SM74601 Datasheet, PDF (9/14 Pages) Texas Instruments – SOT-23 Precision Micropower Series Voltage Reference

SM74601
www.ti.com
SNOSBA5A – SEPTEMBER 2011 – REVISED APRIL 2013
APPLICATION INFORMATION
THEORY OF OPERATION
The foundation of any voltage reference is the band-gap circuit. While the reference in the SM74601 is
developed from the gate-source voltage of transistors in the IC, principles of the band-gap circuit are easily
understood using a bipolar example. For a detailed analysis of the bipolar band-gap circuit, please refer to
Application Note AN-56 SNVA514.
SUPPLY AND ENABLE VOLTAGES
To ensure proper operation, V_EN and V_IN must be within a specified range. An acceptable range of input voltages
is
V_IN > V_REF + 400 mV (I_LOAD ≤ 10 mA)
(1)
The enable pin uses an internal pull-up current source (I_{PULL_UP} ≊ 2 µA) that may be left floating or triggered by
an external source. If the part is not enabled by an external source, it may be connected to V_IN. An acceptable
range of enable voltages is given by the enable transfer characteristics. See the Electrical Characteristics section
and Enable Transfer Characteristics figure for more detail. Note, the part will not operate correctly for V_EN > V_IN.
COMPONENT SELECTION
A small ceramic (X5R or X7R) capacitor on the input must be used to ensure stable operation. The value of C_IN
must be sized according to the output capacitor value. The value of C_IN must satisfy the relationship C_IN ≥ C_OUT.
When no output capacitor is used, C_IN must have a minimum value of 0.1 µF. Noise on the power-supply input
may affect the output noise. Larger input capacitor values (typically 4.7 µF to 22 µF) may help reduce noise on
the output and significantly reduce overshoot during startup. Use of an additional optional bypass capacitor
between the input and ground may help further reduce noise on the output. With an input capacitor, the
SM74601 will drive any combination of resistance and capacitance up to V_REF/20 mA and 10 µF respectively.
The SM74601 is designed to operate with or without an output capacitor and is stable with capacitive loads up to
10 µF. Connecting a capacitor between the output and ground will significantly improve the load transient
response when switching from a light load to a heavy load. The output capacitor should not be made arbitrarily
large because it will effect the turn-on time as well as line and load transients.
While a variety of capacitor chemistry types may be used, it is typically advisable to use low esr ceramic
capacitors. Such capacitors provide a low impedance to high frequency signals, effectively bypassing them to
ground. Bypass capacitors should be mounted close to the part. Mounting bypass capacitors close to the part will
help reduce the parasitic trace components thereby improving performance.
SHORT CIRCUITED OUTPUT
The SM74601 features indefinite short circuit protection. This protection limits the output current to 75 mA when
the output is shorted to ground.
TURN ON TIME
Turn on time is defined as the time taken for the output voltage to rise to 90% of the preset value. The turn on
time depends on the load. The turn on time is typically 33.2 µs when driving a 1µF load and 78.8 µs when driving
a 10 µF load. Some users may experience an extended turn on time (up to 10 ms) under brown out conditions
and low temperatures (-40°C).
THERMAL HYSTERESIS
Thermal hysteresis is defined as the change in output voltage at 25ºC after some deviation from 25ºC. This is to
say that thermal hysteresis is the difference in output voltage between two points in a given temperature profile.
An illustrative temperature profile is shown in Figure 17.
Copyright © 2011–2013, Texas Instruments Incorporated
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