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PDF LT1776 Data sheet ( Hoja de datos )
| Número de pieza | LT1776 | |
| Descripción | Wide Input Range/ High Efficiency/ Step-Down Switching Regulator | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LT1776
Wide Input Range,
High Efficiency, Step-Down
Switching Regulator
FEATURES
s Wide Input Range: 7.4V to 40V
s Tolerates Input Transients to 60V
s 700mA Peak Switch Rating
s Adaptive Switch Drive Maintains Efficiency at High
Load Without Pulse Skipping at Light Load
s True Current Mode Control
s 200kHz Fixed Operating Frequency
s Synchronizable to 400kHz
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 Cellular Phone Battery Charger Accessories
s IEEE 1394 Step-Down Converters
DESCRIPTIO
The LT®1776 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 delivers excellent dynamic input supply
rejection and short-circuit protection.
The LT1776 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 LT1776 switch circuitry is also load depen-
dent. At medium to high loads, the output switch circuitry
maintains fast rise time for good efficiency. At light loads,
rise time is deliberately reduced to avoid pulse skipping
behavior.
The available SO-8 package and 200kHz 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 40V
+
5
1 VIN 2
SHDN VCC
3
VSW
39µF
63V
6
LT1776
SYNC
FB
VC
7
8
100µH*
+
MBR160
100µF
10V
2200pF
GND 22k 100pF
4
*43T #30 ON MAGNETICS
MPP #55030
Figure 1
5V
400mA
36.5k
1%
12.1k
1%
1776 F01
Efficiency vs VIN and ILOAD
90
80
70
60
50
40
VIN = 10V
30 VIN = 20V
VIN = 30V
20 VIN = 40V
1
10
100
LOAD CURRENT (mA)
1000
1776 TA01
1
1 page  
TYPICAL PERFORMANCE CHARACTERISTICS
VC Pin Switching Threshold,
Boost Threshold, Clamp Voltage
vs Temperature
2.2
2.0
CLAMP
1.8 VOLTAGE
1.6
BOOST
1.4 THRESHOLD
1.2 SWITCHING
THRESHOLD
1.0
0.8
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
LT1776 G10
Feedback Amplifier Output
Current vs FB Pin Voltage
100
25°C
–55°C
50 125°C
0
–50
–100
–150
1.0
1.1 1.2 1.3 1.4
FB PIN VOLTAGE (V)
1.5
1776 G11
LT1776
Error Amplifier Transconductance
vs Temperature
750
700
650
600
550
500
450
400
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
LT1776 G12
PIN FUNCTIONS
SHDN (Pin 1): When pulled below the shutdown mode
threshold, nominally 0.30V, this pin turns off the regulator
and reduces VIN input current to a few tens of microam-
peres (shutdown mode).
When this pin is held above the shutdown mode threshold,
but below the lockout threshold, the part will be opera-
tional with the exception that output switching action will
be inhibited (lockout mode). A user-adjustable undervolt-
age 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 one 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 which meets the
input ripple current requirements is needed here. (See
Applications 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
5
5 Page 
LT1776
APPLICATIONS INFORMATION
output controls the shutdown pin allows high efficiency at
light loads through Burst Mode operation behavior (see
Typical Applications and Figure 8).
Maximum Load/Short-Circuit Considerations
The LT1776 is a current mode controller. It uses the VC
node voltage as an input to a current comparator which
turns off the output switch on a cycle-by-cycle basis as
this peak current is reached. The internal clamp on the VC
node, nominally 2V, then acts as an output switch peak
current limit. This action becomes the switch current limit
specification. The maximum available output power is
then determined by the switch current limit.
A potential controllability problem could occur under
short-circuit conditions. If the power supply output is
short circuited, the feedback amplifier responds to the low
output voltage by raising the control voltage, VC, to its
peak current limit value. Ideally, the output switch would
be turned on, and then turned off as its current exceeded
the value indicated by VC. However, there is finite response
time involved in both the current comparator and turnoff
of the output switch. These result in a minimum ON time
tON(MIN). When combined with the large ratio of VIN to
(VF + I • R), the diode forward voltage plus inductor I • R
voltage drop, the potential exists for a loss of control.
Expressed mathematically the requirement to maintain
control is:
f
•
tON
≤
VF
+I•
VIN
R
where:
f = switching frequency
tON = switch ON time
VF = diode forward voltage
VIN = Input voltage
I • R = inductor I • R voltage drop
If this condition is not observed, the current will not be
limited at IPK, but will cycle-by-cycle ratchet up to some
higher value. Using the nominal LT1776 clock frequency
of 200KHz, a VIN of 40V and a (VF + I • R) of say 0.7V, the
maximum tON to maintain control would be approximately
90ns, an unacceptably short time.
The solution to this dilemma is to slow down the oscillator
when the FB pin voltage is abnormally low thereby indicat-
ing some sort of short-circuit condition. Figure 2 shows
the typical response of Oscillator Frequency vs FB divider
Thevenin voltage and impedance. Oscillator frequency is
unaffected until FB voltage drops to about 2/3 of its normal
value. Below this point the oscillator frequency decreases
roughly linearly down to a limit of about 30kHz. This lower
oscillator frequency during short-circuit conditions can
then maintain control with the effective minimum ON time.
A further potential problem with short-circuit operation
might occur if the user were operating the part with its
oscillator slaved to an external frequency source via the
SYNC pin. However, the LT1776 has circuitry that auto-
matically disables the sync function when the oscillator is
slowed down due to abnormally low FB voltage.
200
RTH = 22k
150
RTH = 10k
RTH = 4.7k
100
50
RTH LT1776
FB
0
0 0.25 0.50 0.75 1.00 1.25
FB DIVIDER THEVENIN VOLTAGE (V)
1776 F02
Figure 2. Oscillator Frequency vs FB Divider
Thevenin Voltage and Impedance
Feedback Divider Considerations
An LT1776 application typically includes a resistive divider
between VOUT and ground, the center node of which drives
the FB pin to the reference voltage VREF. This establishes
a fixed ratio between the two resistors, but a second
degree of freedom is offered by the overall impedance level
of the resistor pair. The most obvious effect this has is one
of efficiency—a higher resistance feedback divider will
waste less power and offer somewhat higher efficiency,
especially at light load.
11
11 Page | ||
| Páginas | Total 20 Páginas | |
| PDF Descargar | [ Datasheet LT1776.PDF ] | |
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