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PDF LT1777 Data sheet ( Hoja de datos )
| Número de pieza | LT1777 | |
| Descripción | Low Noise Step-Down Switching Regulator | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de LT1777 (archivo pdf) en la parte inferior de esta página. Total 24 Páginas | ||
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FEATURES
s Programmable dI/dt Limit
s Internally Limited dV/dt
s High Input Voltage: 48V Max
s 700mA Peak Switch Rating
s True Current Mode Control
s 100kHz Fixed Operating Frequency
s Synchronizable to 250kHz
s Low Supply Current in Shutdown: 30µA
s Low Thermal Resistance 16-Pin SO Package
U
APPLICATIO S
s Automotive Cellular and GPS Receivers
s Telecom Power Supplies
s Industrial Instrument Power Supplies
LT1777
Low Noise Step-Down
Switching Regulator
DESCRIPTIO
The LT®1777 is a Buck (step-down) regulator designed for
noise sensitive applications. It contains a dI/dt limiting
circuit programmed via a small external inductor in the
switching path. Internal circuitry also generates controlled
dV/dt ramp rates.
The monolithic die includes all oscillator, control and
protection circuitry. The part can accept operating input
voltages as high as 48V, and contains an output switch
rated at 700mA peak current. Current mode control offers
excellent dynamic input supply rejection and short-circuit
protection. The internal control circuitry is normally pow-
ered via the VCC pin, thereby minimizing power drawn
directly from the VIN supply (see Applications Informa-
tion). The fused-lead SO16 package and 100kHz switch-
ing frequency allow for minimal PC board area
requirements.
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATIO
Low Noise 5V Step-Down Supply
VIN
24V +
10
39µF
63V
VIN
34
SHDN VCC
100pF
LT1777
1µH*
100pF 12k
12
SYNC
14
VC
6
VSW
5
VD
13
FB
SGND
2200pF
7
220µH
+
MBRS1100
*PROGRAMS dI/dT
100µF
10V
VOUT
5V
400mA
36.5k
1%
12.1k
1%
1777 TA01
VSW
VOLTAGE
10V/DIV
VSW
CURRENT
200mA/DIV
VSW Switching Waveforms
500ns/DIV
1777 TA02
1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
LT1777
Switching Frequency
vs Temperature
106
104
102
100
98
96
94
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1777 G07
Output dI/dt Sense Voltage
vs Temperature
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1777 G09
Feedback Amplifier Output
Current vs FB Pin Voltage
100
25°C
125°C
50 –55°C
0
–50
–100
–150
1.0
1.1 1.2 1.3 1.4
FB PIN VOLTAGE (V)
1.5
1777 G11
Minimum Synchronization
Voltage vs Temperature
2.25
2.00
1.75
1.50
1.25
1.00
0.75
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1777 G08
VC Pin Switching Threshold,
Clamp Voltage vs Temperature
2.2
2.0
1.8
CLAMP
1.6 VOLTAGE
1.4
1.2 SWITCHING
THRESHOLD
1.0
0.8
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1777 G10
Error Amplifier Transconductance
vs Temperature
750
700
650
600
550
500
450
400
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1777 G12
5
5 Page 
LT1777
APPLICATIONS INFORMATION
current waveform. The four sense inductor values of 0µH,
0.47µH, 1µH and 2.2µH yield dI/dt rates of roughly
4.5A/µs, 2.2A/µs, 1.4A/µs and 0.6A/µs, respectively.
These photos show that there is a minimum effective value
for sense inductance, which is 0.47µH for a typical part at
room temperature as shown. This value inductor has a
small effect on the trailing edge rate, but essentially no
effect on the rising edge. Minimum effective sense induc-
tance value means that inductors much smaller than this
value will have substantially the same performance as zero
inductance, such that these inductors serve no useful
purpose.
In summary,
1. The LT1777 uses an external sense inductor to set a
theoretical limit for current ramp rate according to the
formula:
Max dI / dt = 2VBE
LSENSE
2. Allowable range for the sense inductor runs from a
minimum of 0 to a maximum of:
Max
LSENSE
=
LMAIN Max
0.5V
VIN –
VOUT
3. The minimum effective inductor size is typically 0.47µH.
Harmonic Behavior
The LT1676 is a high efficiency “cousin” to the LT1777. An
additional set of oscilloscope photographs in Figure 3
show the leading edge and trailing edge of the current
waveform when this part is substituted for the LT1777.
(No sense inductor is used with the LT1676.) The leading
and trailing edges of the LT1676 current waveform are
much faster than that of the LT1777, even when the
LT1777 uses a sense inductor of 0µH. The 10% to 90%
rise time/fall time is on the order of 10ns to 20ns, too fast
to measure accurately at the horizontal sweep rate of
200ns/DIV.
While this time-based analysis demonstrates that the
current waveform of the LT1777 is quieter than standard
high efficiency buck converters, some users may prefer to
see a direct comparison on a frequency domain basis.
Figures 4a, 4b, and 4c show a spectral analysis of the
current waveforms. The horizontal axis is 2MHz/DIV (0MHz
to 20MHz), and the vertical axis is 10dB/DIV. All photos
were taken with VIN = 24V and VOUT = 5V at 400mA. Figure
4a is of the LT1676 and is for comparison purposes.
Figures 4b and 4c are of the LT1777 with a sense inductor
of 0µH and 2.2µH, respectively. A decrease in high fre-
quency energy is seen when going from the LT1676 to the
LT1777 with no sense inductor, and a further improve-
ment with a 2.2µH sense inductor. For example, at 10MHz,
the LT1777 shows an improvement of about –10dB with
0µH and perhaps – 25dB with 2.2µH.
200ns/DIV
(a) Leading Edge
1777 F03a
200ns/DIV
(b) Trailing Edge
Figure 3. LT1676 Current Behavior for Comparison Purposes Only
1777 F03b
11
11 Page | ||
| Páginas | Total 24 Páginas | |
| PDF Descargar | [ Datasheet LT1777.PDF ] | |
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