19-1426; Rev 0; 2/99 NUAL KIT MA ATION EET H S A EVALU T WS DA FOLLO Step-Up DC-DC Converters with Precise, Adaptive Current Limit for GSM The MAX1687/MAX1688 step-up DC-DC converters deliver up to 2W from a single Li-Ion or three NiMH cells. The devices are ideal for burst-load applications such as GSM cell phones and wireless LANs, where the RF power amplifiers require short, high current bursts. The MAX1687/MAX1688 reduce battery surge current by slowly charging a reservoir capacitor, which supplies the necessary peak energy for the load current burst. As a result, the peak battery current is limited, thus maximizing battery life and minimizing battery voltage sag and transient dips. An internal synchronous rectifier provides over 90% conversion efficiency and eliminates the need for an external Schottky diode. A logic shutdown mode reduces the shutdown current to only 3µA. The devices can be disabled during current bursts (RF transmit mode) to eliminate switching noise. The switching frequency of the MAX1687/MAX1688, controlled by the selected inductor, can exceed 1MHz. Two external resistors set the output voltage from 1.25V to 6V. The MAX1687 controls peak battery current, while the MAX1688 features a more advanced, adaptive constantrecharge-time algorithm that maximizes battery life. The MAX1687/MAX1688 are available in thin 16-pin TSSOP (1.1mm max height) or standard 8-pin SO packages. Features ♦ Low 450mA Peak Battery Current Provides 2A, 5V GSM Burst ♦ 90% Efficiency ♦ Internal Power MOSFETs and Current-Sense Resistor ♦ Output Disconnects from Input During Shutdown ♦ 3µA Shutdown Current ♦ Precise Voltage-Controlled Current Limit (MAX1687) ♦ Adaptive Constant-Recharge-Time Capability (MAX1688) ♦ 1.25V to 6V Adjustable Output ♦ 2.7V to 6V Input Range (1 Li-Ion cell or 3 NiMH cells) ♦ Switching Frequency Can Exceed 1MHz ♦ Standby Mode Disables DC-DC During Transmission Burst ♦ Low Inrush Current at Start-Up Ordering Information TEMP. RANGE PIN-PACKAGE -40°C to +85°C 16 TSSOP -40°C to +85°C MAX1687ESA MAX1688EUE -40°C to +85°C MAX1688ESA -40°C to +85°C *U.S. and foreign patents pending. 8 SO 16 TSSOP 8 SO PART* MAX1687EUE Applications GSM Phones Wireless Handsets PC Cards (PCMCIA) Typical Operating Circuit Pin Configurations 2.7V TO 6V TOP VIEW IN IN 1 16 OUT IN 2 15 OUT LX1 3 14 LX2 LX1 4 LIM [CHG] 5 MAX1687 MAX1688 1 Li-lon OR 3 NiMH OR 3 ALKALINE LX2 OUT MAX1687 MAX1688 13 LX2 ON 12 PGND FB 6 11 PGND REF 7 10 AGND N.C. 8 9 OFF 0 TO 1V CONTROL INPUT VOUT UP TO 6V FB ON (LIM) [CHG] REF ON TSSOP Pin Configurations continued at end of data sheet. GND [ ] ARE FOR MAX1688 †Patent pending LX1 ( ) ARE FOR MAX1687 [ ] ARE FOR MAX1688 ________________________________________________________________ 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 1-800-835-8769. MAX1687 †/MAX1688 † General Description MAX1687/MAX1688 Step-Up DC-DC Converters with Precise, Adaptive Current Limit for GSM ABSOLUTE MAXIMUM RATINGS IN, ON, LX1, CHG, LIM, FB, OUT, REF to GND .......-0.3V to +7V LX2 to GND ..............................................................-0.3V to +8V IN, LX1 Average Current..........................................................1A Continuous Power Dissipation (TA = +70°C) TSSOP (derate 5.7mW/°C above +70°C) ....................457mW SO (derate 5.88mW/°C above +70°C) .........................471mW Operating Temperature Range ...........................