LMV225/LMV226/LMV228 RF Power Detector for CDMA and WCDMA General Description Features The LMV225/LMV226/LMV228 are 30 dB RF power detectors intended for use in CDMA and WCDMA applications. The device has an RF frequency range from 450 MHz to 2 GHz. It provides an accurate temperature and supply compensated output voltage that relates linearly to the RF input power in dBm. The circuit operates with a single supply from 2.7V to 5.5V. The LMV225/LMV226/LMV228 have an integrated filter for low-ripple average power detection of CDMA signals with 30 dB dynamic range. Additional filtering can be applied using a single external capacitor. The LMV225 has an RF power detection range from –30 dBm to 0 dBm and is ideally suited for direct use in combination with resistive taps. The LMV226/LMV228 have a detection range from –15 dBm to 15 dBm and are intended for use in combination with a directional coupler. The LMV226 is equipped with a buffered output which makes it suitable for GSM, EDGE, GPRS and TDMA applications. n n n n n n 30 dB linear in dB power detection range Output voltage range 0.2 to 2V Logic low shutdown Multi-band operation from 450 MHz to 2000 MHz Accurate temperature compensation Packages: — micro SMD package 1.0 mm x 1.0 mm x 0.6 mm — LLP package 2.2 mm x 2.5 mm x 0.8 mm (LMV225 and LMV228) Applications n n n n CDMA RF power control WCDMA RF power control CDMA2000 RF power control PA modules The device is active for Enable = HI, otherwise it is in a low power consumption shutdown mode. During shutdown the output will be LOW. The output voltage ranges from 0.2V to 2V and can be scaled down to meet ADC input range requirements. The LMV225/LMV226/LMV228 power detectors are offered in the small 1.0 mm x 1.0 mm X 0.6 mm micro SMD package. The LMV225 and the LMV228 are also offered in the 2.2 mm x 2.5 mm x 0.8 mm LLP package. Typical Application LMV226/LMV228 LMV225 20076001 20076046 © 2005 National Semiconductor Corporation DS200760 www.national.com LMV225/LMV226/LMV228 RF Power Detector for CDMA and WCDMA March 2005 LMV225/LMV226/LMV228 Absolute Maximum Ratings (Note 1) Junction Temperature (Note 3) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Mounting Temperature Infrared or convection (20 sec) Supply Voltage VDD - GND Supply Voltage ESD Tolerance (Note 2) 2.7V to 5.5V Temperature Range 2000V Machine Model −40˚C to +85˚C RF Frequency Range 200V Storage Temperature Range 235˚C Operating Ratings (Note 1) 6.0V Max Human Body Model 150˚C Max 450 MHz to 2 GHz −65˚C to 150˚C 2.7 DC and AC Electrical Characteristics Unless otherwise specified, all limits are guaranteed to VDD = 2.7V; TJ = 25˚C. Boldface limits apply at temperature extremes. (Note 4) Symbol IDD Parameter Supply Current Condition Active Mode: RFIN/EN = VDD (DC), No RF Input Power Present Min Typ Max LMV225 4.8 7 8 LMV226 4.9 6.2 8 LMV228 4.9 6.2 8 0.44 4.5 µA 0.8 V Shutdown: RFIN/EN = GND (DC), No RF Input Power Present VLOW EN Logic Low Input Level (Note 6) VHIGH EN Logic High Input Level (Note 6) ton Turn-on-Time (Note 9) tr Rise Time (Note 7) IEN Current into RFIN/EN Pin PIN Input Power Range (Note 5) 1.8 No RF Input Power Present, Output Loaded with 10 pF 2.1 LMV226 1.2 LMV228 1.7 Step from no Power to 0 dBm Applied, Output Loaded with 10 pF LMV225 4.5 Step from no Power to 15 dBm Applied, Output Loaded with 10 pF LMV226 1.8 LMV228 4.8 LMV228 www.national.com µs µs 1 LMV226 2 mA V LMV225 LMV225 