LT1579
300mA Dual Input Smart
Battery Backup Regulator
U
DESCRIPTION
FEATURES
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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
Loss of RegulationDaisy-Chained Control Outputs
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APPLICATIONS
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Dual Battery Systems
Battery Backup Systems
Automatic Power Management for
Battery-Operated Systems
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 additional 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 protection, current limiting, thermal limiting and reversecurrent 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.
, LTC and LT are registered trademarks of Linear Technology Corporation.
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TYPICAL APPLICATION
5V Dual Battery Supply
2.7M
+
5V
300mA
4.7µF
LBI1
1M
LT1579-5
SS
SHDN
IN2
+
1µF
LBO1
2.7M
LB02
LBI2
1M
TO
POWER
MANAGEMENT
BACKUP
DROPOUT
BIASCOMP
GND
0.01µF
1579 TA01
VIN1
10
SWITCHOVER
POINT
8
IIN1
6
VIN2 = 10V
ILOAD = 50mA
80
IIN2
60
4
40
2
20
0
0
INPUT CURRENT (mA)
1µF
OUT
OUTPUT VOLTAGE (V) INPUT VOLTAGE (V)
IN1
+
Automatic Input Switching
12
5.05
5.00
4.95
0
2
4
6
8 10 12 14 16 18 20
TIME (ms)
1578 TA02
1
LT1579
W W
U
W
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.
U
W
U
PACKAGE/ORDER I FOR ATIO
ORDER PART
NUMBER
TOP VIEW
POWER
INPUTS
GND 1
16 GND
VIN1 2
15 OUT
VIN2 3
14 BACKUP
13 DROPOUT
SS 4
12 LBO1
CONTROL SHDN 5
INPUTS
LBI1 6
LOGIC
OUTPUTS
11 LBO2
LBI2 7
10 BIASCOMP
GND 8
9
GND
GN PACKAGE
S PACKAGE
16-LEAD PLASTIC SSOP 16-LEAD PLASTIC SO
SEE APPLICATION INFORMATION SECTION
TJMAX = 125°C, θJA = 95°C/W (GN)
TJMAX = 125°C, θJA = 68°C/W (S)
LT1579CGN-3
LT1579CGN-3.3
LT1579CGN-5
LT1579CS-3
LT1579CS-3.3
LT1579CS-5
GN PART MARKING
15793
157933
15795
POWER
INPUTS
VIN1 1
8
OUT
VIN2 2
7
BACKUP
CONTROL
SHDN 3
INPUT
GND 4
6
DROPOUT
5
BIASCOMP
LOGIC
OUTPUTS
S8 PACKAGE
8-LEAD PLASTIC SO
SEE APPLICATION INFORMATION SECTION
TJMAX = 125°C, θJA = 90°C/W
LT1579CS8-3
LT1579CS8-3.3
LT1579CS8-5
S8 PART MARKING
15793
157933
15795
Consult factory for Industrial and Military grade parts.
2
GND 1
16 GND
POWER
INPUTS
VIN1 2
15 OUT
VIN2 3
14 ADJ
CONTROL
INPUTS
SHDN 5
12 LBO1
LBI1 6
11 LBO2
LBI2 7
10 BIASCOMP
GND 8
9
LT1579CGN
LT1579CS
13 BACKUP
SS 4
LOGIC
OUTPUTS
GND
GN PACKAGE
S PACKAGE
16-LEAD PLASTIC SSOP 16-LEAD PLASTIC SO
SEE APPLICATION INFORMATION SECTION
TJMAX = 125°C, θJA = 95°C/W (GN)
TJMAX = 125°C, θJA = 68°C/W (S)
ORDER PART
NUMBER
TOP VIEW
ORDER PART
NUMBER
TOP VIEW
ORDER PART
NUMBER
TOP VIEW
POWER
INPUTS
VIN1 1
8
OUT
VIN2 2
7
ADJ
6
LOGIC
BACKUP OUTPUT
5
BIASCOMP
CONTROL
SHDN 3
INPUT
GND 4
GN PART MARKING
1579
S8 PACKAGE
8-LEAD PLASTIC SO
SEE APPLICATION INFORMATION SECTION
TJMAX = 125°C, θJA = 90°C/W
LT1579CS8
S8 PART MARKING
1579
LT1579
ELECTRICAL CHARACTERISTICS
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Regulated Output
Voltage (Note 1)
LT1579-3
●
2.950
2.900
3.000
3.000
3.050
3.100
V
V
●
3.250
3.200
3.300
3.300
3.350
3.400
V
V
VIN1 = VIN2 = 5.5V, ILOAD = 1mA, TJ = 25°C
6V < VIN1 < 20V, 6V < VIN2 < 20V, 1mA < ILOAD < 300mA
●
4.925
4.850
5.000
5.000
5.075
5.150
V
V
VIN1 = VIN2 = 3.2V, ILOAD = 1mA, TJ = 25°C (Note 2)
3.7V < VIN1 < 20V, 3.7V < VIN2 < 20V, 1mA < ILOAD < 300mA
●
1.475
1.450
1.500
1.500
1.525
1.550
V
V
∆VIN1 = 3.5V to 20V, ∆VIN2 = 3.5V to 20V, ILOAD = 1mA
●
1.5
10
mV
LT1579-3.3 ∆VIN1 = 3.8V to 20V, ∆VIN2 = 3.8V to 20V, ILOAD = 1mA
LT1579-5 ∆VIN1 = 5.5V to 20V, ∆VIN2 = 5.5V to 20V, ILOAD = 1mA
●
1.5
10
mV
VIN1 = VIN2 = 3.5V, ILOAD = 1mA, TJ = 25°C
4V < VIN1 < 20V, 4V < VIN2 < 20V, 1mA < ILOAD < 300mA
LT1579-3.3 VIN1 = VIN2 = 3.8V, ILOAD = 1mA, TJ = 25°C
4.3V < VIN1 < 20V, 4.3V < VIN2 < 20V, 1mA < ILOAD < 300mA
LT1579-5
Adjust Pin Voltage
Line Regulation
Load Regulation
LT1579
LT1579-3
●
1.5
10
mV
LT1579
∆VIN1 = 3.2V to 20V, ∆VIN2 = 3.2V to 20V, ILOAD = 1mA (Note 2)
●
1.5
10
mV
LT1579-3
VIN1 = VIN2 = 4V, ∆ILOAD = 1mA to 300mA, TJ = 25°C
VIN1 = VIN2 = 4V, ∆ILOAD = 1mA to 300mA
3
●
12
25
mV
mV
3
12
25
mV
mV
5
15
35
mV
mV
2
10
20
mV
mV
0.10
0.28
0.39
V
V
0.18
0.35
0.45
V
V
0.25
0.47
0.60
V
V
0.34
0.60
0.75
V
V
50
100
400
µA
µA
100
200
500
µA
µA
LT1579-3.3 VIN1 = VIN2 = 4.3V, ∆ILOAD = 1mA to 300mA, TJ = 25°C
VIN1 = VIN2 = 4.3V, ∆ILOAD = 1mA to 300mA
LT1579-5
LT1579
Dropout Voltage
(Notes 3, 4)
VIN1 = VIN2 =
VOUT(NOMINAL)
●
VIN1 = VIN2 = 6V, ∆ILOAD = 1mA to 300mA, TJ = 25°C
VIN1 = VIN2 = 6V, ∆ILOAD = 1mA to 300mA
●
VIN1 = VIN2 = 3.7V, ∆ILOAD = 1mA to 300mA, TJ = 25°C (Note 2)
VIN1 = VIN2 = 3.7V, ∆ILOAD = 1mA to 300mA
●
ILOAD = 10mA, TJ = 25°C
ILOAD = 10mA
●
ILOAD = 50mA, TJ = 25°C
ILOAD = 50mA
●
ILOAD = 150mA, TJ = 25°C
ILOAD = 150mA
●
ILOAD = 300mA, TJ = 25°C
ILOAD = 300mA
●
ILOAD = 0mA, TJ = 25°C
ILOAD = 0mA
●
ILOAD = 1mA, TJ = 25°C
