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PDF LM25005 Data sheet ( Hoja de datos )
| Número de pieza | LM25005 | |
| Descripción | STEP-DOWN SWITCHING REGULATOR | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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www.DataSheet4U.com
February 2006
LM25005
42V, 2.5A Step-Down Switching Regulator
General Description
The LM25005 switching regulator features all of the func-
tions necessary to implement an efficient, high voltage buck
regulator using a minimum of external components. This
easy to use regulator includes a 42V, 160mΩ, N-channel
MOSFET, with an output current capability of 2.5 Amps. The
regulator control method is based upon current mode control
utilizing an emulated current ramp. Current mode control
provides inherent line voltage feed-forward, cycle-by-cycle
current limiting and ease of loop compensation. The use of
an emulated control ramp reduces noise sensitivity of the
pulse-width modulation circuit, allowing reliable control of
very small duty cycles necessary in high input voltage appli-
cations. The operating frequency is programmable from
50kHz to 500kHz. An oscillator synchronization pin allows
multiple LM25005 regulators to self-synchronize or to be
synchronized to an external clock. Additional protection fea-
tures include: current limit, thermal shutdown and remote
shutdown capability. The device is available in a power
enhanced TSSOP-20 package featuring an exposed die
attach pad to aid thermal dissipation.
Features
n Integrated 42V, 160mΩ N-channel MOSFET
n Ultra-wide input voltage range from 7V to 42V
n Internal bias regulator
n Adjustable output voltage from 1.225V
n 1.5% feedback reference accuracy
n Current mode control with emulated inductor current
ramp
n Single resistor oscillator frequency setting
n Oscillator synchronization input
n Programmable soft-start
n Shutdown / Standby input
n Wide bandwidth error amplifier
n Thermal Shutdown
Package
n TSSOP-20EP (Exposed Pad)
Simplified Application Schematic
© 2006 National Semiconductor Corporation DS201700
20170001
www.national.com
1 page  
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
VIN to GND
BST to GND
PRE to GND
SW to GND (Steady State)
BST to VCC
VCC to GND
BST to SW
45V
60V
45V
-1.5V
45V
14V
14V
OUT to GND
SD, SYNC, SS, FB to GND
ESD Rating (Note 2)
Human Body Model
Storage Temperature Range
Limited to Vin
7V
2kV
-65˚C to +150˚C
Operating Ratings (Note 1)
VIN
Operation Junction Temperature
7V to 42V
−40˚C to + 125˚C
Electrical Characteristics Specifications with standard typeface are for TJ = 25˚C, and those with boldface
type apply over full Operating Junction Temperature range. VIN = 24V, RT = 32.4kΩ unless otherwise stated. (Note 3)
Symbol
Parameter
STARTUP REGULATOR
VccReg Vcc Regulator Output
Vcc LDO Mode turn-off
Vcc Current Limit
VCC SUPPLY
Vcc UVLO Threshold
Vcc Undervoltage Hysteresis
Bias Current (Iin)
Shutdown Current (Iin)
SHUTDOWN THRESHOLDS
Shutdown Threshold
Shutdown Hysteresis
Standby Threshold
Standby Hysteresis
SD Pull-up Current Source
SWITCH CHARACTERSICS
Buck Switch Rds(on)
BOOST UVLO
BOOST UVLO Hysteresis
Pre-charge Switch Rds(on)
Pre-charge Switch on-time
CURRENT LIMIT
Cycle by Cycle Current Limit
Cycle by Cycle Current Limit Delay
SOFT-START
SS Current Source
OSCILLATOR
Frequency1
Frequency2
SYNC Source Impedance
SYNC Sink Impedance
SYNC Threshold (falling)
Upper SYNC Frequency
SYNC Pulse Width Minimum
Conditions
Vcc = 0V
(Vcc increasing)
FB = 1.3V
SD = 0V
(SD Increasing)
(Standby Increasing)
RAMP = 0V
RAMP = 2.5V
RT = 11kΩ
Min Typ
6.85
7.15
9
20
5.95
6.35
1
3
50
0.5
1.18
0.7
0.1
1.225
0.1
5
160
3.8
0.56
75
275
3 3.5
100
7 10
180 200
425 485
10
160
1.4
15
Max
7.45
6.75
4.5
85
0.9
1.27
320
4.25
14
220
525
550
Units
V
V
mA
V
V
mA
µA
V
V
V
V
µA
mΩ
V
V
Ω
ns
A
ns
µA
KHz
KHz
kΩ
Ω
V
KHz
ns
5 www.national.com
5 Page 
Shutdown / Standby
The LM25005 contains a dual level Shutdown (SD) circuit.
When the SD pin voltage is below 0.7V, the regulator is in a
low current shutdown mode. When the SD pin voltage is
greater than 0.7V but less than 1.225V, the regulator is in
standby mode. In standby mode the Vcc regulator is active
but the output switch is disabled. When the SD pin voltage
exceeds 1.225V, the output switch is enabled and normal
operation begins. An internal 5µA pull-up current source
configures the regulator to be fully operational if the SD pin is
left open.
An external set-point voltage divider from Vin to GND can be
used to set the operational input range of the regulator. The
divider must be designed such that the voltage at the SD pin
will be greater than 1.225V when Vin is in the desired
operating range. The internal 5µA pull-up current source
must be included in calculations of the external set-point
divider. Hysteresis of 0.1V is included for both the shutdown
and standby thresholds. The voltage at the SD pin should
never exceed 8V. When using an external set-point divider, it
may be necessary to clamp the SD pin to limit its voltage at
high input voltage conditions.
The SD pin can also be used to implement various remote
enable / disable functions. Pulling the UVLO pin below the
0.7V threshold totally disables the controller. If the SD pin
voltage is above 1.225V the regulator will be operational.
Oscillator and Sync Capability
The LM25005 oscillator frequency is set by a single external
resistor connected between the RT pin and the AGND pin.
The RT resistor should be located very close to the device
and connected directly to the pins of the IC (RT and
AGND).To set a desired oscillator frequency (F), the neces-
sary value for the RT resistor can be calculated from the
following equation:
20170006
FIGURE 4. Sync from Multiple Devices
Multiple LM25005 and/or LM5005 devices can be synchro-
nized together simply by connecting the SYNC pins together.
In this configuration all of the devices will be synchronized to
the highest frequency device. The diagram in Figure 5 illus-
trates the SYNC input/output features of the LM25005. The
internal oscillator circuit drives the SYNC pin with a strong
pull-down / weak pull-up inverter. When the SYNC pin is
pulled low either by the internal oscillator or an external
clock, the ramp cycle of the oscillator is terminated and a
new oscillator cycle begins. Thus, if the SYNC pins of sev-
eral LM25005 IC’s are connected together, the IC with the
highest internal clock frequency will pull the connected
SYNC pins low first and terminate the oscillator ramp cycles
of the other IC’s. The LM25005 or LM5005 with the highest
programmed clock frequency will serve as the master and
control the switching frequency of the all the devices with
lower oscillator frequency.
The SYNC pin can be used to synchronize the internal
oscillator to an external clock. The external clock must be of
higher frequency than the free-running frequency set by
the RT resistor. A clock circuit with an open drain output is the
recommended interface from the external clock to the SYNC
pin. The clock pulse duration should be greater than 15 ns.
20170005
FIGURE 3. Sync from External Clock
11
www.national.com
11 Page | ||
| Páginas | Total 21 Páginas | |
| PDF Descargar | [ Datasheet LM25005.PDF ] | |
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