Units µA −30 0 dBm −43 −13 dBV −15 15 dBm −28 2 dBV −15 15 dBm −28 2 dBV (Continued) Unless otherwise specified, all limits are guaranteed to VDD = 2.7V; TJ = 25˚C. Boldface limits apply at temperature extremes. (Note 4) Symbol Parameter Logarithmic Slope (Note 8) Condition 900 MHz 1800 MHz 1900 MHz 2000 MHz Logarithmic Intercept (Note 8) 900 MHz 1800 MHz 1900 MHz 2000 MHz VOUT Output Voltage No RF Input Power Present Min Typ LMV225 44.0 LMV226 44.5 LMV228 44.0 LMV225 39.4 LMV226 41.6 LMV228 41.9 LMV225 38.5 LMV226 41.2 LMV228 41.6 LMV225 38.5 LMV226 41.0 LMV228 41.2 LMV225 −45.5 LMV226 −24.5 LMV228 −27.2 −46.6 LMV226 −25.1 LMV228 −28.2 LMV225 −46.3 LMV226 −24.9 LMV228 −28.0 LMV225 −46.7 LMV226 −24.7 LMV228 −28.0 LMV225 214 350 LMV226 223 350 LMV228 228 350 Output Current Sourcing/Sinking LMV226 Only ROUT Output Impedance LMV225/LMV228 only, no RF Input Power Present 19.8 en Output Referred Noise RF Input = 1800 MHz, −10 dBm for LMV225 and 5 dBm for LMV226/LMV228, Measured at 10 kHz 700 4.5 Units mV/dB LMV225 IOUT 3 Max dBm 5.3 mV mA 29 34 kΩ nV/ www.national.com LMV225/LMV226/LMV228 2.7 DC and AC Electrical Characteristics LMV225/LMV226/LMV228 2.7 DC and AC Electrical Characteristics (Continued) Unless otherwise specified, all limits are guaranteed to VDD = 2.7V; TJ = 25˚C. Boldface limits apply at temperature extremes. (Note 4) Symbol Parameter Variation Due to Temperature Condition Min Typ 900 MHz, RFIN = 0 dBm Referred to 25˚C LMV225 +0.64 −1.07 900 MHz, RFIN = 15 dBm Referred to 25˚C LMV226 +0.05 −0.02 LMV228 +0.22 −0.36 1800 MHz, RFIN = 0 dBm Referred to 25˚C LMV225 +0.09 −0.86 1800 MHz, RFIN = 15 dBm Referred to 25˚C LMV226 +0.07 −0.10 LMV228 +0.29 −0.57 1900 MHz, RFIN = 0 dBm Referred to 25˚C LMV225 +0 −0.69 1900 MHz, RFIN = 15 dBm Referred to 25˚C LMV226 +0 −0.10 LMV228 +0.23 −0.64 2000 MHz, RFIN = 0 dBm Referred to 25˚C LMV225 +0 −0.86 2000 MHz, RFIN = 15 dBm Referred to 25˚C LMV226 +0 −0.29 LMV228 +0.27 −0.65 Max Units dB 5.0 DC and AC Electrical Characteristics Unless otherwise specified, all limits are guaranteed to VDD = 5.0V; TJ = 25˚C. Boldface limits apply at temperature extremes. (Note 4) Symbol IDD Parameter Supply Current Condition Active Mode: RFIN/EN = VDD (DC), no RF Input Power Present. Typ Max LMV225 Min 5.3 7.5 9 LMV226 5.3 6.8 9 LMV228 5.4 6.8 9 0.32 4.5 µA 0.8 V Shutdown: RFIN/EN = GND (DC), no RF Input Power Present. VLOW EN Logic Low Input Level (Note 6) VHIGH EN Logic High Input Level (Note 6) ton Turn-on-Time (Note 9) tr Rise Time (Note 7) www.national.com 1.8 No RF Input Power Present, Output Loaded with 10 pF mA V LMV225 2.1 LMV226 1.0 LMV228 1.7 Step from no Power to 0 dBm Applied, Output Loaded with 10 pF LMV225 4.5 Step from no Power to 15 dBm Applied, Output Loaded with 10 pF LMV226 1.4 LMV228 4.8 4 Units µs µs (Continued) Unless otherwise specified, all limits are guaranteed to VDD = 5.0V; TJ = 25˚C. Boldface limits apply at temperature extremes. (Note 4) Symbol Parameter IEN Current Into RFIN/EN Pin PIN Input Power Range (Note 5) Condition Min LMV225 LMV226 LMV228 Logarithmic Slope (Note 8) 900 MHz 1800 MHz 1900 MHz 2000 MHz Logarithmic Intercept (Note 8) 900 MHz 1800 MHz 1900 MHz 2000 MHz VOUT Output Voltage No RF Input Power Present Typ Units 1 µA −30 0 dBm −43 −13 dBV −15 15 dBm −28 2 dBV −15 15 dBm −28 2 dBV LMV225 44.6 LMV226 44.6 LMV228 44.2 LMV225 40.6 LMV226 42.2 LMV228 42.4 LMV225 39.6 LMV226 41.8 LMV228 42.2 LMV225 39.7 