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LM22672 Scheda tecnica(PDF) 9 Page - Texas Instruments

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Il numero della parte LM22672
Spiegazioni elettronici  SIMPLE SWITCHER짰 Step-Down Voltage Regulator with Features
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EN
Vin
RENB
RENT
LM22672, LM22672-Q1
www.ti.com
SNVS588M – SEPTEMBER 2008 – REVISED NOVEMBER 2014
7.3 Feature Description
7.3.1 Precision Enable and UVLO
The precision enable input (EN) is used to control the regulator. The precision feature allows simple sequencing
of multiple power supplies with a resistor divider from another supply. Connecting this pin to ground or to a
voltage less than 1.6 V (typ) will turn off the regulator. The current drain from the input supply, in this state, is 25
µA (typ) at an input voltage of 12 V. The EN input has an internal pullup of about 6 µA. Therefore this pin can be
left floating or pulled to a voltage greater than 2.2 V (typ) to turn the regulator on. The hysteresis on this input is
about 0.6 V (typ) above the 1.6-V (typ) threshold. When driving the enable input, the voltage must never exceed
the 6 V absolute maximum specification for this pin.
Although an internal pullup is provided on the EN pin, it is good practice to pull the input high, when this feature
is not used, especially in noisy environments. This can most easily be done by connecting a resistor between
VIN and the EN pin. The resistor is required, because the internal zener diode, at the EN pin, will conduct for
voltages above about 6 V. The current in this zener must be limited to less than 100 µA. A resistor of 470 k
Ω will
limit the current to a safe value for input voltages as high 42 V. Smaller values of resistor can be used at lower
input voltages.
The LM22672 device also incorporates an input undervoltage lock-out (UVLO) feature. This prevents the
regulator from turning on when the input voltage is not great enough to properly bias the internal circuitry. The
rising threshold is 4.3 V (typ) while the falling threshold is 3.9 V (typ). In some cases these thresholds may be too
low to provide good system performance. The solution is to use the EN input as an external UVLO to disable the
part when the input voltage falls below a lower boundary. This is often used to prevent excessive battery
discharge or early turn-on during start-up. This method is also recommended to prevent abnormal device
operation in applications where the input voltage falls below the minimum of 4.5 V. Figure 12 shows the
connections to implement this method of UVLO. Equation 1 and Equation 2 can be used to determine the correct
resistor values.
(1)
(2)
Where:
Voff is the input voltage where the regulator shuts off.
Von is the voltage where the regulator turns on.
Due to the 6 µA pullup, the current in the divider should be much larger than this. A value of 20 k
Ω, for RENB is a
good first choice. Also, a zener diode may be needed between the EN pin and ground in order to comply with the
absolute maximum ratings on this pin.
Figure 12. External UVLO Connections
7.3.2 Soft-Start
The soft-start feature allows the regulator to gradually reach steady-state operation, thus reducing start-up
stresses. The internal soft-start feature brings the output voltage up in about 500 µs. This time can be extended
by using an external capacitor connected to the SS pin. Values in the range of 100 nF to 1 µF are recommended.
The approximate soft-start time can be estimated from Equation 3.
Copyright © 2008–2014, Texas Instruments Incorporated
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