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19-1561; Rev 1; 10/99
3V, 15kV ESD-Protected, AutoShutdown Plus RS-232 Transceiver for PDAs and Cell Phones
General Description
The MAX3387E 3V powered EIA/TIA-232 and V.28/V.24 is a communications interface with low power requirements, high data-rate capabilities, and enhanced electrostatic discharge (ESD) protection. The MAX3387E has three receivers and three transmitters. All RS-232 inputs and outputs are protected to 15kV using the IEC 1000-4-2 Air-Gap Discharge method, 8kV using the IEC 1000-4-2 Contact Discharge method, and 15kV using the Human Body Model. A proprietary low-dropout transmitter output stage enables true RS-232 performance from a +3.0V to +5.5V supply with a dual charge pump. The charge pump requires only four small 0.1F capacitors for operation from a +3.3V supply. The MAX3387E is capable of running at data rates up to 250kbps while maintaining RS-232 compliant output levels. The MAX3387E has a unique VL pin that allows interoperation in mixed-logic voltage systems. Both input and output logic levels are pin programmable through the VL pin. The MAX3387E is available in a space-saving TSSOP package.
Features
o VL Pin for Compatibility with Mixed-Voltage Systems o 15kV ESD Protection on Rx Inputs and Tx Outputs o Low 300A Supply Current o Guaranteed 250kbps Data Rate o 1A AutoShutdown PlusTM with Receivers Active o Meets EIA/TIA-232 Specifications Down to 3.0V
MAX3387E
Ordering Information
PART MAX3387ECUG MAX3387EEUG TEMP. RANGE 0C to +70C -40C to +85C PIN-PACKAGE 24 TSSOP 24 TSSOP
Typical Operating Circuit
+3.3V CBYPASS C1 0.1F 24 FORCEOFF 1 C1+ 3 4 C2 0.1F 5 C1C2+ C2T1OUT 21 T2OUT 20 T3OUT 19 VL 14 R1OUT R1IN 18 5k R2IN 17 5k VL 12 9 R3OUT INVALID FORCEON 11 GND 22 R3IN 16 5k RS-232 INPUTS RS-232 OUTPUTS 23 VCC 15 VL V+ 2 C3 0.1F 6 C4 0.1F
Applications
Subnotebook/Palmtop Computers PDAs and PDA Cradles Cell Phone Data Cables Battery-Powered Equipment Hand-Held Equipment Peripherals
MAX3387E
V-
Pin Configuration
TOP VIEW
C1+ 1 V+ 2 C1- 3 C2+ 4 C2- 5 V- 6 T1IN 7 T2IN 8 INVALID 9 T3IN 10 FORCEON 11 R3OUT 12 24 FORCEOFF 23 VCC 22 GND 21 T1OUT
7 T1IN TTL/CMOS INPUTS 8 T2IN 10 T3IN
MAX3387E
20 T2OUT 19 T3OUT 18 R1IN 17 R2IN 16 R3IN 15 VL 14 R1OUT 13 R2OUT
VL TTL/CMOS OUTPUTS 13 R2OUT
TSSOP
AutoShutdown Plus is a trademark of Maxim Integrated Products.
________________________________________________________________ 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.
3V, 15kV ESD-Protected, AutoShutdown Plus RS-232 Transceiver for PDAs and Cell Phones MAX3387E
ABSOLUTE MAXIMUM RATINGS
VCC to GND ..............................................................-0.3V to +6V VL to GND...................................................-0.3V to (VCC + 0.3V) V+ to GND ................................................................-0.3V to +7V V- to GND .................................................................+0.3V to -7V V+ + V-(Note 1) .............................................................. +13V Input Voltages T_IN, FORCEON, FORCEOFF to GND ..................-0.3V to +6V R_IN to GND .....................................................................25V Output Voltages T_OUT to GND...............................................................13.2V R_OUT........................................................-0.3V to (VL + 0.3V) Short-Circuit Duration T_OUT to GND........................Continuous Continuous Power Dissipation (TA = +70C) 24-Pin TSSOP (derate 7.8mW/C above +70C) ..........625mW Operating Temperature Ranges MAX3387ECUG ...................................................0C to +70C MAX3387EEUG ................................................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10sec) .............................+300C
Note 1: V+ and V- can have maximum magnitudes of 7V, but their absolute difference cannot exceed 13V.
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.
