EVALUATION KIT MANUAL FOLLOWS DATA SHEET Step-Up DC-DC Converters with Precise, Adaptive Current Limit for GSM PART* MAX1687EUE MAX1687ESA MAX1688EUE

Transcription

1 ; Rev 0; 2/99 EALUATI KIT MANUAL FOLLOWS DATA SHEET Step-Up DC-DC Converters with General Description The / 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 / 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 /, controlled by the selected inductor, can exceed 1MHz. Two external resistors set the output voltage from 1.25 to 6. The controls peak battery current, while the features a more advanced, adaptive constantrecharge-time algorithm that maximizes battery life. The / are available in thin 16-pin TSSOP (1.1mm max height) or standard 8-pin SO packages. GSM Phones Wireless Handsets PC Cards (PCMCIA) TOP IEW LIM [CHG] 5 Applications N.C AGND [ ] ARE FOR TSSOP Pin Configurations continued at end of data sheet. Pg Pin Configurations Features Low 450mA Peak Battery Current Provides 2A, 5 GSM Burst 90% Efficiency Internal Power MOSFETs and Current-Sense Resistor Output Disconnects from Input During Shutdown 3µA Shutdown Current Precise oltage-controlled Current Limit () Adaptive Constant-Recharge-Time Capability () 1.25 to 6 Adjustable Output 2.7 to 6 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 PART* EUE ESA EUE 2.7 TO 6 1 Li-lon OR 3 NiMH OR 3 ALKALE OFF 0 TO 1 CTROL PUT ( ) ARE FOR [ ] ARE FOR Typical Operating Circuit (LIM) Ordering Information TEMP. RANGE -40 C to +85 C -40 C to +85 C -40 C to +85 C GND [CHG] P-PACKAGE 16 TSSOP 8 SO ESA -40 C to +85 C 8 SO *Ug. 16 TSSOP UP TO 6 / Maxim Integrated Products 1 For free samples & the latest literature: or phone For small orders, phone

2 / ABSOLUTE MAXIMUM RATGS,,, CHG, LIM,,, to GND to +7 to GND to +8, Average Current...1A Continuous Power Dissipation (T A = +70 C) TSSOP (derate 5.7mW/ C above +70 C)...457mW SO (derate 5.88mW/ C above +70 C)...471mW 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 Operating Temperature Range C to +85 C Storage Temperature Range C to +150 C Lead Temperature (soldering, 10sec) C ( = = +3, LIM = 1 (), CHG = 1 (), = 1.5, = 6, T A = 0 C to +85 C, unless otherwise noted. Typical values are at T A = +25 C.) PARAMETER Input oltage Range Input Undervoltage Lockout Output oltage Range Input Supply Current Shutdown Delay Reference oltage Set oltage Transconductance I CHG Source Current Peak Current Ripple Current Sense Resistor Input Low oltage Input High oltage Input Current N-Channel On-Resistance P-Channel On-Resistance Precharge On-Resistance Leakage Current SYMBOL t DELAY g m I PEAK I RIPPLE R SENSE IL IH I I I LIM rising, 1% hysteresis = 1.5 CDITIS Shutdown, = 4.2, connected to, = 0, = GND I = 0 to 10µA rising, 2% hysteresis = 1.125, = 3 () = 0, = 3 () LIM = CHG = 1 LIM = CHG = 0.65 LIM = CHG = 1 = 2.7 = 6 = 4.2 = 1.5 = 0 or 3 T A = +25 C LIM = 1 T A = 0 C to +85 C = 2.7 = 2.7 = 4, = 0, = 0 = = 6, = = 0 M TYP MAX UNITS ma µa ms mmho µa A ma µa µa 2

