LT1579
300mA Dual Input Smart
Battery Backup Regulator
FEATURES
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DESCRIPTION
The LT
®
1579 is a dual input, single output, low dropout
regulator. This device is designed to provide an
uninterruptible output voltage from two independent input
voltage sources on a priority basis. All of the circuitry
needed to switch smoothly and automatically between
inputs is incorporated.
The LT1579 can supply 300mA of output current from
either input at a dropout voltage of 0.4V. Quiescent current
is 50µA, dropping to 7µA in shutdown. Two comparators
are included to monitor input voltage status. Two addi-
tional status flags indicate which input is supplying power
and provide an early warning against loss of output
regulation when both inputs are low. A secondary select
pin is provided so that the user can force the device to
switch from the primary input to the secondary input.
Internal protection circuitry includes reverse-battery pro-
tection, current limiting, thermal limiting and reverse-
current protection.
The device is available in fixed output voltages of 3V, 3.3V
and 5V, and as an adjustable device with a 1.5V reference
voltage. The LT1579 regulators are available in narrow
16-lead SO and 16-lead SSOP packages with all features,
and in SO-8 with limited features.
Maintains Output Regulation with Dual Inputs
Dropout Voltage: 0.4V
Output Current: 300mA
50µA Quiescent Current
No Protection Diodes Needed
Two Low-Battery Comparators
Status Flags Aid Power Management
Adjustable Output from 1.5V to 20V
Fixed Output Voltages: 3V, 3.3V and 5V
7µA Quiescent Current in Shutdown
Reverse-Battery Protection
Reverse Current Protection
Remove, Recharge and Replace Batteries Without
Daisy-Chained Control Outputs
Loss of Regulation
APPLICATIONS
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Dual Battery Systems
Battery Backup Systems
Automatic Power Management for
Battery-Operated Systems
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATION
5V Dual Battery Supply
OUTPUT VOLTAGE (V) INPUT VOLTAGE (V)
Automatic Input Switching
+
5V
300mA
4.7µF
12
10
8
6
4
2
0
5.05
5.00
4.95
0
2
4
6
8 10 12 14 16 18 20
TIME (ms)
1578 TA02
+
1µF
IN1
2.7M
LBI1
1M
LT1579-5
OUT
V
IN1
I
IN1
SS
+
1µF
IN2
2.7M
LBI2
1M
SHDN
LBO1
LB02
BACKUP
DROPOUT
BIASCOMP
GND
0.01µF
1579 TA01
TO
POWER
MANAGEMENT
U
U
U
SWITCHOVER
POINT
V
IN2
= 10V
I
LOAD
= 50mA
80
I
IN2
60
40
20
0
INPUT CURRENT (mA)
1
LT1579
ABSOLUTE
MAXIMUM
RATINGS
Power Input Pin Voltage ......................................
±20V*
Output Pin Voltage
Fixed Devices............................................. 6.5V, – 6V
Adjustable Device ............................................
±20V*
Output Pin Reverse Current .................................... 5mA
ADJ Pin Voltage .............................................. 2V, – 0.6V
ADJ Pin Current ...................................................... 5mA
Control Input Pin Voltage ............................ 6.5V, – 0.6V
Control Input Pin Current ....................................... 5mA
BIASCOMP Pin Voltage ............................... 6.5V, – 0.6V
BIASCOMP Pin Current .......................................... 5mA
Logic Flag Output Voltage ............................ 6.5V, – 0.6V
Logic Flag Input Current ......................................... 5mA
Output Short-Circuit Duration .......................... Indefinite
Storage Temperature Range ................. – 65°C to 150°C
Operating Junction Temperature Range .... 0°C to 125°C
Lead Temperature (Soldering, 10 sec).................. 300°C
*For applications requiring input voltage ratings greater than 20V,
consult factory.