DC ELECTRICAL CHARACTERISTICS
(VCC = VL = +3.0V to +5.5V; C1-C4 = 0.1F, tested at +3.3V 10%; C1 = 0.047F, C2-C4 = 0.33F, tested at +5.0V 10%; TA = TMIN to TMAX, unless otherwise noted. Typical values are at VCC = VL = +3.3V, TA = +25C.) PARAMETER Supply Current, AutoShutdown Plus Supply Current LOGIC INPUTS Input Logic Threshold Low T_IN, FORCEON, FORCEOFF VL = 3.3V or 5.0V VL = 2.5V VL = 5.0V Input Logic Threshold High T_IN, FORCEON, FORCEOFF VL = 3.3V VL = 2.5V VL = 1.8V Transmitter Input Hysteresis Input Leakage Current RECEIVER OUTPUTS Output Voltage Low Output Voltage High RECEIVER INPUTS Input Voltage Range Input Threshold Low Input Threshold High TA = +25C TA = +25C VL = 5.0V VL = 2.5V or 3.3V VL = 5.0V VL = 2.5V or 3.3V -25 0.8 0.6 1.5 1.2 1.8 1.5 2.4 2.4 +25 V V V IOUT = 1.6mA IOUT = -1mA VL 0.6 VL 0.1 0.4 V V T_IN, FORCEON, FORCEOFF 2.4 2.0 1.4 0.9 0.5 0.01 1 V A V 0.8 0.6 V SYMBOL CONDITIONS All R_IN idle, FORCEON = GND, FORCEOFF = VCC, all T_IN idle FORCEOFF = FORCEON = VCC, no load MIN TYP MAX UNITS DC CHARACTERISTICS (VCC = +3.3V or +5V, TA = +25C) 1.0 0.3 10 1 A mA
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3V, 15kV ESD-Protected, AutoShutdown Plus RS-232 Transceiver for PDAs and Cell Phones
DC ELECTRICAL CHARACTERISTICS (continued)
(VCC = VL = +3.0V to +5.5V; C1-C4 = 0.1F, tested at +3.3V 10%; C1 = 0.047F, C2-C4 = 0.33F, tested at +5.0V 10%; TA = TMIN to TMAX, unless otherwise noted. Typical values are at VCC = VL = +3.3V, TA = +25C.) PARAMETER Input Hysteresis Input Resistance TRANSMITTER OUTPUTS Output Voltage Swing Output Resistance Output Short-Circuit Current Output Leakage Current ESD PROTECTION Human Body Model R_IN, T_OUT ESD Protection IEC 1000-4-2 Air-Gap Discharge method IEC 1000-4-2 Contact Discharge method AutoShutdown Plus (FORCEON = GND, FORCEOFF = VCC) Receiver Input Threshold to INVALID Output High Receiver Input Threshold to INVALID Output Low INVALID Output Voltage Low INVALID Output Voltage High Receiver Positive or Negative Threshold to INVALID High Receiver Positive or Negative Threshold to INVALID Low Receiver or Transmitter Edge to Transmitters Enabled tINVH tINVL tWU Figure 3a Figure 3a IOUT = -1.6mA IOUT = -1.0mA VCC = 5V, Figure 3b VCC = 5V, Figure 3b VCC = 5V, Figure 3b 15 VL - 0.6 1 30 100 30 60 Positive threshold Negative threshold -2.7 -0.3 0.3 0.4 2.7 V V V V s s s sec 15 15 8 kV All transmitter outputs loaded with 3k to ground VCC = V+ = V- = 0, transmitter output = 2V VT_OUT = 0 VT_OUT = 12V, transmitters disabled; VCC = 0 or 3.0V to 5.5V 5 300 5.4 10M 60 25 V mA A TA = +25C 3 SYMBOL CONDITIONS MIN TYP 0.5 5 7 MAX UNITS V k
MAX3387E
Receiver or Transmitter Edge to tAUTOSHDN VCC = 5V, Figure 3b Transmitters Shutdown
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3V, 15kV ESD-Protected, AutoShutdown Plus RS-232 Transceiver for PDAs and Cell Phones MAX3387E
TIMING CHARACTERISTICS
(VCC = VL = +3V to +5.5V; C1-C4 = 0.1F, tested at +3.3V 10%; C1 = 0.047F, C2-C4 = 0.33F, tested at +5.0V 10%; TA = TMIN to TMAX, unless otherwise noted. Typical values are at VCC = VL = +3.3V, TA = +25C.) PARAMETER Maximum Data Rate Receiver Propagation Delay Time to Exit Shutdown Transmitter Skew Receiver Skew Transition-Region Slew Rate tPHL tPLH SYMBOL CONDITIONS RL = 3k, CL = 1000pF, one transmitter switching Receiver input to receiver output, CL = 150pF MIN 250 0.15 0.15 100 100 50 CL = 150pF to 1000pF CL = 150pF to 2500pF 6 4 30 V/s 30 TYP MAX UNITS kbps s s ns ns
VT_OUT > 3.7V tPHL - tPLH (Note 2) tPHL - tPLH VCC = 3.3V, TA = +25C, RL = 3k to 7k, measured from +3V to -3V or -3V to +3V
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3V, 15kV ESD-Protected, AutoShutdown Plus RS-232 Transceiver for PDAs and Cell Phones MAX3387E
Typical Operating Characteristics
(VCC = VL = +3.3V, TA = +25C, unless otherwise noted.)
