Datasheet

LM34919B
LM34919B-Q1
www.ti.com SNVS623A –MAY 2010–REVISED FEBRUARY 2013
N-Channel Buck Switch and Driver
The LM34919B integrates an N-Channel buck switch and associated floating high voltage gate driver. The peak
current allowed through the buck switch is 1.5A, and the maximum allowed average current is 1A. 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 charges from V
CC
through the internal diode. The
minimum off-time forced by the LM34919B 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 V
CC
reaches the under-voltage threshold, an internal
10.5 µA current source charges up the external capacitor at the SS pin to 2.5V. 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 V
CC
is below the under-voltage lockout threshold, or if the RON/SD pin is
grounded.
Thermal Shutdown
The LM34919B should be operated so the junction temperature does not exceed 125°C. If the junction
temperature increases, an internal Thermal Shutdown circuit, which activates (typically) at 175°C, takes 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.
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