1
Linear Technology Chronicle •
October 1997
A Showcase of Linear Technology’s Focus Products
Linear Technology Chronicle
October 1997 Vol. 6 No. 10
Product of the Month
The switching frequency may be exter-
nally set with a single capacitor up to
250kHz and may be synchronized to an
external clock source of up to 375kHz. Pro-
tection features include cycle-by-cycle
current limiting, undervoltage lockout and
thermal shutdown. For unregulated DC/DC
conversion, the LT1533 may be configured
to operate in a fixed 50% duty cycle mode.
The LT1533 is available from stock in
a 16-lead narrow SO package, screened to
the commercial and industrial temperature
ranges. For a data sheet and evaluation
samples, contact your local Linear
Technology sales office or visit our web
site at www.linear-tech.com for more
information.
, LTC and LT are registered trademarks of Linear Technology
Corporation. Hot Swap is a trademark of Linear Technology
Corporation.
Inside This Issue:
LTC1422: Hot Swap
TM
Controller IC in SO-8 Permits Safe PC Board Insertion and Removal ..........2
LTC1536: Triple Reset Generator for PCI Applications Offers 0.75% Threshold Accuracy ..............2
LT1300: A 4-Cell to 5V Power Supply ............................................................................................... 3
LTC1329 and LTC1428: Push-Button DACs in SO-8 Packages Tweak Adjustable Regulators ...... 4
The LT1533 can be configured in low
noise step-up/step-down, inverting and iso-
lated DC/DC converters to regulate both
positive and negative output voltages.
Because the voltage and current slew rates
are independently adjustable using external
resistors, the designer is given the flexibility
to select the optimum noise vs efficiency
operating point for any application.
The LT1533 operates from an input
supply voltage of 2.7V to 23V (input volt-
age is limited to 15V for push-pull
configurations). It draws 9mA (typ) quies-
cent current and shuts down to only 12µA,
making the part suitable for portable,
battery-powered applications.
Ultralow Noise Switching Regulator Is
Quieter Than Linear Regulators
The LT
®
1533 is a DC/DC converter
that produces less than 100µV peak-to-peak
output noise—far less than linear regulators.
The first in a new class of switching regula-
tors, the LT1533 dramatically reduces both
conducted and radiated noise by accurately
controlling the voltage and current slew rates
of the two internal power switches. Using
this novel technique, high frequency har-
monic noise can be reduced by as much as
40dB over typical switching regulators, with
a relatively minor impact on power conver-
sion efficiency. Figure 1 shows the
comparison in output noise between a con-
ventional switcher and the LT1533. Its
ability to provide high efficiency voltage
conversion with very low noise makes it
ideal in such noise-sensitive applications as
precision instrumentation and wireless com-
munications.
The LT1533 is a complete current mode
switching regulator that includes an oscilla-
tor, error amplifier, protection circuitry and
two 1A power switches (Figure 2). By
employing a push-pull switching topology to
drive a center-tapped transformer, it signifi-
cantly reduces the RMS ripple current
observed at the converter’s input and output.
This topology and the ability to control
switching harmonics can greatly reduce and
even eliminate power supply shielding
requirements in sensitive applications.
Figure 2. The LT1533 as a 5V to 12V Push-Pull Converter for
Low EMI Applications
SHDN
DUTY
SYNC
COL A
COL B
PGND
R
VSL
R
CSL
FB
NFB
LT1533
GND
V
IN
5V
14
2
15
16
13
12
7
11
3
4
5
6
10
C
T
3300pF
R
T
,
18k
C
VC
0.01µF
R
VSL
,15k
Note 1
C
IN
4.7µF
R
CSL
,15k
+
T1* D
S1
1N4148 BA
L2**
100µH
10 : 36
L3**
100µH
D
S2
1N4148
R2
2.49k
1% *
**
NOTE1:
89
R1
21.5k
1%
C3
47µF
+
C4
47µF
COILTRONICS CTX02-13716-X1
COILTRONICS CTX100-3
25nH TRACE INDUCTANCE OR COILCRAFT B10T
12V
200mA
1533 TA01
+
C
T
R
T
V
C
10mV/DIV
10mV/DIV
LT1533 TYPICAL SWITCHERS
Figure 1. The LT1533 Produces Less Than 100
µ
V Peak-to-Peak Output Noise over
a 100MHz Bandwidth, Considerably Less Than Conventional Switchers
2
Linear Technology Chronicle •
October 1997
Hot Swap
Controller IC in SO-8 Permits
Safe PC Board Insertion and Removal
failures. All that is required is an external
N-channel MOSFET along with resistors
and capacitors to set the timing and reset
thresholds as shown in Figure 1. The
LTC1422 is flexible enough to cover supply
voltages ranging from 2.7V to 12V as well
as –48V applications.