LMV226 41.6 LMV228 41.8 LMV225 −47.0 LMV226 −25.0 mV/dB LMV228 −27.7 LMV225 −48.5 LMV226 −25.7 LMV228 −28.9 LMV225 −48.2 LMV226 −25.6 LMV228 −28.7 LMV225 −48.9 LMV226 −25.5 LMV228 −28.7 LMV225 222 400 LMV226 231 400 LMV228 244 400 IOUT Output Current Sourcing/Sinking LMV226 Only ROUT Output Impedance No RF Input Power Present 23.7 en Output Referred Noise RF Input = 1800 MHz, −10 dBm for LMV225 and 5 dBm for LMV226/LMV228, Measured at 10 kHz 700 4.5 5 Max dBm 5.3 mV mA 29 31 kΩ nV/ www.national.com LMV225/LMV226/LMV228 5.0 DC and AC Electrical Characteristics LMV225/LMV226/LMV228 5.0 DC and AC Electrical Characteristics (Continued) Unless otherwise specified, all limits are guaranteed to VDD = 5.0V; TJ = 25˚C. Boldface limits apply at temperature extremes. (Note 4) Symbol Parameter Variation Due to Temperature Condition Min Typ 900 MHz, RFIN = 0 dBm Referred to 25˚C LMV225 +0.89 −1.16 900 MHz, RFIN = 15 dBm Referred to 25˚C LMV226 +0.25 −0.16 LMV228 +0.46 −0.62 1800 MHz, RFIN = 0 dBm Referred to 25˚C LMV225 +0.3 −0.82 1800 MHz, RFIN = 15 dBm Referred to 25˚C LMV226 +0.21 −0.09 LMV228 +0.55 −0.78 1900 MHz, RFIN = 0 dBm Referred to 25˚C LMV225 +0.34 −0.63 1900 MHz, RFIN = 15 dBm Referred to 25˚C LMV226 +0.21 −0.19 LMV228 +0.55 −0.93 2000 MHz RFIN = 0 dBm Referred to 25˚C LMV225 +0.22 −0.75 2000 MHz RFIN = 15 dBm Referred to 25˚C LMV226 +0.25 −0.34 LMV228 +0.61 −0.91 Max Units dB Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is intended to be functional, but specific performance is not guaranteed. For guaranteed specifications and the test conditions, see the Electrical Characteristics. Note 2: Human body model: 1.5 kΩ in series with 100 pF. Machine model, 0Ω in series with 100 pF. Note 3: The maximum power dissipation is a function of TJ(MAX) , θJA and TA. The maximum allowable power dissipation at any ambient temperature is PD = (TJ(MAX) - TA)/θJA. All numbers apply for packages soldered directly into a PC board Note 4: Electrical Table values apply only for factory testing conditions at the temperature indicated. Factory testing conditions result in very limited self-heating of the device such that TJ = TA. No guarantee of parametric performance is indicated in the electrical tables under conditions of internal self-heating where TJ > TA. Note 5: Power in dBV = dBm + 13 when the impedance is 50Ω. Note 6: All limits are guaranteed by design or statistical analysis Note 7: Typical values represent the most likely parametric norm. Note 8: Device is set in active mode with a 10 kΩ resistor from VDD to RFIN/EN. RF signal is applied using a 50Ω RF signal generator AC coupled to the RFIN/EN pin using a 100 pF coupling capacitor. Note 9: Turn-on time is measured by connecting a 10 kΩ resistor to the RFIN/EN pin. Be aware that in the actual application on the front page, the RC-time constant of resistor R2 and capacitor C adds an additional delay. www.national.com 6 LMV225/LMV226/LMV228 Connection Diagrams 4-Bump micro SMD 6-pin LLP 20076063 Top View 20076002 Top View Pin Description Pin Name micro SMD LLP6 A2 4 B1 A1 B2 Power Supply Output Description VDD Positive Supply Voltage 1 GND Power Ground 3 RFIN/EN 6 Out DC voltage determines enable state of the device (HIGH = device active). AC voltage is the RF input signal to the detector (beyond 450 MHz). The RFIN/EN pin is internally terminated with 50Ω in series with 45 pF. Ground referenced detector output voltage (linear