ILOAD = 1mA
●
ILOAD = 50mA
●
0.7
1.5
mA
ILOAD = 150mA
●
2
4
mA
ILOAD = 300mA
●
5.8
12
mA
Standby Current
(Note 6) ILOAD = 0mA
IVIN2: VIN1 = 20V, VIN2 = VOUT(NOMINAL) + 0.5V, VSS = Open (HI)
IVIN1: VIN1 = VOUT(NOMINAL) + 0.5V, VIN2 = 20V, VSS = 0V
●
●
3.3
2.0
7.0
7.0
µA
µA
Shutdown Threshold
VOUT = Off to On
VOUT = On to Off
●
●
0.9
0.75
2.8
V
V
Shutdown Pin Current
(Note 7)
VSHDN = 0V
●
1.3
5
µA
Quiescent Current in
Shutdown (Note 9)
IVIN1: VIN1 = 20V, VIN2 = 6V, VSHDN = 0V
IVIN2: VIN1 = 6V, VIN2 = 20V, VSHDN = 0V
ISRC: VIN1 = VIN2 = 20V, VSHDN = 0V
●
●
5
5
3
12
12
µA
µA
µA
Ground Pin Current
(Note 5)
VIN1 = VIN2 =
VOUT(NOMINAL) + 1V
0.25
3
LT1579
ELECTRICAL CHARACTERISTICS
PARAMETER
CONDITIONS
Adjust Pin Bias Current
(Notes 2, 7)
TJ = 25°C
Minimum Input Voltage
(Note 8)
ILOAD = 0mA
MIN
●
Minimum Load Current
LT1579
Secondary Select
Threshold
Switch from VIN2 to VIN1
Switch from VIN1 to VIN2
VIN1 = VIN2 = 3.2V
●
●
Secondary Select Pin
Current (Note 7)
VSS = 0V
●
Low-Battery Trip Threshold
VIN1 = VIN2 = VOUT(NOMINAL) + 1V, High-to-Low Transition
●
Low-Battery Comparator
Hysteresis
VIN1 = VIN2 = 6V, ILBO = 20µA (Note 11)
●
Low-Battery Comparator
Bias Current (Notes 7, 10)
VIN1 = VIN2 = 6V, VLBI = 1.4V, TJ = 25°C
Logic Flag Output Voltage
ISINK = 20µA
ISINK = 5mA
Ripple Rejection
VIN1 – VOUT = VIN2 – VOUT = 1.2V (Avg), VRIPPLE = 0.5VP-P
fRIPPLE = 120Hz, ILOAD = 150mA
Current Limit
VIN1 = VIN2 = VOUT(NOMINAL) + 1V, ∆VOUT = – 0.1V
●
Input Reverse Leakage
Current
VIN1 = VIN2 = –20V, VOUT = 0V
●
Reverse Output Current
LT1579-3 VOUT = 3V, VIN1 = VIN2 = 0V
LT1579-3.3 VOUT = 3.3V, VIN1 = VIN2 = 0V
LT1579-5 VOUT = 5V, VIN1 = VIN2 = 0V
The ● 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 VIN – VDROPOUT.
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,
VOUT(NOMINAL) = 3.3V.
Note 5: Ground pin current will rise at TJ > 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.
4
TYP
MAX
6
30
nA
2.7
3.2
V
3
µA
1.2
0.75
2.8
V
V
1
1.5
µA
1.500
1.550
V
18
30
mV
2
5
nA
0.17
0.97
0.45
1.3
V
V
●
0.25
1.440
●
●
UNITS
55
70
dB
320
400
mA
3
3
3
1.0
mA
12
12
12
µA
µA
µA
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
IVIN1 + IVIN2 – ISRC. Both IVIN1 and IVIN2 are specified for worst-case
conditions. IVIN1 is specified under the condition that VIN1 > VIN2 and IVIN2
is specified under the condition that VIN2 > VIN1. ISRC 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 ISRC. For example, if VIN1 = 20V, VIN2 =
6V then IVIN1 = 5µA and IVIN2 = 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.
LT1579
U W
TYPICAL PERFORMANCE CHARACTERISTICS
= TEST POINTS
0.7
0.6
TJ ≤ 125°C
0.6
DROPOUT VOLTAGE (V)
DROPOUT VOLTAGE (V)
Dropout Voltage
0.7
0.5
TJ = 25°C
0.4
0.3
0.2
A: ILOAD = 300mA
B: ILOAD = 150mA
C: ILOAD = 100mA
D: ILOAD = 50mA
E: ILOAD = 10mA
F: ILOAD = 1mA
C
0.3
D
0.2
E
0
50
100
150
200
250
OUTPUT CURRENT (mA)
0
– 50 – 25
300
50
25
75
0
TEMPERATURE (°C)
VIN1 = 20V
VIN2 = 6V
VSHDN = 0V
40
30
2
IVIN2
1
100
ILOAD = 1mA
3.36
3.04
3.34
3.02
3.00
2.98
2.96
75
50
25
TEMPERATURE (°C)
0
100
2.28
2.26
1.53
4.94
1.52
1.51
1.50
1.49
1.48
100
125
1579 G05
1.46
– 50 – 25
40
IIN1
VOUT = 5V
VIN2 = 6V
ILOAD = 0
30
20
10
1.47
4.91
125
IIN2
50
INPUT CURRENT (µA)
ADJUST PIN VOLTAGE (V)
4.97
100
Input Current
60
ILOAD = 1mA
ILOAD = 1mA
5.09
5.00
50
75
25
TEMPERATURE (°C)
1579 G04
Adjust Pin Voltage
5.03
0
1579 G03
5.12
50
75
25
TEMPERATURE (°C)
3.30
2.22
– 50 – 25
125
1.54
0
3.32
2.24
1579 G36
LT1579-5 Output Voltage
125
LT1579-3.3 Output Voltage
3.06
2.92
– 50 – 25
125
5.06
100
3.38
2.94
50
25
75
0
TEMPERATURE (°C)
50
25
0
75
TEMPERATURE (°C)
1579 G02
OUTPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
IVIN1
3
STANDBY
QUIESCENT
CURRENT
20
ILOAD = 1mA
4
4.88
– 50 – 25
50
LT1579-3 Output Voltage
5
OPERATING
QUIESCENT
CURRENT
60
0
– 50 –25
125
3.08
0
– 50 – 25
70
1579 G01
Quiescent Current in Shutdown
QUIESCENT CURRENT (µA)
100
80
VIN = 6V
RL = ∞ (FIXED)
RL = 500k (ADJUSTABLE)
10
F
7
OUTPUT VOLTAGE (V)
B
0.4
1579 G35
6
90
A
0.1
0.1
0
0.5
Quiescent Current
100
QUIESCENT CURRENT (µA)
Guaranteed Dropout Voltage
0.8
0
50
75
25
TEMPERATURE (°C)
100
125
1579 G05
0
–0.2 –0.1 0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
VIN1 – VOUT (V)
1579 G07
5
LT1579
U W
TYPICAL PERFORMANCE CHARACTERISTICS
Input and Ground Pin Current
Input and Ground Pin Current
1.2
250
200
0.6
150
IGND
100
0.2
10
INPUT CURRENT (mA)
VOUT = 5V
VIN2 = 6V
ILOAD = 1mA
0.8
0.4
50
0
–0.2 –0.1 0
600
IIN1
IIN2
IIN1
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
VIN1 – VOUT (V)
VOUT = 5V
VIN2 = 6V
ILOAD = 10mA
8
IGND
6
4
200
2
100
0
–0.2 –0.1 0
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
VIN1 – VOUT (V)
1579 G09
Input and Ground Pin Current
Input and Ground Pin Current
60
100
0.6
20
0.4
10
0.2
0
–0.2 –0.1 0
INPUT CURRENT (mA)