-40°C to +85°C Storage Temperature Range .............................-65°C to +150°C Lead Temperature (soldering, 10sec) ............................+300° C 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 (VIN = VON = +3V, VLIM = 1V (MAX1687), VCHG = 1V (MAX1688), VFB = 1.5V, VOUT = 6V, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL CONDITIONS Input Voltage Range IN rising, 1% hysteresis Output Voltage Range VREF gmFB ICHG Source Current Peak Current IPEAK IRIPPLE Sense Resistor RSENSE ON Input High Voltage Input Current VIL VIH UNITS 6 V 2.6 V 6 V 2 4 mA Shutdown, VIN = 4.2V, LX2 connected to LX1, VOUT = 0, ON = GND 3 10 µA 0.7 1.2 1.8 ms IREF = 0 to 10µA 1.225 1.25 1.275 V VFB rising, 2% hysteresis 1.212 1.250 1.288 V VFB = 1.125V, VOUT = 3V (MAX1688) 0.18 0.2 0.22 mmho 60 110 VFB = 0, VOUT = 3V (MAX1688) Ripple Current ON Input Low Voltage 2.5 MAX VFB = 1.5V tDELAY FB Set Voltage FB Transconductance 2.4 VREF Input Supply Current Reference Voltage TYP 2.7 Input Undervoltage Lockout Shutdown Delay MIN µA VLIM = VCHG = 1V 0.744 0.8 0.856 VLIM = VCHG = 0.65V 0.46 0.5 0.54 VLIM = VCHG = 1V 170 200 230 mA 0.1 0.18 Ω 0.6 V VIN = 2.7V VIN = 6V 1.8 VIN = 4.2V 1.5 V IFB VFB = 1.5V 0.05 0.2 ION VON = 0 or 3V 0.02 0.1 0.02 0.1 ILIM VLIM = 1V TA = +25°C A TA = 0°C to +85°C µA 2 N-Channel On-Resistance VIN = 2.7V 0.4 0.8 Ω P-Channel On-Resistance VIN = 2.7V 0.3 0.7 Ω Precharge On-Resistance VIN = 4V, VFB = 0, VOUT = 0 30 70 Ω LX2 Leakage Current VIN = VLX2 = 6V, VOUT = VON = 0 0.05 10 µA 2 _______________________________________________________________________________________ Step-Up DC-DC Converters with Precise, Adaptive Current Limit for GSM (VIN = VON = +3V, VLIM = 1V (MAX1687), VCHG = 1V (MAX1688), VFB = 1.5V, VOUT = 6V, TA = -40°C to +85°C, unless otherwise noted.) (Note 1) PARAMETER SYMBOL CONDITIONS MIN MAX UNITS 2.7 6 V 2.35 2.65 V VREF 6 V VFB = 1.5V 5 mA Shutdown VIN = 4.2V, LX2 connected to LX1, VOUT = 0, ON = GND 10 µA Input Voltage Range Input Undervoltage Lockout IN rising, 1% hysteresis Output Voltage Range IIN Input Supply Current ISHDN Shutdown Delay 0.6 2 ms IREF = 0 to 10µA 1.212 1.288 V FB rising, 2% hysteresis 1.20 1.30 V VFB = 1.125V, VOUT = 3V (MAX1688) 0.16 0.24 mmho VLIM = VCHG = 1V 0.73 0.90 VLIM = VCHG = 0.65V 0.44 0.57 VLIM = VCHG = 1V 145 240 mA 0.18 Ω 0.6 V TDELAY Reference Voltage VREF FB Set Voltage FB Transconductance gmFB Peak Current IPEAK Ripple Current IRIPPLE Sense Resistor RSENSE ON Input Low Voltage VIL ON Input High Voltage TYP VIN = 2.7V VIH VIN = 6V 1.8 VIN = 4.2V 1.5 A V N-Channel On-Resistance VIN = 2.7V 0.8 Ω P-Channel On-Resistance VIN = 2.7V 0.7 Ω Precharge On-Resistance VIN = 4V, VFB = 0, VOUT = 0 70 Ω Note 1: Specifications to -40°C are guaranteed by design, not production tested. Typical Operating Characteristics (VIN = +3.3V, VOUT = 5V, VLIM = 1V, Figures 6b and 7, TA = +25°C, unless otherwise noted.) EFFICIENCY vs. GSM BURST LOAD (VOUT = 5.5V) 90 VIN = 5V 90 75 VIN = 2.7V 65 80 VIN = 3.3V 75 VIN = 2.7V 70 VIN = 6V 100 150 200 250 LOAD CURRENT (mA) 300 350 85 80 75 70 65 60 50 90 VIN = 6V 65 60 0 95 EFFICIENCY (%) EFFICIENCY (%) EFFICIENCY (%) VIN = 3.3V 70 VIN = 5V 85 85 80 100 MAX1687/88 toc02 95 MAX1687/88 toc01 95 EFFICIENCY vs. LOAD CURRENT (VIN = 2.7V, VOUT = 3.3V) MAX1687/88 toc03 EFFICIENCY vs. DC LOAD CURRENT (VOUT = 5.5V) 60 0 500 1000 1500 2000 LOAD CURRENT (mA) 2500 3000 0 50 100 150 200 250 300 350 LOAD CURRENT (mA) _______________________________________________________________________________________ 3 MAX1687/MAX1688 ELECTRICAL CHARACTERISTICS _____________________________Typical Operating Characteristics (continued) (VIN = +3.3V, VOUT = 5V, VLIM = 1V, Figures 6b and 7, TA = +25°C, unless otherwise noted.) MAX1688 PEAK BATTERY CURRENT vs. RCHG (1A GSM LOAD) 400 300 3.0 1.210 1.205 VIN = 2.7V 20 25 30 35 40 1.200 -40 RCHG (kΩ) -20 0 20 40 60 80 1 100 10 1.249 MAX1687/88 toc08 1200 1000 FREQUENCY (kHz) 1.251 1000 SWITCHING FREQUENCY vs. INDUCTANCE (VIN = 3.3V, VOUT = 5V, ILOAD = 100mA, VLIM = 1V) MAX1687/88 toc07 1.253 100 IREF (µA) TEMPERATURE (°C) REFERENCE VOLTAGE vs. TEMPERATURE (VIN = 3.3V, VOUT = 5V) REFERENCE VOLTAGE (V) 1.225 1.215 VIN = 3.3V 1.5 15 1.230 1.220 VIN = 5V 2.5 2.0 0 MAX1687/88 toc06 1.235 3.5 200 100 1.245 1.240 VREF (V) 500 VIN = 6V 4.0 1.250 MAX1687/88 toc05 600 4.5 SUPPLY CURRENT (mA) 700 REFERENCE VOLTAGE vs. REFERENCE CURRENT (VIN = 3.3V, VOUT = 5V) NO-LOAD BATTERY INPUT CURRENT vs. TEMPERATURE (VOUT = 5V, VLIM = 1V ) MAX1687/88 toc04 800 PEAK BATTERY CURRENT (mA) 800 600 400 1.247 200 0 1.245 0 20 40 60 80 100 10 100 INDUCTANCE (µH) MAX1688 IPEAK vs. VOUT DROOP MAX1688 PEAK INDUCTOR CURRENT vs. RCHG (1A GSM LOAD) MAX1687/88 toc09 800 750 700 650 600 550 500 450 400 RCHG = 40.2k 350 800 700 600 500 400 300 200 100 0 300 100 150 200 250 300 VOUT DROOP (mV) 4 1 TEMPERATURE (°C) MAX1687/88 toc10 -20 PEAK INDUCTOR CURRENT (mA) -40 IPEAK (mA) MAX1687/MAX1688 Step-Up DC-DC Converters with Precise, Adaptive Current Limit for GSM 350 400 15 20 25 30 RCHG (kΩ) _______________________________________________________________________________________ 35 40 Step-Up DC-DC Converters with Precise, Adaptive Current Limit for GSM (VIN = +3.3V, VOUT = 5V, VLIM = 1V, Figures 6b and 7, TA = +25°C, unless otherwise noted.) MAX1688 SWITCHING WAVEFORMS (GSM PULSED LOAD 1A, RCHG = 40.2kΩ) SWITCHING WAVEFORMS (FIXED ILOAD = 300mA) MAX1687/88 toc10a MAX1687/88 toc11 500mA/div ILX 500mA/div ILOAD ILX 500mA/div VOUT VOUT 200mV/div 100mV/div 1ms/div 500µs/div RCHL = 40.2kΩ, L = 10µH MAX1688 SWITCHING WAVEFORMS (GSM PULSED LOAD 1A, RCHG = 18kΩ) INDUCTOR CURRENT MAX1687/88 toc12 MAX1687/88 toc13 500mA/div ILX 500mA/div ILOAD ILX VOUT 200mV/div VLIM = 1V ILX 200mA/div VLIM = 0 0A 1ms/div 2µs/div RCHG = 18kΩ, L = 10µH POWER-UP WAVEFORM (RLOAD = 15Ω COUT = 2000µF) VON vs. BATTERY CURRENT MAX1687/88 toc14 MAX1687/88 toc15 VON VOUT 2V/div VON 1V/div IBATTERY 200mA/div 1V/div 5ms/div 10µs/div _______________________________________________________________________________________ 5 MAX1687/MAX1688 Typical Operating Characteristics (continued) Step-Up DC-DC Converters with Precise, Adaptive Current Limit for GSM MAX1687/MAX1688 Pin Description PIN MAX1687 MAX1688 NAME FUNCTION Supply Voltage Input. Connect Battery to IN. Bypass to GND with a 47µF minimum capacitor. SO TSSOP SO TSSOP 1 1, 2 1 1, 2 IN 2 3, 4 2 3, 4 LX1 Internal Current-Sense Resistor Output. Connect the inductor between LX1 and LX2. 