TM Technology for Innovators Digital Bus Switch Selection Guide Digital Bus Switch Introduction Texas Instruments (TI) has a long history As core system components are migrat- family is the only pure solution available Introduction in the digital bus switch market. TI was ing to low-voltage supplies, TI has been with the capability to translate from 5 V the first to introduce the 3.3-V low- actively expanding its portfolio into next- down to 2.5 V. voltage bus switch (CBTLV) and continues generation, low-voltage bus switches. To complement its broad line of bus to make major technology advances in The CBTLV family, the first FET switch switch families, TI offers packaging this market. TI’s digital switches (also devices designed for 3.3 V, supports options to satisfy the most space-critical referred to as bus switches) are designed next-generation, low-voltage systems. The applications. TI offers bus switch functions to quickly turn on/off the connection to a new CB3Q is a 3.3-V family that offers in its single-gate Little Logic portfolio, high-speed digital line or bus. TI bus low flat ron for high-performance systems. Decision Tree housed in the small packages SC-70 switches provide industry-standard func- The 2.5-V/3.3-V voltage translator family, and SOT-23; as well as Widebus and tions and pinouts (i.e., ’244, ’245) in a full CB3T, provides a high-performance, Widebus+™ functions in space-saving range of bit widths (from 1-bit Little Logic low-power voltage translation interface BGA options. to 32-bit Widebus™). TI bus switches solution for a variety of applications. The feature subnanosecond propagation delays, low on-resistance, low input/ Bus Switch Supply Voltage and Signal Frequency output capacitance and bandwidths up to 500 MHz. Technical Overview Technical 5 CBT/ TI offers over 70 different functions in its CBT-C broad line of FET switches, bus switches, 4 bus exchanges and multiplexer/demulti- plexer switches. Whether it is for isola- 3 CB3T CBTLV CB3Q tion, translation or bus-exchange needs, TI’s crossbar technology portfolio has 2 Supply Voltage (V) Supply Voltage Applications the right device. TI’s 5-V bus switch product line includes 1 the general-purpose CBT family, TI’s original bus switch family; as well as the 0 new CBT-C family, which provides 100 200 500 undershoot protection down to –2 V. Signal Frequency (MHz) Selection Guide Bus Switch Function and Bit Width Two-Port MUX/DeMUX Bus Exchange A B 1A 11B 1A1 1B1 1B2 OE Cross-Reference OE 1A2 1B2 CBT CBT-C CB3T CBTLV CB3Q Control 1 Bit S Logic 2 Bit 4 Bit 8 Bit OE Packaging 10 Bit Control 16 Bit CBT CBT-C CB3T CBTLV CB3Q S 20 Bit 4 x 1:2 Bit Logic 24 Bit 12 x 1:2 Bit 32 Bit PPP16 x 1:2 Bit P = Planned 12 x 1:3 Bit 2 x 1:4 Bit CBT CBT-C CB3T CBTLV CB3Q 1 x 1:8 Bit 10 Bit P 1 x 16:32 Bit 18 Bit 24 Bit P Resources P = Planned www.ti.com/signalswitches 2 Texas Instruments Digital Bus Switch Selection Guide Digital Bus Switch Introduction (Continued) Comparison of Typical ron vs. VI for the CBT, CBTLV and CB3Q Bus Switch Families Introduction 120 TA = 25°C 100 I = –15 mA Ω O on 80 Decision Tree CBT/CBT-C 60 (5-V VCC ) 40 CBTLV On-State Resistance, r20 ( ) (3.3-V V ) CB3Q CC Overview Technical (3.3-V VCC ) 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 VI (V) Applications Digital Signal Considerations VCC — There are a number of to the line impedance to allow for proper Number of Bits Required To Be Switched — considerations and tradeoffs here. What matching and to prevent unwanted signal With TI’s wide variety of signal switches, it voltage levels are present on the board? reflections. For nontransmission-line connec- is possible to switch between 1 to 32 bits at What is the amplitude of the signal levels to tions, the switch ron and the load resistance the same time with a single device. For be passed? Is level translation required? form an undesired voltage divider. In this instance, the LVC1G66 or CBT1G125 can case, a switch with a r small enough to be used to switch a single bit, while the Selection Guide V /V — Switch control (enable). How will on IH IL ensure that the switch output is not reduced CBTLV16211 is capable of switching a total the switch be controlled? Logic level output? below a valid input high level (V ) for the 24 bits in banks of 12. Or, by tying the Comparator? ASIC? Should the switch turn IH connected load. As mentioned previously, adjacent