VOUT = 5V
VIN2 = 6V
ILOAD = 50mA
GROUND PIN CURRENT (mA)
INPUT CURRENT (mA)
1.0
3.0
IIN2
IIN1
80
2.0
60
IGND
1.5
40
1.0
20
0.5
0
–0.2 –0.1 0
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
VIN1 – VOUT (V)
2.5
VOUT = 5V
VIN2 = 6V
ILOAD = 100mA
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
VIN1 – VOUT (V)
1579 G10
1579 G11
Input and Ground Pin Current
Input and Ground Pin Current
160
4.0
IIN1
3.5
3.0
100
IGND
2.5
80
VOUT = 5V
VIN2 = 6V
ILOAD = 150mA
2.0
60
40
1.0
20
0.5
0
–0.2 –0.1 0
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
VIN1 – VOUT (V)
1579 G12
6
1.5
300
12
250
10
IGND
200
150
100
8
6
VOUT = 5V
VIN2 = 6V
ILOAD = 300mA
4
50
0
–0.2 –0.1 0
2
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
VIN1 – VOUT (V)
1579 G13
GROUND PIN CURRENT (mA)
120
14
IIN1
IIN2
INPUT CURRENT (mA)
IIN2
350
GROUND PIN CURRENT (mA)
INPUT CURRENT (mA)
140
GROUND PIN CURRENT (mA)
120
0.8
IGND
30
1.2
IIN1
50
40
400
300
1579 G08
IIN2
500
GROUND PIN CURRENT (µA)
INPUT CURRENT (mA)
IIN2
12
GROUND PIN CURRENT (µA)
1.0
300
LT1579
U W
TYPICAL PERFORMANCE CHARACTERISTICS
Ground Pin Current
Minimum Input Voltage
VIN1 = VIN2 = VOUT(NOMINAL) + 1V
MINIMUM INPUT VOLTAGE (V)
GROUND PIN CURRENT (mA)
ILOAD = 1mA
2.9
7
6
5
4
3
2
1
SHUTDOWN PIN THRESHOLD (V)
8
0
Shutdown Pin Threshold
1.0
3.0
2.8
2.7
2.6
2.5
2.4
2.3
2.2
0
50
100
200
250
150
OUTPUT CURRENT (mA)
2.0
– 50 –25
300
50
25
0
75
TEMPERATURE (°C)
100
SECONDARY SELECT PIN THRESHOLD (V)
2.0
1.5
1.0
0.5
0
– 50 – 25
50
0
75
25
TEMPERATURE (°C)
100
1.8
1.4
1.2
1.0
0.8
0.6
0.4
0.2
50
25
0
75
TEMPERATURE (°C)
100
1.8
1.4
1.0
0.8
0.6
0.4
0.2
0.4
0.3
0.2
0.1
50
25
0
75
TEMPERATURE (°C)
100
125
1579 G18
Logic Flag Output Voltage
(Output Low)
1.2
0.9
LOGIC FLAG OUTPUT VOLTAGE (V)
LOGIC FLAG OUTPUT VOLTAGE (V)
0.5
ILOAD = 1mA
1.2
0
– 50 –25
125
1.0
0.6
ILOAD = 300mA
1.6
Logic Flag Output Voltage
(Output Low)
VSS = 0V
125
1579 G15
1579 G17
Secondary Select Pin Current
0.7
100
2.0
1.6
0
– 50 –25
125
0.8
50
0
75
25
TEMPERATURE (°C)
Secondary Select Threshold
(Switch to VIN1)
ILOAD = 1mA
1579 G16
0
– 50 –25
0
– 50 – 25
125
SECONDARY SELECT PIN THRESHOLD (V)
2.0
VSHDN = 0V
SHUTDOWN PIN CURRENT (µA)
0.2
Secondary Select Threshold
(Switch to VIN2)
2.5
SECONDARY SELECT PIN CURRENT (µA)
0.4
1579 G14
Shutdown Pin Current
0.9
0.6
2.1
1579 G37
1.0
0.8
0.8
0.7
0.6
0.5
0.4
0.3
0.2
1.0
ISINK = 5mA
0.8
0.6
0.4
0.2
ISINK = 20µA
0.1
50
25
0
75
TEMPERATURE (°C)
100
125
1579 G19
0
1µA
10µA
100µA
1mA
LOGIC FLAG SINK CURRENT
10mA
1579 G20
0
– 50 – 25
50
25
75
0
TEMPERATURE (°C)
100
125
1579 G21
7
LT1579
U W
TYPICAL PERFORMANCE CHARACTERISTICS
Logic Flag Input Current
(Output High)
25
20
20
15
10
5
0
0
1
7
3
2
5
6
4
LOGIC FLAG VOLTAGE (V)
8
COMPARATOR HYSTERESIS (mV)
25
CONTROL PIN INPUT CURRENT (mA)
LOGIC FLAG INPUT CURRENT (mA)
Low-Battery Comparator
Hysteresis
Control Pin Input Current
20
15
10
5
0
1
7
3
2
5
6
4
CONTROL PIN VOLTAGE (V)
8
1579 G22
15
10
5
TJ = 25°C
VIN1 = VIN2 = 0V
CURRENT FLOWS INTO
OUTPUT PIN
20
18
15
10
LT1579-3.3
LT1579-3
5
0
1
2
14
12
10
8
6
4
7
3
5
6
4
OUTPUT VOLTAGE (V)
8
0
– 50 –25
9
0.6
TJ = 25°C
VIN1 = VIN2 = 0V
0.5
0.4
0.3
0.2
125
Current Limit
VOUT = 0V
0.6
CURRENT LIMIT (A)
CURRENT LIMIT (A)
0.6
100
0.7
0.5
0.8
0.7
50
25
0
75
TEMPERATURE (°C)
1579 G27
Current Limit
Adjust Pin Input Current
ADJUST PIN INPUT CURRENT (mA)
16
VIN1 = VIN2 = 0V
VOUT = 3V (LT1579-3)
VOUT = 3.3V (LT1579-3.3)
VOUT = 5V (LT1579-5)
1579 G26
1.0
50
2
0
125
1579 G25
0.9
30
40
20
ILBO SINK CURRENT (µA)
Reverse Output Current
LT1579-5
100
10
1579 G24
REVERSE OUTPUT CURRENT (µA)
REVERSE OUTPUT CURRENT (µA)
20
0
20
25
ILBO(SINK) = 50µA
COMPARATOR HYSTERESIS (mV)
9
Reverse Output Current
25
50
0
75
25
TEMPERATURE (°C)
5
1579 G23
Low-Battery Comparator
Hysteresis
0
– 50 – 25
10
0
0
9
15
0.4
0.3
0.2
0.1
VIN1 = VIN2 = VOUT(NOMINAL) + 1V
∆VOUT = – 0.1V
TYPICAL
0.5
0.4
GUARANTEED
0.3
0.2
0.1
0.1
0
0
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
ADJUST PIN VOLTAGE (V)
1579 G28
8
0
1
4
3
5
2
INPUT VOLTAGE (V)
6
7
1579 G29
0
– 50 – 25
50
25
75
0
TEMPERATURE (°C)
100
125
1579 G38
LT1579
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TYPICAL PERFORMANCE CHARACTERISTICS
Ripple Rejection
100
90
0
ILOAD = 150mA
VIN = 6V + 50mVRMS RIPPLE
LOAD REGULATION (mV)
70
COUT = 47µF
SOLID
TANTALUM
60
50
40
COUT = 4.7µF
SOLID
TANTALUM
30
20
∆ILOAD = 1mA TO 300mA
LT1579
–2
80
RIPPLE REJECTION (dB)
LT1579-5 “Hot” Plugging and
Unplugging Transient Response
Load Regulation
–4
6V
LT1579-3
VIN1
LT1579-5
5V
–6
–8
VOUT
50mV/DIV
LT1579-3.3
–10
–12
–14
10
0
10
100
1k
10k
FREQUENCY (Hz)
100k
1M
–16
– 50 – 25
UNPLUG
VIN1
0
50
75
25
TEMPERATURE (°C)
100
1579 G30
OUTPUT VOLTAGE
DEVIATION (mV)
50
0
VIN = 6V
CIN = 1µF CERAMIC
COUT = 4.7µF TANTALUM
100
75
50
25
0
0
50 100 150 200 250 300 350 400 450 500
