LM4128 datasheet(12/33 Pages) TI

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LM4128, LM4128Q

SNVS475E – NOVEMBER 2006 – REVISED APRIL 2013

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APPLICATION INFORMATION

THEORY OF OPERATION

The foundation of any voltage reference is the band-gap circuit. While the reference in the LM4128 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, see the AN-56 Application Report

(SNVA514).

SUPPLY AND ENABLE VOLTAGES

To ensure proper operation, VEN and VIN must be within a specified range. An acceptable range of input voltages

VIN > VREF + 400 mV (ILOAD ≤ 10 mA)

(1)

The enable pin uses an internal pull-up current source (IPULL_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 VIN. 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 VEN > VIN.

COMPONENT SELECTION

A small ceramic (X5R or X7R) capacitor on the input must be used to ensure stable operation. The value of CIN

must be sized according to the output capacitor value. The value of CIN must satisfy the relationship CIN ≥ COUT.

When no output capacitor is used, CIN 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 LM4128

will drive any combination of resistance and capacitance up to VREF/20 mA and 10 µF respectively.

The LM4128 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 LM4128 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 16.

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