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PDF LM2694 Data sheet ( Hoja de datos )
| Número de pieza | LM2694 | |
| Descripción | 600 mA Step Down Switching Regulator | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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May 2006
LM2694
30V, 600 mA Step Down Switching Regulator
General Description
The LM2694 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
available in the 3 x 3 thermally enhanced LLP-10 package
and a TSSOP-14 package. The feedback regulation scheme
requires no loop compensation, results in fast load transient
response, and simplifies circuit implementation. The operat-
ing frequency remains constant with line and load variations
www.DataSdhueeetot4tUh.ecoinmverse relationship between the input voltage and
the on-time. The valley current limit results in a smooth
transition from constant voltage to constant current mode
when current limit is detected, reducing the frequency and
output voltage, without the use of foldback. Additional fea-
tures include: VCC under-voltage lockout, thermal shut-
down, gate drive under-voltage lockout, and maximum duty
cycle limiter.
n No loop compensation required
n Ultra-Fast transient response
n Operating frequency remains constant with load current
and input voltage variations
n Maximum Duty Cycle Limited During Start-Up
n Adjustable output voltage
n Valley Current Limit At 0.6A
n Maximum switching frequency: 1 MHz
n Precision internal reference
n Low bias current
n Highly efficient operation
n Thermal shutdown
Typical Applications
n High Efficiency Point-Of-Load (POL) Regulator
n Non-Isolated Telecommunication Buck Regulator
n Secondary High Voltage Post Regulator
Features
n Integrated N-Channel buck switch
n Integrated start-up regulator
n Input Voltage Range: 8V to 30V
Package
n LLP-10 (3 mm x 3 mm) w/Exposed Pad
n TSSOP-14
Basic Step Down Regulator
© 2006 National Semiconductor Corporation DS201870
20187001
www.national.com
1 page  
Electrical Characteristics Specifications with standard type are for TJ = 25˚C only; limits in boldface type
apply over the full Operating Junction Temperature (TJ) range. Minimum and Maximum limits are guaranteed through test,
design, or statistical correlation. Typical values represent the most likely parametric norm at TJ = 25˚C, and are provided for
reference purposes only. Unless otherwise stated the following conditions apply: VIN = 24V, RON = 200kΩ. See (Note
5). (Continued)
Symbol
Parameter
Conditions
Min
Typ
Max
Units
FB bias current
1 nA
Thermal Shutdown
TSD Thermal shutdown
temperature
175 ˚C
Thermal shutdown hysteresis
20 ˚C
Thermal Resistance
θJA Junction to Ambient
0 LFPM Air Flow
LLP Package
TSSOP Package
33 ˚C/W
40
θJC Junction to Case
www.DataSheet4U.com
LLP Package
TSSOP Package
8.8 ˚C/W
5.2
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 plastic TSSOP and LLP packages, refer to the Packaging Data Book available from National Semiconductor
Corporation.
Note 5: Typical specifications represent the most likely parametric norm at 25˚C operation.
5 www.national.com
5 Page 
Start-Up Regulator, VCC (Continued)
www.DataSheet4U.com
FIGURE 9. Self Biased Configuration
20187014
Regulation Comparator
The feedback voltage at FB is compared to the voltage at the
Softstart pin (2.5V). In normal operation (the output voltage
is regulated), an on-time period is initiated when the voltage
at FB falls below 2.5V. The buck switch stays on for the
programmed on-time, causing the FB voltage to rise above
2.5V. After the on-time period, the buck switch stays off until
the FB voltage falls below 2.5V. Input bias current at the FB
pin is less than 100 nA over temperature.
Over-Voltage Comparator
The voltage at FB is compared to an internal 2.9V reference.
If the voltage at FB rises above 2.9V the on-time pulse is
immediately terminated. This condition can occur if the input
voltage or the output load changes suddenly, or if the induc-
tor (L1) saturates. The buck switch remains off until the
voltage at FB falls below 2.5V.
ON-Time Timer, and Shutdown
The on-time for the LM2694 is determined by the RON resis-
tor and the input voltage (VIN), and is calculated from:
In high frequency applications the minimum value for tON is
limited by the maximum duty cycle required for regulation
and the minimum off-time of 265 ns, ±15%. The minimum
off-time limits the maximum duty cycle achievable with a low
voltage at VIN. The minimum allowed on-time to regulate the
desired VOUT at the minimum VIN is determined from the
following:
(6)
The LM2694 can be remotely shut down by taking the
RON/SD pin below 0.8V. See Figure 10. In this mode the SS
pin is internally grounded, the on-timer is disabled, and bias
currents are reduced. Releasing the RON/SD pin allows
normal operation to resume. The voltage at the RON/SD pin
is normally between 1.5V and 3.0V, depending on VIN and
the RON resistor.
(4)
See Figure 4. The inverse relationship with VIN results in a
nearly constant frequency as VIN is varied. To set a specific
continuous conduction mode switching frequency (FS), the
RON resistor is determined from the following:
(5)
11
20187019
FIGURE 10. Shutdown Implementation
www.national.com
11 Page | ||
| Páginas | Total 19 Páginas | |
| PDF Descargar | [ Datasheet LM2694.PDF ] | |
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