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LinearTechnologyChronicle
A Showcase of Linear Technology’s Focus Products
November 1999
Positive High Voltage
Hot Swap Controller—
LT1641
Product of the Month
Low Noise, Micropower, Low Dropout Regulators
Withstand Reverse Voltage—LT1762 and LT1763
The LT®1762 and LT1763 are very low
noise, low dropout linear regulators. The
LT1762 is rated for 150mA of output current, whereas the LT1763 is rated for
500mA. For supply flexibility and good efficiency, the input voltage of these regulators
can be as low as 350mV (maximum dropout
voltage) above the output at maximum rated
current. The regulators are designed for use
in battery-powered systems with 25µA operating current for the LT1762 and 30µA for
the LT1763. Both regulators feature a 0.1µA
shutdown state. Quiescent current is well
controlled for these devices; it does not rise
in dropout—when the input to output voltage
differential is below the dropout rating—as
is the case with many other regulators.
The LT1762 and LT1763 regulators
feature low noise operation. With the addition of an external 0.01µF bypass capacitor,
output voltage noise over the 10Hz to
100kHz bandwidth is reduced to < 30µVRMS
(Figures 1 and 2). Both regulators can operate with output capacitors as low in value as
2.2µF for the LT1762 and 4.7µF for the
LT1763. Small ceramic capacitors can be
used with either device without the need for
additional series resistance as is common
VIN
3.7V TO
20V
IN
1µF
with other regulators. Internal protection
circuitry on both regulators includes reversebattery protection, current limiting, thermal
limiting and reverse-current protection. No
protection diodes are required.
Both regulators are available in fixed
output voltages of 2.5V, 3V, 3.3V and 5V or
as an adjustable device with an output range
of 1.22V to 20V. The LT1762 is packaged in
an 8-lead MSOP and the LT1763 is available in an 8-lead SO package. These regulators can be paralleled for more output
current if required.
160
COUT = 10µF
IL = 150mA
f = 10Hz TO 100kHz
140
OUTPUT NOISE (µVRMS)
LT1762-5
120
LT1762-3.3
100
LT1762-3
80
60
LT1762
40
LT1762-2.5
20
0
10
100
1000
CBYP (pF)
Vol. 8 No. 11
When a circuit board is inserted into a
live backplane, the supply bypass capacitors
on the board draw high peak currents from
the backplane power bus as they charge. The
transient currents can permanently damage
the connector pins and cause glitches on the
system supply, causing other boards in the
system to reset.
The LT1641 is designed to turn on a
board’s supply voltage in a controlled manner (Figure 1), allowing the board to be
safely inserted into or removed from a live
backplane. A high side switch driver controls an N-channel pass transistor in the
power path (Q1 in Figure 2) for supply voltages ranging from 9V to 80V. The LT1641
is tested and guaranteed to be safe from
damage with supply voltages up to 100V and
may be connected directly to 24V and 48V
supplies without floating grounds or level
shifting of logic signals. The device provides
undervoltage and overcurrent protection and
a power good signal that indicates when the
output supply voltage is ready. It comes in
an 8-lead SO package.
10000
1762 G42
Figure 2. LT1762 RMS Output Noise
vs Bypass Capacitor
OUT
+
SENSE
LT1762-3.3
3.3V AT 150mA
20µVRMS NOISE
10µF
0.01µF
SHDN
BYP
GND
1762 TA01
Figure 1. 3.3V Low Noise Regulator
Inside This Issue:
Micropower Current Sense Amp Operates from 2.5V to 60V
with 40µV Offset Error—LT1787 .............................................................................................
High Efficiency Linear and Switching Solutions for Splitting
a Digital Supply—LT1118 and LTC1504 ................................................................................
High Efficiency PolyPhaseTM DC/DC Current Mode Controllers Provide 20A to 200A
Using Surface Mount Components—LTC1629/LTC1929 .......................................................
A 10-Bit Voltage Output Micropower DAC in a Tiny SOT-23 Package—LTC1663 ................
Linear Technology Chronicle • November 1999
Figure 1. LT1641 Power-Up Waveforms
2
Continued on page 2
3
3
4
, LTC and LT are registered trademarks of Linear Technology
Corporation. Hot Swap, Over-The-Top, PolyPhase and
OPTI-LOOP are trademarks of Linear Technology Corporation.
I2C is a trademark of Philips Electronics N.V.