3 ELECTRICAL CHARACTERISTICS ( = = +3, LIM = 1 (), CHG = 1 (), = 1.5, = 6, T A = -40 C to +85 C, unless otherwise noted.) (Note 1) PARAMETER Input oltage Range Input Undervoltage Lockout Output oltage Range Input Supply Current Shutdown Delay Reference oltage Set oltage Transconductance Peak Current Ripple Current Sense Resistor Input Low oltage Input High oltage N-Channel On-Resistance P-Channel On-Resistance Precharge On-Resistance SYMBOL I I SHDN T DELAY gm I PEAK I RIPPLE R SENSE IL IH rising, 1% hysteresis = 1.5 Shutdown = 4.2, connected to, = 0, = GND I = 0 to 10µA rising, 2% hysteresis = 1.125, = 3 () LIM = CHG = 1 LIM = CHG = 0.65 LIM = CHG = 1 CDITIS = 2.7 = 6 = 4.2 = 2.7 = 2.7 = 4, = 0, = 0 M TYP MAX UNITS ma µa ms mmho A ma / Note 1: Specifications to -40 C are guaranteed by design, not production tested. Typical Operating Characteristics ( = +3.3, = 5, LIM = 1, Figures 6b and 7, T A = +25 C, unless otherwise noted.) EFFICIENCY vs. DC LOAD CURRENT ( = 5.5) = 5 /88 toc EFFICIENCY vs. GSM BURST LOAD ( = 5.5) = 5 /88 toc EFFICIENCY vs. LOAD CURRENT ( = 2.7, = 3.3) /88 toc03 EFFICIENCY (%) = 3.3 = 2.7 = 6 EFFICIENCY (%) = 3.3 = 2.7 = 6 EFFICIENCY (%) LOAD CURRENT (ma) LOAD CURRENT (ma) LOAD CURRENT (ma) 3

4 Precise, Adaptive Current Limit for GSM / Typical Operating Characteristics (continued) ( = +3.3, = 5, LIM = 1, Figures 6b and 7, T A = +25 C, unless otherwise noted.) PEAK BATTERY CURRENT (ma) PEAK BATTERY CURRENT vs. R CHG (1A GSM LOAD) R CHG (k) /88 toc04 SUPPLY CURRENT (ma) NO-LOAD BATTERY PUT CURRENT vs. TEMPERATURE ( = 5, LIM = 1 ) = 6 = 5 = 3.3 = TEMPERATURE ( C) /88 toc05 () ERENCE OLTAGE vs. ERENCE CURRENT ( = 3.3, = 5) I (µa) /88 toc06 ERENCE OLTAGE () ERENCE OLTAGE vs. TEMPERATURE ( = 3.3, = 5) /88 toc07 FREQUENCY (khz) SWITCHG FREQUENCY vs. DUCTANCE ( = 3.3, = 5, I LOAD = 100mA, LIM = 1) /88 toc TEMPERATURE ( C) DUCTANCE (µh) IPEAK (ma) I PEAK vs. DROOP R CHG = 40.2k DROOP (m) /88 toc09 PEAK DUCTOR CURRENT (ma) PEAK DUCTOR CURRENT vs. R CHG (1A GSM LOAD) R CHG (k) /88 toc10 4

5 Typical Operating Characteristics (continued) ( = +3.3, = 5, LIM = 1, Figures 6b and 7, T A = +25 C, unless otherwise noted.) 500mA/div 100m/div SWITCHG WAEFORMS (FIXED I LOAD = 300mA) /88 toc10a 500µs/div I LX 500mA/div 500mA/div 200m/div SWITCHG WAEFORMS (GSM PULSED LOAD 1A, R CHG = 40.2k) 1ms/div R CHL = 40.2k, L = 10µH /88 toc11 I LX I LOAD / SWITCHG WAEFORMS (GSM PULSED LOAD 1A, R CHG = 18k) /88 toc12 DUCTOR CURRENT /88 toc13 500mA/div I LX I LX 500mA/div I LOAD LIM = 1 200m/div 200mA/div 0A I LX LIM = 0 1ms/div R CHG = 18k, L = 10µH POWER-UP WAEFORM (R LOAD = 15 C = 2000µF) /88 toc14 2µs/div vs. BATTERY CURRENT /88 toc15 2/div 1/div I BATTERY 200mA/div 1/div 5ms/div 10µs/div 5

