LM34919_14 Datasheet, PDF (11/21 Pages) Texas Instruments – Ultra-Small 40-V 600-mA Constant On-Time Buck Switching Regulator

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
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 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
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 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:
R1/R2 = (V_OUT/2.5V) - 1
(8)
For this example, R1/R2 = 1. R1 and R2 should be chosen from standard value resistors in the range of 1.0 kΩ
to 10 kΩ which satisfy the above ratio. For this example, 2.49kΩ is chosen for R1 and R2.
R_ON: This resistor sets the on-time, and (by default) the switching frequency. The switching frequency must be
less than 1.6 MHz to ensure the minimum forced off-time does not interfere with the circuit's proper operation.
The R_ON resistor is calculated from the following equation, using the minimum input voltage.
V_OUT x (V_IN(min) - 1.5V)
R_ON = _{FS x 1.13 x 10-10 x VIN(min)} ^{-1.4 k: = 43.5 k:}
(9)
Check that this value resistor does not set an on-time less than 120 ns at maximum V_IN.
A standard value 43.2 kΩ resistor is used, resulting in a nominal frequency of 806 kHz. The minimum on-time is
≊231 ns at Vin = 40V, and the maximum on-time is ≊875 ns at Vin = 8V. Alternately, R_ON can be determined
using Equation 5 if a specific on-time is required.
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