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| 19-1372; Rev 1; 6/98 KIT ATION EVALU ABLE AVAIL Low-Noise Bias Supply in MAX with Power-OK for GaAsFET PA General Description Features o Small MAX Package o 1mVp-p Output Voltage Ripple and Noise o Power-OK Signal to Control GaAsFET Drain Switch o 0.05A Logic-Controlled Shutdown o 1ms Guaranteed Start-Up o 2.5V to 5.5V Input o -0.5V to (-VIN + 0.6V) Output at up to 4mA o Operates with One 4.7F and Three 0.22F Capacitors (no inductors needed) MAX881R The MAX881R low-noise, inverting power supply is designed for biasing GaAsFET power amplifiers in portable wireless applications. This device is a chargepump inverter followed by a negative linear regulator. The input voltage range is 2.5V to 5.5V. The output is preset at -2.0V or can be set, using two resistors, to any voltage from -0.5V to (-VIN + 0.6V). It can deliver up to 4mA. The internal linear regulator also filters the output to 1mVp-p ripple and noise. Other features include a power-OK (POK) output that signals when the negative voltage is within 7.5% of its set point. It protects the GaAsFET by not allowing power to be applied to the GaAsFET's drain until it is properly biased. The signal can be routed either to a microcontroller or directly to a switch at the GaAsFET drain. The MAX881R is available in a space-saving 10-pin MAX package. Applications Cell Phones PCS Phones PHS Phones Wireless Handsets Wireless Modems Two-Way Pagers Mobile Radios Wireless Computers PART MAX881REUB Ordering Information TEMP. RANGE -40C to +85C PIN-PACKAGE 10 MAX Typical Operating Circuit TOP VIEW INPUT 2.5V TO 5.5V IN OUT C1+ 1 OUTPUT -2V (OR ADJ) 4mA, 1mVp-p RIPPLE Pin Configuration 10 IN 9 N.C. GND OUT FB MAX881R ON OFF SHDN C1NEGOUT 2 3 4 5 MAX881R 8 7 6 C1+ POK C1NEGOUT FB GND CONTROLS GaAsFET PA's DRAIN SWITCH POK SHDN MAX N.C. = NOT INTERNALLY CONNECTED ________________________________________________________________ Maxim Integrated Products 1 For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800. For small orders, phone 408-737-7600 ext. 3468. Low-Noise Bias Supply in MAX with Power-OK for GaAsFET PA MAX881R ABSOLUTE MAXIMUM RATINGS IN to GND .................................................................-0.3V to +6V SHDN to GND...........................................................-0.3V to +6V POK to GND ...........................................................-0.3V to +12V C1+ to GND .................................................-0.3V to (VIN + 0.3V) C1-, NEGOUT, OUT, FB to GND ....................-6V to (VIN + 0.3V) Continuous Power Dissipation (TA = +70C) 10-Pin MAX (derate 5.6mW/C above +70C) ...........444mW Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +165C Lead Temperature (soldering, 10sec) .............................+300C Note 1: The output may be shorted to NEGOUT or GND if the package power dissipation is not exceeded. Typical short-circuit current is 35mA. Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. ELECTRICAL CHARACTERISTICS (Circuit of Figure 3, VIN = +3.6V, FB = GND, RL = , SHDN = IN, TA = -40C to +85C, unless otherwise noted. Typical values are at TA = +25C.) (Note 2) PARAMETER Supply Voltage Range Preset Output Voltage Adjustable Output Voltage Range FB Voltage FB Input Current Supply Current Shutdown Supply Current Output Load Regulation Output Ripple Oscillator Frequency POK Threshold POK Output Level POK Off Leakage Current SHDN Input High Voltage SHDN Input Low Voltage SHDN Input Current SHDN Input Capacitance Start-Up Time VIH VIL ISHDN CIN tSTART VIN = 