Dec 2002 Ideal Diode Controller Eliminates Energy Wasting Diodes in Power OR-ing Applications

LINEAR TECHNOLOGY
VOLUME XII NUMBER 4
DECEMBER 2002
COVER ARTICLE
Ideal Diode Controller Eliminates
Energy Wasting Diodes in Power
OR-ing Applications ...................... 1
David Laude
Issue Highlights ............................ 2
LTC® in the News ........................... 2
DESIGN FEATURES
High Performance 400MHz
Quadrature IF Demodulator Runs
from 1.8V Supply .......................... 5
Min Zou and Vladimir Dvorkin
New Power for Ethernet—Powered
Devices (Part 2 of a 3-Part Series) .. 9
Dave Dwelley
2500V/µs Slew Rate Op Amps Process
Large Signals with Low Distortion at
High Frequencies ........................ 11
Kris Lokere and Glen Brisebois
SOT-23 Digitally Controlled Amp Puts
Programmable Gain Anywhere .... 16
Max W. Hauser
Programmable Quad Supervisors Offer
Unparalleled Flexibility for MultiVoltage Monitoring Applications
................................................... 19
Bob Jurgilewicz
Tiny 1.25MHz Monolithic Boost
Regulator Has 1.5A Switch and
Wide Input Voltage Range ........... 25
Keith Szolusha
Secondary Side Synchronous Post
Regulator Provides Precision
Regulation and High Efficiency for
Multiple Output Isolated Power
Supplies ...................................... 29
Charlie Y. Zhao, Wei Chen and Chiawei Liao
DESIGN INFORMATION
A 14-Bit ADC that is Both Fast and
Low Noise .................................... 33
Richard Reay and Dave Thomas
DESIGN IDEAS
.............................................. 34–36
(complete list on page 34)
New Device Cameos ..................... 37
Design Tools ................................ 39
Sales Offices ............................... 40
Ideal Diode Controller
Eliminates Energy
Wasting Diodes in Power
OR-ing Applications
by David Laude
Introduction
Many modern electronic devices need
a means to automatically and
smoothly switch between power
sources when prompted by the insertion or removal of any source. The
LTC4412 permits low loss OR-ing of
multiple power sources for extended
battery life and low self-heating. The
LTC4412 controls external P-channel MOSFET power switches to create
a near ideal diode function for power
switchover applications or load
sharing PowerPath™ management applications. When conducting, the
voltage drop across the MOSFET is
typically only 20mV. It also provides
power monitoring circuitry and external control for integration with
other parts of the power management
system. The low component count
results in a low overall system cost
and, with its ThinSOT™ 6-pin package, a compact design solution. It’s
versatile enough to be used in a variety of diode OR-ing applications.
For battery powered applications
that also can be powered from a wall
adapter or other “auxiliary” power
source, the load is automatically disconnected from the battery when the
auxiliary source is connected, so that
no current is drawn from the battery.
When the auxiliary source is disconnected, operation reverts back to the
1
CONSTANT
RON
LTC4412
CURRENT (A)
IN THIS ISSUE…
CONSTANT
VOLTAGE
SCHOTTKY
DIODE
0
0.02
0.5
FORWARD VOLTAGE (V)
4412 F01b
Figure 1. LTC4412 Ideal diode controller
vs Schottky diode characteristics
battery. An AC adapter present signal
is available. The LTC4412 also has
built in reverse supply protection.
Multiple LTC4412s can be ganged
together to provide load sharing between multiple batteries, or to allow
multiple batteries to be charged from
a single battery charger. The precisely controlled ideal diode behavior
of the LTC4412 is crucial to maintain
current balance when multiple batteries are discharged or charged while
connected to a single load or source,
The advantages of the LTC4412
ideal diode PowerPath controller are
shown in Figure 1. The forward voltage drop of the ideal diode is far less
than that of a conventional diode and
the reverse current leakage can be
continued on page 3
, LTC, LT, Burst Mode, OPTI-LOOP, Over-The-Top and PolyPhase are registered trademarks of Linear Technology
Corporation. Adaptive Power, C-Load, DirectSense, FilterCAD, Hot Swap, LinearView, Micropower SwitcherCAD,
Multimode Dimming, No Latency ∆Σ, No Latency Delta-Sigma, No RSENSE, Operational Filter, PowerPath, PowerSOT,
SoftSpan, SwitcherCAD, ThinSOT and UltraFast are trademarks of Linear Technology Corporation. Other product
names may be trademarks of the companies that manufacture the products.
