'I

Normal

Operation

Load Current

Decreases

Current

Limited

Load

Current

Increases

Normal

Operation

Current Limit

Threshold

2.5V

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

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

voltage at SS (and the non-inverting input of the regulation comparator) ramps up the output voltage in a

controlled manner.

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:

10

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