in dBm) Ordering Information Package 4-Bump micro SMD 6-pin LLP Part Number LMV225TL LMV225TLX LMV225SD LMV225SDX LMV226TL 4-Bump micro SMD LMV226TLX LMV228TL LMV228TLX 6-pin LLP LMV228SD LMV228SDX Package Marking I A90 I I A89 Transport Media 250 Units Tape and Reel 3k Units Tape and Reel 2k Units Tape and Reel 9k Units Tape and Reel NSC Drawing Status TLA04AAA Released SDB06A Preliminary TLA04AAA Released SDB06A Preliminary 250 Units Tape and Reel 3k Units Tape and Reel 250 Units Tape and Reel 3k Units Tape and Reel 2k Units Tape and Reel 9k Units Tape and Reel Note: This product is offered both with leaded and lead free bumps. 7 www.national.com LMV225/LMV226/LMV228 Block Diagrams 20076064 LMV225 20076049 LMV226 20076047 LMV228 www.national.com 8 Unless otherwise specified, VDD = 2.7V, TJ = 25˚C. Supply Current vs. Supply Voltage (LMV225) Supply Current vs. Supply Voltage (LMV226) 20076004 20076051 Supply Current vs. Supply Voltage (LMV228) Output Voltage vs. RF Input Power (LMV225) 20076034 20076005 Output Voltage vs. RF Input Power (LMV226) Output Voltage vs. RF Input Power (LMV228) 20076052 20076035 9 www.national.com LMV225/LMV226/LMV228 Typical Performance Characteristics LMV225/LMV226/LMV228 Typical Performance Characteristics Unless otherwise specified, VDD = 2.7V, TJ = 25˚C. (Continued) Output Voltage and Log Conformance vs. RF Input Power @ 900 MHz (LMV225) Output Voltage and Log Conformance vs. RF Input Power @ 900 MHz (LMV226) 20076053 20076006 Output Voltage and Log Conformance vs. RF Input Power @ 1800 MHz (LMV225) Output Voltage and Log Conformance vs. RF Input Power @ 900 MHz (LMV228) 20076036 20076007 Output Voltage and Log Conformance vs. RF Input Power @ 1800 MHz (LMV228) Output Voltage and Log Conformance vs. RF Input Power @ 1800 MHz (LMV226) 20076054 www.national.com 20076037 10 Output Voltage and Log Conformance vs. RF Input Power @ 1900 MHz (LMV225) Output Voltage and Log Conformance vs. RF Input Power @ 1900 MHz (LMV226) 20076055 20076008 Output Voltage and Log Conformance vs. RF Input Power @ 2000 MHz (LMV225) Output Voltage and Log Conformance vs. RF Input Power @ 1900 MHz (LMV228) 20076038 20076009 Output Voltage and Log Conformance vs. RF Input Power @ 2000 MHz (LMV228) Output Voltage and Log Conformance vs. RF Input Power @ 2000 MHz (LMV226) 20076056 20076039 11 www.national.com LMV225/LMV226/LMV228 Typical Performance Characteristics Unless otherwise specified, VDD = 2.7V, TJ = 25˚C. (Continued) LMV225/LMV226/LMV228 Typical Performance Characteristics Unless otherwise specified, VDD = 2.7V, TJ = 25˚C. (Continued) Logarithmic Slope vs. Frequency (LMV225) Logarithmic Slope vs. Frequency (LMV226) 20076057 20076010 Logarithmic Slope vs. Frequency (LMV228) Logarithmic Intercept vs. Frequency (LMV225) 20076040 20076011 Logarithmic Intercept vs. Frequency (LMV226) Logarithmic Intercept vs. Frequency (LMV228) 20076058 www.national.com 20076041 12 Output Variation vs. RF Input Power Normalized to 25˚C @ 900 MHz (LMV225) Output Variation vs. RF Input Power Normalized to 25˚C @ 900 MHz (LMV226) 20076012 20076059 Output Variation vs. RF Input Power Normalized to 25˚C @ 1800 MHz (LMV225) Output Variation vs. RF Input Power Normalized to 25˚C @ 900 MHz (LMV228) 20076042 20076013 Output Variation vs. RF Input Power Normalized to 25˚C @ 1800 MHz (LMV228) Output Variation vs. RF Input Power Normalized to 25˚C @ 1800 MHz (LMV226) 20076060 20076043 13 