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LM5008 データシート(PDF) 8 Page - Texas Instruments

部品番号 LM5008
部品情報  High Voltage (100V) Step Down Switching Regulator
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ホームページ  http://www.ti.com
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LM5008 データシート(HTML) 8 Page - Texas Instruments

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TOFF =
VFB
(6.35 x 10
-6
x RCL)
0.285 +
10
-5
STOP
RUN
RON
LM5008
RON/SD
VIN
Input
Voltage
LM5008
SNVS280F – MAY 2004 – REVISED APRIL 2011
www.ti.com
Regulation Comparator
The feedback voltage at FB is compared to an internal 2.5V reference. In normal operation (the output voltage is
regulated), an on-time period is initiated when the voltage at FB falls below 2.5V. The buck switch will stay on for
the on-time, causing the FB voltage to rise above 2.5V. After the on-time period, the buck switch will stay off until
the FB voltage again falls below 2.5V. During start-up, the FB voltage will be below 2.5V at the end of each on-
time, resulting in the minimum off-time of 300 ns. Bias current at the FB pin is nominally 100 nA.
Over-Voltage Comparator
The feedback voltage at FB is compared to an internal 2.875V reference. If the voltage at FB rises above 2.875V
the on-time pulse is immediately terminated. This condition can occur if the input voltage, or the output load,
change suddenly. The buck switch will not turn on again until the voltage at FB falls below 2.5V.
On-Time Generator and Shutdown
The on-time for the LM5008 is determined by the RON resistor, and is inversely proportional to the input voltage
(Vin), resulting in a nearly constant frequency as Vin is varied over its range. The on-time equation for the
LM5008 is:
TON = 1.25 x 10
-10 x R
ON / VIN
(4)
See Figure 4. RON should be selected for a minimum on-time (at maximum VIN) greater than 400 ns, for proper
current limit operation. This requirement limits the maximum frequency for each application, depending on VIN
and VOUT. See Figure 5.
The LM5008 can be remotely disabled by taking the RON/SD pin to ground. See Figure 12. The voltage at the
RON/SD pin is between 1.5 and 3.0 volts, depending on Vin and the value of the RON resistor.
Figure 12. Shutdown Implementation
Current Limit
The LM5008 contains an intelligent current limit OFF timer. If the current in the Buck switch exceeds 0.5A the
present cycle is immediately terminated, and a non-resetable OFF timer is initiated. The length of off-time is
controlled by an external resistor (RCL) and the FB voltage (see Figure 6). When FB = 0V, a maximum off-time is
required, and the time is preset to 35µs. This condition occurs when the output is shorted, and during the initial
part of start-up. This amount of time ensures safe short circuit operation up to the maximum input voltage of 95V.
In cases of overload where the FB voltage is above zero volts (not a short circuit) the current limit off-time will be
less than 35µs. Reducing the off-time during less severe overloads reduces the amount of foldback, recovery
time, and the start-up time. The off-time is calculated from the following equation:
(5)
The current limit sensing circuit is blanked for the first 50-70ns of each on-time so it is not falsely tripped by the
current surge which occurs at turn-on. The current surge is required by the re-circulating diode (D1) for its turn-
off recovery.
8
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Copyright © 2004–2011, Texas Instruments Incorporated
Product Folder Links: LM5008


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