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PDF LTC1701B Data sheet ( Hoja de datos )
| Número de pieza | LTC1701B | |
| Descripción | 1MHz Step-Down DC/DC Converters | |
| Fabricantes | Linear | |
| Logotipo |  |
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LTC1701/LTC1701B
1MHz Step-Down
DC/DC Converters in SOT-23
FEATURES
s Tiny 5-Lead SOT-23 Package
s Uses Tiny Capacitors and Inductor
s High Frequency Operation: 1MHz
s High Output Current: 500mA
s Low RDS(ON) Internal Switch: 0.28Ω
s High Efficiency: Up to 94%
s Current Mode Operation for Excellent Line
and Load Transient Response
s Short-Circuit Protected
s Low Quiescent Current: 135µA (LTC1701)
s Low Dropout Operation: 100% Duty Cycle
s Ultralow Shutdown Current: IQ < 1µA
s Peak Inductor Current Independent of Inductor Value
s Output Voltages from 5V Down to 1.25V
U
APPLICATIO S
s PDAs/Palmtop PCs
s Digital Cameras
s Cellular Phones
s Portable Media Players
s PC Cards
s Handheld Equipment
DESCRIPTIO
The LTC®1701/LTC1701B are the industry’s first SOT-23
step-down, current mode, DC/DC converters. Intended for
low to medium power applications, they operate from 2.5V
to 5.5V input voltage range and switch at 1MHz, allowing
the use of tiny, low cost capacitors and inductors 2mm or
less in height. The output voltage is adjustable from 1.25V
to 5V. A built-in 0.28Ω switch allows up to 0.5A of output
current at high efficiency. OPTI-LOOPTM compensation
allows the transient response to be optimized over a wide
range of loads and output capacitors.
The LTC1701 incorporates automatic power saving Burst
ModeTM operation to reduce gate charge losses when the
load current drops below the level required for continuous
operation. The LTC1701B operates continuously to very
low load currents to provide low ripple at the expense of
light load efficiency. With no load, the LTC1701 draws only
135µA. In shutdown, both devices draw less than 1µA,
making them ideal for current sensitive applications.
Their small size and switching frequency enables a
complete DC/DC converter function to consume less than
0.3 square inches of PC board area.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode and OPTI-LOOP are trademarks of Linear Technology Corporation.
TYPICAL APPLICATION
VIN
2.5V TO
5.5V
C1 +
10µF
VIN SW
R4
1M LTC1701
ITH/RUN
R3 GND
5.1k
C3
330pF
VFB
L1
4.7µH
D1
R2
121k
+
R1
121k
VOUT
(2.5V/
500mA)
C2
47µF
C1: TAIYO YUDEN JMK316BJ106ML
C2: SANYO POSCAP 6TPA47M
D1: MBRM120L
L1: SUMIDA CD43-4R7
1701 F01a
Figure 1. 2.5V/500mA Step-Down Regulator
Efficiency Curve
100
95
VIN = 3.3V
VOUT = 2.5V
90
85 LTC1701
80
75
LTC1701B
70
65
60
55
50
1 10 100
LOAD CURRENT (mA)
1000
1701 F01b
1
1 page  
U
OPERATIO
The LTC1701 uses a contant off-time, current mode archi-
tecture. The operating frequency is then determined by the
off-time and the difference between VIN and VOUT.
The output voltage is set by an external divider returned to
the VFB pin. An error amplfier compares the divided output
voltage with a reference voltage of 1.25V and adjusts the
peak inductor current accordingly.
Main Control Loop
During normal operation, the internal PMOS switch is
turned on when the VFB voltage is below the reference
voltage. The current into the inductor and the load in-
creases until the current limit is reached. The switch turns
off and energy stored in the inductor flows through the
external Schottky diode into the load. After the constant
off-time interval, the switch turns on and the cycle repeats.
The peak inductor current is controlled by the voltage on
the ITH/RUN pin, which is the output of the error
amplifier.This amplifier compares the VFB pin to the 1.25V
reference. When the load current increases, the FB voltage
decreases slightly below the reference. This decrease
causes the error amplifier to increase the ITH/RUN voltage
until the average inductor current matches the new load
current.
The main control loop is shut down by pulling the ITH/RUN
pin to ground. When the pin is released an external resistor
is used to charge the compensation capacitor. When the
voltage at the ITH/RUN pin reaches 0.8V, the main control
LTC1701/LTC1701B
loop is enabled and the error amplifier drives the ITH/RUN
pin. Soft-start can be implemented by ramping the voltage
on the ITH/RUN pin (see Applications Information sec-
tion).
