Design Feature: June 22, 1995

While the PCMCIA PC Card standard provides a framework for system design, it also allows considerable latitude to interpret such aspects as power management. Even the latest specification, introduced in March, requires reading between the lines to successfully design a PC Card slot into a system.

When choosing a PCMCIA power controller, consider

• Functionality: V_CC selection (3.3 or 5V); V_PP selection (12, 5, 3.3, or 0V)
• Acceptably low R_ON
• Protection: linear current limiting and overtemperature shutdown
• Controller interface without external components
• Error-indicator feedback to logic controller
• Controlled rise times to ensure no cross conduction or ringing.

The first item, functionality, presents you with a basic decision. Two types of systems exist today: those that have a single V_CC supply (generally 5V) and those with dual supplies (3.3 and 5V). If your target system uses only cards with a single V_CC, then you need not worry about 3.3/5V selection. However, you then need to consider the compatibility issues that can arise because your system lacks a "standard" feature. The same argument applies to the various V_PP voltages.

Acceptably low R_ON is a more difficult consideration that requires interpreting the PCMCIA specification. Protection features are important for a long service life and a good warranty record. Fumbling fingers, probing pencils, and shorted cards can destroy an unprotected slot. A "glueless" interface with the logic controller minimizes board space, complexity, and cost. An error flag feeds back slot-fault-status information that enables the logic controller to take protective action. Controlled rise times prevent damaging voltage overshoot that could destroy sensitive card electronics and prevent power-wasting "shoot through."

PCMCIA specifications

The following discussion is valid for any system, from a handheld data-acquisition system to a high-performance workstation. We'll start by examining the PCMCIA PC Card specification for V_CC. This supply, previously 5V only, is the main power source for the card. The new low-voltage option mandates a "cold socket" with V_CC disabled until you insert the card. A combination of voltage-select pins and mechanical keys on the card and socket differentiates standard (5V) and low-voltage cards. The keys prevent low-voltage cards from fitting into 5V sockets. The voltage-select pins signal which initial voltage the card accepts.

The current requirement for V_CC is 1A max. The specification allows systems using small batteries incapable of supplying large loads to simply reject high-current cards. The PC Card standard limits rise and fall times (measured between the 10 and 90% points), as well as transition timing. Figs 1 and 2 show these limits. Rise time, t₁, must be between 100 µsec and 100 msec. Fall time, t₂, must be between 3 and 300 msec.

A dual-V_CC voltage capability creates a new problem: the need to change from one V_CC level to another. The timing diagram inFig 2 shows the sequence. Upon receipt of the command to change V_CC, the power-control circuit must turn off the present V_CC and let it drop below 0.8V. After at least 100 msec, the circuit enables the new supply voltage, following the same rise-time specification: 100 µsec to 100 msec from the 10 to 90% points.

V_PP requirements

The latest PC Card standard maintains the poorly defined specification for the "program and peripheral" power supply, V_PP, intended for flash-memory write operations. The V_PP voltage is generally either V_CC or 12V. The spec defines two independent V_PP pins, V_PP1 and V_PP2. Many manufacturers combine these pins and drive them together. No known PC Card has a problem with this short cut, but it technically violates the specification.

Though we could justify the same amount of current for V_PP as for V_CC, 12V at 1A represents a power of 12W, which would seriously cut battery life. A more realistic approach is to evaluate the predominant V_PP client's requirement: the flash-memory card. Standard flash cards require 12V at a current between 30 and 120 mA for erasing and programming, to evaluate the predominate V_PP client's requirement: the flash-memory card. Standard flash cards require 12V at a current between 30 and 120 mA for erasing and programming, and demand 5% V_PP accuracy for these delicate operations. The spec does not define V_PP switching times. For flash cards, writing is prohibited until the V_PP supply is stable, so supply stabilization effectively determines the write-access time of a flash "disk." However, excessively fast rise times lead to ringing that can destroy sensitive flash memories. Therefore, you should strive for maximum rise time with zero overshoot. Practical circuits perform this task in 10 to 100 msec.

PC Card slot implementation

A critical requirement for compatibility, performance, and reliability in the field is a low voltage drop at the maximum expected supply current. The V_CC supply, with its 1A output-current specification, requires an extremely low on-resistance, as shown in Table 1. This table shows the highest switch resistance that allows pin voltage to meet the required V_MIN at a given power-supply accuracy.

The key point to remember is that slot voltage depends on a combination of factors: input-supply accuracy, card power draw, pin resistance, and switch resistance. Reasonable values of V_CC5V switch resistance and power-supply tolerance indicate that a good match exists with a ±2% V_CC supply and a 100-mOhms switch. The reduced accuracy requirements of the 3.3V supply allow the use of a lower cost ±5% supply with a moderate-performance 90-mOhms switch or a ±2% supply with a 190-mOhms switch. Small systems that do not support hard-disk drives or other high-current cards may accommodate higher switch resistance, thereby trading lower power-system cost for reduced compatibility.