PACKAGE/ORDER I FOR ATIO
TOP VIEW
GND 1
POWER
INPUTS
VIN1 2
VIN2 3
SS 4
CONTROL SHDN 5
INPUTS
LBI1 6
LBI2 7
GND 8
16 GND
15 OUT
14 BACKUP
13 DROPOUT
12 LBO1
11 LBO2
10 BIASCOMP
9
GND
LOGIC
OUTPUTS
ORDER PART
NUMBER
LT1579CGN-3
LT1579CGN-3.3
LT1579CGN-5
LT1579CS-3
LT1579CS-3.3
LT1579CS-5
GN PART MARKING
15793
157933
15795
ORDER PART
NUMBER
8
7
6
5
OUT
BACKUP
DROPOUT
BIASCOMP
LOGIC
OUTPUTS
GN PACKAGE
S PACKAGE
16-LEAD PLASTIC SSOP 16-LEAD PLASTIC SO
SEE APPLICATION INFORMATION SECTION
T
JMAX
= 125°C,
θ
JA
= 95°C/W (GN)
T
JMAX
= 125°C,
θ
JA
= 68°C/W (S)
TOP VIEW
POWER
INPUTS
V
IN1
1
V
IN2
2
CONTROL
SHDN 3
INPUT
GND 4
LT1579CS8-3
LT1579CS8-3.3
LT1579CS8-5
S8 PART MARKING
15793
157933
15795
S8 PACKAGE
8-LEAD PLASTIC SO
SEE APPLICATION INFORMATION SECTION
T
JMAX
= 125°C,
θ
JA
= 90°C/W
Consult factory for Industrial and Military grade parts.
2
U
U
W
W W
U
W
TOP VIEW
GND 1
POWER
INPUTS
VIN1 2
VIN2 3
SS 4
CONTROL
INPUTS
SHDN 5
LBI1 6
LBI2 7
GND 8
16 GND
15 OUT
14 ADJ
13 BACKUP
12 LBO1
11 LBO2
10 BIASCOMP
9
GND
LOGIC
OUTPUTS
ORDER PART
NUMBER
LT1579CGN
LT1579CS
GN PACKAGE
S PACKAGE
16-LEAD PLASTIC SSOP 16-LEAD PLASTIC SO
SEE APPLICATION INFORMATION SECTION
T
JMAX
= 125°C,
θ
JA
= 95°C/W (GN)
T
JMAX
= 125°C,
θ
JA
= 68°C/W (S)
GN PART MARKING
1579
TOP VIEW
POWER
INPUTS
V
IN1
1
V
IN2
2
8
7
6
5
OUT
ADJ
LOGIC
BACKUP OUTPUT
BIASCOMP
ORDER PART
NUMBER
LT1579CS8
CONTROL
SHDN 3
INPUT
GND 4
S8 PACKAGE
8-LEAD PLASTIC SO
S8 PART MARKING
1579
SEE APPLICATION INFORMATION SECTION
T
JMAX
= 125°C,
θ
JA
= 90°C/W
LT1579
ELECTRICAL CHARACTERISTICS
PARAMETER
Regulated Output
Voltage (Note 1)
CONDITIONS
LT1579-3
V
IN1
= V
IN2
= 3.5V, I
LOAD
= 1mA, T
J
= 25°C
4V < V
IN1
< 20V, 4V < V
IN2
< 20V, 1mA < I
LOAD
< 300mA
q
q
q
q
q
q
q
q
q
MIN
2.950
2.900
3.250
3.200
4.925
4.850
1.475
1.450
TYP
3.000
3.000
3.300
3.300
5.000
5.000
1.500
1.500
1.5
1.5
1.5
1.5
3
3
MAX
3.050
3.100
3.350
3.400
5.075
5.150
1.525
1.550
10
10
10
10
12
25
12
25
15
35
10
20
0.28
0.39
0.35
0.45
0.47
0.60
0.60
0.75
100
400
200
500
1.5
4
12
7.0
7.0
2.8
5
12
12
UNITS
V
V
V
V
V
V
V
V
mV
mV
mV
mV
mV
mV
mV
mV
mV
mV
mV
mV
V
V
V
V
V
V
V
V
µA
µA
µA
µA
mA
mA
mA
µA
µA
V
V
µA
µA
µA
µA
LT1579-3.3 V
IN1
= V
IN2
= 3.8V, I
LOAD
= 1mA, T
J
= 25°C
4.3V < V
IN1
< 20V, 4.3V < V
IN2
< 20V, 1mA < I
LOAD
< 300mA
LT1579-5
Adjust Pin Voltage
Line Regulation
LT1579
LT1579-3
LT1579-5
LT1579
Load Regulation
LT1579-3
V
IN1
= V
IN2
= 5.5V, I
LOAD
= 1mA, T
J
= 25°C
6V < V
IN1
< 20V, 6V < V