1
LT1641 from page 1
Short-Circuit Protection
To prevent excessive power dissipation
in the pass transistor and voltage spikes on
the input supply during short-circuit and
overcurrent conditions at the output, the
LT1641 features a programmable foldback
current limit with an electronic circuit
breaker. The current limit is set by placing a
Q1
IRF530
RS
0.01Ω
VIN
24V
D1
CMPZ
5248B
R5
10Ω
5%
R1
49.9k
1%
R3
59k
1%
sense resistor between the VCC and SENSE
pins. The current folds back as a function of
the output voltage which is sensed at the FB
(feedback) pin. If the chip remains in current limit beyond a programmable time
limit, the pass transistor latches off. The
device may be set to automatically restart
after a time-out delay.
A variable overcurrent response time
provides additional protection under severe
fault conditions. The larger the
fault voltage across the sense
resistor, the faster the gate will be
VOUT
pulled down, protecting the load.
CL
C1
R6, 10nF
1k, 5%
8
VCC
1
7
6
SENSE
GATE
Table 1. Hot Swap Controllers
R4
3.57k
1%
LT1641
R2
3.4k
1%
GND
3
PWRGD
GND
5
4
LTC®1421
2
FB
TIMER
Device
R7
24k
5%
ON
Power Good Detection
The LT1641 includes a comparator for
monitoring the output voltage. The noninverting input (the FB pin) which includes
80mV of hysteresis is compared against an
internal 1.233V precision reference. The
comparator’s output (the PWRGD pin) is an
open collector capable of operating from a
pull-up as high as 100V. PWRGD can be
used to directly enable and disable a power
converter module.
Table 1 shows the entire line of
Hot SwapTM controllers from Linear
Technology.
PWRGD
C2
0.68µF
#Supplies
Supplies
Features
2
3V to 12V
Controls 3rd Rail to – 12V
LTC1422
1
3V to 12V
LT1640
1
–10V to – 80V
SO-8
LT1641
1
9V to 80V
SO-8, Foldback ILIMIT
LTC1642
1
3V to 16.5V
Fault Protected to 33V
LTC1643
4
3.3V, 5V, ±12V
– 48V in SO-8
Four Supplies, Tiny Package
1641 TA01
Figure 2. 24V Input Voltage Application
Micropower Current Sense Amp Operates from 2.5V
to 60V with 40µV Offset Error—LT1787
The LT1787 is a high resolution current
sense amplifier designed for applications
such as precise monitoring of the current
into or out of a battery (gas gauging) in cellular telephones, portable instruments, computers and wireless telecom devices. The
LT1787 has an operating voltage range of
2.5V to 36V so that the device can operate
from a single Li-Ion cell and from most
other sources. The LT1787HV version can
operate on supplies up to 60V, making it
useful for high voltage telecommunication
and industrial systems.
TO
LOAD
I = 100A
1
2
3
4
FIL+
8
VS+
7
LT1787HV
VS–
VEE
RSENSE
VS–
RSENSE
0.0016Ω
FIL–
DNC
The LT1787 monitors bidirectional currents via the voltage across an external sense
resistor (Figure 1). A current or voltage output from the device depicts the direction and
magnitude of the sensed current. The
LT1787 allows the use of a selectable sense
resistor, the value of which depends on the
current to be monitored. This allows the
input voltage to be adjusted to a 128mV fullscale range, allowing a maximum dynamic
range. The part is generally used at ±128mV
full scale, although it is specified for a
± 500mV minimum full scale. The 40µV
input offset voltage of the LT1787 translates
into a 12-bit minimum dynamic range in
resolving currents.
The LT1787 has a fixed voltage gain of
8 from input to output, set by onboard precision resistors (Figure 2). It is self-powered
from the supply that it is monitoring and
requires only 60µA supply current. Additional features include provisions for input
noise filtering (both differential and common
mode) and a PSRR of greater than 120dB.
The device is available in both 8-lead SO
and MSOP packages.
–
60V
RG1A
1.25k
RG2A
1.25k
RG1B
1.25k
RG2B
1.25k
VOUT = (8 • VSENSE) + VREF
OUTPUT STEP = 13mV/Amp
FIL+
R1
15k
C1
1µF
5V
–
+
A1
VOUT 5
C2
0.1µF
VS+
FIL
VBIAS 6
ROUT
20k
ISENSE
VREF VCC
CS
+IN
LTC1286 CLK
–IN
D
GND OUT
LT1634-1.25
IOUT
TO µP
Q1
Q2
VBIAS
ROUT
20k
VOUT
1787 TA01
VEE
1:1 MIRROR
1787 F 01
Figure 1. 12-Bit Dynamic Resolution Bidirectional Output to an ADC
Figure 2. LT1787 Functional Diagram
Contact your local Linear Technology sales office for a data sheet and evaluation samples. For more information, visit our web site at www.linear-tech.com.