Table 2 shows V_PP on-resistances. For V_PP12V, acceptable switch resistance is just over 1 Ohms if the power system can guarantee +4% accuracy. Finite pin and pc-board trace resistances preclude the use of a ±5% supply, because the pin-voltage limit is also ±5%. When V_PPV_CC, the current demand changes. Programmable 5V flash cards need supply currents as high as 60 mA at 5V±5%. Using the same ±2 to ±3% supply required to meet the 5V V_CC specification, the 5V V_PP on-resistance should range from 1 to 2.4Ohms. No available card requires any appreciable current when V_PP3.3V, so the on-resistance for this switch is not critical. Systems with V_PP3.3V and 5Ohms on-resistance are common, with no user complaints of incompatibility.

Figure 3

The R_ON-vs-tolerance trade-off

The PC Card specification provides the required pin-voltage tolerance, pin and socket resistance, and the maximum allowable current. Using these numbers, you can determine the maximum allowable power-switch on-resistance and power-supply tolerance. Note that, in many cases, a standard ±5% supply is inadequate to meet the specifications. A more expensive, tighter tolerance supply is necessary, and the tolerance requirement becomes more stringent when you use switches with higher on-resistance. Generally, you can avoid the higher cost of a high-precision power supply by using a slightly more expensive, lower-resistance switch. In general, a power supply with a ±2% tolerance is about $2 per output more expensive than a ±5% supply. This cost increases rapidly when you require even tighter tolerances.

Figure 4

Design examples

Consider three specific system applications with differing power requirements. Proper electronic design requires taking account of the expected worst-case conditions, so we'll assume a 70øC die temperature for the switch. It's not wise to use room-temperature specifications for portable systems.

The first example involves a subnotebook computer. A user considers such a computer, equipped with a 486 or better processor, as a full-featured machine. Expectations are that the computer will support any PC Card, including modems and hard-disk drives. Therefore, the machine must provide the full 1A V_CC output capability. Here, we choose a Micrel Semiconductor MIC2560 V_CC and V_PP power controller for switching. Using the on-resistance specs in Table 3, let's examine the requirements for the power-supply specifications:

So the minimum V_CC input for 4.75V at the card is 4.75 +(1A×145 mOhms)=4.895V. The system power supply must provide 4.895 to 5.25V. You could state this as 5V+5%, -2.1%, or 5.073V±3.5%. This 5.073V nominal supply voltage is well within the acceptable tolerance of a standard 5V±5% supply, so it can power all the 5V components in the system.

The minimum V_CC input for 3.0V at the card is 3.0V+(1A×111 mOhms)=3.111V. The required power-supply voltage is from 3.111 to 3.6V. A 3.3V+9%, -5.7%, or a 3.356V±7.3% supply is suitable.

In the second example, we consider a handheld, or palmtop, computer. These machines generally require a communications capability via a PC Card modem. They usually don't accommodate a hard-disk drive because of battery-capacity constraints. Assuming a modem draws 300 mA from a 5 or 3.3V supply, we select the MIC2561 V_CC and V_PP power controller. The palmtop's supply requirements are:

The minimum V_CC imput for 4.5V at the card is 4.75V+(300 mA×345 mOhms)=4.835V. The system power supply must provide 4.835 to 5.25V. You can state this as 5V+5%, -3.4% or 5.043V±4.0%.

The minimum V_CC input is 3.111V, which denotes a 3.3V+9%, -5.7% or a 3.356V±7.3% supply. For V_CC, we note that the on-resistances of the MIC2561 are suitable for the intended PC Card load, given 5V±3.4% and 3.3V±5.7% supplies. On the other hand, the MIC2560 would do the same job with 5V±4.1% and 3.3V±8% supplies. You must weigh the additional cost of the MIC2560 against the (usually much larger) added cost of a more accurate supply.

In the third example, we consider equipment used for data-taking, such as handheld loggers, DVMs, and inventory checkers. This type of equipment does not require extensive compatibility with various types of PC Cards. The equipment generally uses either SRAM or flash-memory cards and operates with a single V_CC voltage. If this V_CC is 5V, the PCMCIA specification allows hard-wiring the supply to the pin. Otherwise, a simple, single-pole MOSFET switch can enable V_CC to the card.

You can achieve V_PP control by using the MIC2557 power controller for systems that tie V_PP1 and V_PP2 together or the MIC2558 to support separate V_PP pins. Some 5V-only flash cards draw as much as 60 mA from V_PP. You can accommodate these cards by connecting the two halves of the MIC2558 in parallel and using a 5V+5%, -3.4% (or 5.04V±4.1%) supply. The MIC2559 has improved V_CCV_PP on-resistance and allows the use of a 5V+5%, -4% (or 5.025V±4.4%) supply.

When selecting a power controller for PC Card slots, remember that both specified and nonspecified parameters affect compatibility. A controller must switch all required supply voltages for both V_CC and V_PP with acceptable on-resistance across the operating-temperature range and without cross conduction or ringing. It should also protect the rest of the system from any fault condition and signal the logic controller when such a fault exists. Table 4 lists the features available in several PC Card power switches.

The switches you use for V_CC selection must provide a few features not mentioned in the PCMCIA specification. They must properly sequence the 3.3 and 5V supplies so no shoot-through paths exist, and they must exhibit minimal damaging voltage ringing. A price/performance trade-off exists between switch on-resistance and the required V_CC supply accuracy. For any given output current, the switch with lower on-resistance allows use of a lower cost, lower accuracy supply. In many cases, you realize the lowest system cost by using the best available power switch.