Why use a buck-boost for RF systems?

A single-cell lithium-ion battery discharges from 4.2V to 3.0V. If the RF transceiver requires a 3.3V supply, a buck converter works only above 3.3V and a boost converter works only below 3.3V. The buck-boost operates across the entire range, maintaining a stable 3.3V output through the full discharge cycle. This is critical for RF PAs and PLLs where supply voltage variations cause gain drift, frequency pulling, and phase noise degradation. The four-switch buck-boost automatically transitions between buck mode (when Vin is greater than Vout) and boost mode (when Vin drops below Vout) with no output discontinuity.

What are the different buck-boost topologies?

The inverting buck-boost uses a single switch, single inductor, and single diode but produces a negative output voltage relative to input ground, limiting its usefulness. The SEPIC (Single-Ended Primary-Inductor Converter) uses two inductors (or one coupled inductor) and a coupling capacitor to produce a non-inverting positive output, with the advantage of input-output isolation through the coupling capacitor. The Cuk converter is the dual of the SEPIC with capacitive energy transfer but produces an inverted output. The four-switch (H-bridge) buck-boost uses two half-bridges to provide non-inverting operation with the highest efficiency (95+ percent) by operating in pure buck or pure boost mode when possible and only entering buck-boost mode near the crossover point.

How does switching noise affect RF performance?

Switching converters generate conducted and radiated EMI at the switching frequency (typically 0.5 to 4 MHz) and its harmonics. For RF receivers with sensitivity of minus 100 dBm or better, even minus 80 dBm of switching noise at a harmonic that falls in the receive band can desensitize the receiver by 20 dB. Mitigation includes using switching frequencies that avoid RF band harmonics, adding ferrite bead and capacitor pi-filters on the supply rails, using spread-spectrum frequency modulation (SSFM) to spread the switching energy across a wider bandwidth (reducing the peak spectral density by 10 to 20 dB), and placing the converter on the opposite side of the PCB from sensitive RF circuits with a solid ground plane between them.

Power Management

Buck-Boost Converter

/buk-boost kun-vur-tur/

A DC-DC switching converter that regulates output voltage both above and below the input voltage, ideal for battery-powered RF systems where the battery voltage crosses through the desired output during discharge. The four-switch architecture achieves >95% efficiency by automatically transitioning between buck and boost modes, maintaining stable supply for RF PAs, PLLs, and transceivers across the full battery cycle.

Category: Power Management

Efficiency: > 95% (4-switch)

Application: Battery-powered RF

Understanding Buck-Boost Converters

A single-cell Li-ion battery discharges from 4.2V to 3.0V. If the RF transceiver needs 3.3V, a buck converter only works above 3.3V, a boost only below. The buck-boost maintains 3.3V across the entire range. For RF circuits, supply voltage stability is critical: a 100 mV variation on a PA supply causes measurable gain drift and EVM degradation; on a VCO supply, it causes frequency pulling and phase noise spurs.

The four-switch (H-bridge) buck-boost is the preferred architecture for modern RF systems. It uses two half-bridges: when V_in > V_out, it operates in buck mode (top half-bridge switches, bottom passes through); when V_in < V_out, it operates in boost mode (bottom switches, top passes through). Only near the crossover point does it enter true buck-boost mode with all four switches active. This maximizes efficiency compared to always-buck-boost SEPIC or Cuk topologies.

Converter Equations

Inverting Buck-Boost Duty Cycle:
V_out = −V_in × D / (1 − D)

SEPIC (non-inverting):
V_out = V_in × D / (1 − D)

4-Switch Modes:
Buck mode: D = V_out / V_in (V_in > V_out)
Boost mode: D = 1 − V_in / V_out (V_in < V_out)

DC-DC Topology Comparison for RF

Topology	V_out vs V_in	Polarity	Efficiency	Components	Noise
Buck	V_out < V_in	Non-inverting	> 95%	1L, 1C, 1 switch	Moderate
Boost	V_out > V_in	Non-inverting	> 93%	1L, 1C, 1 switch	Higher
Inverting B-B	Both	Inverting	~90%	1L, 1C, 1 switch	Higher
SEPIC	Both	Non-inverting	~88%	2L, 2C, 1 switch	Lower (isolated)
4-Switch B-B	Both	Non-inverting	> 95%	1L, 2C, 4 switch	Lowest

Common Questions

Frequently Asked Questions

Why use buck-boost for RF?

Li-ion battery (4.2V to 3.0V) crosses 3.3V during discharge. Buck-boost maintains stable output across the entire range. Critical for RF PAs (gain drift), PLLs (frequency pulling), and transceivers (EVM degradation) that are sensitive to supply voltage variations.

What are the topologies?

Inverting (single switch, negative output). SEPIC (two inductors, non-inverting, capacitor-isolated). Cuk (inverted output, capacitor transfer). Four-switch H-bridge (highest efficiency, auto buck/boost mode transitions, preferred for RF).

How does switching noise affect RF?

Switching harmonics (0.5 to 4 MHz) can desensitize receivers by 20+ dB if they land in the receive band. Mitigate with: harmonic-avoidance frequency selection, ferrite bead pi-filters, spread-spectrum modulation (10 to 20 dB peak reduction), and solid ground plane isolation.

Related Terms

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