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ST890: A HIGH SIDE SWITCH FOR PCMCIA AND USB APPLICATIONS

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Last Updated: 15/10/2003

Pages: 9

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AN1441 APPLICATION NOTE
ST890: A HIGH SIDE SWITCH FOR PCMCIA AND USB APPLICATIONS
A. Randazzo

1. INTRODUCTION ST890 is a low voltage, P-Channel MOSFET power switch, intended for high side load switching applications. Its main applications are PCMCIA slots, Portable Equipment and Access bus slots. ST890 is useful in all the applications that need a supply with short-circuit protection and programmable current limitation, like the slots in which the cards can be connected and disconnected without turning off the power supply. 2. FUNCTIONAL DESCRIPTION 2.1. Switch Resistance.

Figure 1 shows the internal block diagram of the ST890. The high side Power MOSFET, used as a switch, has an RON lower than 90m at Vcc=3.0V. In the case of a load current of 1A, the drop voltage between the IN and OUT pins is lower than 1A x 0.1=0.1V and, for a switch, it is very important to keep this drop voltage low. Figure 1: ST890 Internal Block Diagram

IN FAULT

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ON + + ON

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OUT

SET RSET VREF 1.24V

-

Control Circuitry

GND

October 2003 (Rev. 1)

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AN1441 - APPLICATION NOTE Figure 2.1 shows the RON versus the VIN supply voltage. The RON depends on the VIN because the PChannel MOS is driven by the Vout of the internal error amplifier. Figure 2.1: ON Resistance Versus VIN Supply Voltage

ON Resistance vs VIN
10 0 95

R(on ) (m ohm)

90 85 80 75 70 65 2 .5 3 3.5 4 4 .5 5 5 .5

VI N (V)

TA = 25C

2.2. Short Circuit Protection

The low resistance of the MOS channel is important; however, the control circuitry that must quickly drive the MOS, in order to provide enough voltage VGS to turn on the MOS channel, is more important. If the control circuitry does not drive the MOS quick enough, the current of the card could be broken, as shown in figure 2.2. Figure 2.2: ST890 In A Short Circuit Condition

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CH1: VIN voltage, AC Coupled. CH2: Output current CH3: VOUT waveform.

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AN1441 - APPLICATION NOTE When in normal function, the OUTPUT current, divided by 1110 by the current mirror circuitry (see figure 1), flows in the external RSET resistor creating a V SET voltage. VSET is compared to the internal reference voltage (1.24V), and the error amplifier provides the VGS voltage to drive the High Side P-Channel MOS. Thanks to this feedback it is possible to limit the output current at ILIMIT. When a short circuit occurs, the VSET drops because of the internal current mirror and the V + input of the error amplifier becomes lower than 1.24V (internal reference voltage). In this case the error amplifier can not work in the linear area and current control is not possible. To ensure current limitation, even in short circuit conditions, the error amplifier is supported by a correction circuitry and the result is shown in figure 2.2. After the rising of the output current, the feedback circuitry begins to run and the output current is then equal or less than 1.2 times the ILIMIT value. This measurment was made with the test circuit seen in figure 2.3. Figure 2.3: Test Circuit
+2.7÷5.5V

ST890
100F

IN 1
VIN (CH1)

8 7 6 5

FAULT OUT OUT SET IOUT (CH2) VOUT (CH3)

IN 2 ON 3

GND 4

Note: To make this measurment, the Input capacitor is 100F instead of 1F. Typically a 1F input capacitor, like in the demoboard circuit, is enough. RSET is calculated to obtain ILIMIT =1A and R LOAD is set to have 1A to GND. To generate the short circuit condition the switch on the right side of figure 3b was used. 2.3. Programming The Current Limit

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ST890's current limit can be programmed with the SET pin. This pin is internally connected both to the current mirror, that divides the IOUT by 1110, and to the error amplifier (figure 1) that realizes the difference between the reference voltage and the voltage VSET of the SET pin. Figure 2.3.1 shows the ILIMIT vs I SET c ur v e.

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100nF Short

RLOAD 0÷10ohm

circuit resistor 0.3

Set to obtain IOUT=1A

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AN1441 - APPLICATION NOTE Figure 2.3.1: Iset vs Isc Characteristics

2 1.8 1.6 1.4

1.2 1 0.8 0.6 0.4 0.2 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Vin=2.7V Vin=3.3V Vin=5.0V Vin=6.0V Vin=7.0V

Isc (A)

Iset (mA)

The formula to calculate RSET is:

RS E T = VS E T / IS E T
t hen:

I S E T = I L I M I T / 1110

R S E T = 1.24 × 1110 / I L I M I T

Where VSET is the pin 5 voltage; in the case of current limitation, this voltage corresponds to the internal VREF (see figure 1). ISET is the current flowing into the RSET resistor. Figure 2.3.2 shows the programmable current range joined with the RSET. The minimum ISC value is up to 200mA.. Figure 2.3.2: Rset vs Isc Characteristics
2000 1800 1600 1400

1200 1000 800 600 400 200 0

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Vin=2.7V Vin=3.3V Vin=5.0V Vin=6.0V Vin=7.0V

1000

2000

3000

4000

5000

6000

7000

Rset (ohms)