TRANSMITTER OUTPUT VOLTAGE vs. LOAD CAPACITANCE
MAX3387E-01
SLEW RATE vs. LOAD CAPACITANCE
14 12 SLEW RATE (V/s) SLEW RATE 10 8 6 4 SLEW RATE +
MAX3387E-02
7.5 5.0 OUTPUT VOLTAGE (V) 2.5 0 -2.5 -5.0 -7.5 0 1000 2000 3000 4000 DATA RATE = 250kbps LOAD = 3k IN PARALLEL WITH CL
16
2 0 5000 0 1000 2000 3000 4000 5000 LOAD CAPACITANCE (pF) LOAD CAPACITANCE (pF)
TRANSMITTER OUTPUT VOLTAGE vs. DATA RATE
MAX3387E-03
SUPPLY CURRENT vs. LOAD CAPACITANCE
LOAD = 3k, ONE TRANSMITTER SWITCHING AT DATA RATE, OTHER TRANSMITTERS 250kbps AT 1/8 DATA RATE 120kbps
MAX3387E-04
7.5 5.0 OUTPUT VOLTAGE (V) 2.5 0 -2.5 -5.0 -7.5 0 50 100 150 200 LOAD = 3k, 1000pF ONE TRANSMITTER SWITCHING AT DATA RATE, OTHER TRANSMITTERS AT 1/8 DATA RATE
60 50 SUPPLY CURRENT (mA) 40 30 20 10 0
20kbps
250
0
1000
2000
3000
4000
5000
DATA RATE (kbps)
LOAD CAPACITANCE (pF)
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3V, 15kV ESD-Protected, AutoShutdown Plus RS-232 Transceiver for PDAs and Cell Phones MAX3387E
Pin Descriptions
PIN 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 NAME C1+ V+ C1C2+ C2VT1IN T2IN INVALID T3IN FORCEON R3OUT R2OUT R1OUT VL R3IN R2IN R1IN T3OUT T2OUT T1OUT GND VCC FORCEOFF Ground +3.0V to +5.5V Supply Voltage Force-Off Input. Drive low to shut down transmitters and on-board power supply. This overrides all automatic circuitry and FORCEON (Table 1). RS-232 Transmitter Outputs RS-232 Receiver Inputs Logic-Level Supply. All CMOS inputs and outputs are referenced to this supply. TTL/CMOS Receiver Outputs. Swing between 0 and VL. FUNCTION Positive Terminal of the Voltage-Doubler Charge-Pump Capacitor +5.5V Supply Generated by the Charge Pump Negative Terminal of the Voltage-Doubler Charge-Pump Capacitor Positive Terminal of the Inverting Charge-Pump Capacitor Negative Terminal of the Inverting Charge-Pump Capacitor -5.5V Generated by the Charge Pump TTL/CMOS Transmitter Inputs Output of the Valid Signal Detector. INVALID is high if a valid RS-232 signal is present on the receiver inputs. TTL/CMOS Transmitter Inputs Force-On Input. Drive high to override automatic circuitry keeping transmitters on (FORCEOFF must be high) (Table 1).
