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PDF LT1611CS5 Data sheet ( Hoja de datos )
| Número de pieza | LT1611CS5 | |
| Descripción | Inverting 1.4MHz Switching Regulator in SOT-23 | |
| Fabricantes | Linear Technology | |
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LT1611
Inverting 1.4MHz Switching
Regulator in SOT-23
FEATURES
s Very Low Noise: 1mVP–P Output Ripple
s – 5V at 150mA from a 5V Input
s Better Regulation Than a Charge Pump
s Effective Output Impedance: 0.14Ω
s Uses Tiny Capacitors and Inductors
s Internally Compensated
s Fixed Frequency 1.4MHz Operation
s Low Shutdown Current: <1µA
s Low VCESAT Switch: 300mV at 300mA
s Tiny 5-Lead SOT-23 Package
U
APPLICATIO S
s MR Head Bias
s Digital Camera CCD Bias
s LCD Bias
s GaAs FET Bias
s Positive-to-Negative Conversion
DESCRIPTIO
The LT®1611 is the industry’s first inverting 5-lead SOT-23
current mode DC/DC converter. Intended for use in small,
low power applications, it operates from an input voltage
as low as 1.1V and switches at 1.4MHz, allowing the use
of tiny, low cost capacitors and inductors 2mm or less in
height. Its small size and high switching frequency enable
the complete DC/DC converter function to consume less
than 0.25 square inches of PC board area. Capable of
generating – 5V at 150mA from a 5V supply or – 5V at
100mA from a 3V supply, the LT1611 replaces nonregulated
“charge pump” solutions in many applications.
The LT1611 operates in a dual inductor inverting topology
which filters the input side as well as the output side of the
DC/DC converter. Fixed frequency switching ensures a
clean output free from low frequency noise typically present
with charge pump solutions. No load quiescent current of
the LT1611 is 3mA, while in shutdown quiescent current
drops to 0.5µA. The 36V switch allows VIN to VOUT
differential of up to 33V.
The LT1611 is available in the 5-lead SOT-23 package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATIO
L1A
VIN 22µH
5V
VIN SW
+
C1
SHDN
LT1611
22µF
NFB
GND
C2
1µF
L1B
22µH
R1
29.4k
R2
10k
D1
1200pF
C1: AVX TAJB226M010
C2: TAIYO YUDEN LMK212BJ105MG
C3: TAIYO YUDEN JMK325BJ226MM (1210 SIZE)
D1: MBR0520
L1: SUMIDA CLS62-220 OR 2× MURATA LQH3C220 (UNCOUPLED)
VOUT
–5V
150mA
C3
22µF
VOUT
20mV/DIV
AC COUPLED
150mA
LOAD CURRENT
50mA
1611 TA01
Figure 1. 5V to – 5V, 150mA Low Noise Inverting DC/DC Converter
Transient Response
100µs/DIV
1611 F10
1
1 page  
U
OPERATIO
boost converter, generating a negative output voltage,
which is directly regulated. The circuit schematic is de-
tailed in Figure 3. Only one inductor is required, and the
two diodes can be in a single SOT-23 package. Output
noise is the same as in a boost converter, because current
is delivered to the output only during the time when the
LT1611’s internal switch is off.
If D2 is replaced by an inductor, as shown in Figure 4, a
higher performance solution results. This converter topol-
ogy was developed by Professor S. Cuk of the California
Institute of Technology in the 1970s. A low ripple voltage
results with this topology due to inductor L2 in series with
the output. Abrupt changes in output capacitor current are
eliminated because the output inductor delivers current to
the output during both the off-time and the on-time of the
LT1611 switch. With proper layout and high quality output
capacitors, output ripple can be as low as 1mVP–P.
The operation of Cuk’s topology is shown in Figures 5
and␣ 6. During the first switching phase, the LT1611’s
switch, represented by Q1, is on. There are two current
loops in operation. The first loop begins at input capacitor
C1, flows through L1, Q1 and back to C1. The second loop
flows from output capacitor C3, through L2, C2, Q1 and
back to C3. The output current from RLOAD is supplied by
L2 and C3. The voltage at node SW is VCESAT and at node
SWX the voltage is –(VIN + |VOUT|). Q1 must conduct both
L1 and L2 current. C2 functions as a voltage level shifter,
with an approximately constant voltage of (VIN + |VOUT|)
across it.