6 / P SO TSSOP 1 1, 2 2 3, SO TSSOP 1 1, 2 2 3, NAME LIM CHG FUNCTI Pin Description Supply oltage Input. Connect Battery to. Bypass to GND with a 47µF minimum capacitor. Internal Current-Sense Resistor Output. Connect the inductor between and. oltage-controlled Current-Limit Adjust Input. Apply a voltage between 0 and 1 to vary the current limit. LIM is internally clamped to 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) Feedback Input. Connect a resistor-divider from to GND to set the output voltage. regulates to a nominal Reference oltage Output nominal. 8 8 N.C. No Connection. Not internally connected Logic /OFF Input. When is high, the device operates in normal mode. When goes low, the device goes into standby mode. If remains low for greater than 1.2ms, the device shuts down (see Standby/Shutdown section). The supply current falls to 3µA in shutdown mode. 6 6 GND Ground AGND Analog Ground 11, 12 11, 12 Power Ground 7 13, , 14 N-Channel and P-Channel MOSFET Drain 8 15, , 16 Output Detailed Description The and 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 / 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 / provide a voltage-controlled current limit. The is a 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 / 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 / are gentle to the battery during initial power-up, as well. 6

7 g m ZERO CROSSG PEAK/ TROUGH DUCTOR- CURRENT DETECT Q1 P-SWITCH CSTANT HYSTERETIC DUCTOR-CURRENT CTROL LOGIC N-SWITCH Q3 Q2 P-SWITCH / (LIM) [CHG] g m TIMER PRECHARGE - DIODE ( ) ARE FOR [ ] ARE FOR (ALSO DASHED LES) Figure 1. Functional Diagram When starting up, the / 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 30 precharge PMOS device (Figure1, Q1). The transition from Linear Mode to Pseudo Buck Mode occurs when = - 3. The transition from Pseudo Buck Mode to Pseudo Boost Mode occurs when = The transition from Pseudo Boost Mode to Boost Mode occurs when >. Due to these mode changes, the battery input current remains relatively constant, and changes slope as it rises. Hysteretic Inductor-Current Control Logic circuits in the / control the inductor ripple current to typically 200mA (Figure 2). The voltage at LIM (CHG) programs I PEAK. The inductor current oscillates between I PEAK - 200mA and I PEAK. Standby/Shutdown When goes low, the device enters Standby Mode, inductor current ramps to zero, and the output disconnects from the input. If remains low for greater than 1.2ms (typ), the device shuts down and quiescent current drops to 3µA (typ). 7

8 / CURRENT I PEAK SET BY LIM ( CHG ) I PEAK - 200mA ( ) ARE FOR TIME Figure 2. Hysteretic Inductor Current R1 = R2 ( ) - HYSTERESIS BAND Synchronized Pin If desired, drive low during periods of high current demand to eliminate switching noise from affecting sensitive RF circuitry. During the periods when is low, the output reservoir capacitor provides current to the load (Figure 4). Buck Capability Although the IC is not intended for this application, the / operate as a buck converter when the input voltage is higher than the output voltage. The / are not optimally efficient in this mode (see Typical Operating Characteristics for efficiencies at 2.7, 3.3, 5, and 6 input supply voltages). Figure 3. Setting the Output oltage R1 R2 I LOAD CTROL PUT TIME Figure 4. Timing Diagram of Applications Information Adjusting the Output oltage Adjust the / output voltage with two external resistors (Figure 3). Choose R2 to be between 10k to 100k. Calculate R1 as follows: R1 = R2 ( - ) / where is the feedback threshold voltage, 1.25 nominal. Adjusting Current Limit () The 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. 8

