application note SKY65124: WCMDA PA Bias Method For Lower Junction Temperature Introduction Active Bias Circuit This application note describes how SKY65124 may be used with reduced bias control to obtain better thermal performance. It is especially useful for customers who may not have a suitable heat sink available directly below the PA module. Circuit Description To achieve high Gain (>24 dB), the SKY65124 consists of two stages of amplification. The base current of each amplification stage is controlled by an on-die active bias circuit. Each active bias circuit has its own reference voltage supply, Vref1 and Vref2. The bias voltage (Vc_Bias) is shared by the two active bias circuits. This application note describes the active bias circuit used by SKY65124. The note explains the use of external supply voltages (Vc_Bias and Vref) and setting limits available to the customer. Test results of key parameters are also given over temperature. The active circuit is designed to maintain a constant supply current to the base of its respective transistor as supply voltage and temperature is changed. Each active bias circuit consists of three HBT transistors and three resistors, connected as shown in Figure 2 below. VC_BIAS R1 R2 Q1 VREF1 (5 V) I_Supply (to Base of PA) VREF2 (5 V) VC_BIAS (5 V) Q2 Q3 RREF2 RREF1 R3 Active Bias 1 RF In VREF RREF (On PCB) Active Bias 2 Interstage Match Input Match Stage 1 Output Match RF Out Figure 2. Schematic Diagram of Active Bias Circuit. Stage 2 VCC1 (5 V) VCC2 (5 V) Figure 1. Functional Diagram of SKY65124 PA Module Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 201020 Rev. A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • February 26, 2009 1 Application Note • SKY65124: WCMDA PA Bias Method For Lower Junction Temperature Transistor Q3 is used as a thermal reference VT monitor, and is located on-die close to the amplifier transistor. This configuration allows stable PA module operation as the amplifier temperature changes. and Q2 switches off, and effectively remove the supply current to the associated amplifier stage. Reference resistors, Rref1 and Rref2 (located on the customer PCB) can be used to trim the base reference current. Alternately, the Rref can be fixed and Vref voltages can be modified. For Vref1 = Vref2 = 5 V, the recommended values for these reference resistors are: The main supply voltage to the bias circuits, Vc_Bias, should be set to 5 V, and is internally connected to both bias circuits. Reference voltages, Vref1 and Vref2 are made available to the user to allow PA ON/OFF switching; and a certain amount of gain and current control. When the Vref voltages are set to zero, Q1 Rref1 = 330 Ohms, Rref2 = 150 Ohms. J1 RF_IN C13 10 µF C1 8.2 pF C2 1200 pF VREF1 5 R1 330 Ω R2 150 Ω VREF2 6 GND6 NC5 GND7 RF_IN N/C4 GND3 N/C3 SKY65124 VC_BIAS VCC1 N/C1 N/C2 VREF1 VCC2 7 8 9 16 15 14 13 C6 3300 pF C7 10 pF C8 10 µF C10 10 pF C11 1500 pF C12 10 µF VCC1 12 11 GND5 4 VC_BIAS 17 GND2 RF_OUT 3 18 GND4 2 GND1 VREF2 1 19 GND8 20 10 VCC2 J2 RF_OUT C3 1200 pF Figure 3. SKY65124 Evaluation Board Schematic Effect Of Reducing Vref The second stage transistor consumes the most current, whereas the first stage transistor has the most impact of overall gain. Therefore, to reduce junction temperature without severely impacting module gain, it is best to modify Vref2. Table 1 below presents the results of changing Vref2 from 4.0 V to 5.0 V. from 5.0 V to 4.0 V. However, the operating current is reduced by 86 mA, and case temperature is reduced from 71 °C to 64 °C. Note: All measurements taken at 25 °C, with the heat-sink removed from the evaluation board. All other supply voltages fixed at 5 V. The results show maximum output power (for ACLR = -45 dBc) has only slight degradation (i.e. 0.2 dB), when Vref is reduced 2 Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com February 26, 2009 • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • 201020 Rev. A Application Note • SKY65124: WCMDA PA Bias Method For Lower Junction Temperature Table 1: Maximum Pout, Gain, Icc and Case Temperature vs. Vref2 VREF2 (V) PIN (dBm) POUT (dBm) Gain (dB) ICC (mA) Case Temp (°C) 5.0 -0.2 24.80 25.00 572 70.6 4.8 -0.2 24.75 24.95 555 69.6 4.5 -0.2 24.75 24.95 528 67.3 4.3 -0.1 24.71 24.81 511 65.3 4.0 -0.1 24.63 24.73 486 64.2 Measured Junction Temperature The temperature data shows the lowest junction temperature and current consumption are achieved when Vref1 = Vref2 = 4 V. IR-scans were conducted on devices with case temperature set to 85 °C, with various combinations of Vref1 and Vref2. At each setting, output power was adjusted to 21 dBm. Table 2 shows the measured peak maximum junction temperature at each Vref1 and Vref2 combination. IR-scan plot for Vref1 = Vref2 = 4 V is shown in Figure 4 below. Table 2: Maximum Junction Temperature vs. Vref (Case Temperature = 85 °C) Vcc1; Vcc2 (V) Vc_bias (V) Vref1 (V) Vref2 (V) Icc1 (mA) Icc2 (mA) Icc_Total (mA) Pout (dBm) Measured Peak Tjmax (°C) 5 5 5.0 