TIME (µs)
1579 G33
LOAD CURRENT (mA)
OUTPUT VOLTAGE
DEVIATION (mV)
LOAD CURRENT (mA)
LT1579-5 Transient Response
100
–100
125
1579 G40
LT1579-5 Transient Response
– 50
REPLACE
VIN1
100
50
0
VIN = 6V
CIN = 1µF CERAMIC
COUT = 22µF TANTALUM
– 50
–100
300
200
100
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1,0
TIME (ms)
1579 G34
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PIN FUNCTIONS
VIN1: The primary power source is connected to VIN1. A
bypass capacitor is required on this pin if the device is
more than six inches away from the main input filter
capacitor. In general, the output impedance of a battery
rises with frequency, so it is advisable to include a bypass
capacitor in battery-powered circuits. A bypass capacitor
in the range of 1µF to 10µF is sufficient.
VIN2: The secondary power source is connected to VIN2.
A bypass capacitor is required on this pin if the device is
more than six inches away from the main input filter
capacitor. In general, the output impedance of a battery
rises with frequency, so it is advisable to include a bypass
capacitor in battery-powered circuits. A bypass capacitor
in the range of 1µF to 10µF is sufficient.
OUT: The output supplies power to the load. A minimum
output capacitor of 4.7µF is required to prevent oscillations. Larger output capacitors will be required for applications with large transient loads to limit peak voltage
transients.
ADJ: For the adjustable LT1579, this is the input to the
error amplifier. This pin is internally clamped to 7V and
– 0.6V (one VBE). It has a bias current of 6nA which flows
9
LT1579
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PIN FUNCTIONS
out of the pin (see curve of Adjust Pin Bias Current vs
Temperature in the Typical Performance Characteristics).
A DC load of 3µA is needed on the output of the adjustable
part to maintain regulation. The adjust pin voltage is 1.5V
referenced to ground and the output voltage range is 1.5V
to 20V.
SHDN: The shutdown pin is used to put the LT1579 into
a low power shutdown state. All functions are disabled if
the shutdown pin is pulled low. The output will be off, all
logic outputs will be high impedance and the voltage
comparators will be off when the shutdown pin is pulled
low. The shutdown pin is internally clamped to 7V and
– 0.6V (one VBE), allowing the shutdown pin to be driven
either by 5V logic or open collector logic with a pull-up
resistor. The pull-up resistor is only required to supply
the pull-up current of the open collector gate, normally
several microamperes. If unused, the shutdown pin can
be left open circuit. The device is active if the shutdown
pin is not connected.
SS: The secondary select pin forces the LT1579 to switch
power draw to the secondary input (VIN2). This pin is
active low. The current drawn out of VIN1 is reduced to
3µA when this pin is pulled low. The secondary select pin
is internally clamped to 7V and – 0.6V (one VBE), allowing
the pin to be driven directly by either 5V logic or open
collector logic with a pull-up resistor. The pull-up resistor
is required only to supply the leakage current of the open
collector gate, normally several microamperes. If secondary select is not used, it can be left open circuit. The
LT1579 draws power from the primary first if the secondary select pin is not connected.
BACKUP: The backup flag is an open collector output
which pulls low when the LT1579 starts drawing power
from the secondary input (VIN2). The BACKUP output
voltage is 1V when sinking 5mA, dropping to under
200mV at 20µA (see curve of Logic Flag Voltage vs
Current in the Typical Performance Characteristics). This
makes the BACKUP pin equally useful in driving both
bipolar and CMOS logic inputs with the addition of an
external pull-up resistor. It is also capable of driving
higher current devices, such as LEDs. This pin is inter-
10
nally clamped to 7V and – 0.6V (one VBE). If unused, this
pin can be left open circuit. Device operation is unaffected
if this pin is not connected.
DROPOUT: The dropout flag is an open collector output
which pulls low when both input voltages drop sufficiently for the LT1579 to enter the dropout region. This
signals that the output is beginning to go unregulated.
The DROPOUT output voltage is 1V when sinking 5mA,
dropping to under 200mV at 20µA (see curve of Logic
Flag Voltage vs Current in the Typical Performance Characteristics). This makes the DROPOUT pin equally useful
in driving both bipolar and CMOS logic inputs with the
addition of an external pull-up resistor. It is also capable
of driving higher current devices, such as LEDs. This pin
is internally clamped to 7V and – 0.6V (one VBE). If
unused, this pin can be left open circuit. Device operation
is unaffected if this pin is not connected.