9 a) LIM c) R3 R4 LIM Figure 5. Current-Limit Adjust To set the current limit, apply a voltage of 0 to 1 at LIM. The current limit is 200mA when LIM = 0 to Use the following equation to calculate I LIM : I LIM = LIM (0.86A/) 0.06A where LIM = 0.25 to 1. LIM is internally clamped to 1.25 when the voltage applied at LIM is above Generate LIM by one of three methods: an externally applied voltage, the output of a DAC, or a resistor-divider using as the supply voltage (TSSOP packages) (Figure 5). Note that can supply up to 10µA. Determine LIM as follows: LIM = (I LX(PEAK) A) / 0.86 where I LX(PEAK) = [(I LOAD ) / ] + 0.1A (see the Inductor Current parameter in the Typical Operating Characteristics). Setting Recharge Time () The has a recharging feature employing a sample-and-hold, which sets the maximum time to recharge the reservoir capacitor. Synchronize the 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. 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 DAC b) LIM LIM(CHG) = R4 R4 + R3 R3 + R4 > 125k 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: R = CHG ( ) I D BURST GSM D (M) ( GSM ) (M) gm gm 1 - tol DROOP CHG ( ) where: R CHG is the external resistor I BURST is the peak burst current expected D GSM is the duty cycle of GSM is the input voltage is the output voltage = 1.25 DROOP is the drop in output voltage during the current burst g mchg is the internal transconductance = 0.8A/ g m is the feedback transconductance = 200µA/ tol is the tolerance of the R CHG resistor For example, for I BURST = 2.66A, DROOP = 0.36, = +2.7, and = 3.6, then R CHG = 31.5k, using a 5% tolerance resistor. The recovery time for a 40.2k R CHG is shorter than that with an 18k R CHG, but the peak battery current is higher. See Switching Waveforms (GSM Pulsed Load 1A, R CHG = 40.2k) and Switching Waveforms (GSM Pulsed Load 1A, R CH = 18k) in Typical Operating Characteristics. Inductor Selection The value of the inductor determines the switching frequency. Calculate the switching frequency as: f = [1 - ( / )] / (L I RIPPLE ) where f is the switching frequency, is the input voltage, is the output voltage, L is the inductor value, and I RIPPLE 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

10 / Output (Reservoir) Capacitor 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 is synchronized to the RF burst: C = D I t GSM BURST GSM - I ESR 1 - tol DROOP BURST PUT CAPACITOR where C is the output capacitor, I BURST is the peak power amplifier burst current, t GSM is the current pulse period, D GSM is the duty cycle, tol is the capacitor tolerance, and DROOP is the acceptable drop in the output during the current burst. For example, when used in a typical GSM system, t GSM = 4.62ms, I BURST = 2.66A for a +3.6 system (1.42A for a +5.5 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 ( connected to CC ) ripple voltage. The output ripple is: RIPPLE = I RIPPLE ESR (PUT CAPACITOR) where I RIPPLE is typically 0.2A. ( )( ) Typical Application Circuits The current limit of the can be set by an external DAC (Figure 6a), making it variable by using a microcontroller. The is the choice for systems interfacing with a microcontroller, but may also be used with fixed current limit (Figure 6b). The can monitor the droop of the output voltage to set the current limit, maximizing battery life. The is suitable for systems demanding variable burst currents (Figures 6a, 6b, and 7) as well as variable input voltages. Layout The / 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 and should also be kept as short as possible. Use a lowinductance ground plane. Connect the ground leads of the input capacitor, output capacitor, and pins in a star configuration to the ground plane. Table 1 lists suggested suppliers. Refer to the / evaluation kit manual for a suggested surface-mount layout and a list of suggested components. 10µH 2.7 TO 6 0.1µF 47µF = 5 2A AT 12% DUTY CYCLE R2 187k 2000µF DAC PUT 0 TO 1 LIM OFF AGND R1 61.9k Figure 6a. Typical Application Circuit (GSM Pulsed Load) 10

11 2.7 TO 6 0.1µF 47µF OFF * 10µH AGND R2 187k R1 61.9k 47µF = 5 350mA / LIM *TSSOP PACKAGE LY Figure 6b. Typical Application Circuit (Fixed Non-Pulsed Load) 10µH 2.7 TO 6 0.1µF 47µF R2 187k 2000µF = 5 2A AT 12% DUTY CYCLE CHG R CHG 40.2k OFF AGND R1 61.9k Figure 7. Typical Application Circuit (GSM Pulsed Load) 11

12 / Pin Configurations (continued) TOP IEW LIM [CHG] [ ] ARE FOR SO GND Table 1. Component Suppliers COMPANY FAX PHE AX CoilCraft Coiltronics Murata-Erie Sumida TRANSISTOR COUNT: 1920 Chip Information Package Information TSSOP.EPS 12