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LM34919 数据表(PDF) 10 Page - Texas Instruments

部件名 LM34919
功能描述  Ultra-Small 40-V 600-mA Constant On-Time Buck Switching Regulator
Download  21 Pages
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制造商  TI1 [Texas Instruments]
网页  http://www.ti.com
标志 TI1 - Texas Instruments

LM34919 数据表(HTML) 10 Page - Texas Instruments

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0.64A
Load Current
Increases
IPK
IOCL
IO
'I
Normal Operation
Current Limited
STOP
RUN
RON/SD
Input
Voltage
LM34919
VIN
RON
LM34919
SNOSAY2D – MAY 2007 – REVISED JUNE 2008
www.ti.com
Figure 5. Shutdown Implementation
Current Limit
Current limit detection occurs during the off-time by monitoring the recirculating current through the free-wheeling
diode (D1). Referring to the Block Diagram, when the buck switch is turned off the inductor current flows through
the load, into SGND, through the sense resistor, out of ISEN and through D1. If that current exceeds 0.64A the
current limit comparator output switches to delay the start of the next on-time period. The next on-time starts
when the current out of ISEN is below 0.64A and the voltage at FB is below 2.5V. If the overload condition
persists causing the inductor current to exceed 0.64A during each on-time, that is detected at the beginning of
each off-time. The operating frequency is lower due to longer-than-normal off-times.
Figure 6 shows the inductor current waveform. During normal operation the load current is Io, the average of the
ripple waveform. When the load resistance decreases the current ratchets up until the lower peak reaches 0.64A.
During the Current Limited portion of Figure 6, the current ramps down to 0.64A during each off-time, initiating
the next on-time (assuming the voltage at FB is <2.5V). During each on-time the current ramps up an amount
equal to:
ΔI = (VIN - VOUT) x tON / L1
(7)
During this time the LM34919 is in a constant current mode, with an average load current (IOCL) equal to 0.64A +
ΔI/2.
Generally, in applications where the switching frequency is higher than
≊300 kHz and uses a small value
inductor, the higher dl/dt of the inductor's ripple current results in an effectively lower valley current limit threshold
due to the response time of the current limit detection circuit. However, since the small value inductor results in a
relatively high ripple current amplitude (
ΔI in Figure 6), the load current (IOCL) at current limit is typically in excess
of 640 mA.
Figure 6. Inductor Current - Current Limit Operation
10
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Copyright © 2007–2008, Texas Instruments Incorporated
Product Folder Links: LM34919


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