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LM34914 Arkusz danych(PDF) 10 Page - Texas Instruments

Numer części LM34914
Szczegółowy opis  Ultra Small 1.25A Step-Down Switching Regulator with Intelligent Current Limit
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LM34914 Arkusz danych(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
LM34914
SNVS453A – MAY 2007 – REVISED DECEMBER 2007
www.ti.com
Figure 4. Inductor Current - Normal and Current Limit Operation
N - Channel Buck Switch and Driver
The LM34914 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
12.5 µA current source charges up the external capacitor at the SS pin to 2.5V (t2 in Figure 2). 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, or if the RON/SD pin is
grounded.
Thermal Shutdown
The LM34914 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 and the on-timer. 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 following guidelines can be used to select the external components (see the Block Diagram). First determine
the following operating parameters:
- Output voltage (VOUT)
- Minimum and maximum input voltage (VIN(min) and VIN(max))
10
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Product Folder Links: LM34914


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