DESIGN FEATURES
LT1970, continued from page 1
smaller for the ideal diode as well. The
tiny forward voltage drop lowers power
losses and self-heating, resulting in
extended battery life. The very low
reverse leakage, when compared to
Schottky diodes, is also beneficial in
some applications.
The wide supply operating range of
2.5V to 28V supports operation with
one to six Li-Ion batteries in series.
The types of power sources that can
be used include all those that are
within the supply operating range.
The low quiescent current of 11µA
with a 3.6V supply is independent of
the load current. The LTC4412 also
features a status pin that can be used
to enable an auxiliary MOSFET power
switch for additional power savings
when an auxiliary input is utilized. It
may also be used to indicate to a
microcontroller that an auxiliary supply, such as a wall adapter, is present.
A control input pin is provided to
extend applications to those that can
benefit from external control, such as
from a microcontroller.
Applications include anything that
must take power from several inputs,
including cellular phones, portable
computers, PDAs, MP3 players and
electronic video and still cameras,
USB peripherals, wire-ORed multipowered equipment, uninterruptible
power supplies for alarm and emergency systems, systems with standby
capabilities, systems that use load
sharing between two or more batteries, multi-battery chargers, and logic
controlled power switches.
1N5819
WALL
ADAPTER
INPUT
BATTERY
CELL(S)
*
TO LOAD
LTC4412
6
VIN SENSE
5
2
GND GATE
4
3
CTL STAT
1
Linear Technology Magazine • December 2002
Q1
VCC
470k
4412 F01
STATUS OUTPUT LOW
WHEN WALL ADAPTER IS
SUPPLYING LOAD CURRENT
*PARASITIC DRAIN-SOURCE DIODE OF MOSFET
Q1: FAIRCHILD SEMICONDUCTOR FDN306P (408) 822-2126
Figure 2. Automatic power switching between a battery and a wall adapter
is absent. At about 0.6V the drainsource diode of the P-channel
MOSFET transistor begins to forward
bias and pulls the output up (time
B1). Once the primary input reaches
a voltage sufficient to drive the MOSFET gate and power the LTC4412, the
forward regulation mode is achieved
(B2). Now the output voltage is regulated to typically 20mV (10mV min)
“The forward voltage drop
of the ideal diode is far less
than that of a conventional
diode…”
below the input. If the load current
varies, the GATE pin voltage is controlled to maintain 20mV unless the
load current exceeds the P-channel
MOSFET’s ability to deliver the current with a 20mV VDS. If the RON is not
low enough to maintain forward regulation then the gate voltage reaches
ground or clamps 7V below the higher
of the voltages on the VIN or SENSE
B1 B2
pins. Once clamped the MOSFET behaves as a constant low value resistor,
and the forward voltage increases
slightly. During this forward regulation mode of operation the STAT pin is
an open circuit and the 470k resistor
pulls the voltage up to the VCC supply,
which can be as high as 28V.
When a wall adapter or other supply connected to the auxiliary input is
applied the SENSE pin voltage rises.
The battery voltage also rises slightly
from being unloaded. As the SENSE
voltage pulls above VIN – 20mV the
LTC4412 pulls the GATE voltage up
to turn off the P-channel MOSFET
(A1). When the voltage on SENSE
exceeds VIN + 20mV the STAT pin
sinks 10µA of current to indicate that
an AC wall adapter is present. The
system is now in the reverse turn off
mode. Power to the load is being delivered through the external diode and
no current is drawn from the battery.
Should the primary input now be
removed there is no effect (A2). When
the primary input is applied and the
B3
A1
A2
5
How It Works
Figure 2 shows a circuit that automatically switches the power supply
between a battery and a wall adapter
(or other types of power inputs). The
supply inputs are slowly ramped, as
seen in Figure 3, to illustrate operation of the circuit. For the sake of this
discussion the load is purely resistive, and the terms primary and
auxiliary are arbitrary and interchangeable.
