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PDF LTC1622 Data sheet ( Hoja de datos )
| Número de pieza | LTC1622 | |
| Descripción | Low Input Voltage Current Mode Step-Down DC/DC Controller | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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FEATURES
s High Efficiency
s Constant Frequency 550kHz Operation
s VIN Range: 2V to 10V
s Multiampere Output Currents
s OPTI-LOOPTM Compensation Minimizes COUT
s Selectable, Burst Mode Operation
s Low Dropout Operation: 100% Duty Cycle
s Synchronizable up to 750kHz
s Current Mode Operation for Excellent Line and Load
Transient Response
s Low Quiescent Current: 350µA
s Shutdown Mode Draws Only 15µA Supply Current
s ±1.9% Reference Accuracy
s Available in 8-Lead MSOP
U
APPLICATIO S
s 1- or 2-Cell Li-Ion Powered Applications
s Cellular Telephones
s Wireless Modems
s Portable Computers
s Distributed 3.3V, 2.5V or 1.8V Power Systems
s Scanners
s Battery-Powered Equipment
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode and OPTI-LOOP are a trademarks of Linear Technology Corporation.
LTC1622
Low Input Voltage
Current Mode Step-Down
DC/DC Controller
DESCRIPTIO
The LTC®1622 is a constant frequency current mode step-
down DC/DC controller providing excellent AC and DC load
and line regulation. The device incorporates an accurate
undervoltage feature that shuts the LTC1622 down when
the input voltage falls below 2V.
The LTC1622 boasts a ±1.9% output voltage accuracy and
consumes only 350µA of quiescent current. For applica-
tions where efficiency is a prime consideration and the
load current varies from light to heavy, the LTC1622 can
be configured for Burst ModeTM operation. Burst Mode
operation enhances low current efficiency and extends
battery run time. Burst Mode operation is inhibited during
synchronization or when the SYNC/MODE pin is pulled low
to reduce noise and possible RF interference.
High constant operating frequency of 550kHz allows the
use of a small inductor. The device can also be synchro-
nized up to 750kHz for special applications. The high
frequency operation and the available 8-lead MSOP pack-
age create a high performance solution in an extremely
small amount of PCB area.
To further maximize the life of the battery source, the
P-channel MOSFET is turned on continuously in dropout
(100% duty cycle). In shutdown, the device draws a mere
15µA.
TYPICAL APPLICATIO
VIN
2.5V TO 8.5V
R1
10k
C3
220pF
8
2
ITH
VIN
SENSE–
PDRV
1
7
LTC1622
5
SYNC/MODE
470pF
4
RUN/SS
VFB
3
6
GND
R2
0.03Ω
Si3443DV L1
4.7µH
D1
IR10BQ015
C1
10µF
10V
R3
159k +
R4
75k
VOUT
2.5V
1.5A
C2
47µF
6V
C1: TAIYO YUDEN CERAMIC EMK325BJ106MNT L1: MURATA LQN6C-4R7
C2: SANYO POSCAP 6TPA47M
R2: DALE WSL-1206 0-03Ω
D1: INTERNATIONAL RECTIFIER IR10BQ015
1622 F01a
Figure 1. High Efficiency Step-Down Converter
Efficiency vs Load Current with
Burst Mode Operation Enabled
100
VIN = 4.2V
90 VIN = 3.3V
80
VIN = 6V
70 VIN = 8.4V
60
50
40
1
VOUT = 2.5V
RSENSE = 0.03Ω
10 100 1000
LOAD CURRENT (mA)
5000
1622 F01b
1
1 page  
W
FUNCTIONAL DIAGRA
VIN
1µA
BURST DEFEAT
Y
X
Y = “0” ONLY WHEN X IS A CONSTANT “1”
OTHERWISE Y = “1”
VCC
LTC1622
SYNC/
MODE
5
SLOPE
COMP
OSC
0.3V
VFB 3
–
+
VIN
0.8V
REFERENCE
VREF
0.8V
UVLO
TRIP = 1.97V
SHUTDOWN
FREQ
SHIFT
0.8V
VREF
VIN 2.5µA
RUN/SS 4
–
+
+
EA
– gm = 0.5mΩ
RUN/
SOFT-START
+
OV
6
VREF + 60mV
–
GND
0.36V
0.12V
2
ITH
EN
–
+
BURST
S
RQ
RS1
SENSE– 1
8 VIN
SLEEP
SWITCHING
LOGIC
AND
BLANKING
CIRCUIT
+–
ICOMP
VIN
PDRV
7
1622 BD
U
OPERATIO (Refer to Functional Diagram)
Main Control Loop
The LTC1622 is a constant frequency current mode switch-
ing regulator. During normal operation, the external
P-channel power MOSFET is turned on each cycle when
the oscillator sets the RS latch (RS1) and turned off when
the current comparator (ICOMP) resets the latch. The peak
inductor current at which ICOMP resets the RS latch is
controlled by the voltage on the ITH pin, which is the output
of the error amplifier EA. An external resistive divider
connected between VOUT and ground allows EA to receive
an output feedback voltage VFB. When the load current
increases, it causes a slight decrease in VFB relative to the
0.8V reference, which in turn causes the ITH voltage to
increase until the average inductor current matches the
new load current.
