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PDF LTC3775 Data sheet ( Hoja de datos )
| Número de pieza | LTC3775 | |
| Descripción | High Frequency Synchronous Step-Down Voltage Mode DC/DC Controller | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LTC3775
FEATURES
High Frequency
Synchronous Step-Down
Voltage Mode DC/DC
Controller
DESCRIPTION
n Wide VIN Range: 4.5V to 38V
n Line Feedforward Compensation
n Low Minimum On-Time: tON(MIN) < 30ns
n Powerful Onboard MOSFET Drivers
n Leading Edge Modulation Voltage Mode Control
n ±0.75%, 0.6V Reference Voltage Accuracy Over
Temperature
n VOUT Range: 0.6V to 0.8VIN
n Programmable, Cycle-by-Cycle Peak Current Limit
n Sense Resistor or RDS(ON) Current Sensing
n Programmable Soft-Start
n Synchronizable Fixed Frequency from 250kHz to 1MHz
n Selectable Pulse-Skipping or Forced Continuous
Modes of Operation
n Low Shutdown Current: 14μA Typical
n Thermally Enhanced 3mm × 3mm QFN Package
APPLICATIONS
n Automotive Systems
n Telecom and Industrial Power Supplies
wwwn.DaPtaoSihneteot4fUL.cooamd Applications
The LTC®3775 is a high efficiency synchronous step-down
switching DC/DC controller that drives an all N-channel
power MOSFET stage from a 4.5V to 38V input supply
voltage. A patented line feedforward compensation circuit
and a high bandwidth error amplifier provide very fast line
and load transient response.
High step-down ratios are made possible by a low 30ns
minimum on-time, allowing extremely low duty cycles
MOSFET RDS(ON) current sensing maximizes efficiency.
Alternatively, a sense resistor can be used for higher cur-
rent limit accuracy. Continuous monitoring of the voltages
across the top and bottom MOSFETs allows cycle-by-cycle
control of the inductor current, configurable by external
resistors.
The soft-start function controls the duty cycle during
start-up, providing a smooth output voltage ramp up. The
operating frequency is user programmable from 250kHz
to 1MHz and can be synchronized to an external clock.
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other
trademarks are the property of their respective owners. Protected by U.S. Patents, including
5408150, 5481178, 5705919, 6580258, 5847554, 5055767.
TYPICAL APPLICATION
VIN
330μF 5V TO 28V
35V
3.16k
57.6k
4.7μF
0.01μF
39.2k
ILIMT VIN
TG
ILIMB
SENSE
LTC3775
INTVCC BOOST
SS
FREQ
SW
BG
0.1μF
0.36μH
PGND
FB
330pF
MODE/SYNC
3.9nF
10k 10k
4.7k
COMP RUN/SHDN
SGND
VOUT
1.2V
470μF 15A
2.5V
s2
3775 TA01a
Efficiency and Power Loss vs Load Current
100
90
VIN = 12V
VOUT = 1.2V
80 CONTINUOUS MODE
SW FREQ = 500kHz
70
5
4
60
EFFICIENCY
50
3
40 2
30
20 1
POWER LOSS
10
0
0.01
0.1 1
10
LOAD CURRENT (A)
0
100
3775 TA01b
3775f
1
1 page  
TYPICAL PERFORMANCE CHARACTERISTICS
LTC3775
Efficiency vs Load Current
100
90
VIN = 12V
VOUT = 1.2V
80
SW FREQ = 500kHz
FIRST PAGE CIRCUIT
70
60 PULSE-SKIPPING
50 MODE
40
30
CONTINUOUS
MODE
20
10
0
0.01
0.1 1
10
LOAD CURRENT (A)
100
3775 G01
Line Regulation
1.206
1.204
VOUT = 1.2V
LOAD = 1A
FIRST PAGE CIRCUIT
1.202
1.200
1.198
1.196
Efficiency vs Input Voltage
100
90
15A LOAD
80
Load Regulation
1.206
1.204
VIN = 12V
SW FREQ = 500kHz
FIRST PAGE CIRCUIT
1.202
70
1A LOAD
60
VOUT = 1.2V
50 CONTINUOUS MODE
SW FREQ = 500kHz
FIRST PAGE CIRCUIT
40
4 8 12 16
20
INPUT VOLTAGE (V)
24 28
3775 G02
FB Voltage vs Temperature
603
1.200
1.198
1.196
1.194
0 2 4 6 8 10 12 14 16
LOAD CURRENT (A)
3775 G03
Load Step in Forced Continuous
Mode
602 VOUT(AC)
100mV/DIV
601
600
IL
10A/DIV
599
ILOAD
598 10A/DIV
1.194
4
8 12 16 20
www.DataSheet4U.comINPUT VOLTAGE (V)