IN2
< 20V, 1mA < I
LOAD
< 300mA
V
IN1
= V
IN2
= 3.2V, I
LOAD
= 1mA, T
J
= 25°C (Note 2)
3.7V < V
IN1
< 20V, 3.7V < V
IN2
< 20V, 1mA < I
LOAD
< 300mA
∆V
IN1
= 3.5V to 20V,
∆V
IN2
= 3.5V to 20V, I
LOAD
= 1mA
∆V
IN1
= 5.5V to 20V,
∆V
IN2
= 5.5V to 20V, I
LOAD
= 1mA
∆V
IN1
= 3.2V to 20V,
∆V
IN2
= 3.2V to 20V, I
LOAD
= 1mA (Note 2)
V
IN1
= V
IN2
= 4V,
∆I
LOAD
= 1mA to 300mA, T
J
= 25°C
V
IN1
= V
IN2
= 4V,
∆I
LOAD
= 1mA to 300mA
LT1579-3.3
∆V
IN1
= 3.8V to 20V,
∆V
IN2
= 3.8V to 20V, I
LOAD
= 1mA
LT1579-3.3 V
IN1
= V
IN2
= 4.3V,
∆I
LOAD
= 1mA to 300mA, T
J
= 25°C
V
IN1
= V
IN2
= 4.3V,
∆I
LOAD
= 1mA to 300mA
LT1579-5
LT1579
Dropout Voltage
(Notes 3, 4)
V
IN1
= V
IN2
=
V
OUT(NOMINAL)
V
IN1
= V
IN2
= 6V,
∆I
LOAD
= 1mA to 300mA, T
J
= 25°C
V
IN1
= V
IN2
= 6V,
∆I
LOAD
= 1mA to 300mA
V
IN1
= V
IN2
= 3.7V,
∆I
LOAD
= 1mA to 300mA, T
J
= 25°C (Note 2)
V
IN1
= V
IN2
= 3.7V,
∆I
LOAD
= 1mA to 300mA
q
5
q
2
q
I
LOAD
= 10mA, T
J
= 25°C
I
LOAD
= 10mA
I
LOAD
= 50mA, T
J
= 25°C
I
LOAD
= 50mA
I
LOAD
= 150mA, T
J
= 25°C
I
LOAD
= 150mA
I
LOAD
= 300mA, T
J
= 25°C
I
LOAD
= 300mA
0.10
q
0.18
q
0.25
q
0.34
q
Ground Pin Current
(Note 5)
V
IN1
= V
IN2
=
V
OUT(NOMINAL)
+ 1V
I
LOAD
= 0mA, T
J
= 25°C
I
LOAD
= 0mA
I
LOAD
= 1mA, T
J
= 25°C
I
LOAD
= 1mA
I
LOAD
= 50mA
I
LOAD
= 150mA
I
LOAD
= 300mA
50
q
100
q
q
q
q
q
q
q
q
q
q
q
0.7
2
5.8
3.3
2.0
0.25
0.9
0.75
1.3
5
5
3
Standby Current
(Note 6) I
LOAD
= 0mA
Shutdown Threshold
Shutdown Pin Current
(Note 7)
Quiescent Current in
Shutdown (Note 9)
I
VIN2
: V
IN1
= 20V, V
IN2
= V
OUT(NOMINAL)
+ 0.5V, V
SS
= Open (HI)
I
VIN1
: V
IN1
= V
OUT(NOMINAL)
+ 0.5V, V
IN2
= 20V, V
SS
= 0V
V
OUT
= Off to On
V
OUT
= On to Off
V
SHDN
= 0V
I
VIN1
: V
IN1
= 20V, V
IN2
= 6V, V
SHDN
= 0V
I
VIN2
: V
IN1
= 6V, V
IN2
= 20V, V
SHDN
= 0V
I
SRC
: V
IN1
= V
IN2
= 20V, V
SHDN
= 0V
3
LT1579
ELECTRICAL CHARACTERISTICS
PARAMETER
Adjust Pin Bias Current
(Notes 2, 7)
Minimum Input Voltage
(Note 8)
Minimum Load Current
Secondary Select
Threshold
Secondary Select Pin
Current (Note 7)
Low-Battery Trip Threshold
Low-Battery Comparator
Hysteresis
Low-Battery Comparator
Bias Current (Notes 7, 10)
Logic Flag Output Voltage
Ripple Rejection
Current Limit
Input Reverse Leakage
Current
Reverse Output Current
CONDITIONS
T
J
= 25°C
I
LOAD
= 0mA
LT1579
V
IN1
= V
IN2
= 3.2V
q
q
q
q
q
q
q
MIN
TYP
6
2.7
MAX
30
3.2
3
UNITS
nA
V
µA
V
V
µA
V
mV
nA
V
V
dB
mA
Switch from V
IN2
to V
IN1
Switch from V
IN1
to V
IN2
V
SS
= 0V
V