Hot swap means plugging a component
into a socket that is already powered. The
inrush current required to charge up all of
the bypass capacitors can be several amps
which can damage the connector and other
in-line components as well as cause the sys-
tem supply to glitch. The LTC1422
eliminates glitches on the system supply and
allows the user to set the maximum current
limit. Similarly, a user-programmable
RESET threshold notifies the processor
when supply voltage drops below the
selected voltage.
The LTC1422 is available in the 8-pin
PDIP and SO packages. Parts are screened to
the commercial and industrial temperature
ranges and are available from stock. For a
data sheet and evaluation samples, contact
your local Linear Technology sales office or
visit our web site at www.linear-tech.com
for more information.
+
V
CC
SENSE GATE
LTC1422
GNDTIMER
ON
8
34
76
R2
10Ω
5%
R3
6.81k
1%
R4
2.43k
1%
R1
0.005ΩQ1
MTB56N06V
C1
0.1µF
C4
2200µF
V
CC
5V
5A
V
CC
ON/RESET
GND
1422 TA01
PLUG-IN CARDBACKPLANE
C2
0.33µF
52
1
FB
RESET
RESET
µP
CONNECTOR 1
CONNECTOR 2
Figure 1. The LTC1422 Ramps Power Supply Voltages in a Controlled
Fashion When a Board is Plugged or Unplugged into a Live Backplane
The LTC
®
1422 Hot Swap controller is
designed to provide fail-safe insertion and
removal of PC cards without interrupting
system operation. The device controls a
single supply voltage and limits potentially
damaging inrush currents when a circuit
board is plugged into a live backplane. It
also eliminates glitches that disrupt the sys-
tem bus when the board’s connector makes
or breaks contact, protecting against system
Triple Reset Generator for PCI Applications Offers
0.75% Threshold Accuracy
The LTC1536 is the industry’s first
micropower precision triple supply monitor
for multiple voltage systems that meets the
fast PCI timing specifications for reset. The
LTC1536 offers the ability to monitor 5V,
3.3V and a third voltage input that is adjust-
able down to 1V, with a 0.75% threshold
accuracy and glitch immunity. It draws only
100µA (typ) supply current and its RST out-
put is guaranteed to be in the correct state for
VCC5 or V CC3 down to 1V. These features
make the LTC1536 ideal for precision sys-
tem monitoring chores in PCI-based systems
such as desktop and notebook computers,
intelligent instruments and network servers
as shown in Figure 1. The LTC1536 is avail-
able in the 8-lead MSOP—two-thirds the
size of an SO-8.
PCI compliant means a RESET signal is
generated in less than 500ns if either power
rail falls ≥500mV below spec and ensures
reset operation will occur in less than 100ns
if the 5V supply falls below the 3.3V rail by
≥300mV (Figure 2). Add-in cards, in par-
ticular, require critical voltage monitoring
because the supplies may be removed inde-
pendently of any action occurring on the
motherboard or backplane. It is vital that the
processor be notified and the information
stored before the power is completely lost.
For non-PCI applications, the LTC1326
micropower triple supply monitor is also
available with a supply current of just 20µA
for power-conscious system designs. (See
May’97 Chronicle for more details). The
LTC1536 and LTC1326 both offer RST
(active HIGH reset output) and RST (active
Figure 1. The LTC1536 Triple Reset Monitor is Designed for
PCI Local Bus Applications with Multiple Supply Voltage
That Require Accurate Supply Monitoring
1
2
3
4
8
7
6
5
V
CC3
V
CC5
V
CC A
GND
PBR
SRST
RST
RST
LTC1536
PUSH-BUTTON
RESET
PCI
LOCAL
BUS
R
PU
SELECTED TO MEET RISE TIME 
SLEW RATE REQUIREMENTS (1k MIN)
5V ±5%
3.3V ±0.3V
0.1µF
R
PU
RST
1536 TA01
PWR GOOD
0.1µF
Continued on page 3
3
Linear Technology Chronicle •
October 1997
Application of the Month
A 4-Cell to 5V Power Supply
A 4-cell pack is a convenient, popular
battery size. Alkaline cells are sold in
4-packs at retail stores and four cells usu-
ally provide sufficient energy to keep
battery replacement frequency reasonable.
Generating 5V from four cells, however, is
a bit tricky. A fresh 4-cell pack has a termi-
nal voltage of 6.4V but at the end of its life,
the pack’s terminal voltage is around 3.2V;
hence, the DC/DC converter must step the
voltage either up or down, depending on the
state of the batteries.