3V, RL = 500, V SHDN = 0 to VIN, POK goes low fOSC FB = OUT VIN 2.5V, sinking 1mA V POK = 11V VIN = 5.5V VIN = 2.5V Connected to IN or GND 10 1 2.2 0.35 1 IQ ISHUT SHDN = GND VIN 2.7V, IOUT = 0 to 4mA IOUT = 4mA, circuit of Figure 3b 80 90 SYMBOL VIN VOUT VOUT VIN 2.7V, IOUT = 0 to 4mA VIN 2.5V, IOUT = 0 to 4mA VIN 2.5V, IOUT = 0 to 4mA VFB = -0.5V TA = +25C TA = 0C to +85C TA = -40C to +85C CONDITIONS MIN 2.5 -2.1 -(VIN - 0.6) -0.515 -0.525 -0.535 -10 500 0.05 2 1 100 92.5 120 95 100 1 -0.5 -2.0 TYP MAX 5.5 -1.9 -0.5 -0.485 -0.475 -0.465 -100 950 1 6 nA A A mV/mA mVp-p kHz % of VOUT mV A V V A pF ms V UNITS V V V VFB Note 2: Specifications to -40C are guaranteed by design, not production tested. 2 _______________________________________________________________________________________ Low-Noise Bias Supply in MAX with Power-OK for GaAsFET PA MAX881R Typical Operating Characteristics (Circuit of Figure 3, VIN = 3.6V, TA = +25C, unless otherwise noted.) OUTPUT VOLTAGE vs. OUTPUT CURRENT CIRCUIT OF FIGURE 3a -1.86 -1.88 OUTPUT VOLTAGE (V) 1.90 -1.92 -1.94 -1.96 -1.98 -2.00 -2.02 0 1 2 3 4 5 6 7 8 9 10 OUTPUT CURRENT (mA) VIN 3.6V VIN = 2.5V MAX881R TOC01 OUTPUT VOLTAGE vs. OUTPUT CURRENT -1.86 -1.88 OUTPUT VOLTAGE (V) -1.90 -1.92 -1.94 -1.96 -1.98 -2.00 -2.02 0 2 4 6 8 10 12 14 16 OUTPUT CURRENT (mA) VIN 3.6V VIN = 2.5V CIRCUIT OF FIGURE 3b MAX881R TOC02 OUTPUT VOLTAGE vs. INPUT VOLTAGE -1.96 OUTPUT VOLTAGE (V) -1.97 -1.98 -1.99 -2.00 -2.01 -2.02 -2.03 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 INPUT VOLTAGE (V) ILOAD = 4mA MAX881R TOC03 -1.84 -1.84 -1.95 MAXIMUM OUTPUT CURRENT vs. INPUT VOLTAGE MAX881r TOC04 MAXIMUM NEGOUT CURRENT vs. INPUT VOLTAGE MAX881R TOC05 NO-LOAD SUPPLY CURRENT vs. INPUT VOLTAGE MAX881R TOC06 45 40 OUTPUT CURRENT (mA) 35 30 25 20 15 10 5 0 VOUT -1.8V CIRCUIT OF FIGURE 3b 24 20 OUTPUT CURRENT (mA) 16 12 8 4 0 VNEGOUT = 0.9 x VNEGOUT(NO LOAD) 900 800 SUPPLY CURRENT (A) 700 600 500 400 300 VOUT -1.8V CIRCUIT OF FIGURE 3a VOUT < 2mVp-p RIPPLE CIRCUIT OF FIGURE 3b VOUT < 2mVp-p RIPPLE CIRCUIT OF FIGURE 3a CIRCUIT OF FIGURE 3b CIRCUIT OF FIGURE 3a 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 INPUT VOLTAGE (V) INPUT VOLTAGE (V) INPUT VOLTAGE (V) NO-LOAD SUPPLY CURRENT vs. TEMPERATURE MAX881R TOC07 START-UP TIME vs. INPUT VOLTAGE MAX881R TOC08 OSCILLATOR FREQUENCY vs. INPUT VOLTAGE TA = -40C 102 FREQUENCY (kHz) 101 100 99 98 97 TA = +25C TA = +85C MAX881R TOC09 900 800 SUPPLY CURRENT (A) 700 600 500 400 VIN = 2.5V 300 -60 -40 -20 0 20 40 60 80 VIN = 3.6V 800 700 START-UP TIME (s) 600 500 400 300 200 100 CIRCUIT OF FIGURE 3a NO LOAD SHDN TO POK DELAY 103 VIN = 5.5V 100 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 TEMPERATURE (C) INPUT VOLTAGE (V) INPUT VOLTAGE (V) _______________________________________________________________________________________ 3 Low-Noise Bias Supply in MAX with Power-OK for GaAsFET PA MAX881R Typical Operating Characteristics (continued) (Circuit of Figure 3, VIN = 3.6V, TA = +25C, unless otherwise noted.) START-UP FROM SHUTDOWN (NO LOAD) MAX881R-TOC10 OUTPUT NOISE AND RIPPLE (CIRCUIT OF FIGURE 3a) MAX881R-TOC11 OUTPUT NOISE AND RIPPLE (CIRCUIT OF FIGURE 3b) MAX881R-TOC12 SHDN +5V 0 VIN = 3.6V VOUT = -2V IOUT = 2.5mA RL = 500 VOUT = 0 VIN = 0 1mV/div AC COUPLED VOUT = -2V VIN = 3.6V 1mV/div AC COUPLED NOISE FLOOR VOUT 0 -2V POK VIN = 3.6V CIRCUIT OF FIGURE 3b 100s/div VIN 0 VOUT 1mV/div AC COUPLED OUTPUT NOISE 5s/div 10s/div OUTPUT NOISE SPECTRUM (CIRCUIT OF FIGURE 3a) MAX881R TOC13 OUTPUT NOISE SPECTRUM (CIRCUIT OF FIGURE 3b) 50 40 30 NOISE (dBV) 