Application Hint 15 - Micrel Semiconductor, Inc.

Application Hint 15 Micrel

2-80 1997

Introduction

Some applications require a multiplexer that can deliver two

or more supply voltages at 1A or greater. An example is the

VCC multiplexer for the PCMCIA interface. A low cost

multiplexer for high current loads can be made using the

Micrel MIC5014 and a few discrete power MOSFETs. A

simple 3.3V and 5V switch is shown in Figure 1. Since low

cost discrete MOSFETs are available with ON resistances of

a few milliohms, these multiplexers can manage currents

exceeding several tens of amperes.

The MIC5014

Making this solution possible is the MIC5014 MOSFET

driver. This driver is designed to provide gate enhancement

above the positive rail for an N-channel FET. N-channel

FETs have the advantages of lower cost and lower RDS(ON)

than similar P-channel FETs. The MIC5014 consumes a

maximum of 1µA in the OFF state and typically 100µA in the

ON state while powered from a 5V supply. The MIC5014 does

not require its supply to be the input logic supply since the

control input threshold is approximately 1.2V and is indepen-

dent of supply voltage. Likewise, the MIC5014 does not

require its supply to be the MOSFET drain supply voltage

because the voltage supplied to the gate is regulated and will

not exceed 16V above the source voltage and is also inde-

pendent of the supply voltage. The MIC5014 is available in

an 8-pin SOIC package which helps minimizes the size of the

complete switch matrix.

The Switch Matrices

Figure 1 shows the basic switching matrix configurations. Q1

through Q3 can be any low impedance N-channel power

FETs. Table 1 shows the expected VOUT for several FETs

with different RDS(ON) values.

Application Hint 15

A High Current VCC Switching Matrix

by Brenda Kovacevic

Each FET has its body internally connected to its source,

resulting in an intrinsic diode between the body and the drain

known as a “body diode.” Figure 1 shows that the body diode

does not present a problem for the Q1 switch, because it is

always reverse biased. If Q3 were not in the circuit and the

source of Q2 connected directly to the output, then Q2’s body

diode would be reverse biased when both Q1 and Q2 are OFF

and the output voltage is zero. However, Q2’s body diode

would be forward biased when Q1 is ON and the 5V supply

would be shorted to the 3.3V supply through Q1 and the

forward biased body diode of Q2. Similarly, if Q2 were not in

the circuit and the source of Q3 connected to 3.3V, then Q3’s

body diode would be reverse biased when Q1 is ON but

forward biased when both Q1 and Q3 are OFF and the load

would be held one diode drop below 3.3V. With two MOSFETs

connected back to back, both body diodes will be reverse

biased when all switches are OFF as long as the output

voltage remains positive with respect to ground. Although Q3

conducts current in the reverse direction when it is ON, the

body diode will not conduct because it is shorted by Q3’s ON

resistance.

FET Part RDS(ON) VOUT at 1A

Number 5.0V Input 3.3V Input

IRFZ20 100mΩ4.9V 3.1V

IRFZ30 50mΩ4.95V 3.20V

SMP06N06-14 14mΩ4.99V 3.27V

Table 1. Power FETs and Expected

Output Voltages at 1A

Low Current PCMCIA VPP

Switching Matrices

If VPP programming currents are switched, the new MIC2557/

2558 devices provide all level shifting, timing, and high

current switches for this function in one package. The

MIC2557 serves as a single channel and is available in an

8-pin SOIC package (see Figure 2). The MIC2558 is a dual

channel device, and is available in a 14-pin SOIC package

(see Figure 3). See the MIC2557 or MIC2558 data sheets for

full details.

3.3V Control

5V or 12V 3.3V

Q2 Q3

330k

Control

Input

Gnd Source

GateV

MIC5014

5V Control

5V or 12V 5VQ1

Switched V

0V / 3.3V / 5V

Control

Input

Gnd Source

GateV

MIC5014

Figure 1. Switch Matrix for 0V, 3.3V, and 5V Figure 2. Typical MIC2557 Application

MIC2557

Hi-Z / LOW Control

EN0

EN1

PP(n)

OUT

(+3.3V or +5V)

(+12V)