LM311 Differential Comparator IC

LM311 is a dedicatedly designed high speed differential comparator responsible for comparing voltages. The operating supply voltage can range from -15 to +15V for OP-AMPs. This voltage is 5V if LM311 is to be used for logical operations. The compatibility of LM311 output with TTL and MOS circuits is quite good. LM311 can be used for operating relays and lamps. At a current of 50 mA, up to 50V can be switched.

The isolation of system ground from all outputs and inputs is one of the key features of LM 311. The functionality of offset balancing along with strobe capability is easily possible. The available packages of LM311 are 8-pin SO, 8-pin SOIC, 8-pin PDIP, and 8-pin TSSOP. The input current of LM311 is around one thousand times smaller than its standard counterparts. The operating temperature range is from 0 to 70°C.

LM311 Pinout Details

LM311 is available as 8-pin SO, 8-pin SOIC, 8-pin PDIP, and 8-pin TSSOP packages. The 8-pin packages have pin-outs as shown:

LM311 pinout diagram

Equivalent op-amp ICs

LM211, LM111

Pin Configuration Details

1 (EMIT OUT)This pin is for obtaining output at emitter comparator
2 ( IN+ non-inverting Input)This pin is for applying non-inverting input voltage
3 ( IN- Inverting Input)This pin is for applying inverting input voltage
4 (Vcc-)This pin is for connecting negative end of the supply voltage
5 ( BALANCE)This pin is for balancing or adjusting
6 ( BAL/STRB)This pin is for balancing and strobe adjustment
7 ( COL OUT)This pin is for obtaining output at collector emitter
8 (Vcc+)This pin is for connecting positive end of supply voltage

LM311 Differential Comparator I Features

The operational features of LM311 are shown as:

ParametersLM 311
Input offset voltage (mV)7.5
Input bias current (nA)300
Input Offset Current (nA)70
Input Common Mode Voltage Range (V)-14.7 to 13.8
Large signal voltage gain (V/mV)200
Supply  current form Vcc+ output high (mA)7.5
Supply  current form Vcc- output high (mA)-5
High Level (Collector) Output Leakage Current (nA)50
Low Level (Collector to Emitter) Output Voltage (V)1.5
Low Level Strobe Current (mA)3
Supply Voltage Vcc+ (V)18
Supply Voltage Vcc- (V)-18
Input Volatge (V)±15
Electrostatic Discharge (V)±500
Junction-to-ambient thermal Resistance (°C/W)260-300
Differential Input voltage (V)±30
Output Short Circuit to Ground Duration (s)10
Lead temperature PDIP (Soldering, 10s) (°C)57.5
Operating Free Air Temperature Range (°C)70

Alternative options for an op-amp

LM324, NTE922, LM709, LM201 , OP07,

Where and How to Use LM311

LM311 can be operated over the wide range of voltages ranging from -15V to +15V and a single supply of voltage up to 5V for IC logic. Inverting or non-inverting voltage range must lie within input common mode voltage range. Non-inverting input voltage is applied at pin no. 2 and inverting at pin no. 3. Pin no 5 and 6 are meant for off-set balancing and adjusting strobe capability. Negative DC supply voltage is applied at pin no. 4 and the positive at pin no 8. If these pins are not supposed to be used they either be short circuited together or kept open circuited. Both conditions are acceptable. It is always recommended not to connect the strobe pin directly to ground. The output is obtained at COL OUT i.e. pin no. 7 while pin no. 1 is grounded. In order to minimize noise a bypass capacitor is supposed to be attached between supply and ground. This capacitor is not be incorporated at pin no. 4 if LM311 is being operated with single power supply.  It is normally grounded in this case. A simplified schematic of LM311 is shown as:

simplified schematic LM311

Example Circuit LM311

In this section, we will see some examples and use of this differential comparator.

Zero Crossing Detector Example

One of the simplest application LM311 IC is a zero crossing detector circuit which is shown as:

LM311 zero crossing detector exampe

As you know, like other comparators, if the voltage at the non-inverting input is greater than inverting terminal, the output comparator will be high. Otherwise, output will logic low. We can use the same concept to design a zero crossing detector circuit.  In the above schematic, we apply an AC signal at the non-inverting input and connect inverting input with the ground. When  the signal at non-inverting input reaches zero voltage value, voltage magnitude becomes equal at both input pins and you will see a pulse at the output of comparator as shown here:

Zero crosssing detector LM311 proteus simulation

Proteus Simulation

Off Set and Strobing Example

Off set balancing and strobing is possible in case of LM 311. This can easily be achieved by making use of following circuit configurations.

Off set balancing and strobing

Off-set Balancing

Off-set Balancing Example LM311


LM311 Applications

The applications of LM311 are:

  • Free running multivibrator
  • Detector for magnetic transducer
  • Crystal oscillator
  • Comparator and solenoid driver
  • Low voltage adjustable voltage supply
  • MOS logic driver
  • Precision squarer
  • Digital transmission isolator
  • Positive peak detector
  • Negative peak detector
  • Precision photodiode comparator
  • Relay driver with strobe
  • Switching power amplifier
  • Body control modules
  • White goods

2D Diagram

2D dimension diagram

LM311 datasheet download

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