The main control loop is shut down by pulling the RUN/SS
pin low. Releasing RUN/SS allows an internal 2.5µA
current source to charge up the soft-start capacitor CSS.
When CSS reaches 0.7V, the main control loop is enabled
with the ITH voltage clamped at approximately 5% of its
maximum value. As CSS continues to charge, ITH is gradu-
ally released allowing normal operation to resume.
Comparator OV guards against transient overshoots
> 7.5% by turning off the P-channel power MOSFET and
keeping it off until the fault is removed.
Burst Mode Operation
The LTC1622 can be enabled to go into Burst Mode
operation at low load currents simply by leaving the SYNC/
MODE pin open or connecting it to a voltage of at least 2V.
In this mode, the peak current of the inductor is set as if
VITH = 0.36V (at low duty cycles) even though the voltage
at the ITH pin is at lower value. If the inductor’s average
current is greater than the load requirement, the voltage at
5
5 Page 
LTC1622
APPLICATIONS INFORMATION
Transition Loss = 3(VIN)2IO(MAX)CRSS(f)
Other losses including CIN and COUT ESR dissipative
losses, and inductor core losses, generally account for
less than 2% total additional loss.
LTC1622
ITH VFB
R2 +
R1
VOUT
DFB
Run/Soft-Start Function
The RUN/SS pin is a dual purpose pin that provides the
soft-start function and a means to shut down the LTC1622.
Soft-start reduces input surge current from VIN by gradu-
ally increasing the internal current limit. Power supply
sequencing can also be accomplished using this pin.
An internal 2.5µA current source charges up an external
capacitor CSS. When the voltage on the RUN/SS reaches
0.7V the LTC1622 begins operating. As the voltage on
RUN/SS continues to ramp from 0.7V to 1.8V, the internal
current limit is also ramped at a proportional linear rate.
The current limit begins near 0A (at VRUN/SS = 0.7V) and
ends at 0.1/RSENSE (VRUN/SS ≥ 1.8V). The output current
thus ramps up slowly, reducing the starting surge current
required from the input power supply. If the RUN/SS has
been pulled all the way to ground, there will be a delay
before the current limit starts increasing and is given by:
tDELAY = 2.8 • 105 • CSS in seconds
Pulling the RUN/SS pin below 0.4V puts the LTC1622 into
a low quiescent current shutdown (IQ < 15µA).
Foldback Current Limiting
As described in the Output Diode Selection, the worst-
case dissipation occurs with a short-circuited output
when the diode conducts the current limit value almost
continuously. To prevent excessive heating in the diode,
foldback current limiting can be added to reduce the
current in proportion to the severity of the fault.
Foldback current limiting is implemented by adding diode
DFB (1N4148 or equivalent) between the output and the ITH
pin as shown in Figure 5. In a hard short (VOUT = 0V), the
current will be reduced to approximately 50% of the
maximum output current.
1622 F05
Figure 5. Foldback Current Limiting
Design Example
Assume the LTC1622 is used in a single lithium-ion
battery-powered cellular phone application. The VIN will be
operating from a maximum of 4.2V down to a minimum of
2.7V. Load current requirement is a maximum of 1.5A but
most of the time it will be on standby mode, requiring only
2mA. Efficiency at both low and high load current is
important. Output voltage is 2.5V.
In the above application, Burst Mode operation is enabled
by connecting Pin 5 to VIN.
Maximum Duty Cycle = VOUT + VD = 93%
VIN(MIN) + VD
From Figure 2, SF = 57%.
Use the curve of Figure 2 since the operating frequency is
the free running frequency of the LTC1622.
( )( )( ) ( )( )RSENSE = SF = 0.57 = 0.0253Ω
15 IOUT 100 15 1.5A
In the application, a 0.025Ω resistor is used. For the
inductor, the required value is:
LMIN
=
4.2 − 2.5
550kHz
0.036
0.025
2.5 +
4.2 +
0.3
0.3
=
1.33µH
In the application, a 3.9µH inductor is used to reduce
inductor ripple current and thus, output voltage ripple.
For the selection of the external MOSFET, the RDS(ON)
must be guaranteed at 2.5V since the LTC1622 has to work
11
11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LTC1622.PDF ] | |
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