www.national.com LMV225/LMV226/LMV228 Typical Performance Characteristics Unless otherwise specified, VDD = 2.7V, TJ = 25˚C. (Continued) LMV225/LMV226/LMV228 Typical Performance Characteristics Unless otherwise specified, VDD = 2.7V, TJ = 25˚C. (Continued) Output Variation vs. RF Input Power Normalized to 25˚C @ 1900 MHz (LMV225) Output Variation vs. RF Input Power Normalized to 25˚C @ 1900 MHz (LMV226) 20076014 20076061 Output Variation vs. RF Input Power Normalized to 25˚C @ 2000 MHz (LMV225) Output Variation vs. RF Input Power Normalized to 25˚C @ 1900 MHz (LMV228) 20076044 20076015 Output Variation vs. RF Input Power Normalized to 25˚C @ 2000 MHz (LMV228) Output Variation vs. RF Input Power Normalized to 25˚C @ 2000 MHz (LMV226) 20076062 www.national.com 20076045 14 LMV225/LMV226/LMV228 Typical Performance Characteristics Unless otherwise specified, VDD = 2.7V, TJ = 25˚C. (Continued) PSRR vs. Frequency (LMV225 , LMV226 and LMV228 in microSMD) PSRR vs. Frequency (LMV225 and LMV228 in LLP) 20076023 20076065 RF Input Impedance vs. Frequency @ Resistance and Reactance (LMV225, LMV226 and LMV228 in micro SMD) RF Input Impedance vs. Frequency @ Resistance and Reactance (LMV225 and LMV228 in LLP) 20076024 20076066 15 www.national.com LMV225/LMV226/LMV228 Solving this expression for R1, using that RIN = 50Ω, yields: Application Notes CONFIGURING A TYPICAL APPLICATION The LMV225/LMV226/LMV228 are power detectors intended for CDMA and WCDMA applications. Power applied at its input translates to a DC voltage on the output through a linear-in-dB response. The LMV225 detector is especially suited for power measurements via a high-resistive tap, while the LMV226/LMV228 are designed to be used in combination with a directional coupler. The LMV226 has an additional output voltage buffer and therefore a low output impedance. The key features of the devices are shown in table 1. (2) In Figure 1, R1 is set to 1800Ω resulting in an attenuation of 31.4 dB DIRECTIONAL COUPLER APPLICATION The LMV226/LMV228 also has a 50Ω input resistance. However, its input range differs compared to the LMV225, i.e. −15 dBm to +15 dBm. If a typical attenuation of a directional coupler is 20 dB, the LMV226/LMV228 can be directly connected via the directional coupler to the PA without the need of additional external attenuator (Figure 2). Different PA ranges can be configured using couplers with other coupling factors. TABLE 1. DEVICE CHARACTERISTICS Input Range (dBm) Output Buffer Application LMV225 −30 / 0 No High Resistive Tap LMV226 −15 / 15 Yes Directional Coupler LMV228 −15 / 15 No Directional Coupler In order to match the output power range of the power amplifier (PA) with the range of the LMV225’s input, the high resistive tap needs to be configured correctly. In case of the LMV226/LMV228 the coupling factor of the directional coupler needs to be chosen correctly. HIGH RESISTIVE TAP APPLICATION The constant input impedance of the device enables the realization of a frequency independent input attenuation to adjust the LMV225’s range to the range of the PA. Resistor R1 and the 50Ω input resistance (RIN) of the device realize this attenuation (Figure 1). To minimize insertion loss, resistor R1 needs to be sufficiently large. The following example demonstrates how to determine the proper value for R1. 