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PDF LT1676IN8 Data sheet ( Hoja de datos )
| Número de pieza | LT1676IN8 | |
| Descripción | Wide Input Range/ High Efficiency/ Step-Down Switching Regulator | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LT1676
Wide Input Range,
High Efficiency, Step-Down
Switching Regulator
FEATURES
s Wide Input Range: 7.4V to 60V
s 700mA Peak Switch Current Rating
s Adaptive Switch Drive Maintains Efficiency at High
Load Without Pulse Skipping at Light Load
s True Current Mode Control
s 100kHz Fixed Operating Frequency
s Synchronizable to 250kHz
s Low Supply Current in Shutdown: 30µA
s Available in 8-Pin SO and PDIP Packages
U
APPLICATIO S
s Automotive DC/DC Converters
s Telecom 48V Step-Down Converters
s Cellular Phone Battery Charger Accessories
s IEEE 1394 Step-Down Converters
DESCRIPTIO
The LT®1676 is a wide input range, high efficiency Buck
(step-down) switching regulator. The monolithic die in-
cludes all oscillator, control and protection circuitry. The
part can accept input voltages as high as 60V and contains
an output switch rated at 700mA peak current. Current
mode control offers excellent dynamic input supply rejec-
tion and short-circuit protection.
The LT1676 contains several features to enhance effi-
ciency. The internal control circuitry is normally powered
via the VCC pin, thereby minimizing power drawn directly
from the VIN supply (see Applications Information). The
action of the LT1676 switch circuitry is also load depen-
dent. At medium to high loads, the output switch circuitry
maintains high rise time for good efficiency. At light loads,
rise time is deliberately reduced to avoid pulse skipping
behavior.
The available SO-8 package and 100kHz switching fre-
quency allow for minimal PC board area requirements.
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATIO
VIN
8V TO 50V
+
5
1
39µF
63V
VIN
SHDN VCC
VSW
LT1676
2
3
6
SYNC
7
FB
8
VC
GND
4
220µH*
+
MBR160
100µF
10V
5V
400mA
36.5k
1%
2200pF
22k 100pF
12.1k
1%
*65T #30 ON MAGNETICS
MPP #55030
1676 F01
Figure 1
Efficiency vs VIN and ILOAD
90
80
70
60
50
40 VIN = 12V
VIN = 24V
30 VIN = 36V
20 VIN = 48V
1
10 100
ILOAD (mA)
1000
1676 TA01
1
1 page  
LT1676
PIN FUNCTIONS
SHDN (Pin 1): When pulled below the shutdown mode
threshold, nominally 0.30V, this pin turns off the regula-
tor and reduces VIN input current to a few tens of micro-
amperes (shutdown mode).
When this pin is held above the shutdown mode thresh-
old, but below the lockout threshold, the part will be
operational with the exception that output switching
action will be inhibited (lockout mode). A user-adjustable
undervoltage lockout can be implemented by driving this
pin from an external resistor divider to VIN. This action is
logically “ANDed” with the internal UVLO, set at nominally
6.7V, such that minimum VIN can be increased above
6.7V, but not decreased (see Applications Information).
If unused, this pin should be left open. However, the high
impedance nature of this pin renders it susceptible to
coupling from the high speed VSW node, so a small
capacitor to ground, typically 100pF or so is recom-
mended when the pin is left “open.”
VCC (Pin 2): This pin is used to power the internal control
circuitry off of the switching supply output. Proper use of
this pin enhances overall power supply efficiency. During
start-up conditions, internal control circuitry is powered
directly from VIN. If the output capacitor is located more
than an inch from the VCC pin, a separate 0.1µF bypass
capacitor to ground may be required right at the pin.
VSW (Pin 3): This is the emitter node of the output switch
and has large currents flowing through it. This node
moves at a high dV/dt rate, especially when in “boost”
mode. Keep the traces to the switching components as
short as possible to minimize electromagnetic radiation
and voltage spikes.
GND (Pin 4): This is the device ground pin. The internal
reference and feedback amplifier are referred to it. Keep
the ground path connection to the FB divider and the VC
compensation capacitor free of large ground currents.
VIN (Pin 5): This is the high voltage supply pin for the
output switch. It also supplies power to the internal control
circuitry during start-up conditions or if the VCC pin is left
open. A high quality bypass capacitor that meets the input
ripple current requirements is needed here. (See Applica-
tions Information.)
