Linear LTC4355 18v dual input micropower powerpath prioritizer with backup supply monitoring Datasheet

LTC4420
18V Dual Input Micropower
PowerPath Prioritizer with
Backup Supply Monitoring
Features
n
n
n
n
n
n
n
n
n
n
n
n
n
Description
Selects Highest Priority Valid Supply from Two
Inputs
Wide 1.8V to 18V Operating Range
Internal Dual 2Ω, 0.5A Switches
Low 3.6µA Operating Current
Low 320nA V2 Current When V1 Connected to OUT
Blocks Reverse and Cross Conduction Currents
Reverse Supply Protection to –15V
Built-In V2 Test with Optional V2 Disconnect
V2 Freshness Seal/Ship Mode
±1.5% Accurate Adjustable Switchover Threshold
±2.3% Accurate V2 Monitor and Comparator
Overcurrent and Thermal Protection
Thermally Enhanced 12-Pin 3mm × 3mm
DFN and 12-Lead Exposed Pad MSOP Packages
Applications
n
n
n
Low Power Battery Backup
Portable Equipment
Point-of-Sale (POS) Equipment
The LTC®4420 is a dual input monolithic PowerPath™
prioritizer, with low operating current, that provides
backup switchover for keeping critical circuitry alive during brownout and power loss conditions. Unlike diode-OR
products, little current is drawn from the inactive supply
even if its voltage is greater than the active supply.
Internal 2Ω, current limited PMOS switches provide
power path selection from a primary input (V1) or a backup
input (V2) to the output. Two adjustable voltage monitors
set via external resistive dividers provide flexibility in setting V1 to V2 switchover and V2 undervoltage thresholds.
V1 is monitored continuously while V2 supply monitoring
includes controllable low duty cycle UV monitoring. When
primary input V1 drops, the ADJ monitor causes OUT to
be switched to V2. When V2 drops, it is disconnected from
OUT if V2DIS is low. Fast non-overlap switchover circuitry
prevents reverse and cross conduction while minimizing
output droop.
Auxiliary voltage monitor CMP1 provides flexible voltage
monitoring and output V2OK provides V2 undervoltage
status. Freshness seal mode prevents V2 battery discharge
during storage or shipment.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and
PowerPath and ThinSOT are trademarks of Linear Technology Corporation. All other trademarks
are the property of their respective owners.
Typical Application
Typical Switchover Waveforms
5V WALL
ADAPTER
1M
1M
10µF
OUT
OUT
V1
ADJ CMPOUT1
V1UV
V2OK
V2OK
1M
237k
CMP1
121k
+
7.4V
Li-Ion
V2
2V/DIV
V1
2V/DIV
LTC4420
V2
V2 MONITORING
INTERVAL
SWITCHOVER
THRESHOLD
V2 UNDERVOLTAGE
AND DISCONNECT
V2DIS
4.02M
V2UV V2TEST
280k
GNDSW
GND
TYPICAL VALUES:
SWITCHOVER THRESHOLD: V1 < 4V (V1 FALLING)
V1UV THRESHOLD: V1 < 4.4V (V1 FALLING)
4420 TA01a
V2OK THRESHOLD: V2 < 6V (V2 FALLING)
OUT
COUT = 10µF
ILOAD = 100mA
20ms/DIV
4420 TA01b
4420f
For more information www.linear.com/LTC4420
1
LTC4420
Absolute Maximum Ratings
Terminal Voltages
V1, V2.......................................................–15V to 24V
OUT........................................................ –0.3V to 24V
OUT – V2..................................................–24V to 39V
