A 10µF 16V X5R MLCC does not provide 10µF when biased at 12V. Due to the piezoelectric nature of Barium Titanate in Class II dielectrics, the actual capacitance can drop to 4µF under DC bias. For switched capacitor charge pumps, this lost capacitance increases output ripple and drops the maximum load current. Always oversize your voltage rating by at least 2x (e.g., use a 25V rated cap for a 12V rail) or consult the manufacturer's DC bias curves.
Decoding Physical Markings and Safe Substitution
When you are scavenging parts or verifying a BOM, you need to read the physical markings on the MLCC. Because 0603 and 0402 packages are too small for full text, manufacturers use a standardized 3-digit EIA code, often followed by a letter.
How to Read the 3-Digit Code
The first two digits are the significant figures. The third digit is the multiplier (number of zeros to add), expressed in picofarads (pF).
- 104: 10 followed by 4 zeros = 100,000 pF = 100 nF = 0.1 µF.
- 105: 10 followed by 5 zeros = 1,000,000 pF = 1.0 µF. (The standard $C_{FLY}$ for most 5V charge pumps).
- 476: 47 followed by 6 zeros = 47,000,000 pF = 47 µF. (Common for bulk $C_{RES}$).
The trailing letter indicates tolerance: J = ±5%, K = ±10%, M = ±20%. For a flying capacitor in a basic voltage doubler, K (10%) is perfectly acceptable. For an external integrator capacitor in an LTC1068 filter, you need J (5%) or better, which usually forces you into C0G territory.
Rules for Safe Substitution
When the exact BOM part is out of stock, follow these substitution rules to prevent board respins:
- Upgrading Voltage Rating: Always safe electrically, but watch the physical footprint. A 25V 0805 cap will not fit on a 16V 0603 pad. Furthermore, higher voltage caps in the same physical size often use thinner dielectric layers, which can actually worsen the voltage coefficient.
- Swapping Tantalum for MLCC: If a legacy schematic calls for a 10µF Tantalum on an ICL7660, you can substitute a 10µF X7R MLCC. The MLCC's vastly lower ESR will actually improve the output voltage under load. However, you must add a small series resistor (e.g., 0.5Ω) if the IC datasheet specifically requires ESR for loop stability to prevent startup latch-up.
- Swapping X7R for C0G: Safe in almost all charge pump applications, but C0G is physically larger and more expensive. Do not swap X7R into a C0G position in an SC filter; the filter's center frequency will drift wildly as the board heats up.
Failure Modes and Visual Symptoms
Capacitors in switched capacitor circuits endure harsh electrical environments. The flying capacitor experiences full AC square-wave voltage swings at high frequencies, leading to specific failure mechanisms that differ from standard DC decoupling caps.
1. Flex Cracking (Mechanical Stress)
- Visual Symptom: A hairline fracture in the ceramic body, usually running diagonally from the edge of the metal termination pad toward the center of the component. Often invisible to the naked eye; requires a 10x loupe or microscope.
- Electrical Symptom: Dead short across the capacitor. In a charge pump, this shorts the input rail to ground or the flying node, causing the IC to overheat instantly and the input voltage to collapse.
- Cause: PCB bending during depaneling, connector insertion, or thermal shock during wave soldering. MLCCs are brittle. To prevent this, keep flying capacitors at least 2mm away from board edges and V-score lines.
2. Dielectric Aging (Logarithmic Capacitance Drop)
- Visual Symptom: None. The component looks pristine.
- Electrical Symptom: A newly built charge pump works perfectly, but after 3 months on the shelf, the output voltage droops under load, or the ripple exceeds spec.
- Cause: Class II dielectrics (X7R, X5R) naturally lose capacitance over time as their crystal structure aligns. According to Murata's aging characteristics, X7R loses about 1% to 2% of its capacitance per decade hour after being heated above its Curie temperature (approx 125°C). If your circuit is marginal, this aging pushes it over the edge. Fix: Bake the board at 150°C for 1 hour to "de-age" the caps and restore full capacitance, or design with a 20% capacitance margin.
3. Piezoelectric Microphonics (Acoustic Noise)
- Visual Symptom: None.
- Electrical Symptom: An audible high-pitched squeal or whine emanating from the PCB, or excessive low-frequency noise injected into sensitive analog rails.
- Cause: Barium Titanate in X5R/X7R caps is piezoelectric. The rapid voltage steps on the $C_{FLY}$ capacitor cause the ceramic body to physically expand and contract, turning the capacitor into a tiny speaker. This is notorious in audio equipment using switched capacitor voltage inverters for op-amp negative rails. Fix: Use "soft-termination" MLCCs (which have a flexible polymer layer to dampen vibration), switch to C0G if capacitance values allow, or increase the switching frequency above 20 kHz (human hearing limit) using an IC like the TPS60400.
Practical Layout Rules for the Flying Capacitor
The physical placement of the flying capacitor is just as critical as its dielectric. Because the $C_{FLY}$ is switched at high frequencies (often >500 kHz in modern parts like the Analog Devices SC families or modern TI charge pumps), the PCB traces connecting it to the IC act as parasitic inductors.
Parasitic inductance ($L$) opposes rapid changes in current ($V = L \frac{di}{dt}$). If the traces between the IC's $C_{FLY+}$ and $C_{FLY-}$ pins are too long, the inductance will spike the voltage during the MOSFET switching transitions. This causes electromagnetic interference (EMI), ringing, and can exceed the absolute maximum voltage rating of the IC's internal switches, leading to silicon punch-through.
The Golden Rule: Route the flying capacitor as close to the IC pins as physically possible. Use wide, short traces. If you are using a 4-layer board, place the $C_{FLY}$ on the top layer directly adjacent to the IC, and use multiple vias to stitch the ground return of the reservoir capacitor directly to the IC's thermal pad. Never route the flying capacitor traces under the IC or through long vias to the bottom layer.