Low Current Operation
To optimize efficiency when the load is relatively light, the
LTC1701 automatically switches to Burst Mode operation
in which the internal PMOS switch operates intermittently
based on load demand. The main control loop is inter-
rupted when the output voltage reaches the desired regu-
lated value. The hysteretic voltage comparator trips when
ITH/RUN is below 1.5V, shutting off the switch and reduc-
ing the power consumed. The output capacitor and the
inductor supply the power to the load until the output
voltage drops slightly and the ITH/RUN pin exceeds 1.5V,
turning on the switch and the main control loop which
starts another cycle.
For reduced output ripple, the LTC1701B doesn't use
Burst Mode operation and operates continuously down to
very low currents where the part starts skipping cycles.
Dropout Operation
In dropout, the internal PMOS switch is turned on continu-
ously (100% duty cycle) providing low dropout operation
with VOUT at VIN. Since the LTC1701 does not incorporate
an under voltage lockout, care should be taken to shut
down the LTC1701 for VIN < 2.5V.
APPLICATIO S I FOR ATIO
The basic LTC1701 application circuit is shown in
Figure␣ 1. External component selection is driven by the
load requirement and begins with the selection of L1. Once
L1 is chosen, the Schottky diode D1 can be selected
followed by CIN and COUT.
L Selection and Operating Frequency
The operating frequency is fixed by VIN, VOUT and the
constant off-time of about 500ns. The complete expres-
sion for operating frequency is given by:
fO
=
VIN − VOUT
VIN + VD
1
TOFF
Although the inductor does not influence the operating
frequency, the inductor value has a direct effect on ripple
current. The inductor ripple current ∆IL decreases with
higher inductance and increases with higher VIN or VOUT:
∆IL
=
VIN
− VOUT
fL
VOUT + VD
VIN + VD
where VD is the output Schottky diode forward drop.
5
5 Page 
TYPICAL APPLICATIO S
LTC1701/LTC1701B
2mm Nominal Height 1.5V Converter
VIN
2.5V TO
5.5V
+
C1
15µF
R4
1M
C4
1µF
R3
5.1k
VIN SW
LTC1701
ITH/RUN
VFB
GND
C3
330pF
L1
4.7µH
D1
R2
20k
R1
100k
+
C2
22µF
C1: AVX TAJA156M010R C4: TAIYO YUDEN LMK212BJ105MG D1: MBRM120L
C2: AVX TAJA226M006R C5: TAIYO YUDEN JMK212BJ475MG L1: MURATA LQH3C4R7M24
VOUT
(1.5V/0.5A)
C5
4.7µF
1701 TA01a
Efficiency Curve
90
VIN = 2.5V
85
80
VIN = 3.3V
75
70
65
60
55
VOUT = 1.5V
50
1
10
LTC1701
LTC1701B
100 1000
LOAD CURRENT (mA)
1701TA01b
All Ceramic Capacitor 2.5V Converter
VIN
2.5V TO
5.5V
C1
10µF
R4
1M
C4
1µF
R3
5.1k
VIN SW
LTC1701
ITH/RUN
GND
VFB
C3
180pF
R1
121k
L1
4.7µH
D1
R2
121k
C6
33pF
C2
10µF
C1, C2: TAIYO YUDEN JMK316BJ106ML
C4, C5: TAIYO YUDEN LMK212BJ105MG
L1: MURATA LQH3C4R7M24
D1: MBRM120L
VOUT
(2.5V/0.5A)
C5
1µF
1701 TA02
Efficiency Curve
100
VOUT = 2.5V
95
VIN = 3.3V
90
85
VIN = 5.0V
80
75
70
1
LTC1701
LTC1701B
10 100
LOAD CURRENT (mA)
1000
1701 TA02b
5V to 3.3V Converter with Push-Button On/Off
VIN
3.3V TO
5.5V
C1 +
15µF
VIN SW
ON
R4
C4 1M
1µF
R3
OFF R5 5.1k
5.1M
LTC1701
ITH/RUN
VFB
GND
C3
330pF
L1
4.7µH
D1
R2
34k
R1
20.5k
C2 +
22µF
C1: AVX TAJA156M010R
C2: AVX TAJA226M006R
C4, C5: TAIYO YUDEN LMK212BJ105MG
D1: MBRM120L
L1: MURATA LQH3C4R7M24
LTC1701B Low Current Pulse Skip
VOUT
(3.3V/
0.5A)
VOUT
20mV/DIV
C5
1µF
1701 TA03a
IL
50mA/DIV
VIN = 5V
VOUT = 2.5V
5µs/DIV
1701 TA03b
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
11
11 Page | ||
| Páginas | Total 12 Páginas | |
| PDF Descargar | [ Datasheet LTC1701B.PDF ] | |
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