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LM22674 Folha de dados(PDF) 9 Page - Texas Instruments

Nome de Peças LM22674
Descrição Electrónicos  LM22674/-Q1 42 V, 500 mA SIMPLE SWITCHER짰 Step-Down Voltage Regulator with Features
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LM22674 Folha de dados(HTML) 9 Page - Texas Instruments

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EN
Vin
RENB
RENT
LM22674, LM22674-Q1
www.ti.com
SNVS590M – SEPTEMBER 2008 – REVISED NOVEMBER 2014
Feature Description (continued)
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 LM22674 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 10 shows the
connections to implement this method of UVLO. The following equations 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 10. 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 is fixed and can
not be changed. Soft-start is reset any time the part is shut down or a thermal overload event occurs.
7.3.3 Boot-Strap Supply
The LM22674 device incorporates a floating high-side gate driver to control the power MOSFET. The supply for
this driver is the external boot-strap capacitor connected between the BOOT pin and SW. A good quality 10 nF
ceramic capacitor must be connected to these pins with short, wide PCB traces. One reason the regulator
imposes a minimum off-time is to ensure that this capacitor recharges every switching cycle. A minimum load of
about 5 mA is required to fully recharge the boot-strap capacitor in the minimum off-time. Some of this load can
be provided by the output voltage divider, if used.
Copyright © 2008–2014, Texas Instruments Incorporated
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Product Folder Links: LM22674 LM22674-Q1


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