3 5 — — LIM Voltage-Controlled Current-Limit Adjust Input. Apply a voltage between 0 and 1V to vary the current limit. LIM is internally clamped to 1.25V. — — 3 5 CHG Constant-Recharge-Time Input. Set the recharge time of the output reservoir capacitor by connecting a resistor from CHG to GND (see Applications Information section). 4 6 4 6 FB — 7 — 7 REF Reference Voltage Output. 1.25V nominal. — 8 — 8 N.C. No Connection. Not internally connected. Logic ON/OFF Input. When ON is high, the device operates in normal mode. When ON goes low, the device goes into standby mode. If ON remains low for greater than 1.2ms, the device shuts down (see Standby/Shutdown section). The supply current falls to 3µA in shutdown mode. 5 9 5 9 ON 6 — 6 — GND — 10 — 10 AGND Analog Ground — 11, 12 — 11, 12 PGND Power Ground 7 13, 14 7 13, 14 LX2 N-Channel and P-Channel MOSFET Drain 8 15, 16 8 15, 16 OUT Output Detailed Description The MAX1687 and MAX1688 ICs supply power amplifiers in GSM applications where limited input current surge is desirable. For example, GSM systems require high-power, 12% duty-cycle RF bursts. Synchronizing the MAX1687/MAX1688 to enter standby mode during these RF bursts eliminates battery surge current and minimizes switching noise to the power amplifier. In standby mode, the charged output reservoir capacitor delivers power to the power amplifier. Between each burst, the DC-DC converter switches on to charge the output capacitor. To improve efficiency and reduce peak battery current, the MAX1687/MAX1688 provide a volt6 Feedback Input. Connect a resistor-divider from OUT to GND to set the output voltage. FB regulates to a nominal 1.25V. Ground age-controlled current limit. The MAX1688 is a MAX1687 with added self-regulating circuitry that recharges the reservoir capacitor in a fixed time (Figure 1). Start-Up Sequence In a conventional DC-DC converter, when high current is required by the load, the battery voltage droops due to battery series resistance. This may cause other circuitry that depends on the battery to malfunction or be reset. The MAX1687/MAX1688 prevent battery voltage droop by charging the reservoir capacitor during system off-time and isolate the battery from the output during high current demand. The MAX1687/MAX1688 are gentle to the battery during initial power-up, as well. _______________________________________________________________________________________ Step-Up DC-DC Converters with Precise, Adaptive Current Limit for GSM MAX1687/MAX1688 LX2 LX1 VOUT Q1 VIN P-SWITCH Q2 P-SWITCH ZERO CROSSING N-SWITCH CONSTANT HYSTERETIC INDUCTOR-CURRENT CONTROL LOGIC Q3 PEAK/ TROUGH INDUCTORCURRENT DETECT gm REF FB (LIM) [CHG] MAX1687 MAX1688 gm TIMER VPRECHARGE VIN VIN - VDIODE VOUT ON ( ) ARE FOR MAX1687 [ ] ARE FOR MAX1688 (ALSO DASHED LINES) Figure 1. Functional Diagram When starting up, the MAX1687/MAX1688 employ four successive phases of operation to reduce the inrush of current from the battery. These phases are Linear Regulator Mode, Pseudo Buck Mode, Pseudo Boost Mode, and Boost Mode. In Linear Mode, the output connects to the input through a 30Ω precharge PMOS device (Figure1, Q1). The transition from Linear Mode to Pseudo Buck Mode occurs when VOUT = VIN - 3V. The transition from Pseudo Buck Mode to Pseudo Boost Mode occurs when VOUT = VIN - 0.7V. The transition from Pseudo Boost Mode to Boost Mode occurs when VOUT > VIN. Due to these mode changes, the battery input current remains relatively