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LM2854 数据表(PDF) 10 Page - Texas Instruments

部件名 LM2854
功能描述  Synchronous SIMPLE SWITCHER Buck Regulator
Download  33 Pages
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制造商  TI1 [Texas Instruments]
网页  http://www.ti.com
标志 TI1 - Texas Instruments

LM2854 数据表(HTML) 10 Page - Texas Instruments

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PVIN
AVIN
AGND
PGND
FB
SW
LM2854
SS
RT1
RT2
VIN
CIN
Master Power
Supply
VOUT1
VOUT2
LO
CO
VSS
10
LM2854
SNVS560D – MARCH 2008 – REVISED JANUARY 2016
www.ti.com
Product Folder Links: LM2854
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Copyright © 2008–2016, Texas Instruments Incorporated
Feature Description (continued)
In the event of either AVIN or EN decreasing below the falling UVLO or enable threshold respectively, the
voltage on the soft-start pin is collapsed by discharging the soft-start capacitor through a 5-k
Ω transistor to
ground.
7.3.4 Tracking
The LM2854 can track the output of a master power supply during soft-start by connecting a resistor divider to
the SS pin. In this way, the output voltage slew rate of the LM2854 will be controlled by a master supply for loads
that require precise sequencing. When the tracking function is used, a small value soft-start capacitor can be
connected to the SS pin to alleviate output voltage overshoot when recovering from a current limit fault.
Figure 14. Simplified Schematic Showing Use of Tracking
7.3.5 Pre-Biased Start-up Capability
The LM2854 is in a pre-biased state when the device starts up with an output voltage greater than zero. This
often occurs in many multi-rail applications such as when powering an FPGA, ASIC, or DSP. The output can be
pre-biased in these applications through parasitic conduction paths from one supply rail to another. Even though
the LM2854 is a synchronous converter, it will not pull the output low when a pre-bias condition exists. The
LM2854 will not sink current during start up until the soft-start voltage exceeds the voltage on the FB pin. Since
the device can not sink current it protects the load from damage that might otherwise occur if current is
conducted through the parasitic paths of the load.
7.3.6 Feedback Voltage Accuracy
The FB pin is connected to the inverting input of the voltage loop error amplifier and during closed loop operation
its reference voltage is 0.8 V. The FB voltage is accurate to within –1.25% / +1% over temperature. Additionally,
the LM2854 contains error nulling circuitry to substantially eliminate the feedback voltage over temperature drift
as well as the long term aging effects of the internal amplifiers. In addition, the 1/f noise of the bandgap amplifier
and reference are dramatically reduced. The manifestation of this circuit action is that the duty cycle will have two
slightly different but distinct operating points, each evident every other switching cycle. The oscilloscope plot
shown previously of the SW pin with infinite persistence set shows this behavior. No discernible effect is evident
on the output due to LC filter attenuation. For further information, a Texas Instruments white paper is available on
this topic.
7.3.7 Positive Current Limit
The LM2854 employs lossless cycle-by-cycle high-side current limit circuitry to limit the peak current through the
high-side FET. The peak current limit threshold, denoted ICL, is nominally set at 6 A internally. When a current
greater than ICL is sensed through the PFET, its on-time is immediately terminated and the NFET is activated.
The NFET stays on for the entire next four switching cycles (effectively four PFET pulses are skipped). During
these skipped pulses, the voltage on the soft-start pin is reduced by discharging the soft-start capacitor by a
current sink on the soft-start pin of nominally 6 µA or 14 µA for the 500-kHz or 1-MHz options, respectively.
Subsequent overcurrent events will drain more and more charge from the soft-start capacitor, effectively
decreasing the reference voltage as the output droops due to the pulse skipping. Reactivation of the soft-start
circuitry ensures that when the overcurrent situation is removed, the part will resume normal operation smoothly.


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