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PDF LM2695 Data sheet ( Hoja de datos )
| Número de pieza | LM2695 | |
| Descripción | High Voltage Step Down Switching Regulator | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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January 2006
LM2695
High Voltage (30V, 1.25A) Step Down Switching
Regulator
General Description
The LM2695 Step Down Switching Regulator features all of
the functions needed to implement a low cost, efficient, buck
bias regulator capable of supplying 1.25A to the load. This
buck regulator contains a 33V N-Channel Buck Switch, and
is available in the thermally enhanced LLP-10 and TSSOP-
14EP packages. The hysteretic regulation scheme requires
no loop compensation, results in fast load transient re-
www.DataSshpeoenst4eU, .acnodmsimplifies circuit implementation. The operating
frequency remains constant with line and load variations due
to the inverse relationship between the input voltage and the
on-time. The current limit detection is set at 1.25A. Additional
features include: VCC under-voltage lockout, thermal shut-
down, gate drive under-voltage lockout, and maximum duty
cycle limiter.
Features
n Integrated 33V, N-Channel buck switch
n Integrated start-up regulator
n Input Voltage Range: 8V to 30V
n No loop compensation required
n Ultra-Fast transient response
n Operating frequency remains constant with load current
and input voltage
n Maximum Duty Cycle Limited During Start-Up
n Adjustable output voltage
n Valley Current Limit At 1.25A
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
Package
n LLP-10 (4 mm x 4 mm)
n TSSOP-14EP
n Exposed Thermal Pad For Improved Heat Dissipation
Basic Step Down Regulator
© 2006 National Semiconductor Corporation DS201704
20170431
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
FB bias current
Thermal Shutdown
TSD Thermal shutdown
temperature
Thermal shutdown hysteresis
Thermal Resistance
θJA Junction to Ambient
0 LFPM Air Flow
θJC Junction to Case
Conditions
Both Packages
Both Packages
Min
Typ
Max
Units
1 nA
175 ˚C
20 ˚C
37 ˚C/W
6.6 ˚C/W
www.NDoateta1:ShAebseotlu4tUe .Mcaoxmimum 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 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 
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 12.3 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
under-voltage lockout threshold, if a thermal shutdown oc-
curs, or if the RON/SD pin is grounded.
Thermal Shutdown
The LM2695 should be operated so the junction temperature
does not exceed 125˚C. If the junction temperature in-
creases, an internal Thermal Shutdown circuit, which acti-
vates (typically) at 175˚C, takes the controller to a low power
www.raDenasdetatgSrshoteautenedt4ibnUyg.cdtohismeabSloinftgsttahret
buck switch and the on-timer,
pin. This feature helps prevent
catastrophic failures from accidental device overheating.
When the junction temperature reduces below 155˚C (typical
hysteresis = 20˚C), the Softstart pin is released and normal
operation resumes.
Applications Information
EXTERNAL COMPONENTS
The following guidelines can be used to select the external
components.
R1 and R2: The ratio of these resistors is calculated from:
R1/R2 = (VOUT/2.5V) - 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.
RON: The minimum value for RON is calculated from:
Equation 1 can be used to select RON if a specific frequency
is desired as long as the above limitation is met.
L1: The main parameter affected by the inductor is the
output current ripple amplitude (IOR). The limits for IOR must
be determined at both the minimum and maximum nominal
load currents.
a) If the maximum load current is less than the current limit
threshold (1.25A), the minimum load current is used to de-
termine the maximum allowable ripple. To maintain continu-
ous conduction mode the lower peak should not reach 0 mA.
For this case, the maximum ripple current is:
IOR(MAX1) = 2 x IO(min)
(7)
The ripple calculated in Equation 7 is then used in the
following equation:
(8)
where VIN is the maximum input voltage and Fs is deter-
mined from equation 1. This provides a minimum value for
L1. The next larger standard value should be used, and L1
should be rated for the IPK current level.
b) If the maximum load current is greater than the current
limit threshold (1.25A), the LM2695 ensures the lower peak
reaches 1.25A each cycle, requiring that IOR be at least twice
the difference. The upper peak, however, must not exceed
2A. For this case, the ripple limits are:
and
IOR(MAX2) = 2 x (2A - IO(max))
(9)
IOR(MIN1) = 2 x (IO(max) - 1.25A)
(10)
The lesser of Equations 9 and 10 is then used in Equation 8.
If IOR(MAX2) is used, the maximum VIN is used in Equation 8.
The next larger value should then be used for L1. If IOR(MIN1)
is used, the minimum VIN is used in Equation 8. The next
smaller value should then be used for L1. L1 must be rated
for the peak value of the current waveform (IPK in Figure 7).
C3: The capacitor at the VCC output provides not only noise
filtering and stability, but also prevents false triggering of the
VCC UVLO at the buck switch on/off transitions. For this
reason, C3 should be no smaller than 0.1 µF, and should be
a good quality, low ESR, ceramic capacitor.
C2, and R3: Since the LM2695 requires a minimum of 25
mVp-p of ripple at the FB pin for proper operation, the re-
quired ripple at VOUT1 is increased by R1 and R2. This
necessary ripple is created by the inductor ripple current
acting on C2’s ESR + R3. The minimum ripple current is
calculated using equation 8, rearranged to solve for IOR at
minimum VIN. The minimum ESR for C2 is then equal to:
(11)
If the capacitor used for C2 does not have sufficient ESR, R3
is added in series as shown in Figure 3. Generally R3 is less
than 1Ω. C2 should generally be no smaller than 3.3 µF,
although that is dependent on the frequency and the allow-
able ripple amplitude at VOUT1. Experimentation is usually
necessary to determine the minimum value for C2, as the
nature of the load may require a larger value. A load which
creates significant transients requires a larger value for C2
than a non-varying load.
D1: The important parameters are reverse recovery time and
forward voltage. The reverse recovery time determines how
long the reverse current surge lasts each time the buck
switch is turned on. The forward voltage drop is significant in
the event the output is short-circuited as it is mainly this
diode’s voltage (plus the voltage across the current limit
sense resistor) which forces the inductor current to decrease
during the off-time. For this reason, a higher voltage is better,
although that affects efficiency. A reverse recovery time of
)30 ns, and a forward voltage drop of )0.75V are preferred.
The reverse leakage specification is important as that can
significantly affect efficiency. D1’s reverse voltage rating
must be at least as great as the maximum VIN, and its
current rating must equal or exceed IPK Figure 7.
C1 and C5: C1’s purpose is to supply most of the switch
current during the on-time, and limit the voltage ripple at VIN,
on the assumption that the voltage source feeding VIN has
an output impedance greater than zero. If the source’s dy-
namic impedance is high (effectively a current source), it
supplies the average input current, but not the ripple current.
11 www.national.com
11 Page | ||
| Páginas | Total 14 Páginas | |
| PDF Descargar | [ Datasheet LM2695.PDF ] | |
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