voltage clamp circuits

8/11/2019 Voltage Clamp Circuits

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Voltage clamps for protectinglow-voltage dc circuits require anew approach to circuit design.

Many commonly used voltage clampswere built for systems of 5 V or high-er, but lower-voltage systems requiredistinctive clamping abilities. The useof very-low-voltage precision en-hancement-mode MOSFETs play apivotal role in designing voltageclamps in low-voltage applications.

Todays electronic systems ofteninclude many different protectiontechnologies in order to ward offESD, EMI, voltage transients, and

supply faults or fluctuations that canrandomly occur on power supplies,analog signal lines, communicationlines, and data buses.

Laptop computers, for instance,include peripherals, open ports, bus-es, connecting signal cables, andpower cables that are all vulnerable.Electromechanical disk drives cangenerate sudden load changes, andinductive switching often generateshigh levels of transient energy that isradiated around the system.

Transient voltages often resultfrom the sudden release of stored en-ergy. In many systems, circuits sharethe same supply bus and power anddata lines are often bundled togeth-er. Parasitic cable capacitances andinductances can create a path fortransient voltages produced on thepower lines and transferred to datalines. Connecting a USB cable into asocket, or hot-swapping a card or ca-ble, can invisibly generate dangeroustransients. Additionally, portable

systems use dc/dc switching regula-

Voltage clamp circuits

for ultra-low-voltage appsVery-low-voltage enhancement-mode MOSFETs can playa significant role in low-voltage designs

tors that generate both transientsand noise.

This spells trouble for microcon-trollers and other MOS-based ICsand devices that are susceptible todamage from overvoltage. Transientvoltages on low voltage power linesoften attain amplitudes many timesthe nominal voltage level, therebyputting vulnerable components con-stantly at risk. As a result, the needfor reliable overvoltage protectionand voltage clamps is even more im-portant now than ever.

There is, however, a subtle differ-

ence between an overvoltage protec-tion circuit and a voltage clamp.Both types of circuits monitor theinput voltage and control the gate ofan external transistor switch with-out interfering with normal opera-tion of the load circuit.

If the incoming voltage exceeds apreset threshold, the overvoltageprotection circuit will disconnectthe load during the event. In con-trast the clamp circuit will continue

to power the load during the tran-

sient event, but limit the voltage be-ing applied to it. In both cases theprotection circuit must be fastenough to prevent any transientfrom damaging the load.

Alternative protectionmethodsVery-low-voltage operation puts se-vere strain on the existing methodsof overvoltage protection. Low-volt-age Zeners used as voltage clampshave high leakage currents, and theirvoltage ratings are not precise (1.8 V

Z

5% = 90 mV; 2.7 VZ5% = 135 mV,

etc.), while MOVs and most TVS prod-ucts are mostly impractical due totheir >5-V breakdown voltages. Sim-ple diodes have limited forward volt-ages and power-handling capability.

A novel approach uses very low-voltage precision enhancement-mode MOSFETs to improve on theclamping actions of Zener diodes(seeFig. 1a).Figure 1bshows a circuitmade to simulate a low-power Zenershunt regulator, using two parallel-

connected EPAD transistors.

BY BOB CHAO

Advanced Linear Devices

Sunnyvale, CA

http://www.aldinc.com

Fig. 1. The typical low-voltage Zener shunt regulator (a) suffers froma high power-dissipation.

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Under normal conditions the re-sistor R

1will drop the voltage differ-

ence between the supply voltage (3V), and the gate threshold voltage(V

TH= +1.4 V 1.5%). The difference

between each MOSFETs gate thresh-old voltage is typically 10 mV.

You should always ensure that R1

is small enough to supply the mini-mum IZ(5 nA max), even when the

supply voltage is at its minimum (2.5

V) and load current is at its maxi-mum (6 mA). The total current pass-ing through the resistor is (I

L + I

Z).

The value of the resistor will be

R1= 1.1 V/6 mA = 183

Although the gate normally con-nects directly to the drain, the 1-kprotective resistor R

2and the 0.01-F

capacitor C1, may be required for im-

proved stability.Dual-matched enhancement-mode

MOSFETs (b) serve as low-power low-voltage Zener shunt regulators.

Compared with a real Zener di-ode, this circuit illustrates that amuch lower Zener current can beused to establish the Zener voltage(only the EPADs leakage current of1 A).

Under normal operating condi-tions, when the clamp is not activat-ed, it draws virtually zero power be-cause the circuits quiescent powerdissipation consists essentially of thecombined leakage currents (


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voltage clamp circuits

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