BIASCOMP: This is a compensation point for the internal
bias circuitry. It must be bypassed with a 0.01µF capacitor for stability during the switch from VIN1 to VIN2.
LBI1: This is the noninvering input to low-battery comparator LB1 which is used to detect a low input/battery
condition. The inverting input is connected to a 1.5V
reference. The low-battery comparator input has 18mV of
hysteresis with more than 20µA of sink current on the
output (see Applications Information section). This pin is
internally clamped to 7V and – 0.6 (one VBE). If not used,
this pin can be left open circuit, with no effect on normal
circuit operation. If unconnected, the pin will float to 1.5V
and the logic output of LB1 will be high impedance.
LBI2: This is the noninverting input to low-battery comparator LB2 which is used to detect a low input/battery
condition. The inverting input is connected to a 1.5V
reference. The low-battery comparator input has 18mV of
hysteresis with more than 20µA of sink current on the
output (see Applications Information section). This pin is
internally clamped to 7V and – 0.6V (one VBE). If not used,
this pin can be left open circuit, with no effect on normal
circuit operation. If unconnected, the pin will float to 1.5V
and the logic output of LB2 will be high impedance.
LT1579
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PIN FUNCTIONS
LBO1: This is the open collector output of the low-battery
comparator LB1. This output pulls low when the comparator input drops below the threshold voltage. The
LBO1 output voltage is 1V when sinking 5mA, dropping
to under 200mV at 20µA (see curve of Logic Flag Voltage
vs Current in the Typical Performance Characteristics).
This makes the LBO1 pin equally useful in driving both
bipolar and CMOS logic inputs with the addition of an
external pull-up resistor. It is also capable of driving
higher current devices, such as LEDs. This pin is internally clamped to 7V and – 0.6V (one VBE). If unused, this
pin can be left open circuit. Device operation is unaffected
if this pin is not connected.
LBO2: This is the open collector output of the low-battery
comparator LB2. This output pulls low when the comparator input drops below the threshold voltage. The
LBO2 output voltage is 1V when sinking 5mA, dropping
to under 200mV at 20µA (see curve of Logic Flag Voltage
vs Current in the Typical Performance Characteristics).
This makes the LBO2 pin equally useful in driving both
bipolar and CMOS logic inputs with the addition of an
external pull-up resistor. It is also capable of driving
higher current devices, such as LEDs. This pin is
internally clamped to 7V and – 0.6V (one VBE). If unused,
this pin can be left open circuit. Device operation is
unaffected if this pin is not connected.
W
BLOCK DIAGRAM
VIN1
VIN2
VOUT
DROPOUT
DETECT
SHDN
BIAS CURRENT
CONTROL
INTERNAL
RESISTOR DIVIDER
FOR FIXED VOLTAGE
DEVICES ONLY
SS
–
BIASCOMP
OUTPUT DRIVER
CONTROL
E/A
+
1.5V REFERENCE
LBI1
ADJ
WARNING
FLAGS
+
BACKUP
DROPOUT
LBO1
LB1
–
LBI2
+
LB2
LBO2
–
1579 • BD
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LT1579
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APPLICATIONS INFORMATION
Device Overview
The LT1579 is a dual input, single output, low dropout
linear regulator. The 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 in the device. All power supplied to
the load is drawn from the primary input (VIN1) until the
device senses that the primary input is failing. At this point
the LT1579 smoothly switches from the primary input to
the secondary input (VIN2) to maintain output regulation.
The device is capable of providing 300mA from either
input at a dropout voltage of 0.4V. Total quiescent current
when operating from the primary input is 50µA, which is
45µA from the primary input, 2µA from the secondary and
a minimum input current of 3µA which will be drawn from
the higher of the two input voltages.
A single error amplifier controls both output stages so
regulation remains tight regardless of which input is
providing power. Threshold levels for the error amplifier
and low-battery detectors are set by the internal 1.5V
reference. Output voltage is set by an internal resistor
divider for fixed voltage parts and an external divider for
adjustable parts. Internal bias circuitry powers the reference, error amplifier, output driver controls, logic flags
and low-battery comparators.
The LT1579 aids power management with the use of two
independent low-battery comparators and two status flags.
The low-battery comparators can be used to monitor the
input voltage levels. The BACKUP flag signals when any
power is being drawn from the secondary input and the
DROPOUT flag provides indication that both input voltages are critically low and the output is unregulated.
Additionally, the switch to the secondary input from the
primary can be forced externally through the use of the
secondary select pin (SS). This active low logic pin, when
pulled below the threshold, will cause power draw to
switch from the primary input to the secondary input.
Current flowing in the primary input is reduced to only a
few microamperes, while all power draw (load current and
bias currents) switches to the secondary. The LT1579 has
a low power shutdown state which shuts off all bias
12
currents and logic functions. In shutdown, quiescent
currents are 2µA from the primary input, 2µA from the
secondary input and an additional 3µA which is drawn
from the higher of the two input voltages.
Adjustable Operation
The adjustable version of the LT1579 has an output
voltage range of 1.5V to 20V. The output voltage is set by
the ratio of two external resistors as shown in Figure 1. The
device servos the output to maintain the voltage at the
adjust pin at 1.5V. The current in R1 is then equal to 1.5V/
R1 and the current in R2 is the current in R1 minus the
adjust pin bias current. The adjust pin bias current, 6nA at
25°C, flows out of the adjust pin through R1 to ground. The
output voltage can now be calculated using the formula:
R2
VOUT = 1.5V 1 + – (IADJ )(R2)
R1
The value of R1 should be less than 500k to minimize the
error in the output voltage caused by adjust pin bias
current. With 500k resistors for both R1 and R2, the error
induced by adjust pin bias current at 25°C is 3mV or 0.1%
of the total output voltage. With appropriate value and
tolerance resistors, the error due to adjust pin bias current
may often be ignored. Note that in shutdown, the output is
turned off and the divider current is zero. The parallel
combination of R1 and R2 should be greater than 20k to
allow the error amplifier to start. In applications where the
minimum parallel resistance requirement cannot be met,
a 20k resistor may be placed in series with the adjust pin.
This introduces an error in the reference point for the
resistor divider equal to (IADJ)(20k).
OUT
+
R2
CFB
VOUT
COUT
ADJ
GND
R1
1579 • F01
Figure 1. Adjustable Operation
LT1579
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APPLICATIONS INFORMATION
A small capacitor placed in parallel with the top resistor
(R2) of the output divider is necessary for stability and
transient performance of the adjustable LT1579. The
impedance of CFB at 10kHz should be less than the value
of R1.