LT1611
When Q1 turns off during the second phase of switching,
the SW node voltage abruptly increases to (VIN + |VOUT|).
The SWX node voltage increases to VD (about 350mV).
Now current in the first loop, begining at C1, flows through
L1, C2, D1 and back to C1. Current in the second loop flows
from C3 through L2, D1 and back to C3. Load current
continues to be supplied by L2 and C3.
An important layout issue arises due to the chopped
nature of the currents flowing in Q1 and D1. If they are both
tied directly to the ground plane before being combined,
switching noise will be introduced into the ground plane.
It is almost impossible to get rid of this noise, once present
in the ground plane. The solution is to tie D1’s cathode to
the ground pin of the LT1611 before the combined cur-
rents are dumped into the ground plane as drawn in
Figures 4, 5 and 6. This single layout technique can
virtually eliminate high frequency “spike” noise so often
present on switching regulator outputs.
Output ripple voltage appears as a triangular waveform
riding on VOUT. Ripple magnitude equals the ripple current
of L2 multiplied by the equivalent series resistance (ESR)
of output capacitor C3. Increasing the inductance of L1
and L2 lowers the ripple current, which leads to lower
output voltage ripple. Decreasing the ESR of C3, by using
ceramic or other low ESR type capacitors, lowers output
ripple voltage. Output ripple voltage can be reduced to
arbitrarily low levels by using large value inductors and
low ESR, high value capacitors.
C2
L1 1µF
VIN
+ VIN
C1
SW
LT1611
SHUTDOWN
SHDN
NFB
GND
R1
R2
10k
D2
D1
–VOUT
C3
C2
L1 1µF
VIN
+ VIN
C1
SW
LT1611
NFB
GND
R1
R2
10k
L2
D1
–VOUT
C3
Figure 3. Direct Regulation of Negative Output
Using Boost Converter with Charge Pump
1611 F03
1611 F04
Figure 4. L2 Replaces D2 to Make Low Output Ripple
Inverting Topology. Coupled or Uncoupled Inductors Can
Be Used. Follow Phasing If Coupled for Best Results
5
5 Page 
U
OPERATIO
Capacitors
As described previously, ceramic capacitors can be used
with the LT1611 provided loop stability is considered. For
lower cost applications, small tantalum units can be used.
A value of 22µF is acceptable, although larger capacitance
values can be used. ESR is the most important parameter
in selecting an output capacitor. The “flying” capacitor (C2
in the schematic figures) should be a 1µF ceramic type. An
X5R or X7R dielectric should be used to avoid capacitance
decreasing severely with applied voltage. The input by-
pass capacitor is less critical, and either tantalum or
LT1611
ceramic can be used with little trade-off in circuit perfor-
mance. Some capacitor types appropriate for use with the
LT1611 are listed in Table 2.
Diodes
A Schottky diode is recommended for use with the LT1611.
The Motorola MBR0520 is a very good choice. Where the
input to output voltage differential exceeds 20V, use the
MBR0530 ( a 30V diode). If cost is more important than
efficiency, a 1N4148 can be used, but only at low current
loads.
Table 1. Inductor Vendors
VENDOR
PHONE
Sumida
(847) 956-0666
Murata
Coiltronics
(404) 436-1300
(407) 241-7876
Table 2. Capacitor Vendors
VENDOR
PHONE
Taiyo Yuden (408) 573-4150
AVX (803) 448-9411
Murata
(404) 436-1300
URL
www.sumida.com
www.murata.com
www.coiltronics.com
PART
CLS62-22022
CD43-470
LQH3C-220
CTX20-1
COMMENT
22µH Coupled
47µH
22µH, 2mm Height
20µH Coupled, Low DCR
URL
www.t-yuden.com
www.avxcorp.com
www.murata.com
PART
Ceramic Caps
Ceramic Caps
Tantalum Caps
Ceramic Caps
COMMENT
X5R Dielectric
TYPICAL APPLICATIO S
“Charge Pump” Inverting DC/DC Converter
L1
10µH
C2
1µF
3.3V
D1 D2
VIN SW
SHDN
C1 LT1611
1µF NFB
GND
29.4k
10k
–5V
70mA
C3
22µF
C1, C2: TAIYO YUDEN LMK212BJ105MG
C3: TAIYO YUDEN JMK325BJ226MM
D1, D2: MBR0520
L1: MURATA LQH3C-100
1611 TA02
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 LT1611CS5.PDF ] | |
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