enable pins together, it is possible on if the control signal is high or low? the tradeoff for low ron is often higher signal- to control 24 bits with one enable signal. Switch Output Level — The maximum path capacitance, which reduces frequency Special Features — TI offers bus switches signal level that a switch without a charge response. with special features, such as a bus-hold pump can pass is limited to the switch VCC. ten/tdis — These parameters determine how option (CBTH) for holding floating or unused Is there sufficient noise margin on the device Cross-Reference quickly the switch can respond to a desired I/O pins at valid logic levels; an integrated downstream of the switch such that signal on or off state. In general, switch enable and series resistor option (CBTR) to reduce attenuation in the switch will not cause data disable times are not symmetrical. This is not signal-reflection noise; and precharge to errors? For instance, the n-channel transistor usually an issue, as few applications require support hot plugging. of a CBT device clamps the switch output at high control (enable) signal frequencies. a little more than 1 V below the operating VCC, making it unsuitable for 5-V CMOS tpd — This parameter is negligible for all but high-level (VIH = 3.5 V) signal transmission the most critical timing budgets. When the Packaging unless operated from at least 4.5-V VCC. switch is on, the propagation delay through the pass transistor(s) is minimal. TI specifies r — Is the switch connected to a trans- on this number as the mathematical calculation mission line? If so, what is the impedance? of the typical ron times the load capacitance. The switch ron should be less than or equal Resources Texas Instruments Digital Bus Switch Selection Guide 3 www.ti.com/signalswitches Digital Bus Switch Introduction (Continued) Digital Switches—Control Signal Levels (VIH, VIL) Introduction CBT/CBT-CCB3T CBTLV CB3Q = Input Signal Range = Supply Voltage 5 = Input Threshold Decision Tree 4 Supply Voltage (V) Supply Voltage 3 VIH VIH VIH VIH Technical Overview Technical 2 VIH VIH VIH 1 VIL VIL VIL VIL 0.8 0.7 Control Signal Levels VIL VIL VIL Applications 0 TTL TTL 2.5-V TTL2.5-V TTL 2.5-V CMOS CMOS CMOS Digital Switches—Signal Level Through the Switch Output Signal 5 Selection Guide 4 CBT/CBT-C 3 CBT/CBT-C V = 5 V 2 V = 4.3–5 V CC 1 CC 0 CBT/CBT-C –1 Output Signal VCC = 4.3–5 V CBT/CBT-C CBTLV 5 5 VCC = 4.3 V VCC = 3.3 V 5-V Input Signal 4 3.3-V Input Signal 4 2.5-V Input Signal Output Signal 3 3 CBTLV 2 2 5 5 5 1 1 VCC = 2.5–3.3 V 5 4 CB3T 0 4 CB3Q 0 4 4 3 –1 3 –1 3 3 2 VCC = 3.3 V 2 VCC = 2.5–3.3 V 2 2 Cross-Reference 1 1 1 1 0 5 0 0 CB3Q 0 –1 4 –1 –1 –1 CB3T 3 CB3T VCC = 2.5–3.3 V V = 2.5 V 2 VVcc: = 3.33.3 VV CC 1 CC 0 –1 Output Signal CB3T V = 2.5–3.3 V 5 5 CC 4 4 CB3Q 3 CB3T 3 V = 3.3 V 2 V = 2.5 V 2 Packaging CC 1 CC 1 0 0 –1 –1 Resources www.ti.com/signalswitches 4 Texas Instruments Digital Bus Switch Selection Guide Digital Bus Switch Decision Tree Digital Bus Switch Introduction Level Translation Signal Switching Frequency ≤10 MHz ≤20 MHz ≤100 MHz ≤200 MHz ≤200 MHz ≤500 MHz (Data I/O) Decision Tree 2.5 V 2.5 V 2.5 V VCC 5 V 5 V 5 V 3.3 V 3.3 V 3.3 V Beyond ≤≤ Rail-to-Rail VCC V I/O 5-V Not Rail-to-Rail (0 to 4 V) (0 to 5 V With VI/O Level 5 V to 3.3 V 5 V to 3.3 V Translation Rail-to-Rail to V 3.3-V VCC ) CC (0 to 3.3 V With 2.5-V VCC ) Undershoot Overview Technical Yes Yes Yes Yes Protection No (–2 V) No No (–0.5 V) (–2 V) (–2 V) No No (Switch OFF) Undershoot Clamping –0.5 V –0.5 V –0.5 V –0.5 V –0.5 V –0.5 V –0.5 V –0.5 V –0.5 V (Switch ON) Applications µ ICC 1.5 mA 1.5 mA 40 A 3 µA 3 µA 3 µA 3 µA 10 µA 2 mA Yes Yes Yes Yes Yes Yes I OFF No (10 µA) (10 µA) No No (20 µA) (10 µA) (10 µA) (1 µA) • 2 Port • 2 Port • 2 Port • 2 Port • 2 Port • 2 Port • 2 Port • 2 Port • 2 Port • MUX • MUX • MUX • Bus- • MUX • MUX • MUX Configuration • Bus- • Bus- Exchange • Bus- • Bus- • Bus- Selection Guide Exchange Exchange Exchange Exchange Exchange Solution CBTD CBTD-CCB3T CBT CBTS CBTK CBT-C CBTLV CB3Q Cross-Reference Packaging Resources Texas Instruments Digital Bus Switch Selection Guide 5 www.ti.com/signalswitches Digital Bus Switch Technical Overview 3.3-V LOW-VOLTAGE BUS SWITCHES Introduction More systems are moving to low-voltage CBTLV3245 Logic Diagram (3.3-V) supplies, the chief benefits of which are low power consumption and higher performance.