The adjustable LT1579 is tested and specified with the
output pin tied to the adjust pin and a 3µA load (unless
otherwise noted) for an output voltage of 1.5V. Specifications for output voltages greater than 1.5V are proportional to the ratio of the desired output voltage to 1.5V;
(VOUT/1.5V). For example, load regulation for an output
current change of 1mA to 300mA is – 2mV typical at
VOUT = 1.5V. At VOUT = 12V, load regulation is:
12 V
–2mV = –16mV
1.5 V
(
)
Output Capacitance and Transient Response
The LT1579 is designed to be stable with a wide range of
output capacitors. The ESR of the output capacitor affects
stability, most notably with small capacitors. A minimum
output capacitor of 4.7µF with an ESR of 3Ω or less is
recommended to prevent oscillations. Smaller value capacitors may be used, but capacitors which have a low
ESR (i.e. ceramics) may need a small series resistor added
to bring the ESR into the range suggested in Table 1. The
LT1579 is a micropower device and output transient
response is a function of output capacitance. Larger
values of output capacitance decrease the peak deviations
and provide improved output transient response for larger
load current changes. Bypass capacitors, used to decouple
individual components powered by the LT1579, will
increase the effective output capacitor value.
Table 1. Suggested ESR Range
OUTPUT CAPACITANCE
SUGGESTED ESR RANGE
1.5µF
1Ω to 3Ω
2.2µF
0.5Ω to 3Ω
3.3µF
0.2Ω to 3Ω
≥ 4.7µF
0Ω to 3Ω
BIASCOMP Pin Compensation
The BIASCOMP pin is a connection to a compensation
point for the internal bias circuitry. It must be bypassed
with a 0.01µF capacitor for stability during the switch from
VIN1 to VIN2.
“Hot” Plugging and Unplugging of Inputs
The LT1579 is designed to maintain regulation even if one
of the outputs is instantaneously removed. If the primary
input is supplying load current, removal and insertion of
the secondary input creates no noticeable transient at the
output. In this case, the LT1579 continues to supply
current from the primary; no switching is required. However, when load current is being supplied from the primary
input and it is removed, load current must be switched
from the primary to the secondary input. In this case, the
LT1579 sees the input capacitor as a rapidly discharging
battery. If it discharges too quickly, the LT1579 does not
have ample time to switch over without a large transient
occurring at the output. The input capacitor must be large
enough to supply load current during the transition from
primary to secondary input. Replacement of the primary
creates a smaller transient on the output because both
inputs are present during the transition. For a 100mA load,
input and output capacitors of 10µF will limit peak output
deviations to less than 50mV. See the “Hot” Plugging and
Unplugging Transient Response in the Typical Performance Characteristics. Proportionally larger values for
input and output capacitors are needed to limit peak
deviations on the output when delivering larger load
currents.
Standby Mode
“Standby” mode is where one input draws a minimum
quiescent current when the other input is delivering all
bias and load currents . In this mode, the standby current
is the quiescent current drawn from the standby input. The
secondary input will be in standby mode, when the primary input is delivering all load and bias currents. When
the secondary input is in standby mode the current drawn
from the secondary input will be 3µA if VIN1 > VIN2 and 5µA
13
LT1579
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APPLICATIONS INFORMATION
if VIN2 >VIN1, so typically only 3µA. The primary input will
automatically go into standby mode as the primary input
drops below the output voltage. The primary input can also
be forced into standby mode by asserting the SS pin. In
either case, the current drawn from the primary input is
reduced to a maximum of 7µA.
Shutdown
The LT1579 has a low power shutdown state where all
functions of the device are shut off. The device is put into
shutdown mode when the shutdown pin is pulled below
0.7V. The quiescent current in shutdown has three components: 2µA drawn from the primary, 2µA drawn from the
secondary and 3µA which is drawn from the higher of the
two inputs.
Protecting Batteries Using Secondary Select (SS)
Some batteries, such as lithium-ion cells, are sensitive to
deep discharge conditions. Discharging these batteries
below a certain threshold severely shortens battery life. To
prevent deep discharge of the primary cells, the LT1579
secondary select (SS) pin can be used to switch power
draw from the primary input to the secondary. When this
pin is pulled low, current out of the primary is reduced to
2µA. A low-battery detector with the trip point set at the
critical discharge point can signal the low battery condition and force the switchover to the secondary as shown
in Figure 2. The second low-battery comparator can be
used to set a latch to shutdown the LT1579 (see the Typical
Applications).
Low-Battery Comparators
There are two independent low-battery comparators in the
LT1579. This allows for individual monitoring of each
input. The inverting inputs of both comparators are connected to an internal 1.5V reference. The low-battery
comparator trip point is set by an external resistor divider
as shown in Figure 3. The current in R1 at the trip point is
1.5V/R1. The current in R2 is equal to the current in R1.
The low-battery comparator input bias current, 2nA flowing out of the pin, is negligible and may be ignored. The
value of R1 should be less than 1.5M in order to minimize
errors in the trip point. The value of R2 for a given trip point
is calculated using the formulas in Figure 3.
The low-battery comparators have a small amount of
hysteresis built-in. The amount of hysteresis is dependent
upon the output sink current (ISINK) when the comparator
is tripped low. At no load, comparator hysteresis is zero,
increasing to a maximum of 18mV for sink currents above
20µA. See the curve of Low-Battery Comparator Hysteresis in the Typical Performance Characteristics. If larger
amounts of hysteresis are desired, R3 and D1 can be
added. D1 can be any small diode, typically a 1N4148.
Calculating VLBO can be done using a load line on the curve
of Logic Flag Output Voltage vs Sink Current in the Typical
Performance Characteristics.
VTRIP
VOUT
R2
LBI
LBO
–
R1
VCC
R4
+
ISINK
1.5V
RP
R3
D1
LBO
( )
( )
LTC1579 • F02
R2 = (VTRIP – 1.5V)
SS
R1
1.5V
HYSTERESIS = VHYST 1 +
GND
1579 F02
R2
R1
FOR ADDED HYSTERESIS
(1.5V + VHYST – 0.6V – VLBO)(R2)
VHYST(ADDED)
R3 =
Figure 2. Connecting SS to Low-Battery Detector
Output to Prevent Damage to Batteries
FOR ISINK ≥ 20µA, VHYST = 18mV,
FOR ISINK < 20µA, SEE THE TYPICAL
PERFORMANCE CHARACTERISTICS
Figure 3. Low-Battery Comparator Operation
14
LT1579
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Example: The low-battery detector must be tripped at a
terminal voltage of 5.5V. There is a 100k pull-up resistor
to 5V on the output of the comparator and 200mV of
hysteresis is needed to prevent chatter. With a 1M resistor
for R1, what other resistor values are needed?
current supplied to the load from each input. Normal
output deviation during transient load conditions (with
sufficient input voltages) will not set the status flags.
Using the formulas in Figure 3,
The timing diagram for the 5V dual battery supply is shown
in Figure 4. The schematic is the same as the 5V Dual
Battery Supply on the front of the data sheet. All logic flag
outputs have 100k pull-up resistors added. Note that there
is no time scale for the timing diagram. The timing diagram
is meant as a tool to help in understanding basic operation
of the LT1579. Actual discharge rates will be a function of
the load current and the type of batteries used. The load
current used in the example was 100mA DC.