constant, and VOUT changes slope as it rises. Hysteretic Inductor-Current Control Logic circuits in the MAX1687/MAX1688 control the inductor ripple current to typically 200mA (Figure 2). The voltage at LIM (CHG) programs IPEAK. The inductor current oscillates between I PEAK - 200mA and IPEAK. Standby/Shutdown When ON goes low, the device enters Standby Mode, inductor current ramps to zero, and the output disconnects from the input. If ON remains low for greater than 1.2ms (typ), the device shuts down and quiescent current drops to 3µA (typ). _______________________________________________________________________________________ 7 CURRENT MAX1687/MAX1688 Step-Up DC-DC Converters with Precise, Adaptive Current Limit for GSM ILOAD IPEAK SET BY VLIM (VCHG) HYSTERESIS BAND IPEAK - 200mA VOUT “ON” CONTROL INPUT TIME TIME ( ) ARE FOR MAX1688 Figure 2. Hysteretic Inductor Current Figure 4. Timing Diagram of “ON” Applications Information Adjusting the Output Voltage VOUT OUT Adjust the MAX1687/MAX1688 output voltage with two external resistors (Figure 3). Choose R2 to be between 10kΩ to 100kΩ. Calculate R1 as follows: R1 MAX1687 MAX1688 R1 = R2 · (VOUT - VFB ) / VFB FB where VFB is the feedback threshold voltage, 1.25V nominal. R2 Adjusting Current Limit (MAX1687) R1 = R2 V -V ( OUTVFB FB ) Figure 3. Setting the Output Voltage Synchronized ON Pin If desired, drive ON low during periods of high current demand to eliminate switching noise from affecting sensitive RF circuitry. During the periods when ON is low, the output reservoir capacitor provides current to the load (Figure 4). The MAX1687 has an adjustable current limit for applications requiring limited supply current, such as PC card sockets or applications with variable burst loads. For single Li-Ion battery cell applications, the high peak current demands of the RF transmitter power amplifier can pull the battery very low as the battery impedance increases toward the end of discharge. The reservoir capacitor at the output supplies power during load-current bursts; this allows for a lower input current limit. With this feature, the life of the Li-Ion battery versus the reservoir capacitor size trade-off can be optimized for each application. Buck Capability Although the IC is not intended for this application, the MAX1687/MAX1688 operate as a buck converter when the input voltage is higher than the output voltage. The MAX1687/MAX1688 are not optimally efficient in this mode (see Typical Operating Characteristics for efficiencies at 2.7V, 3.3V, 5V, and 6V input supply voltages). 8 _______________________________________________________________________________________ Step-Up DC-DC Converters with Precise, Adaptive Current Limit for GSM MAX1687 REF between CHG and GND controls the output recharge time. A large resistor increases peak inductor current which speeds up recovery time. Calculate the resistor as follows: b) MAX1687 LIM DAC c) LIM ( REF R3 MAX1687 LIM VLIM(CHG) = VREF R4 R4 + R3 IBURST ⋅ VOUT ⋅ DGSM RCHG = VIN(MIN) ⋅ 1 - DGSM VDROOP R3 + R4 > 125kΩ R4 ( ) ) + 0.1 ⋅ VIN(MIN) ⋅ gmCHG ⋅ VREF ⋅ gmFB ⋅ 1 - tol ( ) where: Figure 5. Current-Limit Adjust To set the current limit, apply a voltage of 0 to 1V at LIM. The current limit is 200mA when VLIM = 0 to 0.25V. Use the following equation to calculate ILIM: ILIM = VLIM (0.86A/V) – 0.06A where