IN1
= V
IN2
= V
OUT(NOMINAL)
+ 1V, High-to-Low Transition
V
IN1
= V
IN2
= 6V, I
LBO
= 20µA (Note 11)
V
IN1
= V
IN2
= 6V, V
LBI
= 1.4V, T
J
= 25°C
I
SINK
= 20µA
I
SINK
= 5mA
V
IN1
– V
OUT
= V
IN2
– V
OUT
= 1.2V (Avg), V
RIPPLE
= 0.5V
P-P
f
RIPPLE
= 120Hz, I
LOAD
= 150mA
V
IN1
= V
IN2
= V
OUT(NOMINAL)
+ 1V,
∆V
OUT
= – 0.1V
V
IN1
= V
IN2
= –20V, V
OUT
= 0V
LT1579-3 V
OUT
= 3V, V
IN1
= V
IN2
= 0V
LT1579-3.3 V
OUT
= 3.3V, V
IN1
= V
IN2
= 0V
LT1579-5 V
OUT
= 5V, V
IN1
= V
IN2
= 0V
0.25
1.2
0.75
1
2.8
1.5
1.550
30
5
0.45
1.3
1.440
1.500
18
2
q
q
0.17
0.97
55
70
400
q
q
320
1.0
3
3
3
12
12
12
mA
µA
µA
µA
The
q
denotes specifications which apply over the full operating
temperature range.
Note 1:
Operating conditions are limited by maximum junction
temperature. The regulated output voltage specification will not apply for
all possible combinations of input voltage and output current. When
operating at maximum input voltage, output current must be limited.
When operating at maximum output current, the input voltage range must
be limited.
Note 2:
The LT1579 (adjustable version) is tested and specified with the
adjust pin connected to the output pin and a 3µA DC load.
Note 3:
Dropout voltage is the minimum input-to-output voltage
differential required to maintain regulation at the specified output current.
In dropout, the output voltage will be equal to V
IN
– V
DROPOUT
.
Note 4:
To meet the requirements for minimum input voltage, the LT1579
(adjustable version) is connected with an external resistor divider for a
3.3V output voltage (see curve of Minimum Input Voltage vs Temperature
in the Typical Performance Characteristics). For this configuration,
V
OUT(NOMINAL)
= 3.3V.
Note 5:
Ground pin current will rise at T
J
> 75°C. This is due to internal
circuitry designed to compensate for leakage currents in the output
transistor at high temperatures. This allows quiescent current to be
minimized at lower temperatures, yet maintain output regulation at high
temperatures with light loads. See the curve of Quiescent Current vs
Temperature in the Typical Performance Characteristics.
Note 6:
Standby current is the minimum quiescent current for a given
input while the other input supplies the load and bias currents.
Note 7:
Current flow is out of the pin.
Note 8:
Minimum input voltage is the voltage required on either input to
maintain the 1.5V reference for the error amplifier and low-battery
comparators.
