Selecting DC-DC Step-Down Voltage Regulators For WiFi RF Power Amplifier Applications

Transcription

1 Nvember 14, 2014 Selecting DC-DC Step-Dwn Vltage Regulatrs Fr WiFi RF Pwer Amplifier Applicatins By David Brwn, Skywrks Slutins, Inc. Wireless lcal area netwrk (WLAN) radi frequency (RF) links have been integrated int almst all facets f prtable and mbile prducts tday. Operating in ac requires greater RF pwer amplifier (PA) linearity cmpared t legacy a/b/g/n systems which, in many cases, has reduced the RF PA input supply vltage range while drawing mre input current with increased pwer dissipatin demands. New ac RF PAs ften pse a challenge t prtable prduct designers, because the RF PA input supply range is n lnger cmpatible with direct cnnectin t a Li-In/ply battery supply vltage range. This means that a vltage regulatr must nw be emplyed. Perfrmance fr an ac RF PA can be affected by the vltage regulatr peratin, but regulatr integratin can be simplified if the designer knws which key perfrmance characteristics t evaluate in a DC-DC step-dwn vltage regulatr. An additinal benefit f a DC-DC switching vltage regulatr is reduced system pwer dissipatin, which reduces the WLAN circuit thermal ftprint and saves pwer drawn frm the battery surce supply. Many very small ftprint DC-DC vltage regulatrs are available tday t fit the need f the ac WiFi PA applicatin. The current generatin f ac RF PAs fr use in mbile applicatins typically has ptimized RF perfrmance ver a 2.9 V t 3.6 V range. At the same time, typical Li-In/ply batteries have an perating range f 4.3 V t 2.7 V, but mst systems cut ff the battery arund 3.4 V. This sets an ideal situatin t use a fixed vltage DC-DC vltage regulatr with the utput f 3.3 V. When the battery is at

2 the maximum-t-average vltage levels f 4.2 V t 3.7 V, the RF PA lad current can be reduced by a high efficiency DC-DC switching vltage cnversin. Typically, a gd DC-DC regulatr will have efficiencies greater than 90 percent. At a battery supply level f 4.0 V, the PA current drawn frm the battery is reduced by 20 ma, and the system pwer added efficiency is imprved (Figures 1 and 2). An added side benefit t using a DC-DC vltage regulatr with high pwer cnversin efficiency is that it can reduce the verall system pwer dissipatin and, mre imprtantly, the system thermal ftprint when an RF PA is placed under adverse high VSWR perating cnditins. When the PA input supply vltage is stepped dwn frm the input battery, the current is reduced as utlined in Figure 1. When the RF PA is placed under an extreme 10:1 VSWR utput mismatch cnditin, the input current increases t maintain the utput pwer level. The increased pwer dissipatin is released in the frm f heat. Figures 3 and 4 shw a 7.4 C reductin in RF PA case temperature by stepping the PA input supply vltage dwn t 3.3 V frm 4.0 V.

3 Applicatins in ac typically enable the RF PA fr the duratin f WiFi peratin with a fixed utput pwer level. Mdulated data burst is applied t the PA, taking it frm a stand-by lad cnditin t a maximum lad cnditin in a very shrt perid f time. A DC-DC vltage regulatr must have a very fast lad transient respnse characteristic t maintain the utput vltage regulatr in less than 20 µs. If the regulatr takes lnger than 20 µs fr the utput vltage t settle after a large lad step is applied, the supply vltage t the RF PA will sag at the beginning f transitin and will cmprmise the dynamic errr vectr magnitude (DEVM) f the transmit signal. Fr the example shwn in Figures 5 and 6, an ac RF PA idles near n-lad cnditins and steps up t 170 ma with the applied mdulatin. The DC-DC vltage regulatr utput vltage is able t settle in 12.4 µs, allwing the RF PA t achieve a DEVM f 1.78 percent, which is acceptable fr mst systems. When discussing the use f a DC-DC switching regulatr in RF applicatins, regulatr utput switching nise r ripple-created PWM r PFM switch mde cntrl is ften a cncern. Mst small and cmpact DC-DC step-dwn switching regulatrs use a base switching frequency in the range f 2 MHz t 6 MHz t enable them t use the smallest pssible external cmpnents (inductr, input, and utput capacitrs). The switching nise r ripple frm these regulatrs usually falls within the mdulated bandwidth f the WiFi transmit signal. The end effect f ripple, if t high, wuld be decreased Errr Vectr Magnitude (EVM) perfrmance and increased spurius respnse fr the transmit carrier. Fr this reasn, a lw nise DC-DC switching regulatr shuld be used. An example f a gd lw nise perfrmance DC-DC switching regulatr wuld be the SKY This device replaces a cnventinal lw-side synchrnus switch with a lw nise rectifier circuit that greatly reduces utput switching ripple and subsequent harmnics.

4 Figure 7: DC-DC switching cnverter utput nise spectrum (0 t 10 MHz) fr a cnventinal PWM switch mde regulatr and a specialized lw nise DC-DC switching regulatr Fr mbile applicatins, ne final attribute t cnsider when selecting a DC-DC vltage regulatr fr an ac RF PA is perfrmance under drp-ut cnditins. DC-DC regulatr drp-ut is defined by the pint where the utput can n lnger maintain utput regulatin frm the input supply. The ac RF PA in a mbile applicatin is prne t this cnditin. Since the DC-DC regulatr is typically set fr a fixed 3.3 V utput and the input supply frm the battery supply can drp t 3.4 V r 3.3 V befre it is cut ff, the system can run int a drp-ut state. Because f this, it is imperative t select a DC-DC step-dwn regulatr that is capable f 100 percent duty-cycle peratin and has the ability t transitin in and ut f drp-ut cnditins smthly. Sme regulatrs have increased instability and ripple as they apprach 100 percent duty-cycle peratin, which will lead t adverse RF PA signal integrity in the frm f decreased EVM and ACLR perfrmance.

5 Figure 8: Gd 100 percent duty cycle peratin fr DC-DC vltage regulatr. This device enters and recvers frm an input drp-ut cnditin while maintaining stability and minimum utput ripple r nise. Adding a DC-DC step-dwn switching regulatr t an ac RF PA applicatin in a prtable prduct des nt need t add risk r cmplexity t the system design if the selected regulatr can prvide a few key perfrmance characteristics. When selecting a DC-DC step-dwn regulatr, the device shuld have the fllwing characteristics: High pwer cnversin efficiency Typically better than 90 percent High switching frequency fr minimum external cmpnents sizing 2 MHz t 6 MHz range Very fast lad transient respnse t achieve gd DEVM perfrmance <20 µ fr a n-lad r light lad t full lad current step Specialized fr lw nise and utput switching ripple perfrmance 20 mvpp r less Clean, 100 percent duty-cycle peratin t cver lw battery vltage drp-ut cnditins The additin f a DC-DC switch mde step-dwn pwer cnversin device allws an ac RF PA t perate ver the wide input supply vltage range f a mbile device battery supply. At the same time, the regulatr aids t reduce current cnsumed frm the battery and extends the system life between charge cycles. Reduced current and subsequent pwer dissipatin reduces the thermal ftprint f the PA under adverse perating cnditins.