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3V, 15kV ESD-Protected, AutoShutdown Plus RS-232 Transceiver for PDAs and Cell Phones
Detailed Description
Dual Charge-Pump Voltage Converter
The MAX3387E's internal power supply consists of a regulated dual charge pump that provides output voltages of +5.5V (doubling charge pump) and -5.5V (inverting charge pump), regardless of the input voltage (VCC) over a +3.0V to +5.5V range. The charge pumps operate in a discontinuous mode: if the output voltages are less than 5.5V, the charge pumps are enabled; if the output voltages exceed 5.5V, the charge pumps are disabled. Each charge pump requires a flying capacitor (C1, C2) and a reservoir capacitor (C3, C4) to generate the V+ and V- supplies. These RS-232 output stages are turned off (high impedance) when the device is in shutdown mode. When the power is off, the MAX3387E permits the outputs to be driven up to 12V. The transmitter inputs do not have pull-up resistors. Connect unused inputs to GND or VL.
MAX3387E
RS-232 Receivers
The receivers convert RS-232 signals to CMOS-logic output levels. The MAX3387E's receivers are always active, even when the device is in shutdown. The MAX3387E features an INVALID output that indicates when no signal is present on any RS-232 receiver inputs. INVALID is independent of other control logic functions; it indicates the receiver input conditions only (Figures 2 and 3).
RS-232 Transmitters
The transmitters are inverting level translators that convert CMOS-logic levels to 5.0V EIA/TIA-232 levels. The MAX3387E transmitters guarantee a 250kbps data rate with worst-case loads of 3k in parallel with 1000pF, providing compatibility with PC-to-PC communication software (such as LapLinkTM). Transmitters can be paralleled to drive multiple receivers or mice. Figure 1 shows a complete system connection.
FORCEON FORCEOFF INVALID
+0.3V
R_IN
-0.3V POWERMANAGEMENT UNIT OR KEYBOARD CONTROLLER I/O CHIP POWER SUPPLY
30s COUNTER R
INVALID
TRANSMITTERS ARE DISABLED, REDUCING SUPPLY CURRENT TO 1A IF ALL RECEIVER INPUTS ARE BETWEEN +0.3V AND -0.3V FOR AT LEAST 30s.
VL
Figure 2a. INVALID Function Diagram, INVALID = Low MAX3387E
+2.7V
VCC
I/O CHIP WITH UART CPU
R_IN RS-232 -2.7V
30s COUNTER R
INVALID
TRANSMITTERS ARE ENABLED IF: ANY RECEIVER INPUT IS GREATER THAN +2.7V OR LESS THAN -2.7V; ANY RECEIVER INPUT HAS BEEN BETWEEN +0.3V AND -0.3V FOR LESS THAN 30s.
Figure 2b. INVALID Function Diagram, INVALID = High Figure 1. Interface Under Control of PMU LapLink is a trademark of Traveling Software.
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3V, 15kV ESD-Protected, AutoShutdown Plus RS-232 Transceiver for PDAs and Cell Phones MAX3387E
EDGE DETECT TRANSMITTERS ENABLED, INVALID HIGH FORCEOFF RECEIVER INPUT LEVELS S 30sec TIMER R +2.7V INDETERMINATE +0.3V 0V -0.3V INDETERMINATE -2.7V TRANSMITTERS ENABLED, INVALID HIGH AutoShutdown, TRANSMITTERS DISABLED, 1A SUPPLY CURRENT, INVALID LOW
T_IN
AUTOSHDN
R_IN
EDGE DETECT
FORCEON
Figure 2c. AutoShutdown Plus Logic Figure 3a. Receiver Thresholds for INVALID
FORCEOFF
POWERDOWN
FORCEON AUTOSHDN
POWERDOWN IS ONLY AN INTERNAL SIGNAL. IT CONTROLS THE OPERATIONAL STATUS OF THE TRANSMITTERS AND THE POWER SUPPLIES.
Figure 2d. Power-Down Logic
FORCEOFF states. Figure 2 and Table 1 summarize the MAX3387E's operating modes. FORCEON and FORCEOFF override AutoShutdown Plus circuitry. When neither control is asserted, the IC selects between these states automatically based on the last receiver or transmitter input edge received. By connecting FORCEON to INVALID, the MAX3387E shuts down when no valid receiver level and no receiver or transmitter edge is detected for 30sec, and wakes up when a valid receiver level or receiver or transmitter edge is detected. By connecting FORCEON and FORCEOFF to INVALID, the MAX3387E shuts down when no valid receiver level is detected. A mouse or other system with AutoShutdown Plus may need time to wake up. Figure 4 shows a circuit that forces the transmitters on for 100ms, allowing enough time for the other system to realize that the MAX3387E is awake. If the other system outputs valid RS-232 signal transitions within that time, the RS-232 ports on both systems remain enabled.