4.5 81 337 418 21 124 5 5 5.0 4.0 82 275 357 21 116 5 5 4.5 4.5 73 339 412 21 122 5 5 4.5 4.0 73 276 349 21 114 5 5 4.0 4.5 63 340 403 21 121 5 5 4.0 4.0 64 277 341 21 115 IR-Scan Showing thermal profile across amplifier with Vref1 = Vref2 = 4V, Case Temperature = +85 �C, Pout = 21 dBm CW Figure 4. IR-scan Plot Showing Thermal Profile Across Amplifier with Vref1 = Vref2 = 4 V, Case Temperature = 85 °C, and Pout = 21 dBm Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 201020 Rev. A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • February 26, 2009 3 Application Note • SKY65124: WCMDA PA Bias Method For Lower Junction Temperature Summary and Recommendation RF and IR-scans tests were conducted on the SKY65124 amplifier module to determine the best method for biasing the module to reduce case and junction temperature in conditions where a heat-sink may not be available. The results presented shows Vref2 can be set to 4.0 V with no significant degradation of gain and ACLR performance. The best temperature profile can be obtained with Vref1 = Vref2 = 4.0 V. With no heat-sink and case temperature to 85 °C, the maximum junction temperature was measured at 115 °C. 4 Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com February 26, 2009 • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • 201020 Rev. A Application Note • SKY65124: WCMDA PA Bias Method For Lower Junction Temperature Copyright © 2009, Skyworks Solutions, Inc. All Rights Reserved. Information in this document is provided in connection with Skyworks Solutions, Inc. (“Skyworks”) products or services. These materials, including the information contained herein, are provided by Skyworks as a service to its customers and may be used for informational purposes only by the customer. Skyworks assumes no responsibility for errors or omissions in these materials or the information contained herein. Skyworks may change its documentation, products, services, specifications or product descriptions at any time, without notice. Skyworks makes no commitment to update the materials or information and shall have no responsibility whatsoever for conflicts, incompatibilities, or other difficulties arising from any future changes. No license, whether express, implied, by estoppel or otherwise, is granted to any intellectual property rights by this document. Skyworks assumes no liability for any materials, products or information provided hereunder, including the sale, distribution, reproduction or use of Skyworks products, information or materials, except as may be provided in Skyworks Terms and Conditions of Sale. THE MATERIALS, PRODUCTS AND INFORMATION ARE PROVIDED “AS IS” WITHOUT WARRANTY OF ANY KIND, WHETHER EXPRESS, IMPLIED, STATUTORY, OR OTHERWISE, INCLUDING FITNESS FOR A PARTICULAR PURPOSE OR USE, MERCHANTABILITY, PERFORMANCE, QUALITY OR NON-INFRINGEMENT OF ANY INTELLECTUAL PROPERTY RIGHT; ALL SUCH WARRANTIES ARE HEREBY EXPRESSLY DISCLAIMED. SKYWORKS DOES NOT WARRANT THE ACCURACY OR COMPLETENESS OF THE INFORMATION, TEXT, GRAPHICS OR OTHER ITEMS CONTAINED WITHIN THESE MATERIALS. SKYWORKS SHALL NOT BE LIABLE FOR ANY DAMAGES, INCLUDING BUT NOT LIMITED TO ANY SPECIAL, INDIRECT, INCIDENTAL, STATUTORY, OR CONSEQUENTIAL DAMAGES, INCLUDING WITHOUT LIMITATION, LOST REVENUES OR LOST PROFITS THAT MAY RESULT FROM THE USE OF THE MATERIALS OR INFORMATION, WHETHER OR NOT THE RECIPIENT OF MATERIALS HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. Skyworks products are not intended for use in medical, lifesaving or life-sustaining applications, or other equipment in which the failure of the Skyworks products could lead to personal injury, death, physical or environmental damage. Skyworks customers using or selling Skyworks products for use in such applications do so at their own risk and agree to fully indemnify Skyworks for any damages resulting from such improper use or sale. Customers are responsible for their products and applications using Skyworks products, which may deviate from published specifications as a result of design defects, errors, or operation of products outside of published parameters or design specifications. Customers should include design and operating safeguards to minimize these and other risks. Skyworks assumes no liability for applications assistance, customer product design, or damage to any equipment resulting from the use of Skyworks products outside of stated published specifications or parameters. Skyworks, the Skyworks symbol, and “Breakthrough Simplicity” are trademarks or registered trademarks of Skyworks Solutions, Inc., in the United States and other countries. Third-party brands and names are for identification purposes only, and are the property of their respective owners. Additional information, including relevant terms and conditions, posted at www.skyworksinc.com, are incorporated by reference. Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 201020 Rev. A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • February 26, 2009 5