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PDF LTC1649 Data sheet ( Hoja de datos )
| Número de pieza | LTC1649 | |
| Descripción | 3.3V Input High Power Step-Down Switching Regulator Controller | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LTC1649
3.3V Input High Power
Step-Down Switching
Regulator Controller
FEATURES
s High Power 3.3V to 1.xV-2.xV Switching Regulator
Controller: Up to 20A Output
s All N-Channel External MOSFETs
s Provides 5V MOSFET Gate Drive with 3.3V Input
s Constant Frequency Operation Minimizes
Inductor Size
s Excellent Output Regulation: ±1% Over Line, Load
and Temperature Variations
s High Efficiency: Over 90% Possible
s No Low-Value Sense Resistor Needed
s Available in 16-Lead SO Package
U
APPLICATIONS
s 3.3V Input Power Supply for Low Voltage
Microprocessors and Logic
s Low Input Voltage Power Supplies
s High Power, Low Voltage Regulators
s Local Regulation for Multiple Voltage Distributed
Power Systems
DESCRIPTION
The LTC®1649 is a high power, high efficiency switching
regulator controller optimized for use with very low supply
voltages. It operates from 2.7V to 5V input, and provides
a regulated output voltage from 1.26V to 2.5V at up to 20A
load current. A typical 3.3V to 2.5V application features
efficiency above 90% from 1A to 10A load. The LTC1649
uses a pair of standard 5V logic-level N-channel external
MOSFETs, eliminating the need for expensive P-channel
or super-low-threshold devices.
The LTC1649 shares its internal switching architecture
with the LTC1430, and features the same ±1% line, load
and temperature regulation characteristics. Current limit
is user-adjustable without requiring an external low-value
sense resistor. The LTC1649 uses a 200kHz switching
frequency and voltage mode control, minimizing external
component count and size. Shutdown mode drops the
quiescent current to below 10µA.
The LTC1649 is available in the 16-pin narrow SO package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATION
VIN
3.3V
SHDN
+
10µF
MBR0530
22Ω
C1
220pF
RC
7.5k
CC
0.01µF
3.3V to 2.5V, 15A Converter
RIMAX
50k
PVCC1
G1
PVCC2
IFB
VCC G2
LTC1649
IMAX
FB
SHDN
COMP
SS
VIN
C+
C–
GND CPOUT
1µF
1k
1µF
Q1, Q2 +
IRF7801
TWO IN
PARALLEL
CIN
3300µF
LEXT*
1.2µH
Q3
IRF7801
R1
12.4k
VOUT
2.5V
@15A
+ COUT
4400µF
R2
12.7k
0.1µF
+
MBR0530
10µF
0.33µF
IRF7801 = INTERNATIONAL RECTIFIER
MBR0530 = MOTOROLA
*12TS-1R2HL = PANASONIC
1649 TA01
100
90
80
70
60
50
40
0.1
LTC1649 Efficiency
1
LOAD CURRENT (A)
10
1649 TA02
1
1 page  
BLOCK DIAGRA
VIN
SHDN
DELAY
50µs
INTERNAL
SHUTDOWN
C+ C–
CHARGE
PUMP
LTC1649
CPOUT
COMP
SS
IMAX
VCC
12µA
ILIM
–+
12µA
PWM
FB
+–
+
1.26V
MIN
40mV
MAX
+
40mV
PVCC1
G1
PVCC2
G2
IFB
FB
1649 BD
TEST CIRCUIT
VIN
3.3V
MBR0530
22Ω
SHDN
+
10µF
C1
220pF
RC
7.5k
CC
0.01µF
RIMAX
50k
0.1µF
PVCC1
G1
PVCC2
IFB
VCC G2
LTC1649
IMAX
FB
SHDN
COMP
SS
VIN
C+
C–
GND CPOUT
1µF
1k
1µF
Q1, Q2 +
IRF7801
TWO IN
PARALLEL
CIN
3300µF
LEXT
1.2µH
Q3
IRF7801
R1
12.4k
VOUT
2.5V
+ COUT
4400µF
R2
12.7k
+
MBR0530
10µF
0.33µF
Figure 1
1649 TA03
5
5 Page 
LTC1649
APPLICATIONS INFORMATION
The CPOUT pin can typically supply 50mA at 5V, adequate
to power the VCC and PVCC pins. This supply can also be
used to power external circuitry, but any additional current
drawn from CPOUT subtracts from the current available to
drive the external MOSFETs. Circuits with small external
MOSFETs can draw as much as 20mA or 30mA from
CPOUT without hindering performance. High output cur-
rent circuits with large or multiple external MOSFETs may
need every milliamp they can get from CPOUT, and external
loads should be minimized. The charge pump at PVCC1 is
more limited in its abilities, and should not be connected
to anything except PVCC1. In particular, do not connect a
bypass capacitor from PVCC1 to ground—it will steal
charge from the charge pump and actually degrade perfor-
mance.
Compensation and Transient Response
The LTC1649 voltage feedback loop is compensated at the
COMP pin; this is the output node of the internal gm error
amplifier. The loop can generally be compensated prop-
erly with an RC network from COMP to GND and an
additional small C from COMP to GND (Figure 5). Loop
stability is affected by inductor and output capacitor
values and by other factors. Optimum loop response can
be obtained by using a network analyzer to find the loop
poles and zeros; nearly as effective and a lot easier is to
empirically tweak the RC values until the transient recovery
looks right with an output load step.
Output transient response is set by three major factors: the
time constant of the inductor and the output capacitor, the
ESR of the output capacitor, and the loop compensation
components. The first two factors usually have much
more impact on overall transient recovery time than the
third; unless the loop compensation is way off, more
improvement can be had by optimizing the inductor and
the output capacitor than by fiddling with the loop com-
pensation components. In general, a smaller value induc-
tor will improve transient response at the expense of ripple
and inductor core saturation rating. Minimizing output
capacitor ESR will also help optimize output transient
response. See Input and Output Capacitors for more
information.
Soft Start and Current Limit
The LTC1649 includes a soft start circuit at the SS pin; this
circuit is used both for initial start-up and during current
limit operation. SS requires an external capacitor to GND
with the value determined by the required soft start time.
An internal 12µA current source is included to charge the
external capacitor. Soft start functions by clamping the
maximum voltage that the COMP pin can swing to, thereby
controlling the duty cycle (Figure 6). The LTC1649 will
begin to operate at low duty cycle as the SS pin rises to
about 2V below the VCC pin. As SS continues to rise, the
duty cycle will increase until the error amplifier takes over
and begins to regulate the output. When SS reaches 1V
below VCC the LTC1649 will be in full operation. An internal
switch shorts the SS pin to GND during shutdown.
LTC1649
COMP
RC
CC C1
1659 F05
Figure 5. Compensation Pin Hook-Up
COMP
LTC1649
FB
VCC
SS
CSS
12µA
1649 F06
Figure 6. Soft Start Clamps COMP Pin
11
11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LTC1649.PDF ] | |
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