-10

-1.4 k: = 43.5 k:

LM34919

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SNOSAY2D – MAY 2007 – REVISED JUNE 2008

N-Channel Buck Switch and Driver

The LM34919 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

minimum off-time forced by the LM34919 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

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.

grounded.

Thermal Shutdown

The LM34919 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.

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 40V

•

Minimum load current = 200 mA

•

Maximum load current = 600 mA

•

Switching Frequency = 800 kHz

•

Soft-start time = 5 ms

R1 and R2: These resistors set the output voltage. The ratio of the feedback resistors is calculated from:

(8)

For this example, R1/R2 = 1. R1 and R2 should be chosen from standard value resistors in the range of 1.0 k

Ω

to 10 k

Ω which satisfy the above ratio. For this example, 2.49kΩ is chosen for R1 and R2.

less than 1.6 MHz to ensure the minimum forced off-time does not interfere with the circuit's proper operation.

(9)

A standard value 43.2 k

Ω resistor is used, resulting in a nominal frequency of 806 kHz. The minimum on-time is

using Equation 5 if a specific on-time is required.

Copyright © 2007–2008, Texas Instruments Incorporated

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