A flyback topology with a costly, cus-
tom designed transformer could be
employed, but Figure 1’s circuit gets
around these problems by using a flying
capacitor scheme along with a second
inductor. The circuit also isolates the input
from the output, allowing the output to go
to 0V during shutdown. The circuit can be
divided conceptually into boost and buck
sections. L1 and the LT1300 switch com-
prise the boost or step-up section, and L2,
D1 and C3 comprise the buck or step-down
section. C2 is charged to VIN and acts as a
level shift between the two sections. The
switch node toggles between ground and
VIN + VOUT, and the L2-C2 diode node
toggles between –VIN and VOUT + VD.
Figure 2 shows efficiency versus load cur-
rent for the circuit. All four energy storage
elements must handle power, which
accounts for the lower efficiency of this
circuit compared to a simpler boost circuit.
NC
AN59 • F11
VIN
SW
LT1300
GND PGND
ILIM
SENSESHDN
C1**
100µF
4 ×
AA
CELLS
SHUTDOWN
L2*
27µH
C3**
100µF
5V OR
3.3V
220mA
L1*
27µH
C2**
100µF
1N5817
5V/3.3V
*L1, L2 = GOWANDA GA20-272K (716) 532-2234
**C1, C2, C3 = SANYO OS-CON 16SA100M (619) 661-6835
+
+
+SELECT
LTC1536 from page 2
The LTC1536 and LTC1326 are avail-
able from stock in 8-lead MSOP and SO
packages. Contact your local Linear
Technology sales office for a data sheet and
evaluation samples. Visit our web site at
www.linear-tech.com for more information.
LOW reset output) to accommodate proces-
sors with either requirement. Both have a
“soft” reset which signals the processor
when short duration glitches occur on the
supplies. Processors can use the soft reset to
detect the presence of transients while using
the “hard” reset to detect supply failures.
Both have a push-button input to manually
reset the system.
TIME (20ns/DIV)
VOLTAGE (1V/DIV)
5
4
3
2
1
0
1536 TA02
RST
V
CC5
V
CC5
= 5V TO 3V STEP
V
CC3
= V
CC A
= 3.3V
4.7k PULL-UP FROM
RST TO V
CC3
Figure 2. In the PCI Timing Specification,
Rev. 2.1, a RESET Signal Occurs When
the 5V Supply Falls Below the 3.3V
Supply by 300mV or More
LOAD CURRENT (mA)
1
64
EFFICIENCY (%)
84
10
AN59 • F12
100
78
76
74
72
70
68
66
80
82
V
IN
= 5V
V
IN
= 6V
V
IN
= 4V
V
IN
= 3V
Figure 2. Efficiency of Up/Down
Converter in Figure 1
Efficiency is directly related to the ESR
and DCR of the capacitors and inductors
used. Better capacitors cost more money.
Better inductors do not necessarily cost
more, but they do take up more space.
Worst-case RMS current through C2 occurs
at minimum input voltage and measures
0.4A at full load with a 3V input. C2’s
specified maximum RMS current must be
greater than this worst-case current. The
Sanyo capacitors shown in the schematic
specify a maximum ESR of 0.045Ω with a
maximum ripple current rating of 2.1A. The
Gowanda inductors specify a maximum
DCR of 0.058Ω.
Figure 1. 4-Cell to 3.3V or 5V Converter Output Goes to Zero When
in Shutdown. Inductors May Have, but Do Not Require Coupling; a
Transformer or Two Separate Units Can Be Used
4
Linear Technology Chronicle •
October 1997
© 1997 Linear Technology Corporation/Printed in USA
Linear Technology Corporation • 1630 McCarthy Blvd. • Milpitas, CA 95035-7417 • (408) 432-1900 • FAX: (408) 434-0507 • www.linear-tech.com • For Literature Only: 1-800-4-LINEAR
Linear Technology
Products Are
Distributed By:
Almac/Arrow
Arrow/Schweber
Arrow/Zeus
Digi-Key
Electrosonic
Gerber Electronics
Farnell Electronics
Marshall Industries
Phase 1
Push-Button DACs in
SO-8 Packages Tweak
Adjustable Regulators
in 4LSB steps using the CLK and DIN pin
(to set UP or DOWN). The LTC1426 volt-
age out dual DAC can be controlled using
one of two interface modes: push-button
and pulse (see Figure 2). A pulse mode in-
terface allows the user to increase or
decrease the output by simply sending a
single pulse to the device. It automatically
configures itself into the appropriate mode
at start-up by monitoring the state of the
CLK pins.
The LTC1329 and LTC1428 are
offered in an 8-lead SO package and the
LTC1426 is available in 8-lead MSOP and
SO packages. Parts are screened to the com-
mercial and industrial temperature ranges
and are available from stock. For data
sheets and evaluation samples of this series
of push-button DACs, contact your local
Linear Technology sales office or visit our
web site at www.linear-tech.com for more
information.
plies, in backlight brightness control and for
LCD contrast control.