AutoShutdown Plus Mode
The MAX3387E achieves a1A supply current with Maxim's AutoShutdown Plus feature, which operates when FORCEOFF is high and a FORCEON is low. When these devices do not sense a valid signal transition on any receiver and transmitter input for 30sec, the onboard charge pumps are shut down, reducing supply current to 1A. This occurs if the RS-232 cable is disconnected or if the connected peripheral transmitters are turned off, and if the UART driving the transmitter inputs is inactive. The system turns on again when a valid transition is applied to any RS-232 receiver or transmitter input. As a result, the system saves power without changes to the existing BIOS or operating system. Figures 2a and 2b show valid and invalid RS-232 receiver voltage levels. INVALID indicates the receiver input's condition, and is independent of FORCEON and
VL Logic Supply Input
Unlike other RS-232 interface devices where the receiver outputs swing between 0 and VCC, the MAX3387E features a separate logic supply input (VL) that sets VOH for the receiver outputs and sets thresholds for the receiver inputs. This feature allows a great deal of flexibility in interfacing to many different types of systems with different logic levels. Connect this input to the host logic supply (1.8V VL VCC). Also, see the Typical PDA/Cell-Phone Application section.
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3V, 15kV ESD-Protected, AutoShutdown Plus RS-232 Transceiver for PDAs and Cell Phones MAX3387E
RECEIVER INPUTS
} REGION
INVALID
TRANSMITTER INPUTS
TRANSMITTER OUTPUTS INVALID OUTPUT VCC 0
tINVL
tINVH tAUTOSHDN tWU
tAUTOSHDN tWU
V+ VCC 0 V-
Figure 3b. AutoShutdown Plus/INVALID Timing Diagram
15kV ESD Protection
POWERMANAGEMENT UNIT MASTER SHDN LINE 0.1F 1M
FORCEOFF FORCEON
MAX3387E
Figure 4. AutoShutdown with Initial Turn-On to Wake Up a System
Software-Controlled Shutdown
If direct software control is desired, INVALID can be used to indicate a DTR or ring indicator signal. Connect FORCEOFF and FORCEON together to bypass AutoShutdown so the line acts like a SHDN input.
As with all Maxim devices, ESD-protection structures are incorporated on all pins to protect against electrostatic discharges (ESDs) encountered during handling and assembly. The MAX3387E driver outputs and receiver inputs have extra protection against static electricity. Maxim has developed state-of-the-art structures to protect these pins against ESD of 15kV without damage. The ESD structures withstand high ESD in all states: normal operation, shutdown, and powered down. After an ESD event, Maxim's "E" version devices keep working without latchup, whereas competing RS232 products can latch and must be powered down to remove latchup. ESD protection can be tested in various ways. The transmitter outputs and receiver inputs of this product family are characterized for protection to the following limits: 1) 15kV using the Human Body Model 2) 8kV using the Contact-Discharge method specified in IEC 1000-4-2 3) 15kV using IEC 1000-4-2's Air-Gap method
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3V, 15kV ESD-Protected, AutoShutdown Plus RS-232 Transceiver for PDAs and Cell Phones MAX3387E
Table 1. Output Control Truth Table
OPERATION STATUS FORCEON FORCEOFF VALID RECEIVER LEVEL RECEIVER OR TRANSMITTER EDGE WITHIN 30sec X X Yes No X Yes No X X T_OUT R_OUT
Shutdown (Forced Off) Normal Operation (Forced On) Normal Operation (AutoShutdown Plus) Shutdown (AutoShutdown Plus) Normal Operation Normal Operation Shutdown Normal Operation (AutoShutdown) Shutdown (AutoShutdown)
X 1 0 0 INVALID INVALID INVALID INVALID INVALID
0 1 1 1 1 1 1 INVALID INVALID
X X X X Yes X No Yes No
High-Z Active Active High-Z Active Active High-Z Active High-Z
Active Active Active Active Active Active Active Active Active
X = Don't care
RC 1M CHARGE-CURRENT LIMIT RESISTOR HIGHVOLTAGE DC SOURCE
RD 1500 DISCHARGE RESISTANCE DEVICE UNDER TEST
IP 100% 90% AMPERES
Ir
PEAK-TO-PEAK RINGING (NOT DRAWN TO SCALE)
Cs 100pF
STORAGE CAPACITOR
36.8% 10% 0 0 tRL TIME tDL CURRENT WAVEFORM
Figure 5a. Human Body ESD Test Model
Figure 5b. Human Body Current Waveform
ESD Test Conditions
ESD performance depends on a variety of conditions. Contact Maxim for a reliability report that documents test setup, methodology, and results.