OUT – V1..................................................–24V to 39V
Input Voltages
ADJ, CMP1, V2UV, V2TEST, V2DIS
(Note 3) ................................................. –0.3V to 24V
Output Voltages
CMPOUT1, GNDSW, V2OK (Note 3) ....... –0.3V to 24V
(Notes 1, 2)
Pin Currents (Note 2)
ADJ, CMP1, V2UV, CMPOUT1, GNDSW..............–1mA
V2TEST, V2DIS, V2OK .......................................–1mA
Operating Ambient Temperature Range
LTC4420C................................................. 0°C to 70°C
LTC4420I..............................................–40°C to 85°C
Junction Temperature (Notes 4, 5)......................... 125°C
Storage Temperature Range................... –65°C to 150°C
Lead Temperature (Soldering, 10 sec)
MSOP Package.................................................. 300°C
Pin Configuration
TOP VIEW
TOP VIEW
V1
1
12 V2
V2TEST
2
11 V2DIS
CMP1
3
ADJ
4
13
GND
V1
V2TEST
CMP1
ADJ
GND
CMPOUT1
10 V2UV
9 OUT
GND
5
8 V2OK
CMPOUT1
6
7 GNDSW
1
2
3
4
5
6
12
11
10
9
8
7
13
GND
V2
V2DIS
V2UV
OUT
V2OK
GNDSW
MSE PACKAGE
12-LEAD PLASTIC MSOP
DD PACKAGE
12-LEAD (3mm × 3mm) PLASTIC DFN
TJMAX = 125°C, θJA = 40°C/W
EXPOSED PAD (PIN 13) IS GND, MUST BE SOLDERED TO PCB
TJMAX = 125°C, θJA = 43°C/W
EXPOSED PAD (PIN 13) IS GND, MUST BE SOLDERED TO PCB
Order Information
(http://www.linear.com/product/LTC4420#orderinfo)
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC4420CDD#PBF
LTC4420CDD#TRPBF
LGMR
12-Lead (3mm × 3mm) Plastic DFN
0°C to 70°C
LTC4420IDD#PBF
LTC4420IDD#TRPBF
LGMR
12-Lead (3mm × 3mm) Plastic DFN
–40°C to 85°C
LTC4420CMSE#PBF
LTC4420CMSE#TRPBF
4420
12-Lead Plastic Exposed Pad MSOP
0°C to 70°C
LTC4420IMSE#PBF
LTC4420IMSE#TRPBF
4420
12-Lead Plastic Exposed Pad MSOP
–40°C to 85°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/. Some packages are available in 500 unit reels through
designated sales channels with #TRMPBF suffix.
Electrical Characteristics
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. V1 = 3.6V, V2 = 3.6V unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Supply Voltage and Currents
V1, V2
Operating Voltage Range
IV1
V1 Current, V1 Powering OUT
V1 Current, V2 Powering OUT
l
IOUT = 0, V1 = 8.4V, V2 = 3.6V
V1 = 8.4V, V2 = 3.6V
l
l
1.8
3.6
500
18
V
6.3
800
µA
nA
4420f
2
For more information www.linear.com/LTC4420
LTC4420
Electrical Characteristics
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. V1 = 3.6V, V2 = 3.6V unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
IV2
V2 Current, V2 Powering OUT
V2 Current, V1 Powering OUT
V2 Current in Freshness Seal Mode
IOUT = 0, V1 = 3.6V, V2 = 8.4V
V1 = 3.6V, V2 = 8.4V
V1 = GND, V2 = 5V
l
l
l
3.3
320
120
6
650
220
µA
nA
nA
RON
Switch Resistance
V1 = V2 = 5V, IOUT = –100mA
l
1
2
5
Ω
tVALID(V1)
Input Qualification Time
V1 Rising, ADJ Rising
l
34
64
94
ms
Input Comparators
VTHA
ADJ Threshold
ADJ Falling
l
1.032
1.047
1.062
VHYSTA
ADJ Comparator Hysteresis
ADJ Rising
l
30
50
70
V
VTHC
CMP1, V2UV Threshold
CMP1, V2UV Falling
l
0.378
0.387
0.396
V