First the battery primary input,
which powers the VIN pin, is ramped
up from 0V while the auxiliary input
COUT
PRIMARY INPUT (V)
0
0
7.5
AUXILIARY INPUT (V)
0
7
4.98
6.9
OUTPUT TO LOAD (V)
0
3
GATE (V)
STATUS (V)
0
VCC
0
0.2
Figure 3. Operation waveforms
3
DESIGN FEATURES
Q2
*
WALL
ADAPTER
INPUT
Q1
*
TO LOAD
BATTERY
CELL(S)
COUT
LTC4412
6
VIN SENSE
5
2
GND GATE
4
3
CTL STAT
1
470k
4412 F02
*PARASITIC DRAIN-SOURCE DIODE OF MOSFET
STATUS OUTPUT
DROPS WHEN A
WALL ADAPTER
IS PRESENT AND
SUPPLYING THE
LOAD CURRENT
Q1, Q2: SILICONIX Si4953DY (800) 554-5565
Figure 4. Lowest loss automatic power switching between a battery and a wall adapter
auxiliary input is removed the circuit
reverts to battery operation (B3).
The external diode is used to protect against auxiliary input faults. A
silicon diode could be used instead of
the Schottky, but will result in higher
power dissipation and heating due to
the higher forward voltage drop. The
Lowest Loss Automatic
PowerPath Control
1N5819
WALL
ADAPTER
INPUT
Q1
*
TO LOAD
BAT1
COUT
LTC4412
6
VIN SENSE
5
2
GND GATE
4
3
CTL STAT
1
VCC
470k
STATUS IS HIGH
WHEN BAT1 IS
SUPPLYING
LOAD CURRENT
WHEN BOTH STATUS LINES ARE
HIGH, THEN BOTH BATTERIES ARE
SUPPLYING LOAD CURRENTS. WHEN
BOTH STATUS LINES ARE LOW, THEN
WALL ADAPTER IS PRESENT AND
SUPPLYING FULL LOAD CURRENT
Q2
*
BAT2
LTC4412
6
VIN SENSE
5
2
GND GATE
4
3
CTL STAT
1
VCC
470k
4412 F05
*PARASITIC DRAIN-SOURCE DIODE OF MOSFET
STATUS IS HIGH
WHEN BAT2 IS
SUPPLYING
LOAD CURRENT
Q1, Q2: SILICONIX Si4953DY (800) 554-5565
Figure 5. Dual battery load sharing with automatic
switchover of power from batteries to wall adapter
Q1
*
BATTERY
CHARGER
INPUT
TO LOAD OR
PowerPath
BAT1 CONTROLLER
LTC4412
6
1
VIN SENSE
5
2
GND GATE
4
3
CTL STAT
VCC
470k
STATUS IS HIGH
WHEN BAT1 IS
CHARGING
Q2
*
LTC4412
6
VIN SENSE
5
2
GND GATE
4
3
CTL STAT
1
TO LOAD OR
PowerPath
BAT2 CONTROLLER
VCC
470k
STATUS IS HIGH
WHEN BAT2 IS
CHARGING
*PARASITIC DRAIN-SOURCE DIODE OF MOSFET
4412 F06
Q1, Q2: SILICONIX Si4953DY (800) 554-5565
Figure 6. Automatic dual battery charging from single charging source
4
wall adapter voltage must be high
enough to overcome the diodes forward voltage drop. Note that the
external MOSFET is wired so that the
drain to source diode will reverse bias
when a wall adapter input is applied.
If the control input (CTL pin) is asserted high the GATE voltage is forced
to the higher of the voltages on the VIN
pin or the SENSE pin turning the
MOSFET off. Also the STAT pin will
sink 10µA of current if connected.
This feature is useful for forced switching of the load between two power
sources. The drain to source diode
inherent in MOSFETs require that
series back to back MOSFETs, with
gates tied together, be used to fully
isolate a power source.
Figure 4 illustrates an application
circuit for automatic switchover of
load between a battery and a wall
adapter that features lower power
loss in the auxiliary path than the
circuit of Figure 2. Operation is similar to Figure 2 except that a P-channel
MOSFET (Q2) replaces the diode. The
STAT pin is used to turn on Q2 once
the SENSE pin voltage exceeds the
battery voltage by 20mV. When the
wall adapter input is applied the drainsource diode of Q2 turns on first to
pull up the SENSE pin and load voltages, followed by Q2’s gate turning
on. Once Q2 has turned on, the voltage drop across it can be very low
depending on the MOSFET’s characteristics.