2
Linear Technology Chronicle • November 1999
Application of the Month
High Efficiency Linear and Switching Solutions for
Splitting a Digital Supply—LT1118 and LTC1504
At low current levels the LT1118 is the optimum solution.
The LTC1504 is a synchronous switching regulator that provides as much as 90%
efficiency (Figure 2) while sourcing or sinking up to 500mA. It will draw only 56mA
from the 5V supply while sourcing 100mA
from its 2.5V output. The switching architecture of the LTC1504 requires more external
components than the LT1118 (Figure 3) and
generates a small amount of output noise
D
90
80
VCC
5V
IN
0.1µF
OUT
LT1118-2.5
1µF
NC
C
F
5V
50
A
B
20
10
1
COUT
12.1k
7.5k
220pF
0.01µF
COUT: AVX TAJC476M016R
LEXT: SUMIDA CDRH73-470 (47µH: LOW RIPPLE, HIGH EFFICIENCY)
SUMIDA CDRH73-220 (22µH: FAST TRANSIENT RESPONSE) DN172 F05
10
100
1000
LOAD CURRENT (SINK/SOURCE)(mA)
DN172 F02
COMP
NC
0
GND
+
FB
SS
A: LT1118 SOURCE
B: LT1118 SINK
C: LTC1504/47µH SOURCE
D: LTC1504/47µH SINK
E: LTC1504/22µH SOURCE
F: LTC1504/22µH SINK
SPLIT SUPPLY
2.5V ± 500mA
11.8k
LTC1504
GND
40
LEXT
SW
VCC
4.7µF
CERAMIC
60
2.5V
VIRTUAL GROUND
ZOUT < 0.1Ω
SHDN
IMAX SHDN
E
70
30
0.6mA
(ripple) at its 200kHz switching frequency.
With a 47µH inductor, switching noise is
minimal and the circuit recovers from a
±400mA output load step in 30µs. A shutdown function drops quiescent current
below 10µA when the split supply is not
required. The LTC1504 is the best solution
where efficiency, especially at high current
levels, is the overriding concern. (See
Design Note 172)
100
EFFICIENCY (%)
This note presents two methods of creating a split supply with good transient
response and minimal supply current in a
5V-only system.
The LT1118 is a linear regulator
designed to source or sink current to keep
its output in regulation. It draws 600µA of
quiescent current typically and can source
800mA or sink 400mA, enough to satisfy
most analog subsystems. The LT1118
requires only two external components
(Figure 1) and has a DC output impedance
below 0.1Ω. The LT1118 provides nearly
50% power efficiency over a wide range of
load currents (Figure 2). Load transient
response is excellent with less than 5µs
recovery time from a ± 400mA load step.
DN172 F03
Figure 1. LT1118-2.5 Supply Splitter
Figure 2. Efficiency Comparison
Figure 3. LTC1504 Supply Splitter
High Efficiency PolyPhase DC/DC Current Mode Controllers Provide 20A to
200A Using Surface Mount Components—LTC1629/LTC1929
cancellation occurs in both the input and
output circuits due to properly phased current pulses. Their multiple phase technique
effectively multiplies the fundamental frequency by the number of channels used,
Continued on page 4
10Ω
S
VIN LTC1629 TG1
ITH
S
SGND
M3
TG2
S
BOOST2
SW2
VDIFFOUT
S
BG2
EAIN
INTVCC
–
+
VOS +
SENSE2
VOUT
1.6V/40A
0.47µF
S
VOS
0.002Ω
D1
SENSE1 –
16k
16k
L1
1µH
SENSE1 +
S
100pF
S
M2
×2
S
BG1
PGND
1000pF
S
0.47µF
SW1
RUN/SS
10k
M1
S
BOOST1
0.1µF
VIN
5V TO 28V
10µF ×4
35V
CERAMIC
0.1µF
S
+
The LTC1629 and LTC1929 are high
power true current mode DC/DC controllers.
These dramatically simplify the design of
DC/DC converters for powering today’s
high end microprocessors, large memory
arrays and other high power loads. Using a
unique PolyPhase switching technique, the
LTC1629 can be configured to supply load
currents up to 200A with inherent current
sharing using readily available surface
mount components. This PolyPhase technique allows a significant reduction of input
and output ripple currents, resulting in high
efficiency, low EMI DC/DC converter
designs that require very little PC board
space.