VLIM = 0.25V to 1V. VLIM is internally clamped to 1.25V when the voltage applied at VLIM is above 1.25V. Generate VLIM by one of three methods: an externally applied voltage, the output of a DAC, or a resistor-divider using VREF as the supply voltage (TSSOP packages) (Figure 5). Note that REF can supply up to 10µA. Determine VLIM as follows: VLIM = (ILX(PEAK) + 0.06A) / 0.86 where ILX(PEAK) = [(ILOAD · VOUT) / VIN ] + 0.1A (see the Inductor Current parameter in the Typical Operating Characteristics). Setting Recharge Time (MAX1688) The MAX1688 has a recharging feature employing a sample-and-hold, which sets the maximum time to recharge the reservoir capacitor. Synchronize the ON pin to place the converter in standby during each load current burst. At the end of each load current burst, the output voltage is sampled by the MAX1688. This voltage controls the peak inductor current. The greater the difference between the regulated output voltage and the valley of the sag voltage, the higher the peak current. This results in a constant recharge time that compensates for varying output filter capacitor characteristics as well as a varying input voltage. Therefore, the circuit demands only as much peak current from the battery as output conditions require, minimizing the peak current from the battery. An external resistor RCHG is the external resistor IBURST is the peak burst current expected DGSM is the duty cycle of GSM VIN is the input voltage VOUT is the output voltage VREF = 1.25V VDROOP is the drop in output voltage during the current burst gmCHG is the internal transconductance = 0.8A/V gmFB is the feedback transconductance = 200µA/V tol is the tolerance of the RCHG resistor For example, for IBURST = 2.66A, VDROOP = 0.36V, VIN = +2.7V, and VOUT = 3.6V, then RCHG = 31.5kΩ, using a 5% tolerance resistor. The recovery time for a 40.2kΩ RCHG is shorter than that with an 18kΩ RCHG, but the peak battery current is higher. See Switching Waveforms (GSM Pulsed Load 1A, RCHG = 40.2kΩ) and Switching Waveforms (GSM Pulsed Load 1A, RCH = 18kΩ) in Typical Operating Characteristics. Inductor Selection The value of the inductor determines the switching frequency. Calculate the switching frequency as: f = VIN [1 - (VIN / VOUT)] / (L · IRIPPLE) where f is the switching frequency, VIN is the input voltage, VOUT is the output voltage, L is the inductor value, and IRIPPLE is the ripple current expected, typically 0.2A. Using a lower value inductor increases the frequency and reduces the physical size of the inductor. A typical frequency is from 150kHz to 1MHz (see Switching Frequency vs. Inductance in the Typical Operating Characteristics). _______________________________________________________________________________________ 9 MAX1687/MAX1688 a) MAX1687/MAX1688 Step-Up DC-DC Converters with Precise, Adaptive Current Limit for GSM Output (Reservoir) Capacitor Typical Application Circuits The value of the output capacitor determines the amount of power available to deliver to the power amplifier during the RF burst. A larger output capacitor with low ESR reduces the amount of output voltage droop during an RF burst. Use the following equation to determine capacitor size when ON is synchronized to the RF burst: The current limit of the MAX1687 can be set by an external DAC (Figure 6a), making it variable