20 10 0 -10 -20 -30 -40 VIN = 3.6V VOUT = -2V IOUT = 4mA MAX881R TOC14 60 50 40 30 NOISE (dBV) 20 10 0 -10 -20 -30 -40 1 10 100 VIN = 3.6V VOUT = -2V IOUT = 2.5mA 60 1000 1 10 100 1000 FREQUENCY (kHz) FREQUENCY (kHz) LINE-TRANSIENT RESPONSE MAX881R-TOC15 LOAD-TRANSIENT RESPONSE MAX881R-TOC16 4mA 3.3V VIN 2.7V IOUT 400A VOUT ILOAD = 4mA VOUT = -2V CIRCUIT OF FIGURE 3b 2ms/div 10mV/ div VOUT VIN = 3.6V VOUT = -2V CIRCUIT OF FIGURE 3b 100s/div 10mV/ div 4 _______________________________________________________________________________________ Low-Noise Bias Supply in MAX with Power-OK for GaAsFET PA Pin Description PIN 1 2 3 4 5 NAME C1+ C1NEGOUT POK SHDN Positive Terminal for C1 Negative Terminal for C1 Negative Output Voltage (unregulated) Active-Low, Open-Drain Power-OK Output. Goes low when OUT reaches 92.5% of its set value. Active-Low, Logic-Level Shutdown Input. Connect to IN for normal operation. Do not leave this pin unconnected. Dual-ModeTM Feedback Input. When FB is connected to GND, the output is preset to -2V. To select other voltages, connect FB to an external resistor-divider (Figure 4). Do not leave this pin unconnected. Regulated Negative Output Voltage Ground No Connection. Not internally connected. Positive Power-Supply Input FUNCTION MAX881R 6 7 8 9 10 FB OUT GND N.C. IN Dual-Mode is a trademark of Maxim Integrated Products. SHDN C1+ C1 C1CHARGE PUMP IN C3 NEGOUT C2 FB OUT C4 MAX881R 0.5V 0.4625V POK GND Figure 1. Functional Diagram _______________________________________________________________________________________ 5 Low-Noise Bias Supply in MAX with Power-OK for GaAsFET PA MAX881R OUT C4 TO PA GATE BIAS VCC FOR POWER AMPLIFIER VIN C3 0.22F C1+ 100k POK P TO POWER AMPLIFIER ON OFF C2 0.22F C1 0.22F C1SHDN NEGOUT POK GND FB IN OUT VOUT = -2V C4 4.7F MAX881R MAX881R Figure 2. Using the POK Function ________________Detailed Description The MAX881R, a low-noise, inverting, regulated chargepump power supply, is designed for biasing GaAsFET devices such as power-amplifier modules in cellular handsets. The applied input voltage (VIN) is inverted to a negative voltage at NEGOUT by a capacitive charge pump. This voltage is regulated by an internal linear regulator at OUT (Figure 1). With FB connected to GND, VOUT is regulated at -2V. Alternatively, use a voltage-divider at FB to adjust the output voltage between -0.5V and -(VIN - 0.6V) (see the section Setting the Output Voltage). The internal linear regulator reduces the ripple noise induced by the charge-pump inverter to 1mVp-p at OUT (circuit of Figure 3b). In addition, the excellent AC rejection of the linear regulator attenuates noise from the incoming supply. Figure 3a. Standard Application Circuit for Minimum Capacitor Values VIN C3 1F C1+ C1 1F ON OFF C2 1F C1SHDN NEGOUT POK GND FB IN OUT VOUT = -2V C4 10F MAX881R Power-OK Signal The MAX881R has an active-low, open-drain, power-OK (POK) output. This output goes low when OUT reaches 92.5% of the regulated output voltage. POK can be used to drive the gate of a P-MOSFET that switches power to the GaAsFET power amplifier (Figure 2), thereby ensuring that the power amplifier is not powered before the required negative bias voltage is present. Use a 50k or larger pull-up resistor to signal a logic high when POK goes high impedance. Figure 3b. Standard Application Circuit for Minimum Output Noise Applications Information Setting the Output Voltage Select either a fixed or adjustable output for the MAX881R. Connect FB to GND for a fixed -2V output (Figure 3). Select an alternative output voltage by