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LM22678 Scheda tecnica(PDF) 11 Page - Texas Instruments

Il numero della parte LM22678
Spiegazioni elettronici  SIMPLE SWITCHER Step-Down Voltage Regulator With Features
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LM22678 Scheda tecnica(HTML) 11 Page - Texas Instruments

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LM22678, LM22678-Q1
www.ti.com
SNVS585L – SEPTEMBER 2008 – REVISED NOVEMBER 2014
Feature Description (continued)
In general, hand calculations or simulations can only aid in selecting good power stage components. Good
design practice dictates that load and line transient testing should be done to verify the stability of the application.
Also, Bode plot measurements should be made to determine stability margins. AN-1889 How to Measure the
Loop Transfer Function of Power Supplies (SNVA364) shows how to perform a loop transfer function
measurement with only an oscilloscope and function generator.
7.4 Device Functional Modes
7.4.1 Shutdown Mode
The EN pin provides electrical ON and OFF control for the LM22678 device. When VEN is below 1.6 V, the
device is in shutdown mode. 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. The LM22678 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).
7.4.2 Active Mode
The LM22678 device is in active mode when VEN is above the precision enable threshold and its input voltage is
above its UVLO level. The simplest way to enable the LM22678 is to connect the EN pin to VIN through a
resistor. A resistor of 470 k
Ω will limit the current to a safe value for input voltages as high 42 V.
7.4.3 Current Limit
The LM22678 device has current limiting to prevent the switch current from exceeding safe values during an
accidental overload on the output. This peak current limit is found in the Electrical Characteristics table under the
heading of ICL. The maximum load current that can be provided, before current limit is reached, is determined
from Equation 5.
(5)
Where:
L is the value of the power inductor.
When the LM22678 device enters current limit, the output voltage will drop and the peak inductor current will be
fixed at ICL at the end of each cycle. The switching frequency will remain constant while the duty cycle drops. The
load current will not remain constant, but will depend on the severity of the overload and the output voltage.
For very severe overloads "short-circuit", the regulator changes to a low frequency current foldback mode of
operation. The frequency foldback is about 1/5 of the nominal switching frequency. This will occur when the
current limit trips before the minimum on-time has elapsed. This mode of operation is used to prevent inductor
current "run-away", and is associated with very low output voltages when in overload. Equation 6 can be used to
determine what level of output voltage will cause the part to change to low frequency current foldback.
(6)
Where:
Fsw is the normal switching frequency.
Vin is the maximum for the application.
Copyright © 2008–2014, Texas Instruments Incorporated
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Product Folder Links: LM22678 LM22678-Q1


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