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PDF LTC1553 Data sheet ( Hoja de datos )
| Número de pieza | LTC1553 | |
| Descripción | 5-Bit Programmable Synchronous Switching Regulator Controller for Pentium II Processor | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de LTC1553 (archivo pdf) en la parte inferior de esta página. Total 24 Páginas | ||
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LTC1553
5-Bit Programmable
Synchronous Switching
Regulator Controller for
Pentium® II Processor
FEATURES
DESCRIPTION
s 5-Bit Digitally Programmable 1.8V to 3.5V Fixed
Output Voltage
s Provides All Features Required by the Intel
Pentium® II Processor VRM 8.2 DC/DC
Converter Specification
s Flags for Power Good, Over-Temperature and
Overvoltage Fault
s 19A Output Current Capability from a 5V or 12V Supply
s Dual N-Channel MOSFET Synchronous Driver
s Initial Output Accuracy: ±1.5%
s Excellent Output Accuracy: ±2% Typ Over Line,
Load and Temperature Variations
s High Efficiency: Over 95% Possible
s Adjustable Current Limit Without External Sense
Resistors
s Fast Transient Response
s Available in 20-Lead SSOP and SW Packages
U
APPLICATIONS
s Power Supply for Pentium II, SPARC, ALPHA and
PA-RISC Microprocessors
s High Power 5V or 12V to 1.8V-3.5V Regulators
The LTC®1553 is a high power, high efficiency switching
regulator controller optimized for 5V or 12V input to 1.8V-
3.5V output applications. It features a digitally programmable
output voltage, a precision internal reference and an internal
feedback system that provides output accuracy of ±1.5% at
room temperature and typically ±2% over-temperature, load
current and line voltage shifts. The LTC1553 uses a synchro-
nous switching architecture with two external N-channel
output devices, providing high efficiency and eliminating the
need for a high power, high cost P-channel device. Addition-
ally, it senses the output current across the on-resistance of
the upper N-channel FET, providing an adjustable current
limit without an external low value sense resistor.
The LTC1553 free-runs at 300kHz and can be synchronized
to a faster external clock if desired. It includes all the inputs
and outputs required to implement a power supply conform-
ing to the Intel Pentium® II Processor VRM 8.2 DC/DC
Converter Specification.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Pentium is a registered trademark of Intel Corporation.
TYPICAL APPLICATION
PVCC
12V
VIN
5V
PENTIUM® II
SYSTEM
5.6k 5.6k 5.6k
+
0.1µF
10µF
2.7k
0.1µF
+
10µF
+ CIN**
1200µF
×4
5
C1
150pF
RC
8.2k
CC
0.01µF
PWRGD
VCC IMAX
FAULT
OT
VID0 TO VID4
LTC1553
OUTEN
COMP SS SGND GND
CSS
0.1µF
PVCC
G1
IFB
SENSE
G2
0.1µF
Q1*
LO†
2µH
20Ω 18A
+COUT††
Q2* 330µF
×7
*SILICONIX SUD50N03-10
**SANYO 10MV1200GX
†COILTRONICS CTX02-13198 OR
PANASONIC 12TS-2R5SP
††AVX TPSE337M006R0100
Figure 1. 5V to 1.8V-3.5V Supply Application
VOUT
1.8V TO
3.5V
14A
1553 F01
1
1 page  
TYPICAL PERFORMANCE CHARACTERISTICS
LTC1553
Oscillator Frequency
vs Temperature
350
340
330
320
310
300
290
280
270
260
250
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1553 G10
Maximum G1 Duty Cycle
vs Temperature
92
OSCILLATOR FREQUENCY = 300kHz
90
88
G1, G2 CAPACITANCE = 1100pF
86 2200pF
3300pF
84
82 5500pF
7700pF
80
78
– 50 – 25
0 25 50 75
TEMPERATURE (°C)
100 125
1553 G13
PVCC Supply Current
vs Gate Capacitance
70
PVCC = 12V
60 TA = 25°C
50
40
30
20
10
0
0
2000
4000
6000
8000
GATE CAPACITANCE (pF)
1553 G16
IMAX Sink Current
vs Temperature
220
210
200
190
180
170
160
150
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1553 G11
VCC Operating Supply Current
vs Temperature
1.2
VCC = 5V
1.1 fOSC = 300kHz
1.0
0.9
0.8
0.7
0.6
0.5
– 50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1553 G14
Soft Start Source Current
vs Temperature
–7
–8
–9
– 10
– 11
– 12
– 13
– 50 – 25
0 25 50 75
TEMPERATURE (°C)
100 125
1553 G12
VCC Shutdown Supply Current
vs Temperature
250
225
200
175
150
125
100
75
50
– 50 – 25