Human Body Model
Figure 5a shows the Human Body Model, and Figure 5b shows the current waveform it generates when discharged into a low impedance. This model consists of a 100pF capacitor charged to the ESD voltage of interest, which is then discharged into the test device through a 1.5k resistor.
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3V, 15kV ESD-Protected, AutoShutdown Plus RS-232 Transceiver for PDAs and Cell Phones MAX3387E
RC 50M to 100M CHARGE-CURRENT LIMIT RESISTOR HIGHVOLTAGE DC SOURCE RD 330 DISCHARGE RESISTANCE DEVICE UNDER TEST
I 100% 90%
I PEAK
Cs 150pF
STORAGE CAPACITOR
Figure 6a. IEC 1000-4-2 ESD Test Model
10% t R = 0.7ns to 1ns t 30ns 60ns
IEC 1000-4-2
The IEC 1000-4-2 standard covers ESD testing and performance of finished equipment; it does not specifically refer to ICs. The MAX3387E helps you design equipment that meets Level 4 (the highest level) of IEC 1000-4-2, without the need for additional ESD-protection components. The major difference between tests done using the Human Body Model and IEC 1000-4-2 is higher peak current in IEC 1000-4-2 because series resistance is lower in the IEC 1000-4-2 model. Hence, the ESD withstand voltage measured to IEC 1000-4-2 is generally lower than that measured using the Human Body Model. Figure 6a shows the IEC 1000-4-2 model, and Figure 6b shows the current waveform for the 8kV IEC 1000-4-2 Level 4 ESD Contact Discharge test. The air-gap test involves approaching the device with a charged probe. The contact-discharge method connects the probe to the device before the probe is energized.
Figure 6b. IEC 1000-4-2 ESD Generator Current Waveform
Table 2. Minimum Required Capacitor Values
VCC (V) 3.0 to 3.6 4.5 to 5.5 3.0 to 5.5 C1 (F) 0.1 0.047 0.22 C2, C3, C4 (F) 0.1 0.33 1
Machine Model
The Machine Model for ESD tests all pins using a 200pF storage capacitor and zero discharge resistance. Its objective is to emulate the stress caused by contact that occurs with handling and assembly during manufacturing. All pins require this protection during manufacturing, not just RS-232 inputs and outputs. Therefore, after PC board assembly, the Machine Model is less relevant to I/O ports.
__________Applications Information
Capacitor Selection
The capacitor type used for C1-C4 is not critical for proper operation; polarized or nonpolarized capacitors can be used. The charge pump requires 0.1F capaci-
tors for 3.3V operation. For other supply voltages, see Table 2 for required capacitor values. Do not use values smaller than those listed in Table 2. Increasing the capacitor values (e.g., by a factor of 2) reduces ripple on the transmitter outputs and slightly reduces power consumption. C2, C3, and C4 can be increased without changing C1's value. However, do not increase C1 without also increasing the values of C2, C3, and C4 to maintain the proper ratios (C1 to the other capacitors). When using the minimum required capacitor values, make sure the capacitor value does not degrade excessively with temperature. If in doubt, use capacitors with a larger nominal value. The capacitor's equivalent series resistance (ESR), which usually rises at low temperatures, influences the amount of ripple on V+ and V-.
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3V, 15kV ESD-Protected, AutoShutdown Plus RS-232 Transceiver for PDAs and Cell Phones MAX3387E
Power-Supply Decoupling
In most circumstances, a 0.1F bypass capacitor is adequate. In applications that are sensitive to power-supply noise, decouple VCC to ground with a capacitor of the same value as charge-pump capacitor C1. Connect bypass capacitors as close to the IC as possible.