24 28
3775 G02
597
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3775 G05
VIN = 12V
50μs/DIV
VOUT = 1.2V
LOAD STEP = 0A TO 10A TO 0A
MODE/SYNC = 0V
SW FREQ = 500kHz
FIRST PAGE CIRCUIT
3775 G06
Positive Load Step in Forced
Continuous Mode
VSW
20V/DIV
VOUT(AC)
100mV/DIV
Negative Load Step in Forced
Continuous Mode
VSW
20V/DIV
VOUT(AC)
100mV/DIV
Load Step in Pulse-Skipping Mode
VOUT(AC)
100mV/DIV
IL
10A/DIV
ILOAD
10A/DIV
VIN = 12V
5μs/DIV
VOUT = 1.2V
LOAD STEP = 0A TO 10A
MODE/SYNC = 0V
SW FREQ = 500kHz
FIRST PAGE CIRCUIT
3775 G07
IL
10A/DIV
ILOAD
10A/DIV
VIN = 12V
5μs/DIV
VOUT = 1.2V
LOAD STEP = 10A TO 0A
MODE/SYNC = 0V
SW FREQ = 500kHz
FIRST PAGE CIRCUIT
IL
10A/DIV
ILOAD
10A/DIV
3775 G08
VIN = 12V
50μs/DIV
VOUT = 1.2V
LOAD STEP = 1A TO 11A TO 1A
MODE/SYNC = INTVCC
SW FREQ = 500kHz
FIRST PAGE CIRCUIT
3775 G09
3775f
5
5 Page 
LTC3775
APPLICATIONS INFORMATION
Figure 1 shows a Type 3 amplifier. The transfer function of
this amplifier is given by the following equation:
( )VCOMP =
– 1+ sR2C1 ⎡⎣1+ s(RA + R3)C3⎤⎦
( )( )( )VOUT sRA C1+ C2 1+ s(C1||C2)R2 1+ sC3R3
The RC network across the error amplifier and the feed-
forward components R3 and C3 introduce two pole-zero
pairs to obtain a phase boost at the system unity-gain
frequency, fC. In theory, the zeros and poles are placed
symmetrically around fC, and the spread between the zeros
and the poles is adjusted to give the desired phase boost
at fC. However, in practice, if the crossover frequency
is much higher than the LC double-pole frequency, this
method of frequency compensation normally generates
a phase dip within the unity bandwidth and creates some
concern regarding conditional stability.
If conditional stability is a concern, move the error ampli-
fier’s zero to a lower frequency to avoid excessive phase
dip. The following equations can be used to compute the
feedback compensation component values:
fSW = switching frequency
fLC = 2π
1
LCOUT
www.DatfaESShRe=et42Uπ.cRomES1R COUT
choose:
fC =
crossover frequency
=
fSW
10
fZ1(ERR)
=
fLC
=
1
2πR2C1
( )fZ2(RES) =
fC
5
=
2π
RA
1
+ R3
C3
fP1(ERR)
=
fESR
=
1
2πR2(C1|| C2)
fP2(RES)
=
5fC
=
1
2πR3C3
Required error amplifier gain at frequency fC:
A V(CROSSOVER)
( )≈ 40log
⎛
1+ ⎝⎜
fC ⎞ 2
fLC ⎠⎟
–
20log
1+
⎛
⎝⎜
fC
fESR
⎞
⎠⎟
2
– 20log
AMOD
≈
20log
R2
•
⎛
⎝⎜ 1+
fLC ⎞
fC ⎠⎟
⎛
⎜ 1+
⎝
fP2(RES)
fC
+
fP2(RES) – fZ2(RES)⎞
fZ2(RES)
⎟
⎠
RA
⎛
⎝⎜ 1+
fC
fESR
+
fLC ⎞
fESR – fLC⎠⎟
⎛
⎝⎜ 1+
fP2(RES)⎞
fC ⎠⎟
where AMOD is the modulator and line feedforward gain
and is equal to:
AMOD ≈
VIN(MAX) • DCMAX
VSAW
=
40V • 0.95
1.25V
≈ 30V/V
Once the value of resistor RA and the pole and zero loca-
tions have been decided, the values of C1, R2, C2, R3 and
C3 can be obtained from the above equations.
Compensating a switching power supply feedback loop is
a complex task. The applications shown in this data sheet
show typical values, optimized for the power components
shown. Though similar power components should suffice,
substantially changing even one major power component
may degrade performance significantly. Stability also may
depend on circuit board layout. To verify the calculated
component values, all new circuit designs should be
prototyped and tested for stability.
VOUT
C3
C2
C1
R2
RA R3 –
FB
RB
+
VREF
–1
GAIN
0
COMP
PHASE
+1 –1
BOOST
Figure 1. Type 3 Amplifier Compensation
FREQ
–90
–180
–270
–380
3775 F01
3775f
11
11 Page | ||
| Páginas | Total 30 Páginas | |
| PDF Descargar | [ Datasheet LTC3775.PDF ] | |
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