April 2010 - Dual 500mA µPower LDO Features Independent 1.8V–20V Inputs and Easy Sequencing in a 4mm × 3mm DFN

design ideas
Dual 500mA µPower LDO Features Independent 1.8V–20V
Inputs and Easy Sequencing in a 4mm × 3mm DFN
Molly Zhu
The LT3029 integrates
two independent 500mA
monolithic LDOs in a
tiny 16-lead MSOP or
4mm × 3mm × 0.75mm DFN
package. Both regulators
have a wide 1.8V to 20V
input voltage range with
a 300mV dropout voltage
at full load. The output
voltage is adjustable down
to the 1.215V reference
voltage. With an external
bypass capacitor, the
output voltage noise is less
than 20µVRMS. A complete
power supply requires only
a minimum 3.3µF ceramic
output capacitor for each
channel to be stable.
Quiescent current is 55µA per channel, dropping below 1µA in shutdown.
Reverse-battery protection, reversecurrent protection, current limit foldback and thermal shutdown are all
Figure 1. Completely independent
channels have separate input and
SHDN pins.
VIN1
5V
IN1
1µF
OUT1
LT3029
10nF
BYP1
ADJ1
VIN2
2.5V
IN2
ON
OFF
SHDN1
OUT2
ON
OFF
SHDN2
BYP2
ADJ2
10nF
GND
The LT3029 includes features that simplify
the design of multivoltage systems. Its two
independent regulators present separate input and shutdown pins. It is also
compatible with the LTC2921, LTC2922 and
LTC2923 power supply tracking controllers, allowing for easy multirail power
supply tracking and sequencing design.
TWO INDEPENDENT REGULATORS
The LT3029’s inputs can be used independently or combined. Figure 1 shows
an application generating two output voltages from two different input
voltages, with independent shutdown control for each channel.
10µF
237k
1%
1µF
integrated into the package, making it
ideal for battery-powered systems.
402k
1%
VOUT1
3.3V
500mA
113k
1%
10µF
VOUT2
1.8V
500mA
237k
1%
DIFFERENT START-UP SLEW RATES
Start-up time is roughly proportional
to the bypass capacitance, regardless
of the input and output voltage. The
output capacitance and the load characteristics also have no influence on
the result. Figure 2 shows the regulator
start-up time versus bypass capacitance.
The LT3029 is compatible with LTC292x
series of power supply sequencing and
tracking controllers. Its ADJ pin should be
connected to LTC292x FB pin. By choosing the right resistors, it can track or
sequence the power supply. Please refer
to the LTC2923 data sheet for details.
(continued on page 35)
Table 1. Comparison between dual channel LDOs
V IN RANGE (V)
I OUT (mA)
DROPOUT VOLTAGE @ I OUT (mV)
INDEPENDENT V IN
I Q /CHANNEL (µA)
DFN PACKAGE SIZE
LT3023
1.8~20
100/100
300/300
N
20
3mm × 3mm
LT3024
1.8~20
100/500
300/300
N
30
4mm × 3mm
LT3027
1.8~20
100/100
300/320
Y
25
3mm × 3mm
LT3028
1.8~20
100/500
300/300
Y
30
5mm × 3mm
LT3029
1.8~20
500/500
300/300
Y
55
4mm × 3mm
April 2010 : LT Journal of Analog Innovation | 33
product briefs
The LTC3108 is a highly integrated
DC/DC converter ideal for harvesting and managing surplus energy from
extremely low input voltage sources
such as TEG (thermoelectric generators),
thermopiles and small solar cells. The
step-up topology operates from input
voltages as low as 20mV. Using a small
step-up transformer, the LTC3108 provides
a complete power management solution
for wireless sensing and data acquisition.
