May 2004 Dual Regulators Offer Flexibility with Independent Shutdown Control and Adjustable Start-Up Timing

DESIGN IDEAS
Dual Regulators Offer Flexibility
with Independent Shutdown Control
and Adjustable Start-Up Timing
by Todd Owen
Introduction
DESIGN IDEAS
Dual Regulators Offer Flexibility
with Independent Shutdown Control
and Adjustable Start-Up Timing .. 29
Todd Owen
4A, 4MHz Monolithic Synchronous
Regulator with Tracking
offers a Compact Solution
for Power Supply Sequencing ...... 31
Joey M. Esteves
Synchronous Boost Converter
with Output Disconnect
Delivers 4W from Two Cells ......... 32
Dongyan Zhou
Boost Converter
Drives 1A White LEDs .................. 33
Keith Szolusha
Smart Card
Interfaces Made Easy .................. 34
Steven Martin
LCD Power Supply
Provides ±15V Plus LED Driver..... 35
Mike Shriver
Linear Technology Magazine • May 2004
VIN
3.7V TO 20V
IN
OUT1
0.01µF
1µF
BYP1
10µF
3.3V
AT 100mA
422k
ADJ1
249k
LT3023
OFF ON
SHDN1
OUT2
0.01µF
SHDN2
GND
10µF
2.5V
AT 100mA
261k
BYP2
ADJ2
249k
Figure 1. Noise bypassing slows start-up, allows outputs to track
100mA regulator. Both regulators operate over an input voltage range of
1.8V to 20V with a dropout of 300mV
at full load current. Quiescent current
is less than 30µA for each regulator,
dropping to less than 0.1µA in shutdown. Individual shutdown controls
for each regulator allow for flexibility
in power management. Both devices
are available as adjustable parts with
a 1.22V reference.
The small size of these regulators
simplifies system design. The LT3023
is packaged in the 3mm × 3mm 10-lead
DFN, maintaining the same footprint
as a SOT-23. The LT3023 is also
available in the thermally enhanced
10-lead MSOP package. The LT3024
is offered in the 4mm × 3mm 12-lead
DFN, with a footprint only 33% larger
than a SOT-23, and also in the thermally enhanced 16-lead TSSOP. These
regulators also help minimize external
component size. The 100mA regulators
are stable with output capacitors as
low as 1µF; the 500mA regulator in
the LT3024 requires a minimum of
3.3µF. Small ceramic capacitors can
be used without the series resistance
required by other regulators.
Tracking Supplies
Adding external 0.01µF bypass capacitors to the LT3023 or LT3024 devices
drops output voltage noise for each
regulator to 20µVRMS over a 10Hz to
100kHz bandwidth. This capacitor
improves transient performance of
the regulators and also slows startup of the regulator. Figure 1 shows
100
VOUT1,
VOUT2
100mV/
DIV
START-UP TIME (ms)
Desktop computers to digital cameras
demand more from their power supplies than ever before. Some devices
require more than seven supplies, often complicated further by a unique
set of vital conditions and specifications for power supply start-up timing,
tracking and voltage differentials. In
many cases, the power supplies must
start up in specific order, and track
each other in concert, to avoid the risk
of damage to critical components that
run from the multiple supply rails.
To help meet these conditions,
Linear Technology introduces the
LT3023 and LT3024. Both parts
are dual low dropout, low noise,
micropower regulators based on the
LT1761 and LT1763, single regulators
delivering 100mA and 500mA respectively. The LT3023 combines a pair of
100mA regulators while the LT3024
combines a 500mA regulator with a
IOUT1
50mA/
DIV
IOUT2
50mA/
DIV
2ms/DIV
Figure 2. Output voltages track
independent of load
10
1
0.1
10
100
1000
10000
CBYP (pF)
Figure 3. Start-up time
29
DESIGN IDEAS
VIN
3.7V TO 20V
1µF
IN
3.3V
AT
500mA
OUT1
10µF
0.01µF
LT3024
BYP1
422k
35.7k
249k
28k
ADJ1
VOUT1 –
VOUT4
500mV/
DIV
OFF ON
SHDN1
OUT2
SHDN2
BYP2
10µF
0.01µF
GND
0.47µF
2.5V
AT
100mA
261k
ADJ2
249k
2ms/DIV
Figure 4. Multiple parts provide
consistent start-up
Figure 5. Start-up sequencing
an application that takes advantage
of this slowed start-up in a soft-start
circuit.
In this circuit, two different supply
rails are generated by an LT3023. Both
the SHDN1 and SHDN2 pins are tied
together, driving the regulators simultaneously. As the two regulators are
brought out of shutdown, their output
voltages rise at the same rates. The
rate at which the output voltages rise
is independent of load current—the
regulators can deliver up to the full
rated output current at the intermediate voltages. The size of the output
capacitor also drops out of the equation
when its charging current added to the
load current is less than the regulator current limit. Figure 2 shows the
output voltages and currents of the
regulators as they are brought out of
shutdown.
Figure 3 shows the time for the
regulators to start as the value of the
noise bypass capacitor varies. Minimum time for start-up is 150µs with
no bypass capacitor. Start-up time is
roughly proportional to the size of the
noise bypass capacitor, with 0.01µF
of capacitance giving a time of 15ms.
Two more supply rails are provided
by an LT3024: a 1.5V rail at 500mA,
and a 1.8V rail at 100mA. As shown in
Figure 4, start-up times are consistent
between the two regulators.
Flexibility is an important feature
of this circuit. The regulators can be
operated with differing sizes of noise
capacitor to slew one regulator on
faster and the SHDN1 and SHDN2
pins can also be separated as needed
for independent shutdown control.
Since these regulators are based on
the LT1761/LT1763, the same design
techniques and characteristics apply
to those parts. Supply rails can be
generated in any number, not just
even multiples.
Start-Up Sequencing
Figure 5 shows an LT3024 being
used to sequence the start-up of the
regulators. In this circuit, the 500mA
regulator is turned on and begins to
rise at the rate determined by the noise
bypass capacitor. As the output lifts,
it begins to pull up the SHDN2 pin to
turn on the 100mA side. The 0.47µF
capacitor slows the rise of this pin,
keeping it from turning on until several milliseconds after the 500mA side
begins turning on (see Figure 6).
When the circuit is turned off, the
Schottky diode between SHDN1 and
SHDN2 allows both outputs to be
shutdown simultaneously. This is a
precaution to prevent voltage differences between OUT1 and OUT2 that
may cause application problems or
damage. Figure 7 shows both outputs
turning off together. The resistor divider between OUT1 and SHDN2 is
designed to account for the threshold
voltage of the SHDN2 pin and the current of this pin as well (typically 1µA
at 0.8V, maximum 3µA at 1.4V).
Conclusion
The LT3023 and LT3024 are dual high
performance regulators available in
tiny packages. Both offer independent
channel shutdown control and adjustable start-up timing. These features
offer a high degree of flexibility that
makes it easy to meet demanding
system requirements.
VSHDN1
1V/DIV
VSHDN1
1V/DIV
VOUT1
1V/DIV
VOUT2
1V/DIV
VOUT1
1V/DIV
VOUT2
1V/DIV
2ms/DIV
Figure 6. Turn-on waveforms
2ms/DIV
Figure 7. Turn-off waveforms
To view this and past issues of LT Magazine online, see
http://www.linear.com/go/ltmag
30
Linear Technology Magazine • May 2004