connecting FB to the midpoint of a resistor-divider from OUT to GND (Figure 4). When operating under full load (4mA), the voltage at IN should be at least 0.6V higher than the absolute voltage required at OUT. Note that the minimum input voltage required for operation is 2.5V, regardless of the desired output voltage. Choose R1 to be between 100k and 400k and calculate R2. For greater accuracy, use resistors with 1% or better tolerance. R2 = R1 (2 x |VOUT| - 1) Shutdown Mode The MAX881R features a shutdown mode that reduces supply current to less than 1A over temperature. SHDN is an active-low, logic-level input. Start-up time coming out of shutdown mode is typically 0.5ms. OUT and NEGOUT are switched to GND in shutdown mode. 6 _______________________________________________________________________________________ Low-Noise Bias Supply in MAX with Power-OK for GaAsFET PA MAX881R VIN C3 0.22F C1+ C1 0.22F ON OFF C2 0.22F C1SHDN NEGOUT GND IN OUT R2 FB POK R1 C4 4.7F tance of the internal switches) is typically 10. When regulating, the output resistance of the circuit is simply the linear-regulator load regulation (2mV/mA). C1, C2, and C3 should be 0.22F capacitors with less than 0.4 ESR. C4 should be a 4.7F capacitor with less than 0.1 ESR. Larger capacitor values can be used (C1 = C2 = C3 = 1F, C4 = 10F) to reduce output noise and ripple (1mVp-p), at the expense of cost and board space. All capacitors should be either ceramic or surface-mount chip tantalum (Figures 3a and 3b). MAX881R Layout and Grounding Good layout is important for good noise performance. To optimize the layout: Figure 4. Adjustable Output Configuration Capacitors Use capacitors with a low effective series resistance (ESR) to maintain a low dropout voltage (VIN - |VOUT|). The overall dropout voltage is a function of the output resistance of the charge pump and the voltage drop across the linear regulator (N-channel pass transistor). At the 100kHz charge-pump switching frequency, output resistance is a function of the value of C1 and the ESR of C1 and C2. Therefore, increasing C1 and minimizing the ESR of the charge-pump capacitors minimizes dropout voltage. The output resistance of the entire circuit (in dropout) is approximately: ROUT = RO + 4 x ESRC1 + ESRC2 + 1 / (fS x C1) + R(linear regulator) Where R(linear regulator) (the output impedance of the linear regulator) is approximately 2 and RO (the resis- 1) Mount all components as close together as possible. 2) Keep traces short to minimize parasitic inductance and capacitance, especially connections to FB. 3) Use a ground plane. Noise and Ripple Measurement Accurately measuring the output noise and ripple is a challenge. Slight momentary differences in ground potential between the circuit and the oscilloscope (which result from the charge pump's switching action) cause ground currents in the probes' wires, inducing sharp voltage spikes. For best results, measure directly across the output capacitor (C4). Do not use the ground lead of the probe; instead, remove the probe's tip cover and touch the ground ring on the probe directly to C4's ground terminal. This direct connection gives the most accurate noise and ripple measurement. Chip Information TRANSISTOR COUNT: 413 _______________________________________________________________________________________ 7 Low-Noise Bias Supply in MAX with Power-OK for GaAsFET PA MAX881R Package Information 10LUMAXB.EPS 8 _______________________________________________________________________________________ |
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