20076046 FIGURE 2. Typical LMV226/LMV228 Application with Directional Coupler SHUTDOWN FUNCTIONALITY The LMV225/LMV226/LMV228 RFIN/EN pins have 2 functions combined: • Enable/Shutdown • Power input The capacitor C and the resistor R2 (Figure 1 and Figure 2) separate the DC shutdown functionality from the AC power measurement. The device is active when Enable = HI, otherwise it is in a low power consumption shutdown mode. During shutdown the output will be LOW. Capacitor C should be chosen sufficiently large to ensure a corner frequency far below the lowest input frequency to be measured. In case of the LMV225 the corner frequency can be calculated using: 20076033 FIGURE 1. Typical LMV225 Application with High Resistive Tap Suppose the useful output power of the PA ranges up to +31 dBm. As the LMV225 can handle input power levels up to 0 dBm. R1 should realize a minimum attenuation of 31 - 0 = 31 dB. The attenuation realized by R1 and the effective input resistance RIN of the detector equals: (3) Where RIN = 50Ω, CIN = 45 pF typical. With R1 = 1800Ω and C = 100 pF, this results in a corner frequency of 2.8 MHz. This corner frequency is an indicative (1) www.national.com 16 LMV225/LMV226/LMV228 Application Notes (Continued) number. The goal is to have a magnitude transfer, which is sufficiently flat in the used frequency range; capacitor C should be chosen significantly larger than capacitor CIN to assure a proper performance of the high resistive tap. Capacitor C shouldn’t be chosen excessively large since the RC-time, it introduces in combination with resistor R2, adds to the turn-on time of the device. The LMV226/LMV228 do not use a resistor R1 like the LMV225. Though a resistor is seen on the coupler side (RCOUPLER). Therefore a similar equation holds for the LMV226/LMV228 LF corner frequency, where R1 is replaced with the coupler output impedance (RCOUPLER). With RCOUPLER = 50Ω and C = 100 pF, the resulting corner frequency is 50 MHz. The output voltage is proportional to the logarithm of the input power, often called “linear-in-dB”. Figure 3 shows the typical output voltage versus PA output power of the LMV225 setup as depicted in Figure 1. 20076017 FIGURE 4. AM Modulated RF Signal The ripple observed at the output of the detector equals the detectors response to the power variation at the input due to AM modulation (Figure 4). This signal has a maximum amplitude VIN • (1+µ) and a minimum amplitude VIN • (1-µ), where 1+µ can be maximum 2 and 1-µ can be minimum 0. The amplitude of the ripple can be described with the formula: (5) where VY is the slope of the detection curve (Figure 5) and µ is the modulation index. Equation (5) can be reduced to: (6) Consequently, the ripple is independent of the average input power of the RF input signal and only depends on the logarithmic slope VY and the ratio of the maximum and the minimum input signal amplitude. For CDMA, the ratio of the maximum and the minimum input signal amplitude modulation is typically in the order of 5 to 6 dB, which is equivalent to a modulation index µ of 0.28 to 0.33. A further understanding of the equation above can be achieved via the knowledge that the output voltage VOUT of the LMV225/LMV228 is linear in dB, or proportional to the input power PIN in dBm. As discussed earlier, CDMA has a modulation in the order of 5 to 6 dB. Since the transfer is linear in dB, the output voltage VOUT will vary linearly over about 5 to 6 dB in the curve (Figure 5). 