SYNC (Pin 6): Pin used to synchronize internal oscillator
to the external frequency reference. It is directly logic
compatible and can be driven with any signal between
10% and 90% duty cycle. The sync function is internally
disabled if the FB pin voltage is low enough to cause
oscillator slowdown. If unused, this pin should be grounded.
FB (Pin 7): This is the inverting input to the feedback
amplifier. The noninverting input of this amplifier is inter-
nally tied to the 1.24V reference. This pin also slows down
the frequency of the internal oscillator when its voltage is
abnormally low, e.g., 2/3 of normal or less. This feature
helps maintain proper short-circuit protection.
VC (Pin 8): This is the control voltage pin which is the
output of the feedback amplifier and the input of the
current comparator. Frequency compensation of the over-
all loop is effected by placing a capacitor, (or in most cases
a series RC combination) between this node and ground.
TIMING DIAGRAMS
High dV/dt Mode
VIN
VSW
0
SWDR
SWON
BOOST
SWOFF
1676 TD01
VIN
VSW
0
SWDR
SWON
BOOST
SWOFF
Low dV/dt Mode
1676 TD02
5
5 Page 
LT1676
APPLICATIONS INFORMATION
frequency resonance problems, proper layout of the com-
ponents connected to the IC is essential, especially the
power path. B field (magnetic) radiation is minimized by
keeping output diode, switch pin and intput bypass
capacitor leads as short as possible. E field radiation is
kept low by minimizing the length and area of all traces
connected to the switch pin (VSW). A ground plane should
always be used under the switcher circuitry to prevent
interplane coupling.
The high speed switching current path is shown schemati-
cally in Figure 3. Minimum lead length in these paths is
essential to ensure clean switching and minimal EMI. The
paths containing the input capacitor, output switch and
output diode are the only ones containing nanosecond rise
and fall times. Keep these paths as short as possible.
Additionally, it is possible for the LT1676 to cause EMI
problems by “coupling to itself”. Specifically, this can
occur if the VSW pin is allowed to capacitively couple in an
uncontrolled manner to the part’s high impedance nodes,
+
VIN
C1
VIN
LT1676
VSW
L1
+ VOUT
D1 C2
1676 F03
Figure 3. High Speed Current Switching Paths
i.e., SHDN, SYNC, VC and FB. This can cause erratic
operation such as odd/even cycle behavior, pulse width
“nervousness”, improper output voltage and/or prema-
ture current limit action.
As an example, assume that the capacitance between the
VSW node and a high impedance pin node is 0.1pF, and
further assume that the high impedance node in question
exhibits a capacitance of 1pF to ground. Due to the high
dV/dt, large excursion behavior of the VSW node, this will
couple a nearly 5V transient to the high impedance pin,
causing abnormal operation. (This assumes the “typical”
48VIN to 5VOUT application.) An explicit 100pF capacitor
added to the node will reduce the amplitude of the distur-
bance to more like 50mV (although settling time will
increase).
Specific pin recommendations are as follows:
SHDN: If unused, add a 100pF capacitor to ground.
SYNC: Ground if unused.
VC: Add a capacitor directly to ground in addition to the
explicit compensation network. A value of one-tenth of
the main compensation capacitor is recommended, up
to a maximum of 100pF.
FB: Assuming the VC pin is handled properly, this pin
usually requires no explicit capacitor of its own, but
keep this node physically small to minimize stray ca-
pacitance.
TYPICAL APPLICATIONS
Minimum Component Count Application
Figure 4a shows a basic “minimum component count”
application. The circuit produces 5V at up to 500mA IOUT
with input voltages in the range of 12V to 48V. The typical
POUT/PIN efficiency is shown in Figure 4b. No pulse
skipping is observed down to zero external load. As
shown, the SHDN and SYNC pins are unused, however
either (or both) can be optionally driven by external signals
as desired.
User Programmable Undervoltage Lockout
Figure 5 adds a resistor divider to the basic application.
This is a simple, cost-effective way to add a user-program-
mable undervoltage lockout (UVLO) function. Resistor R5
is chosen to have approximately 200µA through it at the
nominal SHDN pin lockout threshold of roughly 1.25V.
The somewhat arbitrary value of 200µA was chosen to be
significantly above the SHDN pin input current to minimize
its error contribution, but significantly below the typical
3.2mA the LT1676 draws in lockout mode. Resistor R4 is
then chosen to yield this same 200µA, less 2.5µA, with the
11
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| PDF Descargar | [ Datasheet LT1676IN8.PDF ] | |
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