Operation Down to 2.7V
Transmitter outputs will meet EIA/TIA-562 levels of 3.7V with supply voltages as low as +2.7V.
Transmitter Outputs when Exiting Shutdown
Figure 7 shows two transmitter outputs when exiting shutdown mode. As they become active, the two transmitter outputs are shown going to opposite RS-232 levels (one transmitter input is high; the other is low). Each transmitter is loaded with 3k in parallel with 2500pF. The transmitter outputs display no ringing or undesirable transients as they come out of shutdown. Note that the transmitters are enabled only when the magnitude of V- exceeds approximately 3V.
5V/div
T2
2V/div
High Data Rates
The MAX3387E maintains the RS-232 5.0V minimum transmitter output voltage even at high data rates. Figure 8 shows a transmitter loopback test circuit. Figure 9 shows a loopback test result at 120kbps, and Figure 10 shows the same test at 250kbps. For Figure 9, all transmitters were driven simultaneously at 120kbps into RS-232 loads in parallel with 1000pF. For Figure 10, a single transmitter was driven at 250kbps, and all transmitters were loaded with an RS-232 receiver in parallel with 1000pF.
VCC = 3.3V C1-C4 = 0.1F 50s/div
T1
Figure 7. Transmitter Outputs when Exiting Shutdown
VCC 0.1F
Interconnection with 3V and 5V Logic
The MAX3387E can directly interface with various 5V logic families, including ACT and HCT CMOS. The logic voltage power-supply pin (VL) sets the output voltage level of the receivers and the input thresholds of the transmitters.
VCC C1+ C1 C1C2+ C2 C2V+
MAX3387E
V-
C3
C4
T_ IN
T_ OUT
T1IN
5V/div
R_ OUT
R_ IN 5k T1OUT 1000pF
5V/div
FORCEOFF VCC FORCEON
GND
R1OUT VCC = 3.3V 2s/div
5V/div
Figure 8. Loopback Test Circuit
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Figure 9. Loopback Test Results at 120kbps
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3V, 15kV ESD-Protected, AutoShutdown Plus RS-232 Transceiver for PDAs and Cell Phones MAX3387E
The ring indicate (RI) signal is generated when a PDA, phone, or other "cradled" device is plugged into its cradle. This generates a logic-low signal to the RI transmitter input, creating +6V at the ring indicate pin. The PC's UART RI input is the only pin that can generate an interrupt from signals arriving through the RS-232 port. The interrupt routine for this UART will then service the RS232 full-duplex communication between the PDA and the PC. As cell phone design becomes more like that of PDAs, cell phones will require similar docking ability and communication protocol. Cell phones operate on a single lithium-ion (Li+) battery and work with a power-supply voltage of +2.7V to +4V. The baseband logic coming from the phone connector can be as low as 1.8V at the transceivers. To prevent forward biasing of a device internal to the cell phone, the MAX3387E comes with a logic power-supply pin (VL) that limits the logic levels presented to the phone. The receiver outputs will sink to zero for low outputs, but will not exceed VL for logic highs. The input logic levels for the transmitters are also altered, scaled by the magnitude of the VL input. The device will work with V L as low as 1.8V before the charge-pump noise will begin to cause the transmitter outputs to oscillate. This is useful with cell phones and other power-efficient devices with core logic voltage levels that go as low as 1.8V.
T1IN
5V/div
T1OUT
5V/div
R1OUT VCC = 3.3V 2s/div
5V/div
Figure 10. Loopback Test Results at 250kbps
Typical PDA/Cell-Phone Application
The MAX3387E is designed with PDA applications in mind. Two transmitters and two receivers handle standard full-duplex communication protocol, while an extra transmitter allows a ring indicator signal to alert the UART on the PC. Without the ring indicator transmitter, solutions for these applications would require softwareintensive polling of the cradle inputs.
Chip Information
TRANSISTOR COUNT: 1267
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3V, 15kV ESD-Protected, AutoShutdown Plus RS-232 Transceiver for PDAs and Cell Phones MAX3387E
Package Information
TSSOP.EPS
14
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3V, 15kV ESD-Protected, AutoShutdown Plus RS-232 Transceiver for PDAs and Cell Phones MAX3387E
NOTES
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3V, 15kV ESD-Protected, AutoShutdown Plus RS-232 Transceiver for PDAs and Cell Phones MAX3387E
NOTES
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
16 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 1999 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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