0 25 50 75
TEMPERATURE (°C)
100 125
1553 G15
Output Over Current Protection
3.0
2.5 Q1 CASE = 90°C, VOUT = 2.8V
Q1 = 2 × MTD20N03HDL
Q2 = 1 × MTD20N03HDL
2.0 RIMAX = 2.7k, RIFB = 20Ω,
SS CAP = 0.01µF
1.5
1.0
SHORT-CIRCUIT
CURRENT
0.5
0
02
4 6 8 10 12 14 16 18
OUTPUT CURRENT (A)
1553 G17
Transient Response
50mV/DIV
5A/DIV
100µs/DIV
1553 G18
5
5 Page 
LTC1553
APPLICATIONS INFORMATION
Soft Start and Current Limit
The LTC1553 includes a soft start circuit which is used for
initial start-up and during current limit operation. The SS
pin requires an external capacitor to GND with the value
determined by the required soft start time. An internal
10µA current source is included to charge the external SS
capacitor. During start-up, the COMP pin is clamped to a
diode drop above the voltage at the SS pin. This prevents
the error amplifier, ERR, from forcing the loop to maxi-
mum duty cycle. The LTC1553 will begin to operate at low
duty cycle as the SS pin rises above about 1.2V (VCOMP ≈
1.8V). As SS continues to rise, QSS turns off and the error
amplifier begins to regulate the output. The MIN compara-
tor is disabled when soft start is active to prevent it from
overriding the soft start function.
The LTC1553 includes yet another feedback loop to con-
trol operation in current limit. Just before every falling
edge of G1, the current comparator, CC, samples and
holds the voltage drop measured across the external
MOSFET, Q1, at the IFB pin. Note that when VIN = 12V, the
IFB pin requires an external Zener to GND to prevent
voltage transients at the switching node between Q1 and
Q2 from damaging internal structures. CC compares the
voltage at IFB to the voltage at the IMAX pin. As the peak
current rises, the measured voltage across Q1 increases
due to the drop across the RDS(ON) of Q1. When the voltage
at IFB drops below IMAX, indicating that Q1’s drain current
has exceeded the maximum level, CC starts to pull current
out of the external soft start capacitor, cutting the duty
cycle and controlling the output current level. The CC
comparator pulls current out of the SS pin in proportion to
the voltage difference between IFB and IMAX. Under minor
overload conditions, the SS pin will fall gradually, creating
a time delay before current limit takes effect. Very short,
mild overloads may not affect the output voltage at all.
More significant overload conditions will allow the SS pin
to reach a steady state, and the output will remain at a
reduced voltage until the overload is removed. Serious
overloads will generate a large overdrive at CC, allowing it
to pull SS down quickly and preventing damage to the
output components.
By using the RDS(ON) of Q1 to measure the output current,
the current limiting circuit eliminates an expensive dis-
crete sense resistor that would otherwise be required. This
helps minimize the number of components in the high
current path. Due to switching noise and variation of
RDS(ON), the actual current limit trip point is not highly
accurate. The current limiting circuitry is primarily meant
to prevent damage to the power supply circuitry during
fault conditions. The exact current level where the limiting
circuit begins to take effect will vary from unit to unit as the
RDS(ON) of Q1 varies.
For a given current limit level, the external resistor from
IMAX to VIN can be determined by:
( )( )RIMAX =
ILMAX RDS(ON)Q1
IIMAX
where,
IL MAX
=
ILOAD
+
IRIPPLE
2
ILOAD = Maximum load current
IRIPPLE = Inductor ripple current
( )( )= VIN − VOUT VOUT
( )( )( )fOSC LO VIN
fOSC = LTC1553 oscillator frequency = 300kHz
LO = Inductor value
RDS(ON)Q1 = Hot on-resistance of Q1 at ILMAX
IIMAX = Internal 180µA sink current at IMAX
LTC1553
+
CC
–
VIN
180µA
IMAX
7
IFB
8
RIMAX
G1
20Ω
G2
Q1
LO
Q2
+
CIN
VOUT
+
COUT
Figure 5. Current Limit Setting
1553 F05
11
11 Page | ||
| Páginas | Total 24 Páginas | |
| PDF Descargar | [ Datasheet LTC1553.PDF ] | |
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