20076016 FIGURE 3. Typical power detector response, VOUT vs. PA output Power OUTPUT RIPPLE DUE TO AM MODULATION A CDMA modulated carrier wave generally contains some amplitude modulation that might disturb the RF power measurement used for controlling the PA. This section explains the relation between amplitude modulation in the RF signal and the ripple on the output of the LMV225/LMV228. Expressions are provided to estimate this ripple on the output. The ripple can be further reduced by lowpass filtering at the output. This is realized by connecting an capacitor from the output of the LMV225/LMV228 to ground. Estimating Output Ripple The CDMA modulated RF input signal of Figure 3 can be described as: (4) VIN(t) = VIN [1 + µ(t)] cos (2 · π · f · t) In which VIN is the amplitude of the carrier frequency and the amplitude modulation µ(t) can be between -1 and 1. 17 www.national.com LMV225/LMV226/LMV228 Application Notes a 1.5 nF capacitor is then 20 • log (200/12) = 24.4 dB. This is very close to the calculated number of the previous paragraph. (Continued) 20076025 20076018 FIGURE 6. Output Ripple vs. RF Input Power FIGURE 5. VOUT vs. RF Input Power PIN The output voltage variation ∆VOUT is thus identical for RF input signals that fall within the linear range (in dB) of the detector. In other words, the output variation is independent of the absolute RF input signal: (7) ∆VO = VY · ∆PIN In which VYis the slope of the curve. The log-conformance error is usually much smaller than the ripple due to AM modulation. In case of the LMV225/LMV228, VY = 40 mV/ dB. With ∆PIN = 5 dB for CDMA, ∆VOUT = 200 mVPP. This is valid for all VOUT. PRINCIPLE OF OPERATION The logarithmic response of the LMV225/LMV226/LMV228 is implemented by a logarithmic amplifier as shown in Figure 7. The logarithmic amplifier consists of a number of cascaded linear gain cells. With these gain cells, a piecewise approximation of the logarithmic function is constructed. Output Ripple with Additional Filtering The calculated result above is for an unfiltered configuration. When a low pass filter is used by shunting a capacitor of e.g. COUT = 1.5 nF at the output of the LMV225/LMV228 to ground, this ripple is further attenuated. The cut-off frequency follows from: 20076019 (8) With the output resistance of the LMV225/LMV228 RO = 19.8 kΩ typical and COUT = 1.5 nF, the cut-off frequency equals fC = 5.36 kHz. A 100 kHz AM signal then gets attenuated by 5.36/100 or 25.4 dB. The remaining ripple will be less than 20 mV. With a slope of 40 mV/dB this translates into an error of less than ± 0.5 dB. Since the LMV226 has a low output impedance buffer, a capacitor to reduce the ripple will not be effective. FIGURE 7. Logarithmic Amplifier Every gain cell has a response according to Figure 8. At a certain threshold (EK), the gain cell starts to saturate, which means that the gain drops to zero. The output of gain cell 1 is connected to the input of gain cell 2 and so on. Output Ripple Measurement Figure 6 shows the ripple reduction that can be achieved by adding additional capacitance at the output of the LMV225/ LMV228. The RF signal of 900 MHz is AM modulated with a 100 kHz sinewave and a modulation index of 0.3. The RF input power is swept while the modulation index remains unchanged. Without the output capacitor the ripple is about 200 mVPP. Connecting a capacitor of 1.5 nF at the output to ground, results in a ripple of 12 mVPP. The attenuation with www.national.com 