by using a microcontroller. The MAX1687 is the choice for systems interfacing with a microcontroller, but may also be used with fixed current limit (Figure 6b). The MAX1688 can monitor the droop of the output voltage to set the current limit, maximizing battery life. The MAX1688 is suitable for systems demanding variable burst currents (Figures 6a, 6b, and 7) as well as variable input voltages. C OUT = ( DROOP V D GSM - I BURST ⋅ ⋅ I BURST ESR ⋅ t GSM OUTPUT CAPACITOR )(1 - tol) Layout where COUT is the output capacitor, IBURST is the peak power amplifier burst current, tGSM is the current pulse period, DGSM is the duty cycle, tol is the capacitor tolerance, and VDROOP is the acceptable drop in the output during the current burst. For example, when used in a typical GSM system, tGSM = 4.62ms, IBURST = 2.66A for a +3.6V system (1.42A for a +5.5V system), and with a droop of less than 10%, the value of the capacitor is 5.3mF ±20%. The output capacitor also determines the constant-load (ON connected to VCC) ripple voltage. The output ripple is: VRIPPLE = IRIPPLE · ESR(OUTPUT CAPACITOR) The MAX1687/MAX1688’s high-frequency operation and high peak currents make PC board layout critical to minimize ground bounce and noise. Locate input bypass and output filter capacitors as close to the device pins as possible. All connections to OUT and FB should also be kept as short as possible. Use a lowinductance ground plane. Connect the ground leads of the input capacitor, output capacitor, and PGND pins in a star configuration to the ground plane. Table 1 lists suggested suppliers. Refer to the MAX1687/MAX1688 evaluation kit manual for a suggested surface-mount layout and a list of suggested components. where IRIPPLE is typically 0.2A. 10µH VIN 2.7V TO 6V LX1 LX2 LX1 LX2 IN 47µF 0.1µF MAX1687 IN VOUT = 5V 2A AT 12% DUTY CYCLE OUT OUT R2 187k DAC OUTPUT 0 TO 1V LIM ON ON FB AGND OFF PGND REF 2000µF R1 61.9k PGND Figure 6a. MAX1687 Typical Application Circuit (GSM Pulsed Load) 10 ______________________________________________________________________________________ Step-Up DC-DC Converters with Precise, Adaptive Current Limit for GSM MAX1687/MAX1688 10µH VIN 2.7V TO 6V LX1 LX2 LX1 LX2 IN MAX1687 OUT IN 0.1µF 47µF R2 187k REF* ON VOUT = 5V 350mA OUT 47µF FB AGND ON OFF PGND R1 61.9k PGND LIM *TSSOP PACKAGE ONLY Figure 6b. MAX1687 Typical Application Circuit (Fixed Non-Pulsed Load) 10µH VIN 2.7V TO 6V LX1 LX2 LX1 LX2 IN MAX1688 IN 0.1µF OUT 47µF R2 187k CHG RCHG 40.2k ON VOUT = 5V 2A AT 12% DUTY CYCLE OUT ON 2000µF FB AGND OFF PGND REF R1 61.9k PGND Figure 7. MAX1688 Typical Application Circuit (GSM Pulsed Load) ______________________________________________________________________________________________________ 11 Step-Up DC-DC Converters with Precise, Adaptive Current Limit for GSM MAX1687/MAX1688 Pin Configurations (continued) Table 1. Component Suppliers COMPANY TOP VIEW IN 1 8 LX1 2 LIM [CHG] 3 MAX1687 MAX1688 FB 4 OUT 7 LX2 6 GND 5 ON FAX PHONE AVX 207-283-1941 207-282-5111 CoilCraft 708-639-6400 708-639-1469 Coiltronics 561-241-9339 561-241-7876 404-736-3030 404-736-1300 81-3-3607-5428 708-956-0666 Murata-Erie Sumida Chip Information TRANSISTOR COUNT: 1920 SO [ ] ARE FOR MAX1688 TSSOP.EPS Package Information 12 ______________________________________________________________________________________