Ceramic capacitor characteristics make them nearly ideal for use in power supplies. They provide wide operating temperature range, small footprint, high ripple current capacity, high surge current, very low ESR, over-voltage tolerance, long life, and reasonably low cost. So, why do many switcher and LDO linear regulator datasheets stipulate tantalum caps and warn that ceramic caps are inappropriate? The answer is feedback loop stability.
Low dropout linear regulators (LDOs) that use a PNP or PFET pass element have control loops with high DC gain and one internal low frequency pole as illustrated in the simplified schematic of Figure 1. A second pole is formed by Cout in parallel with the load resistance. These device types generally rely on the zero formed by Cout and its ESR to cancel one pole to provide adequate phase margin and a stable loop. This zero must be placed below the loop's unity gain frequency to provide >45degrees phase margin. Figure 2 shows an example Bode plot with 2 poles and a zero placed to provide a stable design.
Figure 1: LDO with control loop and high DC gain
Figure 2: Bode plot with 2 poles and a zero placed to provide a stable design
Many voltage mode switching ICs use a high DC gain GM error amplifier with an internal pole set at low frequency and an internal zero set near the expected LC output double pole location. Like the LDO, this compensation scheme requires a second zero to be placed below the loop's unity gain frequency to provide >45degrees phase margin. The zero is provided by Cout and its ESR.
When one tries to use a regulator of this design type with a Cout having very low ESR he will often see an oscillation at a frequency >50 kHz or prolonged ringing following transients due to low phase margin in the feedback loop. Adding more capacitance in search of a cure may instead make the instability even more pronounced but at a reduced frequency.
This design idea presents a simple method to stabilize voltage regulators that were not designed to be stable with ceramic output capacitors. It requires a small capacitor, a resistor, and access to the feedback pin making it applicable to adjustable regulators.
Figure 3 shows a typical LDO regulator configuration with Cout and its required internal ESR. Figure 4 shows the same regulator but a series resistor and a feed forward capacitor have been added and the Cout has no ESR. By inspection one can see that if Rseries is equal to ESRcout and Cff is large enough to "short out" Rfb1 at frequencies of interest, then from a stability point of view we have similar circuits -- but with a major exception. The ESRcout in Figure 3 is seldom known accurately, has no guaranteed value or range, and can vary >> 2:1 with temperature. In contrast, Rseries in Figure 4 is known and stable making it easy to place the zero created by Rseries and Cout at a frequency of choice. Another benefit is immunity to instability caused by ceramic load capacitance that can destabilize the circuit of Figure 3. The price paid for this stability is two additional parts, slightly increased dropout voltage, and the dissipation introduced by Rseries. Dissipation in Rseries is usually not important when using linear regulators since loss in the resistor means less loss in the IC.
Figure 3: Typical LDO with Cout and internal ESR
Figure 4: Typical LDO with series resistor and feed forward capacitor
Figures 5 and 6 show two popular LDO linear regulator circuits modified to be stable with ceramic capacitors. The series resistor value, 150 milli-ohm, was taken from the datasheet ESR curve and the Cout value, now ceramic, is the minimum suggested on the datasheet. The minimum Cff value was calculated using;
Cff = 1/ (2*pi*Fz*Rf1||Rf2) where Fz = 1/ (2*pi*Rseries*Cout).
Figure 5: Popular LDO linear regulator circuit
Figure 6: Another popular LDO linear regulator circuit
A larger value for Cff will work but if too large will cause a form of soft start for output voltages greater then Vref. This can be good or not so good depending on desired startup and turn off response times.
Figure 7 shows the technique applied to stabilize the LM1117 "floating" 3 terminal adjustable using a 0.5 ohm series resistor and a 22μF ceramic Cout. The LM1117 datasheet advises Cout to be tantalum with ESR of 0.5ohm and 22μF minimum value for low output voltages.
Figure 8 shows a simple switcher circuit using the LM2596 to supply up to 3 amps at 1.5V using a series resistor value of 0.1 ohm with a ceramic Cout value of 100μF. At 3A continuous, Rseries dissipates 0.9W and should be rated accordingly. The combination of values shown for L, C, and Rseries will provide 45degree phase margin. Not only is phase margin well controlled, even at low Vout, but output ripple is reduced to only a few millivolts due to the very low ESR of the ceramic Cout.
Figure 7: Technique applied to stabilize the "floating" adjustable resistor and a ceramic capacitor
Figure 8: Switcher circuit to 3 amps at 1.5V
In summary, adjustable regulators originally designed to use tantalum capacitors can be safely used with ceramic output capacitors by inserting a resistor and a capacitor into the feedback path to provide loop compensation. Use of Rseries along with X5R or X7R ceramic material for Cout can provide a stable feedback loop that is invariant to temperature changes and immune to additional capacitive loading. In addition, when used with a switcher, the output ripple voltage is often reduced by an order of magnitude without adding a second LC.
About the Author
Wayne Rewinkel is a member of National Semiconductor's technical staff in the Chicago area. Wayne joined National in 1979 as an FAE and has held a variety of engineering and sales positions since then. He was a founder of National's Power Applications Design Center in Phoenix, where he designed hundreds of custom DC-DC converters. He began his career as a design engineer with Motorola in 1972 after receiving his BSEE from the University of Nebraska.