18 Figure 10 shows a logarithmic function on a logarithmic scale and the piecewise approximation of the logarithmic function. (Continued) 20076020 FIGURE 8. Gain Cell All gain cell outputs are AM-demodulated with a peak detector and summed together. This results in a logarithmic function. The logarithmic range is about: 20076022 20 · n · log (A) FIGURE 10. Log-Function on Log Scale where, The maximum error for this approximation occurs at the geometric mean of a gain section, which is e.g. for the third segment: n = number of gain cells A = gain per gaincell Figure 9 shows a logarithmic function on a linear scale and the piecewise approximation of the logarithmic function. (9) The size of the error increases with distance between the thresholds. LAYOUT CONSIDERATIONS For a proper functioning part a good board layout is necessary. Special care should be taken for the series resistance R1 (Figure 1) that determines the attenuation. For high resistor values the parasitic capacitance of the resistor may significantly impact the realized attenuation. The effective attenuation will be lower than intended. To reduce the parasitic capacitance across resistor R1, this resistor can be composed of several components in series instead of using a single component. 20076021 FIGURE 9. Log-Function on Lin Scale 19 www.national.com LMV225/LMV226/LMV228 Application Notes LMV225/LMV226/LMV228 Physical Dimensions inches (millimeters) unless otherwise noted NOTES: UNLESS OTHERWISE SPECIFIED 1. EPOXY COATING 2. FOR SOLDER BUMP COMPOSITION, SEE “SOLDER INFORMATION” IN THE PACKAGE SECTION OF THE NATIONAL SEMICONDUCTOR WEB PAGE (www.national.com). 3. RECOMMEND NON-SOLDER MASK DEFINED LANDING PAD. 4. PIN A1 IS ESTABLISHED BY LOWER LEFT CORNER WITH RESPECT TO TEXT ORIENTATION. REMAINING PINS ARE NUMBERED COUNTER CLOCKWISE. 5. XXX IN DRAWING NUMBER REPRESENTS PACKAGE SIZE VARIATION WHERE X1 IS PACKAGE WIDTH, X2 IS PACKAGE LENGTH AND X3 IS PACKAGE HEIGHT. REFERENCE JEDEC REGISTRATION MO-211, VARIATION BC. 4-Bump micro SMD NS Package Number TLA04AAA X1 = 1.014 ± 0.030 mm X2 = 1.014 ± 0.030 mm X3 = 0.600 ± 0.075 mm www.national.com 20 inches (millimeters) unless otherwise noted (Continued) 6-Pin LLP NS Package Number SDB06A National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications. For the most current product information visit us at www.national.com. LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. 2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. BANNED SUBSTANCE COMPLIANCE National Semiconductor manufactures products and uses packing materials that meet the provisions of the Customer Products Stewardship Specification (CSP-9-111C2) and the Banned Substances and Materials of Interest Specification (CSP-9-111S2) and contain no ‘‘Banned Substances’’ as defined in CSP-9-111S2. National Semiconductor Americas Customer Support Center Email: [email protected] Tel: 1-800-272-9959 www.national.com National Semiconductor Europe Customer Support Center Fax: +49 (0) 180-530 85 86 Email: [email protected] Deutsch Tel: +49 (0) 69 9508 6208 English Tel: +44 (0) 870 24 0 2171 Français Tel: +33 (0) 1 41 91 8790 National Semiconductor Asia Pacific Customer Support Center Email: [email protected] National Semiconductor Japan Customer Support Center Fax: 81-3-5639-7507 Email: [email protected] Tel: 81-3-5639-7560 LMV225/LMV226/LMV228 RF Power Detector for CDMA and WCDMA Physical Dimensions