Unlike traditional switching regulators,
the LTC1629 and LTC1929 interleave the
clock signals of their two paralleled power
stages 180° out of phase. Ripple current
M4
×2
D2
L2
1µH
0.002Ω
10µF
+
SENSE2 –
COUT: T510E108K004AS
L1, L2: CEPH149-IROMC
COUT
1000µF ×2
4V
1629 TA01
Figure 1. High Current Dual Phase Step-Down Converter
Contact your local Linear Technology sales office for a data sheet and evaluation samples. For more information, visit our web site at www.linear-tech.com.
Linear Technology Chronicle • November 1999
3
LTC1629/LTC1929 from page 3
improving transient response while operating
each channel at an optimum frequency for
efficiency. This allows a dramatic reduction
in input and output capacitance requirements
and EMI generation while improving overall
efficiency.
The LTC1929 operates in a 2-phase
configuration and can support load currents
up to 40A. The LTC1629 adds additional
internal clock management circuitry that
allows multiple devices to be connected in
parallel to support higher currents. Using
this technique, the LTC1629 can be used in
DC/DC converter circuits incorporating up
to twelve phases to provide load currents up
to 200A.
A wide 4V to 36V input range allows
the LTC1629 and LTC1929 to operate from
a variety of input sources and OPTI-LOOPTM
compensation allows the transient response
to be optimized for the specific output
capacitors used in each application. The
LTC1629 and LTC1929 provide output voltages from 0.8V to 6V. An internal differential amplifier provides true remote sensing of
the regulated supply’s positive and negative
output terminals as required in high current
applications. Internal current foldback with a
shutdown timer and a soft-latch overvoltage
protection feature helps protect the load
against harmful voltage spikes and
overcurrent conditions.
The LTC1629 and LTC1929 are both
offered in a 28-lead SSOP package, in commercial and industrial temperature grades.
Where previous high power applications
required the use of an expensive and bulky
DC/DC converter module, the LTC1629
and LTC1929 offer a smaller and more
efficient alternative that can be easily integrated into even the most space-constrained
system.
A 10-Bit Voltage Output
Micropower DAC in a
Tiny SOT-23 Package—
LTC1663
SMBus and I2CTM, allowing multiple devices to drive and monitor the bus without
contention. In the MS8 package, eight userselectable addresses allow up to eight
LTC1663s to be placed on the same bus. Its
ultralow power consumption and small size
make the LTC1663 ideal for portable, battery-powered applications where supply
current directly affects the operation time
and board space is at a premium.
The LTC1663 allows the user to select
either an internal 1.25V bandgap reference
or the positive supply as the DAC’s reference voltage. Selecting the internal reference
will set the full-scale output voltage to
0V–2.5V. Selecting VCC as the reference
extends the full-scale output to the positive
supply voltage, adding extra flexibility for
ratiometric applications and maximizing the
output voltage range.
The LTC1663 includes a rail-to-rail
output buffer, eliminating the need for external op amps. A DNL of ± 0.75LSB (max)
guarantees true 10-bit monotonic performance over the full temperature range, critical in control loop applications. A power-on
reset function ensures that the DAC output is
at 0V when power is initially applied and all
internal registers are cleared.
The LTC1663 is available in MS8 and
5-lead SOT-23 packages.
The LTC1663 is a single, 10-bit, voltage output DAC with true buffered rail-torail output voltage capability, all in a tiny
SOT-23 package. Operating from supply
voltages between 2.7V and 5.5V, the
LTC1663 consumes only 60µA of supply
current and a mere 10µA in shutdown
mode. The part also features a simple
2-wire serial interface compatible with
5
VCC
1.25V
BANDGAP
REFERENCE
REFERENCE
SELECT
10-BIT
DAC LATCH
+
VOUT 8
–
COMMAND
LATCH
R
INPUT
LATCH
Linear Technology Products
Are Distributed By:
R
6 AD0
2 AD1
3 AD2
2-WIRE INTERFACE
SDA
SCL
GND
1
4
7
Figure 1. LTC1663 Block Diagram
1663 BD
Arrow Electronics
Arrow/Zeus Components
Digi-Key
Gerber Electronics
Marshall Industries
Wyle Electronics
© 1999 Linear Technology Corporation/Printed in USA
Contact your local Linear Technology sales office for a data sheet and evaluation samples. For more information, visit our web site at www.linear-tech.com.
Linear
• www.linear-tech.com
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1999