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AN1441 - APPLICATION NOTE 2.4. Fault The FAULT pin (pin 8) is an Open Drain Output useful to warn the microprocessor that a fault condition has occurred. The fault condition starts in the following conditions: - if the IOUT current exceeds the ILIMIT value set; - if a short circuit occurs; - if the devices goes in thermal protection. The FAULT pin should be connected to the IN pin (Vcc) by a 100K pull-up resistor. This n-channel MOSFET can drive a LED in pull-up configuration as shown in figure 2.3. In a typical USB application the ST890 typically supplys a load up to 44 in parallel with 10F that represents any bypass capacitor directly connected across the power USB line. When the Power Switch is turned ON an inrush current flows through the capacitor and causes an unwanted FAULT warning signal as seen in figure 2.4. To avoid that the controller goes in alarm, an RC filter can be placed on the FAULT pin as seen in figure 2.5. Figure 2.4: Fault Signal When An Inrush Condition Occurs

C H1: Voltage on the FAULT pin (pin 8) C H2: output current

Figure 2.5: Filter For The FAULT Transients

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Supply Voltage 5V

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33k FLT OUT 10k 8

USB CONTROLLER

CHECK
V Bus
22nF

7 0.1 F

3 ON 4
GND

OUT 6
SET

5 RSET

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AN1441 - APPLICATION NOTE 2.5. Thermal Protection Figure 2.6: Thermal Protection Behavior CH1: VIN voltage, AC Coupled. CH2: VOUT waveform. CH3: Output current

Thermal protection occurs when the junction temperature exceeds 135C and the thermal hysteresis is 15C. This feature safeguards the device from dangerous currents or temperatures. Figure 2.6 shows the thermal protection behavior. The pulse width and period of the output current depend on the thermal dissipation. This test was made in a free air temperature condition. 2.6. ON Pin Function Figure 2.7: Turn-on Time

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Figure 2.8: Turn-off Time

The ON pin switches over the N-Channel MOSFET. Figures 2.7 and 2.8 respectively show the turn-on and turn-off times.

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AN1441 - APPLICATION NOTE 3. APPLICATION INFORMATION The ST890 application circuit needs few external components. 3.1. Power Supply Filtering At the input a 1F capacitor to GND can be placed in order to reduce the drop voltage during the switching and short circuit events. On the OUT pin to GND a 100nF capacitor filters the output signal. For example, figures 2.7 and 2.8 show the output voltage in a switching condition, with a load sinked current of 500mA. In order to improve the ST890 performance, the filter capacitors should be placed near the pins. 3.2. PCB Rules Regarding the Printed Circuit Board, some rules should be followed: the IN and OUT pins should be connected using a large metal area to reduce the wire resistance and to reduce the drop voltage between IN and OUT. The SET, FAULT and ON pins are not critical. 3.3. PCB Thermal Dissipation Figure 3.1: ST890 Demoboard (not to scale)

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A ground plane is useful when improving the power dissipation of the ST890 device. In normal conditions the package used for the ST890 device, SO-8, can dissipate the power produced by the ILIMIT current that flows in the switch, P=I2LIMIT x RON. This power is also equivalent to P=V2SW/RON , where VSW is the drop voltage of the switch in ON state. When the OUT pin is shorted to GND the VSW increases as well as the power and the junction temperature. This temperature, that continually increases until the thermal protection occurs, can be dissipated with a ground plain, as shown in figure 3.1. Figure 3.1 and 3.2 display the demoboard PCB and schematic circuit. It was designed for SMD components.

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AN1441 - APPLICATION NOTE 3.4. Demoboard Description Figure 3.2: ST890 Demoboard Schematic Circuit (ILIMIT=500mA)

R3

VIN = +2.7÷5.5V
J1 C1 IN IN J3 ON GND 1 2 3 4

LE D 1

R2

ST890
8 7 6 5 FAULT OUT O UT

FAULT Jumper
J4

External FAULT
J5

ON/OFF Jumper

C2 SET R1

OUTPUT J2

On the left side of the board the power Input connector is found, while the Output connector is found on the right side. A 3 pin connector J3 is found on the left side as well. This connector uses a "jumper" that jumps from either GND or VIN to turn the switch either ON or OFF. The FAULT pin (pin 8) is connected to J4. It can be closed by using the jumper to reach LED1 or it can be used to connect the FAULT to an external microprocessor. The SET pin (pin 5) is connected to GND through a resistor R1 to set the ILIMIT. The bill of material is: R1= 2700 1/4W (to set ILIMIT=500mA) R2=680 1/4W R3=100K 1/4W C1= 1F C2= 0.1F. IC1=ST890 LED1= 3mm LED J1÷J5= straight pin Headers 3.5. ST890 and USB Bus

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An example of ST890's application is the supply of the USB bus, as seen in figure 3.3. Few components are required in ST890; RSET is 2.7k in order to obtain an ILIMIT of 500mA and the filtering capacitance are used to filter the power supply IN and OUT. The USB CONTROLLER is used to control the switch, check the VBUS condition through the FLT pin and drive the USB Data line (D+ and D-); it could even be the microcontroller ST7263.

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AN1441 - APPLICATION NOTE Figure 3.3: ST890 USB Application

ST890
1 1 F IN FLT OUT 8 7
1 Vbus

2 IN 3 4 ON
GND

0.1F
2 D+

OUT 6
SET

USB PORT

3

D-

5
4 GND

RSET 2.7k
USB CONTROLLER
ONOFF CHECK

100k

D+ DVCC

0.1F

1F
GND

Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics.

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Document Number: 8422



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