Note 9:
Total quiescent current in shutdown will be approximately equal to
I
VIN1
+ I
VIN2
– I
SRC
. Both I
VIN1
and I
VIN2
are specified for worst-case
conditions. I
VIN1
is specified under the condition that V
IN1
> V
IN2
and I
VIN2
is specified under the condition that V
IN2
> V
IN1
. I
SRC
is drawn from the
highest input voltage only. For normal operating conditions, the quiescent
current of the input with the lowest input voltage will be equal to the
specified quiescent current minus I
SRC
. For example, if V
IN1
= 20V, V
IN2
=
6V then I
VIN1
= 5µA and I
VIN2
= 5µA – 3µA = 2µA.
Note 10:
The specification applies to both inputs independently
(LBI1, LBI2).
Note 11:
Low-battery comparator hysteresis will change as a function of
current in the low-battery comparator output. See the curve of Low-Battery
Comparator Hysteresis vs Sink Current in the Typical Performance
Characteristics.
4
LT1579
TYPICAL PERFORMANCE CHARACTERISTICS
Guaranteed Dropout Voltage
0.8
0.7
= TEST POINTS
T
J
≤
125°C
DROPOUT VOLTAGE (V)
DROPOUT VOLTAGE (V)
0.6
0.5
T
J
= 25°C
0.4
0.3
0.2
0.1
0
0
50
100
150
200
250
OUTPUT CURRENT (mA)
300
0.5
0.4
0.3
0.2
0.1
QUIESCENT CURRENT (µA)
Quiescent Current in Shutdown
7
6
V
IN1
= 20V
V
IN2
= 6V
V
SHDN
= 0V
3.08
3.06
QUIESCENT CURRENT (µA)
5
4
I
VIN1
3
2
1
0
– 50 – 25
I
VIN2
OUTPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
50
25
75
0
TEMPERATURE (°C)
LT1579-5 Output Voltage
5.12
I
LOAD
= 1mA
5.09
ADJUST PIN VOLTAGE (V)
OUTPUT VOLTAGE (V)
INPUT CURRENT (µA)
5.06
5.03
5.00
4.97
4.94
4.91
4.88
– 50 – 25
0
50
75
25
TEMPERATURE (°C)
100
125
U W
1579 G35
Dropout Voltage
0.7
0.6
100
Quiescent Current
A
90
80
70
60
50
40
30
20
10
STANDBY
QUIESCENT
CURRENT
50
25
0
75
TEMPERATURE (°C)
100
125
OPERATING
QUIESCENT
CURRENT
V
IN
= 6V
R
L
=
∞
(FIXED)
R
L
= 500k (ADJUSTABLE)
A: I
LOAD
= 300mA
B: I
LOAD
= 150mA
C: I
LOAD
= 100mA
D: I
LOAD
= 50mA
E: I
LOAD
= 10mA
F: I
LOAD
= 1mA
B
C
D
E
F
0
– 50 – 25
50
25
75
0
TEMPERATURE (°C)
100
125
0
– 50 –25
1579 G01
1579 G02
LT1579-3 Output Voltage
3.38
I
LOAD
= 1mA
LT1579-3.3 Output Voltage
I
LOAD
= 1mA
3.36
3.34
3.32
3.30
2.28
2.26
2.24
3.04
3.02
3.00
2.98
2.96
2.94
100
125
2.92
– 50 – 25
0
75
50
25
TEMPERATURE (°C)
100
125
2.22
– 50 – 25
0
50
75
25
TEMPERATURE (°C)
100
125
1579 G36
1579 G03
1579 G04
Adjust Pin Voltage
1.54
I
LOAD
= 1mA
1.53
1.52
1.51
1.50
1.49
1.48
1.47
1.46
– 50 – 25
0
50
75
25
TEMPERATURE (°C)
100
125
Input Current
60
50
40
30
20
10
0
–0.2 –0.1 0
I
IN2
I
IN1
V
OUT
= 5V
V
IN2
= 6V
I
LOAD
= 0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
V
IN1
– V
OUT
(V)
1579 G07
1579 G05
1579 G05
5
Analogue and Mixed Signal
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