R2 =
(5.5V – 1.5V)(1MΩ) = 2.67MΩ
1.5V
Use a standard value of 2.7MΩ. With the 100k pull-up
resistor, this gives a sink current and logic flag voltage of
approximately 45µA at 0.4V. The hysteresis in this case
will be:
2.7MΩ
Hysteresis = 18mV 1 +
= 67mV
1MΩ
An additional 133mV of hysteresis is needed, so a resistor
and diode must be added. The value of R3 will be:
R3 =
(1.5V + 18mV – 0.6V – 0.4V)(2.7MΩ) = 10.5MΩ
133mV
A standard value of 10MΩ can be used. The additional
current flowing through R3 into the comparator output is
negligible and can usually be ignored
Timing Diagram
A
B
C
D
E
6V
VIN1
5V
6V
VIN2
5V
VOUT 5V
4.8V
100mA
Logic Flags
The low-battery comparator outputs and the status flags
of the LT1579 are open collector outputs capable of
sinking up to 5mA. See the curve of Logic Flag Output
Voltage vs. Current in the Typical Performance Characteristics.
There are two status flags on the LT1579. The BACKUP
flag and the DROPOUT flag provide information on which
input is supplying power to the load and give early warning
of loss of output regulation. The BACKUP flag goes low
when the secondary input begins supplying power to the
load. The DROPOUT flag signals the dropout condition on
both inputs, warning of an impending drop in output
voltage. The conditions that set either status flag are
determined by input to output voltage differentials and
IIN1
0
100mA
IIN2
0
1
LB01
BACKUP
LB02
DROPOUT
0
1
0
1
0
1
0
LTC1579 • F03
Figure 4. Basic Dual Battery Timing Diagram
15
LT1579
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APPLICATIONS INFORMATION
Five milestones are noted on the timing diagram. Time A
is where the primary input voltage drops enough to trip the
low-battery detector LB1. The trip threshold for LB1 is set
at set at 5.5V, slightly above the dropout voltage of the
primary input. At time B, the BACKUP flag goes low,
signaling the beginning of the transition from the primary
source to the secondary source. Between times B and C,
the input current makes a smooth transition from VIN1 to
VIN2. By time C, the primary battery has dropped below the
point where it can deliver useful current to the output. The
primary input will still deliver a small amount of current to
the load, diminishing as the primary input voltage drops.
By time D, the secondary battery has dropped to a low
enough voltage to trip the second low-battery detector,
LB2. The trip threshold for LB2 is also set at 5.5V, slightly
above where the secondary input reaches dropout. At time
E, both inputs are low enough to cause the LT1579 to enter
dropout, with the DROPOUT flag signaling the impending
loss of output regulation. After time E, the output voltage
drops out of regulation.
Some interesting things can be noted on the timing
diagram. The amount of current available from a given
input is determined by the input/output voltage differential. As the differential voltage drops, the amount of
current drawn from the input also drops, which slows the
discharge of the battery. Dropout detection circuitry will
maintain the maximum current draw from the input for the
given input/output voltage differential. In the case shown,
this causes the current drawn from the primary input to
approach zero, though never actually dropping to zero.
Note that the primary begins to supply significant current
again when the output drops out of regulation. This occurs
because the input/output voltage differential of the primary input increases as the output voltage drops. The
LT1579 will automatically maximize the power drawn
from the inputs to maintain the highest possible output
voltage.
Thermal Considerations
The power handling capability of the LT1579 is limited by
the maximum rated junction temperature (125°C). Power
dissipated is made up of two components:
16
1. The output current from each input multiplied by the
respective input to output voltage differential:
(IOUT)(VIN – VOUT) and
2. Ground pin current from the associated inputs multiplied by the respective input voltage: (IGND)(VIN).
If the primary input is not in dropout, all significant power
dissipation is from the primary input. Conversely, if SS has
been asserted to minimize power draw from the primary,
all significant power dissipation will be from the secondary. When the primary input enters dropout, calculation of
power dissipation requires consideration of power dissipation from both inputs. Worst-case power dissipation is
found using the worst-case input voltage from either input
and the worst-case load current.
Ground pin current is found by examining the Ground Pin
Current curves in the Typical Performance Characteristics. Power dissipation will be equal to the sum of the two
components above for the input supplying power to the
load. Power dissipation from the other input is negligible.
The LT1579 has internal thermal limiting designed to
protect the device during overload conditions. For continuous normal load conditions, the maximum junction
temperature rating of 125°C must not be exceeded. It is
important to give careful consideration to all sources of
thermal resistance from junction to ambient. Additional
heat sources nearby must also be considered.
Heating sinking for the device is accomplished by using
the heat spreading capabilities of the PC board and its
copper traces. Copper board stiffeners and plated throughholes can also be used to spread the heat. All ground pins
on the LT1579 are fused to the die paddle for improved
heat spreading capabilities.
The following tables list thermal resistances for each package. Measured values of thermal resistance for several
different board sizes and copper areas are listed for each
package. All measurements were taken in still air on 3/32”
FR-4 board with one ounce copper. All ground leads were
connected to the ground plane. All packages for the
LT1579 have all ground leads fused to the die attach
paddle to lower thermal resistance. Typical thermal
LT1579
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APPLICATIONS INFORMATION
resistance from the junction to a ground lead is 40°C/W for
16-lead SSOP, 32°C/W for 16-lead SO and 35°C/W for
8-lead S0.
Table 2. 8-Lead SO Package (S8)
COPPER AREA
TOPSIDE*
BACKSIDE
THERMAL RESISTANCE
BOARD AREA (JUNCTION-TO-AMBIENT)
2500 sq mm 2500 sq mm
2500 sq mm
73°C/W
1000 sq mm 2500 sq mm
2500 sq mm
75°C/W
225 sq mm 2500 sq mm
2500 sq mm
80°C/W
100 sq mm 2500 sq mm
2500 sq mm
90°C/W
*Device is mounted on topside.
Table 3. 16-Lead SO Package (S)
COPPER AREA
TOPSIDE*
BACKSIDE
THERMAL RESISTANCE
BOARD AREA (JUNCTION-TO-AMBIENT)
2500 sq mm 2500 sq mm
2500 sq mm
55°C/W
1000 sq mm 2500 sq mm
2500 sq mm
58°C/W
225 sq mm 2500 sq mm
2500 sq mm
60°C/W
100 sq mm 2500 sq mm
2500 sq mm
68°C/W
*Device is mounted on topside.
Table 4. 16-Lead SSOP Package (GN)
COPPER AREA
TOPSIDE*
BACKSIDE
THERMAL RESISTANCE
BOARD AREA (JUNCTION-TO-AMBIENT)
2500 sq mm 2500 sq mm
2500 sq mm
70°C/W
1000 sq mm 2500 sq mm
2500 sq mm
75°C/W
225 sq mm 2500 sq mm
2500 sq mm
80°C/W
100 sq mm 2500 sq mm
2500 sq mm
95°C/W
*Device is mounted on topside.
Calculating Junction Temperature
Example: Given an output voltage of 5V, an input voltage
range of 5V to 7V for VIN1 and 8V to 10V for VIN2, with an
output current range of 10mA to 150mA and a maximum
ambient temperature of 50°C, what will the maximum
junction temperature be?