The 2.2V LDO powers an external microprocessor, while the main output is
programmed to one of four fixed voltages
to power a wireless transmitter or sensors. The power good indicator signals
that the main output voltage is within
regulation. A second output can be
(LTC3855 continued from page 29)
The MOSFET drivers and control circuits
are powered by INTVCC, which by default
is powered through an internal low
dropout regulator from the main input
supply, VIN. If lower power dissipation
in the IC is desired, a 5V supply can be
connected to EXTVCC. When a supply is
detected on EXTVCC, the LTC3855 switches
INTVCC over to EXTVCC, with a drop of
just 50mV. The strong gate drivers with
optimized dead time provide high efficiency. The full load efficiency is 86.7%
and the peak efficiency is 89.4% (Figure 4).
The LTC3855 features a RUN and TRACK/SS pin
for each channel. RUN enables the output
and INTVCC, while TRACK/SS acts as a softstart or allows the outputs to track an
external reference. If a multiphase output is desired, all RUN and TRACK/SS pins
are typically tied to one another.
Peak current limiting is used in this
application, with the peak sense voltage set by the three-state ILIM pin. A
high speed rail-to-rail differential current
enabled by the host. A storage capacitor
provides power when the input voltage source is unavailable. Extremely low
quiescent current and high efficiency
design ensure the fastest possible charge
times of the output reservoir capacitor.
POWERFUL SYNCHRONOUS
N-CHANNEL MOSFET DRIVER IN A
2MM × 3MM DFN
The LTC4449 is high speed synchronous MOSFET driver designed to
maximize efficiency and extend the
operating voltage range in a wide variety of DC/DC converter topologies,
from buck to boost to buck-boost.
The LTC4449’s rail-to-rail driver outputs operate over a range of 4V to
6.5V and can sink up to 4.5A and source
up to 3.2A of current, allowing it to
easily drive high gate capacitance and/
or multiple MOSFETs in parallel for high
sense comparator looks across the current sense element (here the inductor’s
DC resistance, implied from the associated R-C network). If a short circuit
occurs, current limit foldback reduces
the peak current to protect the power
components. Foldback is disabled during start-up, for predictable tracking.
CONCLUSION
The LTC3855 is ideal for converters using
inductor DCR sensing to provide high current outputs. Its temperature compensation and remote output voltage sensing
ensure predictable behavior from light
load to high current. From inputs up to
38V it can regulate two separate outputs
from 0.6V to 12.5V, and can be configured
for higher currents by tying its channels
together, or by paralleling additional
LTC3855 power stages. At low duty cycles,
the short minimum on-time ensures constant frequency operation, and peak current limit remains constant even as duty
cycle changes. The LTC3855 incorporates
these features and more into 6mm × 6mm
QFN or 38-lead TSSOP packages. n
current applications. The high side driver
can withstand voltages up to 38V.
Adaptive shoot-through protection circuitry is integrated to prevent
MOSFET cross-conduction current.
With 14ns propagation delays and
4ns to 8ns transition times driving 3nF loads, the LTC4449 minimizes
power loss due to switching losses and
dead time body diode conduction.
The LTC4449 features a three-state
PWM input for power stage control and
shutdown that is compatible with all controllers that employ a three-state output
feature. The LTC4449 also has a separate
supply input for the input logic to match
the signal swing of the controller IC.
Undervoltage lockout detectors monitor
both the driver and logic supplies and disable operation if the voltage is too low. n
(LT3029 continued from page 35)
CONCLUSION
The LT3029 is a dual 500mA/500mA monolithic LDO with a wide input voltage range
and low noise. The two channels are fully
independent, allowing for flexible power
management. It is ideal for battery-powered systems because of its low quiescent
current, small package and integration of battery protection features. n
100
START-UP TIME (ms)
ULTRALOW VOLTAGE STEP-UP
CONVERTER AND POWER MANAGER
FOR ENERGY HARVESTING
10
1
0.1
0.01
10
1k
100
BYPASS CAPACITANCE (pF)
10k
Figure 2. Start-up time vs bypass capacitor value
April 2010 : LT Journal of Analog Innovation | 35
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