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LM34917A Folha de dados(PDF) 10 Page - Texas Instruments

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Nome de Peças LM34917A
Descrição Electrónicos  Ultra Small 33V, 1.25A Constant On-Time Buck Switching Regulator with Intelligent Current Limit
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Página de início  http://www.ti.com
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LM34917A Folha de dados(HTML) 10 Page - Texas Instruments

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'I
IOUT2
IOUT3
Normal
Operation
Load Current
Decreases
Current
Limited
Load
Current
Increases
Normal
Operation
Current Limit
Threshold
IOUT1
TON
2.5V
TON
2
LM34917A
SNOSAX7C – DECEMBER 2007 – REVISED MAY 2008
www.ti.com
Figure 5. Inductor Current - Normal and Current Limit Operation
N - Channel Buck Switch and Driver
The LM34917A integrates an N-Channel buck switch and associated floating high voltage gate driver. The gate
driver circuit works in conjunction with an external bootstrap capacitor and an internal high voltage diode. A 0.022
µF capacitor (C4) connected between BST and SW provides the voltage to the driver during the on-time. During
each off-time, the SW pin is at approximately -1V, and C4 is recharged for the next on-time from VCC through the
internal diode. The minimum off-time ensures a minimum time each cycle to recharge the bootstrap capacitor.
Softstart
The softstart feature allows the converter to gradually reach a steady state operating point, thereby reducing
start-up stresses and current surges. Upon turn-on, after VCC reaches the under-voltage threshold, an internal
11.6 µA current source charges up the external capacitor at the SS pin to 2.5V (t2 in Figure 3). The ramping
voltage at SS (and the non-inverting input of the regulation comparator) ramps up the output voltage in a
controlled manner.
An internal switch grounds the SS pin if VCC is below the under-voltage lockout threshold, if the RON/SD pin is
grounded, or if VIN exceeds the overvoltage threshold.
Thermal Shutdown
The LM34917A should be operated so the junction temperature does not exceed 125°C. If the junction
temperature increases above that, an internal Thermal Shutdown circuit activates (typically) at 175°C, taking the
controller to a low power reset state by disabling the buck switch. This feature helps prevent catastrophic failures
from accidental device overheating. When the junction temperature reduces below 155°C (typical hysteresis =
20°C), normal operation resumes.
Applications Information
EXTERNAL COMPONENTS
The procedure for calculating the external components is illustrated with the following design example. Referring
to the Block Diagram, the circuit is to be configured for the following specifications:
- VOUT = 5V
- VIN = 8V to 33V
10
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