VHYSTC
CMP1, V2UV Comparator Hysteresis
CMP1, V2UV Rising
l
7.5
10
12.5
mV
tPDA
ADJ Comparator Falling Response Time
10% Overdrive
l
4
7.3
12
µs
tPDC
CMP1, V2UV Comparator Response Times
20% Overdrive
l
30
65
µs
1.1
1.6
A
mV
Power Path Function
ILIM
Output Current Limit
V1, V2 = 8.4V
l
0.5
VREV
Reverse Comparator Threshold
(V1, V2) – VOUT for Power Path Turn-On
l
25
50
75
mV
tSWITCH
Break-Before-Make Switchover Time
V1 = V2 = 5V, IOUT < 10mA
l
1
2.5
5
µs
tMONL
Longest Possible V2UV Monitor Duration
V2TEST ≥ VIH
l
88
128
168
ms
tMONS
Shortest Possible V2UV Monitor Duration
V2TEST ≥ VIH
l
1
2
3
ms
132
180
V2 Monitoring
tLTEST
Time Between V2UV Monitoring Events
V2TEST ≥ VIH
l
80
tHV2T
Minimum Allowed V2TEST High Time
V2TEST Driven Externally
l
10
ms
s
tLV2T
Minimum Allowed V2TEST Low Time
V2TEST Driven Externally
l
10
ms
I/O Specifications
VOL
Output Voltage Low, CMPOUT1, GNDSW and V2OK I = 100µA
I = 1mA
l
l
VOH
V2OK Output High Voltage
I = –1µA, V2 = 5V
l
IOH
V2OK, GNDSW, CMPOUT1 Output High Leakage
CMPOUT1, GNDSW, V2OK = 18V
l
VIL
V2DIS, V2TEST Input Low Voltage
V1 = V2 = 5V
l
VIH
V2DIS, V2TEST Input High Voltage
V1 = V2 = 5V
l
IV2X(IN,Z)
V2DIS, V2TEST Allowable Leakage in Open State
IPU(V2OK)
V2OK Pull-Up Current
V2 = 5V, ADJ = 0V, V2OK = 0V
l
ILEAK
ADJ, CMP1, V2UV Leakage Current
ADJ, CMP1, V2UV = 0V, 1.5V
l
15
120
1.05
2.3
V
±50
±150
nA
0.2
V
0.9
–2.7
mV
mV
1.65
V
0.5
µA
–5
–8
µA
±1
±5
nA
l
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: All currents into pins are positive; all voltages are referenced to
GND unless otherwise noted.
Note 3: These pins can be tied to voltages down to –5V through a resistor
that limits the current to less than –1mA.
Note 4: The LTC4420 includes overtemperature protection that is intended
to protect the device during momentary overload conditions. Junction
50
250
temperature will exceed 125°C when overtemperature protection is active.
Continuous operation above the specified maximum operating junction
temperature may impair device reliability.
Note 5: The LTC4420 is tested under pulsed load conditions such that
TJ ≈ TA. The junction temperature (TJ in °C) is calculated from the ambient
temperature (TA in °C) and power dissipation (PD in Watts) according to
the formula:
TJ = TA + (PD • θJA)
4420f
For more information www.linear.com/LTC4420
3
LTC4420
Typical Performance Characteristics
(TA = 25°C, V1 = V2 = 3.6V unless otherwise indicated)
4.5
V1 Current, V1 Powers OUT
(IOUT = 0)
V2 Current, V2 Powers OUT
(IOUT = 0)
4.0
V1 = 1.8V
V1 = 3.6V
V1 ≥ 6V
V2 Current, V1 Powers Out
450
V2 = 1.8V
V2 = 3.6V
V2 ≥ 6V
V1 = V2
400
V2 CURRENT (µA)
3.5
3.5
V2 CURRENT (nA)
V1 CURRENT (µA)
4.0
3.0
3.0
–25
0
25
50
75
TEMPERATURE (°C)
100
2.5
–50
125
–25
0
25
50
75
TEMPERATURE (°C)
250
–40°C
25°C
90°C
250
V1 = V2
200
450
150
VOL (mV)
500
400
0
5
10
15
V2 VOLTAGE (V)
0
20
4420 G03
0.995
0
0.5
1
1.5
PULL-DOWN CURRENT (mA)
2
0.990
–50
NORMALIZED VTHA
125
4420 G07
100
VADJ = 0V, 1.5V
2.5
50
40
30
–50
125
ADJ Leakage vs Temperature
3.0
ADJ LEAKAGE (nA)
