LM2907N National Semiconductor, LM2907N Datasheet - Page 7

• Manufacturer Part Number: LM2907N
• Description: IC, F/V CONVERTER, 10KHZ, 0.3%, 14-DIP
• Manufacturer: National Semiconductor

Specifications of LM2907N

• Frequency: 5kHz
• Linearity %: 0.3%
• Supply Voltage Range: 28V
• Digital Ic Case Style: DIP
• No. Of Pins: 14
• Converter Type: Frequency/Voltage
• Operating Temperature Range: -40°C To +85°C
• Converter Function: FVC
• Power Supply Requirement: Single
• Single Supply Voltage (typ): 12V
• Single Supply Voltage (max): 28V
• Dual Supply Voltage (typ): Not RequiredV
• Dual Supply Voltage (min): Not RequiredV
• Dual Supply Voltage (max): Not RequiredV
• Operating Temperature (min): -40C
• Operating Temperature (max): 85C
• Operating Temperature Classification: Industrial
• Package Type: MDIP
• Leaded Process Compatible: No
• Rohs Compliant: No
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Applications Information
The LM2907 series of tachometer circuits is designed for
minimum external part count applications and maximum ver-
satility. In order to fully exploit its features and advantages
let's examine its theory of operation. The first stage of oper-
ation is a differential amplifier driving a positive feedback flip-
flop circuit. The input threshold voltage is the amount of
differential input voltage at which the output of this stage
changes state. Two options (LM2907-8, LM2917-8) have one
input internally grounded so that an input signal must swing
above and below ground and exceed the input thresholds to
produce an output. This is offered specifically for magnetic
variable reluctance pickups which typically provide a single-
ended ac output. This single input is also fully protected
against voltage swings to ±28V, which are easily attained with
these types of pickups.
The differential input options (LM2907, LM2917) give the user
the option of setting his own input switching level and still have
the hysteresis around that level for excellent noise rejection
in any application. Of course in order to allow the inputs to
attain common-mode voltages above ground, input protection
is removed and neither input should be taken outside the lim-
its of the supply voltage being used. It is very important that
an input not go below ground without some resistance in its
lead to limit the current that will then flow in the epi-substrate
diode.
Following the input stage is the charge pump where the input
frequency is converted to a dc voltage. To do this requires
one timing capacitor, one output resistor, and an integrating
or filter capacitor. When the input stage changes state (due
to a suitable zero crossing or differential voltage on the input)
the timing capacitor is either charged or discharged linearly
between two voltages whose difference is V
half cycle of the input frequency or a time equal to 1/2 f
change in charge on the timing capacitor is equal to V
C1. The average amount of current pumped into or out of the
capacitor then is:
The output circuit mirrors this current very accurately into the
load resistor R1, connected to ground, such that if the pulses
of current are integrated with a filter capacitor, then V
R1, and the total conversion equation becomes:
Where K is the gain constant—typically 1.0.
V
O
= V
CC
× f
IN
× C1 × R1 × K
CC
/2. Then in one
O
CC
IN
= i
/2 ×
the
c
×
7
The size of C2 is dependent only on the amount of ripple volt-
age allowable and the required response time.
CHOOSING R1 AND C1
There are some limitations on the choice of R1 and C1 which
should be considered for optimum performance. The timing
capacitor also provides internal compensation for the charge
pump and should be kept larger than 500 pF for very accurate
operation. Smaller values can cause an error current on R1,
especially at low temperatures. Several considerations must
be met when choosing R1. The output current at pin 3 is in-
ternally fixed and therefore V
to this value. If R1 is too large, it can become a significant
fraction of the output impedance at pin 3 which degrades lin-
earity. Also output ripple voltage must be considered and the
size of C2 is affected by R1. An expression that describes the
ripple content on pin 3 for a single R1C2 combination is:
It appears R1 can be chosen independent of ripple, however
response time, or the time it takes V
voltage increases as the size of C2 increases, so a compro-
mise between ripple, response time, and linearity must be
chosen carefully.
As a final consideration, the maximum attainable input fre-
quency is determined by V
USING ZENER REGULATED OPTIONS (LM2917)
For those applications where an output voltage or current
must be obtained independent of supply voltage variations,
the LM2917 is offered. The most important consideration in
choosing a dropping resistor from the unregulated supply to
the device is that the tachometer and op amp circuitry alone
require about 3 mA at the voltage level provided by the zener.
At low supply voltages there must be some current flowing in
the resistor above the 3 mA circuit current to operate the reg-
ulator. As an example, if the raw supply varies from 9V to 16V,
a resistance of 470Ω will minimize the zener voltage variation
to 160 mV. If the resistance goes under 400Ω or over 600Ω
the zener variation quickly rises above 200 mV for the same
input variation.
CC
O
, C1 and I
/R1 must be less than or equal
OUT
2
:
to stabilize at a new
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