Minimizing Switching Regulator Residue in Linear Regulator Outputs (Linear Technology) - 2

Application Note 101 spikes, which often have harmonic content approaching The fi gure considers the regulation path with emphasis on 100MHz, result from high energy, rapidly switching power high frequency parasitics. It is important to identify these elements within the switching regulator. The fi lter capacitor parasitic terms because they allow ripple and spikes to is intended to reduce these spikes but in practice cannot propagate into the nominally regulated output. Additionally, entirely eliminate them. Slowing the regulator’s repeti- understanding the parasitic elements permits a measure- tion rate and transition times can greatly reduce ripple ment strategy, facilitating reduction of high frequency out- and spike amplitude, but magnetics size increases and put content. The regulator includes high frequency parasitic effi ciency falls1. The same rapid clocking and fast switch- paths, primarily capacitive, across its pass transistor and ing that allows small magnetics size and high effi ciency into its reference and regulation amplifi er. These terms results in high frequency ripple and spikes presented to combine with fi nite regulator gain-bandwidth to limit high the linear regulator. frequency rejection. The input and output fi lter capacitors include parasitic inductance and resistance, degrading their
Ripple and Spike Rejection
effectiveness as frequency rises. Stray layout capacitance The regulator is better at rejecting the ripple than the very provides additional unwanted feedthrough paths. Ground wideband spikes. Figure 3 shows rejection performance potential differences, promoted by ground path resistance for an LT1763 low dropout linear regulator. There is 40db and inductance, add additional error and also complicate attenuation at 100KHz, rolling off to about 25db at 1MHz. measurement. Some new components, not normally as- The much more wideband spikes pass directly through the sociated with linear regulators, also appear. These additions regulator. The output fi lter capacitor, intended to absorb the include ferrite beads or inductors in the regulator input spikes, also has high frequency performance limitations. and output lines. These components have their own high The regulator and fi lter capacitors imperfect response, frequency parasitic paths but can considerably improve due to high frequency parasitics, reveals Figure 1 to be overall regulator high frequency rejection and will be ad- overly simplistic. Figure 4 restates Figure 1 and includes dressed in following text. the parasitic terms as well as some new components.
Note 1:
Circuitry employing this approach has achieved signifi cant harmonic content reduction at some sacrifi ce in magnetics size and effi ciency. See Reference 1. 80 70 60 50 40 30 IL = 500mA RIPPLE REJECTION (dB) 20 VIN = VOUT(NOMINAL) + 1V + 50mVRMS RIPPLE 10 COUT = 10µF CBYP = 0.01µF 0 10 100 1k 10k 100k 1M FREQUENCY (Hz) AN101 F03
Figure 3. Ripple Rejection Characteristics for an LT1763 Low Dropout Linear Regulator Show 40dB Attenuation at 100kHz, Rolling Off Towards 1MHz. Switching Spike Harmonic Content Approaches 100MHz; Passes Directly From Input to Output
an101f AN101-2