When run from the primary input, current drawn from the
secondary input is negligible and worst-case power dissipation will be:
(IOUT(MAX))(VIN1(MAX) – VOUT) + (IGND)(VIN1(MAX))
Where:
IOUT(MAX) = 150mA
VIN1(MAX) = 7V
IGND at (IOUT = 150mA, VIN1 = 7V) = 2mA
Therefore,
P = (150mA)(7V – 5V) + (2mA)(7V) = 0.31W
When switched to the secondary input, current from the
primary input is negligible and worst-case power dissipation will be:
(IOUT(MAX))(VIN2(MAX) – VOUT) + (IGND)(VIN2(MAX))
Where:
IOUT(MAX) = 150mA
VIN2(MAX) = 10V
IGND at (IOUT = 150mA, VIN2 = 10V) = 2mA
Therefore,
P = (150mA)(10V – 5V) + (2mA)(10V) = 0.77W
Using a 16-lead SO package, the thermal resistance will be
in the range of 55°C/W to 68°C/W dependent upon the
copper area. So the junction temperature rise above
ambient will be approximately equal to:
(0.77W)(65°C/W) = 50.1°C
The maximum junction temperature will then be equal to
the maximum temperature rise above ambient plus the
maximum ambient temperature or:
TJMAX = 50.1°C + 50°C = 100.1°C
Protection Features
The LT1579 incorporates several protection features that
make it ideal for use in battery-powered circuits. In addition to the normal protection features associated with
monolithic regulators, such as current limiting and thermal limiting, the device is protected against reverse input
voltages, reverse output voltages and reverse voltages
from output to input.
Current limit protection and thermal overload protection
are intended to protect the device against current overload
conditions at the output of the device. For normal operation, the junction temperature should not exceed 125°C.
Current limit protection is designed to protect the device
if the output is shorted to ground. With the output shorted
to ground, current will be drawn from the primary input
until it is discharged. The current drawn from VIN2 will not
increase until the primary input is discharged. This prevents a short-circuit on the output from discharging both
inputs simultaneously.
17
LT1579
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APPLICATIONS INFORMATION
The inputs of the device can withstand reverse voltages up
to 20V. Current flow into the device will be limited to less
than 1mA (typically less than 100µA) and no negative
voltage will appear at the output. The device will protect
both itself and the load. This provides protection against
batteries which can be plugged in backwards. Internal
protection circuitry isolates the inputs to prevent current
flow from one input to the other. Even with one input
supplying all bias currents and the other being plugged in
backwards (a maximum total differential of 40V), current
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PACKAGE DESCRIPTION
flow from one input to another will be limited to less than
1mA. Output voltage will be unaffected. In the case of
reverse inputs, no reverse voltages will appear at the load.
Pulling the SS pin low will cause all load currents to come
from the secondary input. If the secondary input is not
present, the output will be turned off. If the part is put into
current limit with the SS pin pulled low, current limit will
be drawn from the secondary input until it is discharged,
at which point the current limit will drop to zero.
Dimensions in inches (millimeters) unless otherwise noted.
GN Package
16-Lead Plastic SSOP (Narrow 0.150)
(LTC DWG # 05-08-1641)
0.189 – 0.196*
(4.801 – 4.978)
16 15 14 13 12 11 10 9
0.229 – 0.244
(5.817 – 6.198)
0.150 – 0.157**
(3.810 – 3.988)
1
0.015 ± 0.004
× 45°
(0.38 ± 0.10)
0.007 – 0.0098
(0.178 – 0.249)
4
5 6
7
8
0.004 – 0.0098
(0.102 – 0.249)
0° – 8° TYP
0.016 – 0.050
(0.406 – 1.270)
* DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
** DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD
FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
18
0.053 – 0.068
(1.351 – 1.727)
2 3
0.008 – 0.012
(0.203 – 0.305)
0.025
(0.635)
BSC
GN16 (SSOP) 1197
LT1579
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PACKAGE DESCRIPTION
Dimensions in inches (millimeters) unless otherwise noted.
S8 Package
8-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
0.189 – 0.197*
(4.801 – 5.004)
7
8
5
6
0.150 – 0.157**
(3.810 – 3.988)
0.228 – 0.244
(5.791 – 6.197)
1
0.010 – 0.020
× 45°
(0.254 – 0.508)
0.008 – 0.010
(0.203 – 0.254)
3
2
4
0.053 – 0.069
(1.346 – 1.752)
0.004 – 0.010
(0.101 – 0.254)
0°– 8° TYP
0.016 – 0.050
0.406 – 1.270
0.050
(1.270)
TYP
0.014 – 0.019
(0.355 – 0.483)
*DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
**DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD
FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
SO8 0996
S Package
16-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
0.386 – 0.394*
(9.804 – 10.008)
16
15
14
13
12
11
10
9
0.150 – 0.157**
(3.810 – 3.988)
0.228 – 0.244
(5.791 – 6.197)
1
0.010 – 0.020
× 45°
(0.254 – 0.508)
2
3
4
5
6
0.053 – 0.069
(1.346 – 1.752)
0.008 – 0.010
(0.203 – 0.254)
0.014 – 0.019
(0.355 – 0.483)
8
0.004 – 0.010
(0.101 – 0.254)
0° – 8° TYP
0.016 – 0.050
0.406 – 1.270
7
0.050
(1.270)
TYP
S16 0695
*DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
**DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD
FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
19
LT1579
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TYPICAL APPLICATION
Additional Logic Forces LT1579 Into Shutdown to Protect Input Batteries
IN1
C1
1µF
IN1
OUT
R2
2.7M
BACKUP
LBI1
R1
1M
D2
C3
4.7µF
VOUT
5V/300mA
R3
1M
D1
R4
10M
D1 TO D3: 1N4148
R10
1M
MAIN GOOD
LBO1
DROPOUT
NC
SS
IN2
C2
1µF
IN2
D3
R6
2.7M
LBI2
R5
1M
R7
1M
R8
330k
D4
5.1V
1N751A
C5
0.1µF
LBO2
VCC
1/4
74C02
LT1579-5
BIASCOMP
C4
0.01µF
1/4
74C02
SHDN
GND
GND
1579 TA03
RESET
R9
1.5M
1/4
74C02
RELATED PARTS
PART NUMBER
LT1175
LTC®1421
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LTC1473
LTC1479
DESCRIPTION
500mA Negative Low Dropout Micropower Regulator
Hot SwapTM Controller
Hot Swap Controller
Dual PowerPathTM Switch Driver
PowerPath Controller for Dual Battery Systems
LT1521
300mA Low Dropout Micropower Regulator with Shutdown
Hot Swap and PowerPath are trademarks of Linear Technology Corporation.
20
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417 ● (408) 432-1900
FAX: (408) 434-0507● TELEX: 499-3977 ● www.linear-tech.com
COMMENTS
Adjustable Current Limit, Shutdown Control
Controls Multiple Supplies, 24-Lead SSOP Package
Controls Single Supply, 8-Lead SO Package
Power Path Management for Systems with Multiple Inputs
Complete Power Path Management for Two Batteries,
DC Power Source, Charger and Backup
12µA IQ, Reverse Battery Protection
1579f LT/TP 0398 4K • PRINTED IN USA
LINEAR TECHNOLOGY CORPORATION 1998
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湘ICP备2023018690号