Load Sharing
Figure 5 illustrates an application
circuit for dual battery load sharing
with automatic switchover of power
between batteries and a wall adapter.
Whichever battery can supply the
higher voltage provides the load current until it is discharged to the voltage
of the other battery. Then, the load is
shared between the two batteries with
the higher capacity battery providing
proportionally higher current to the
load. When a wall adapter input is
applied both MOSFETs turn off and
continued on page 15
Linear Technology Magazine • December 2002
DESIGN FEATURES
fore obtained in the lowest gain configurations. The 2500V/µs slew rate
specified on the data sheet is measured in a noninverting unity gain
configuration. The 1800V/µs production tested slew rate is measured in
an (inverting) gain of –1, which is
equivalent to a noninverting gain of 2.
The internal current generated
across the input resistor can be much
higher than the quiescent supply current (up to 80mA). In normal transient
closed loop operation this does not
present a problem, since after a few
nanoseconds the feedback brings the
differential input signal back to zero.
However, sustained (i.e. open loop)
differential input voltages may result
in excessive power dissipation and
therefore this amplifier should not be
used as a comparator.
The output stage buffers the high
impedance node from the load by
providing current gain. Emitter followers Q17–Q20 provide a current
gain equal to BetaNPN × BetaPNP, but
the effective current gain is greatly
enhanced by the dynamic base current compensation provided by
Q24–Q26 and Q21–Q23. Q24 measures a fraction of the output current
that flows through Q19, and mirror
Q25–Q26 injects the appropriate current back into the base of Q19. This
signal-dependent boost improves the
linearity of the amplifier by reducing
the amount of differential input signal required for a given output current.
An additional advantage is that the
output devices can be smaller, which
requires less quiescent current for a
given amplifier speed.
LTC4412, continued from page 4
High Side Power Switch
voltage, therefore denying power to
the load. The MOSFET is connected
with its source connected to the power
source. This prevents the drain-source
diode from supplying voltage to the
load when the MOSFET is off.
no load current is drawn from the
batteries. The STAT pins provide information as to which input is
supplying the load current. This concept can be applied to as many power
inputs as are needed.
Multiple Battery Charging
Figure 6 shows an application circuit
for automatically charging two batteries from a single charger. Whichever
battery has the lower voltage receives
the charging current until both battery voltages are equal then both will
be charged. When both are charged
simultaneously the higher capacity
battery receives proportionally higher
current from the charger. For Li-Ion
batteries both batteries achieve the
float voltage of the battery charger
minus the forward regulation voltage
of 20mV. This concept can apply to
more than two batteries. The STAT
pins provide information as to which
batteries are being charged.
Figure 7 illustrates an application
circuit for a logic controlled high side
power switch using the control input
pin. When the CTL pin is a logical low
the LTC4412 turns on the MOSFET.
Because the SENSE pin is grounded
the LTC4412’s internal controller
functions as an open-loop comparator and applies maximum gate drive
voltage to the MOSFET. When the
CTL pin is a logical high the LTC4412
turns off the MOSFET by pulling its
gate voltage up to the supply input
Q1
*
SUPPLY
INPUT
LOGIC
INPUT
TO LOAD
LTC4412
6
1
VIN SENSE
5
2
GND GATE
4
3
CTL STAT
COUT
Conclusion
The ultrafast slew rate and high bandwidth allow the LT1818 and LT1819
op amps to process large signals at
high frequencies with low distortion.
Combined with the low noise and
moderate supply current, these amplifiers are a good choice for receivers,
filters, or drivers of cables and ADCs
in high-speed communication or data
acquisition systems.
Conclusion
The LTC4412 provides a simple and
efficient way to implement a low loss
ideal diode controller that extends
battery life and significantly reduces
self-heating. The low external parts
count translates directly to low overall system cost and its ThinSOT 6-pin
package makes for compact design
solutions. It’s versatile enough to be
used in a variety of diode OR-ing
applications covering a wide range of
supply voltages.
4412 F07
*PARASITIC DRAIN-SOURCE DIODE OF MOSFET
Q1: FAIRCHILD SEMICONDUCTOR FDN306P (408) 822-2126
Figure 7. Logic controlled
high side power switch
for
the latest information
on LTC products,
visit
www.linear.com
Linear Technology Magazine • December 2002
15
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