DN121 - New Micropower, Low Dropout Regulators Ease Battery Supply Designs

New Micropower, Low Dropout Regulators Ease Battery
Supply Designs – Design Note 121
Mitchell Lee and John Seago
Three new linear regulators simplify the design of
battery-operated equipment. The LT®1521 is a 300mA,
positive low dropout regulator with micropower quiescent current and shutdown. The LT1175 is a 500mA
negative complement with adjustable current limit. A
third product, the LT1118, has the unique capability of
maintaining output regulation while sourcing or sinking
load current.
The LT1521 contains all of the features associated
with battery-operated applications. In designs where
memory must be powered continuously, the LT1521’s
12μA quiescent current eliminates the need for a
separate micropower backup supply. In shutdown the
quiescent current drops to 6μA. Figure 1 shows an
example application using the LTC ®1477 as a means
of disconnecting all circuitry except for memory and
ON/OFF control logic. The LTC1477 protected high
side switch draws only 10nA in shutdown, eliminating
itself as well as its load as a factor in battery shelf life.
In battery-backed memory applications, the output of
the LT1521 can be held up by the backup battery while
in shutdown or even with the input power removed. No
series output diode is required as reverse current flow
is internally limited to about 5μA.
ON/OFF
LOGIC
MEMORY
8
1μF
NC
≥ 5.5V
IN
OUT
1
3
2
2
LT1521-5
5
SHDN SENSE
GND
3
1.5μF
VIN5
EN
4
LTC1477
1
VOUT
8
VIN1
VOUT
GND
5
μP, ETC
DN121 F01
Figure 1. The LT1521’s 12μA Standby Current Eliminates
the Need for a Separate Memory Backup Supply
01/96/121_conv
A common problem in portable equipment is the chance
of installing the batteries backwards, thereby destroying
the electronics contained within. The LT1521 needs no
reverse protection diode to guard against this condition,
as it is designed to withstand up to –20V input while
also protecting the load. Low dropout is preserved.
Most important for battery applications is low dropout, a characteristic not neglected in the design of the
LT1521. At 300mA the dropout is just 500mV, dropping
to approximately 290mV at a 50mA load. This enables
the LT1521 to maintain regulation while draining the
last drop of power from the battery.
Like the LT1521, the LT1175 also features low dropout,
running 500mV at 500mA load current. Quiescent current is 45μA dropping to 10μA in shutdown.
The LT1175 offers several unique features not available
in other negative regulators. First, the current limit is
adjustable by pin strapping to 200mA, 400mA, 600mA
or 800mA. This allows the current limit to be tailored to
suit normal load requirements while not exceeding the
maximum safe current drain from the battery during
a short-circuit fault.
Shutdown on a negative regulator could be a mixed
blessing, particularly if positive logic was used to control
a negative input. The LT1175 solves this problem by
accepting either positive or negative shutdown signals.
Holding the Shutdown pin within 0.8V of ground disables
the LT1175. If pulled to 2.5V or more positive or negative with respect to ground, the regulator is enabled.
If shutdown is not desired, either float or connect the
Shutdown pin to VIN and the regulator will be enabled
whenever power is applied.
The LT1521 and LT1175 work well together as the basis
for a split supply as shown in Figure 2. Low output
capacitance requirements allow the use of ceramic
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of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
IN
5V
300mA
OUT
LT1521-5
≥ 5.5V
1N4001
SENSE
SHDN GND
< 0.25V = OFF > 2.8V = ON
1.5μF
100nF
SHDN GND
SENSE
≥ 5.5V
1N4001
LT1175-5
IN
OUT
DN121 F02
– 5V
500mA
VO = 2V/DIV
Figure 2. In a Split Supply Application the
Shutdown Pins May Be Commanded in Parallel
Using Positive Logic
LT1521-5
0V
features the ability to both source and sink load current
(+800mA, –400mA), and exhibits an output impedance
of about 16μH across a wide range of frequencies. The
output remains stable irrespective of any bypass capacitance of 220nF or more. An output impedance of less
than 3Ω can be achieved across a 10MHz bandwidth
with the addition of a 1μF bypass; less than 1Ω with
a 10μF bypass.
The LT1118 is available in 5V, 2.85V and 2.5V versions. Where the 5V version might serve as a stand
alone regulator, the 2.5V version is a good choice for
splitting an existing 5V rail (see Figure 4). In addition
to greatly reducing power consumption, the DC output
impedance is less than 0.025Ω—unmatched by any
resistive divider solution. A separate Enable pin shuts
off the LT1118, reducing its supply current to 1μA. Figure 5 shows typical output impedance under a variety
of operating conditions.
LT1175-5
OUT
IN
TIME = 50μs/DIV
RL = 100Ω/SIDE
DN121 F03
≥ 5.5V
5V
1.5μF
LT1521-5
SHDN SENSE
Figure 3. Clean Start-Up and Shutdown is Assured by
Utilizing Ganged Shutdown Control of the LT1521 and
LT1175.
IN
GND
LT1118-2.5
OFF ON
EN
2.5V
ZO < 3Ω
DC to
1μF 10MHz
OUT
GND
Owing to the LT1175’s unique Shutdown pin, the Shutdown pins of both devices can be joined together as
shown and driven from a positive control logic signal.
Behavior of the outputs relative to shutdown control
is shown in Figure 3.
Although the LT1521-5 can tolerate up to –20V forced
output potential with respect to its input, supply reversal
diodes (1N4001) are often required to protect both linear
and digital load circuitry from damage under transient
start-up or fault conditions. The LT1175 is designed to
withstand up to +2V forced output voltage. For both
devices, start-up and recovery from short-circuit or
thermal shutdown is guaranteed under these conditions.
As anyone who has designed and built a “single supply” op amp circuit can attest, few can be implemented
without the use of a mid-supply bias point or resistive
divider providing that same function. The LT1118 serves
as a low power means of obtaining a regulated, low
dropout bias point for critical applications. This device
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DN121 F04
Figure 4. Splitting the Supply Saves Power and Holds
Bias Point DC Resistance to Less Than 0.025Ω
10
NO CURRENT
1
100mA SOURCE
10mA SOURCE
ZOUT (Ω)
units instead of larger, more expensive electrolytic or
tantalum capacitors.
0.1
0.01
100mA SINK
10mA SINK
0.001
102
103
104
105
FREQUENCY (Hz)
106
DN121 • F05
Figure 5. LT1118-2.5 Output Impedance vs Frequency
For applications help,
call (408) 432-1900
dn121f_conv LT/GP 0196 155K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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© LINEAR TECHNOLOGY CORPORATION 1996
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