The LTC1428-50 8-bit current sink
output DAC delivers 50µA ±3% full scale
and can be biased from 2V to 10V. Supply
current is 130µA over a supply range of
3V ≤ VCC ≤ 6.5V. Figure 1 shows it used as
a digitally controlled LCD bias generator.
The LTC1329-50 8-bit current source output
DAC has an output range of 0µA to 50µA
±3% (biased from –15V to 2V or –15V to
2.5V in 3.3V and 5V supply systems,
respectively) and draws a supply current of
95µA. The LTC1329A-50 is a precise (±1%
accuracy) current output DAC, designed to
source 50µA at full scale. For low supply
operation, the LTC1329-10, which sources
10µA at full scale, draws only 75µA supply
current. This part is particularly appropriate
for LCD display voltage bias applications.
The current output DACs (LTC1329-
10, LTC1329-50, LTC1329A-50 and
LTC1428-50) can communicate with exter-
nal circuitry by using one of three interface
modes: standard 3-wire serial mode or one
of two pulse modes. Pulse Mode 1: wire
interface uses the CLK signal to increment
the DAC in 4LSB steps to adjust the output
in a simple manner. Pulse Mode 2: wire in-
terface can increment or decrement the DAC
The LTC1329 and LTC1428 are new
precision current output DACs, designed to
sink or source up to 50µA of output current
at full scale with a guaranteed accuracy as
low as ±1%. These DACs are designed for
trim applications on power supplies—the
current range is ideal for driving the feed-
back pin on adjustable regulators instead of
using a digital potentiometer. Each has a
serial interface that can be used with a
simple push-button to trim the adjustment
point. For applications with PWM feedback
control, the LTC1426 dual 6-bit voltage out
DAC provides a PWM signal that swings
from 0V to VREF and allows the duty cycle
output to be varied by a simple push-button
interface. All these DACs (see Table Push-
Button DACs) have a shutdown mode that
drops the supply current to 0.2µA without
jeopardizing their contents. With push-but-
ton control, there is no need for a micro-
controller to create a serial data stream and
shift it into the device. They are ideal for
adjusting the voltage output of power sup-
1
2
3
4
8
7
6
5
CLK1
CLK2
GND
PWM1
SHDN
V
CC
V
REF
PWM2
LTC1426
PWM1
1426 F05
PWM2
PWM1/PWM2: 0V TO 0.985(V
REF
)
0.1µF
V
REF
0V TO 5.5V
V
CC
2.7V TO 5.5V
V
CC
2.7V TO 5.5V
SHDN
DOWNDOWN
UP UP
RR
LIMITING RESISTOR R PREVENTS SHORTING OF V
CC
AND GND WHEN
BOTH BUTTONS ARE SIMULTANEOUSLY PUSHED. THIS RESISTOR CAN
BE PLACED EITHER IN THE V
CC
OR GND LEG AND THIS DETERMINES
THE FUNCTION WHEN BOTH BUTTONS ARE PUSHED. VALUE OF R < 50k
Figure 2. Push-Button Mode Interface of
the LTC1426 6-Bit DAC
Compliance Supply
Part Number Resolution Type DAC Output Voltage (V) Voltage (V)
LTC1329-10 8-Bit Current Source 10µA ±3% –15V to 2.5V 2.7V ≤ V
CC
≤ 6.5V
LTC1329-50 8-Bit Current Source 50µA ±3% –15V to 2.5V 2.7V ≤ V
CC
≤ 6.5V
LTC1329A-50 8-Bit Current Source 50µA ±1% –15V to 2.5V 2.7V ≤ V
CC
≤ 6.5V
LTC1428-50 8-Bit Current Sink 150µA ±3% 2V to 10V 3V ≤ V
CC
≤ 6.5V
LTC1426 Dual 6-Bit PWM Voltage Out 0V to V
REF
– 2.7V ≤ V
CC
≤ 6.5V
Push-Button DACs
1
2
8
7
D
OUT
D
IN
I
OUT
V
CC
SHDN
CLK
LTC1428-50
GND
CS
1µF
2
CELLS
1428-50 TA01
V
OUT
15.75V TO 27.75V IN STEPS OF 47mV
15mA FROM 2 CELLS
R1
240k
3
4
6
5
C1
0.1µF
LT1307
L1
L1: 4.7µH MURATA-ERIE LQH3C
D1: MBR0530 OR 1N4148
D1
V
IN
FB
V
C
GND
SWSHDN SHDN R2
22k
R3
22k
100k
MPU
(e.g., 8051)
P1.3
P1.2
P1.1
P1.0
V
CC
5V
4700pF
Figure 1. LTC1428-50 8-Bit Current Output DAC Sinks up to 50µA with ±3%
Accuracy. It is Shown Here as a Digitally Controlled LCD Bias Generator