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PDF LM34919 Data sheet ( Hoja de datos )
| Número de pieza | LM34919 | |
| Descripción | 600mA Constant On-Time Buck Switching Regulator | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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June 2, 2008
LM34919
Ultra Small 40V, 600 mA Constant On-Time Buck Switching
Regulator
General Description
The LM34919 Step Down Switching Regulator features all of
the functions needed to implement a low cost, efficient, buck
bias regulator capable of supplying 0.6A to the load. This buck
regulator contains an N-Channel Buck Switch, and is avail-
able in a micro SMD package. The constant on-time feedback
regulation scheme requires no loop compensation, results in
fast load transient response, and simplifies circuit implemen-
tation. The operating frequency remains constant with line
and load variations due to the inverse relationship between
the input voltage and the on-time. The valley current limit re-
sults in a smooth transition from constant voltage to constant
current mode when current limit is detected, reducing the fre-
quency and output voltage, without the use of foldback. Ad-
ditional features include: VCC under-voltage lockout, thermal
shutdown, gate drive under-voltage lockout, and maximum
duty cycle limiter.
Features
■ Integrated N-Channel buck switch
■ Integrated start-up regulator
■ Input Voltage Range: 8V to 40V
■ No loop compensation required
■ Ultra-Fast transient response
■ Operating frequency remains constant with load current
and input voltage
■ Maximum switching frequency: 2.0 MHz
■ Maximum Duty Cycle Limited During Start-Up
■ Adjustable output voltage
■ Valley Current Limit At 0.64A
■ Precision internal reference
■ Low bias current
■ Highly efficient operation
■ Thermal shutdown
Typical Applications
■ High Efficiency Point-Of-Load (POL) Regulator
■ Non-Isolated Telecommunication Buck Regulator
■ Secondary High Voltage Post Regulator
Package
■ micro SMD
Basic Step Down Regulator
© 2008 National Semiconductor Corporation 300044
30004431
www.national.com
1 page  
Symbol
Parameter
Thermal Shutdown
TSD Thermal shutdown temperature
Thermal shutdown hysteresis
Thermal Resistance
θJA Junction to Ambient
0 LFPM Air Flow
Conditions
Min Typ Max Units
175 °C
20 °C
61 °C/W
Note 1: Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions under which operation of the
device is intended to be functional. For guaranteed specifications and test conditions, see the Electrical Characteristics.
Note 2: The human body model is a 100pF capacitor discharged through a 1.5kΩ resistor into each pin.
Note 3: VCC provides self bias for the internal gate drive and control circuits. Device thermal limitations limit external loading
Note 4: For detailed information on soldering micro SMD package, refer to the Application Note AN-1112.
Note 5: Typical specifications represent the most likely parametric norm at 25°C operation.
5 www.national.com
5 Page 
FIGURE 4. Inductor Current - Current Limit Operation
30004414
N - Channel Buck Switch and Driver
The LM34919 integrates an N-Channel buck switch and as-
sociated 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 in-
ternal high voltage diode. A 0.022 µF capacitor (C4) connect-
ed between BST and SW provides the voltage to the driver
during the on-time. During each off-time, the SW pin is at ap-
proximately -1V, and C4 charges from VCC through the inter-
nal 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 VCC 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 VCC is below the un-
der-voltage lockout threshold, or if the RON/SD pin is ground-
ed.
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 (typi-
cally) at 175°C, takes the controller to a low power reset state
by disabling the buck switch. This feature helps prevent catas-
trophic 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 il-
lustrated 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 = (VOUT/2.5V) - 1
For this example, R1/R2 = 1. R1 and R2 should be chosen
from standard value resistors in the range of 1.0 kΩ - 10 kΩ
which satisfy the above ratio. For this example, 2.49kΩ is
chosen for R1 and R2.
RON: This resistor sets the on-time, and (by default) the
switching frequency. The switching frequncy must be less
than 1.6 MHz to ensure the minimum forced off-time does not
interfere with the circuit's proper operation. The RON resistor
is calculated from the following equation, using the minimum
input voltage.
Check that this value resistor does not set an on-time less
than 120 ns at maximum VIN.
A standard value 43.2 kΩ resistor is used, resulting in a nom-
inal 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, RON can be determined using Equation 4 if a
specific on-time is required.
11 www.national.com
11 Page | ||
| Páginas | Total 18 Páginas | |
| PDF Descargar | [ Datasheet LM34919.PDF ] | |
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