60
ADJ HYSTERESIS (mV)
1.005
0.995
0
25
50
75
TEMPERATURE (°C)
4420 G06
ADJ Hysteresis vs Temperature
1.000
–25
4420 G05
70
100
20
1.010
50
1.010
0
25
50
75
TEMPERATURE (°C)
10
15
V2 VOLTAGE (V)
1.000
Normalized Falling ADJ
Threshold vs Temperature
–25
5
1.005
4420 G04
0.990
–50
0
Normalized CMP1 and
V2UV Falling Thresholds vs
Temperature
Open-Drain (CMPOUT1, GNDSW,
V2OK) VOL vs Pull-Down Current
100
–40°C
25°C
90°C
350
150
125
NORMALIZED VTHC
V1 Current, V2 Powers Out
550
100
4420 G02
4420 G01
V1 CURRENT (nA)
300
200
2.5
–50
300
350
2.0
1.5
1.0
–25
0
25
50
75
TEMPERATURE (°C)
100
125
4420 G08
0.5
–50
–25
0
25
50
75
TEMPERATURE (°C)
100
125
4420 G09
4420f
4
For more information www.linear.com/LTC4420
LTC4420
Typical Performance Characteristics
(TA = 25°C, V1 = V2 = 3.6V unless otherwise indicated)
Output Current IOUT Response for
Different Shorting Impedances
3.0
1.30
2.5
1.20
2.0
1.10
IOUT vs VOUT for Different Input
Supply Voltages
1.2
1.2Ω
2.2Ω
3.3Ω
3.9Ω
5Ω
1.5
0.4
0.90
0.5
0.2
0
25
50
75
TEMPERATURE (°C)
100
0
125
0
40µs/DIV
4420 G10
5
250
5V
3.6V
2V
200
V2 CURRENT (nA)
RON (Ω)
1
2
3
VOUT (V)
4
5
4419 G12
Freshness Seal Current vs
V2 Voltage and Temperature
4
3
2
1.8V
3.6V
5V
≥6V
V1 = 0V
150
100
50
–25
0
25
50
75
TEMPERATURE (°C)
100
0
–50
125
4419 G13
Switchover from a Higher to a
Lower Voltage
V2
DISCONNECT FROM V1
V2
CONNECT TO V2
3ms/DIV
–25
0
25
50
TEMPERATURE (°C)
V1
2V/DIV
100
V1 Reverse Voltage Blocking
with V2 Powering Out
V2
5V/DIV
10V
6V
10V
6V
OUT
COUT = 10µF
C1= C2 = 10µF
ILOAD = 50mA
IOUT
0.5A/DIV
4419 G15
75
4420 G14
Output Voltage and Current
Waveforms During Switchover
COUT = 10µF
IOUT = 200mA
V1
OUT
2V/DIV
0
4419 G11
Switch RON vs Temperature
1
–50
OHMIC
0.6
1.0
–25
CURRENT
LIMIT
0.8
1.00
0.80
–50
VIN = 1.8V
VIN = 3.6V
VIN = 5V
FOLDBACK
1.0
IOUT (A)
1.40
IOUT (A)
CURRENT LIMIT (A)
Output Current Limit vs Temperature
10µs/DIV
4420 G16
V1
10V/DIV
–10V
IOUT
0.5A/DIV
ILOAD = 50mA
20ms/DIV
4420 G17
4420f
For more information www.linear.com/LTC4420
5
LTC4420
Pin Functions
ADJ: Adjustable Switchover Threshold Input. ADJ is the
noninverting input to the switchover threshold comparator.
If V1 ≥ 1.55V and ADJ ≥ 1.097V for at least 64ms, OUT
is switched internally to the primary V1 input. When the
ADJ input voltage is lower than 1.047V, OUT is switched
internally to V2 if conditions in Table 1 of the Applications Information section are met. Otherwise, OUT stays
unpowered. Tie ADJ via a resistive divider to V1, in order
to set the V1 to V2 switchover voltage. Do not leave open.
CMP1: Auxiliary Comparator 1 Monitor Input . CMP1 is the
noninverting input to an auxiliary comparator. The inverting input is internally connected to a 0.387V reference.
Connect CMP1 to GND when it is not used.
CMPOUT1: Auxiliary Comparator Output 1. This open-drain
comparator output is pulled low when CMP1 is below
0.387V and during power-up, otherwise it is released.
Once released, connecting a resistor between CMPOUT1
and a desired supply voltage up to 18V causes this pin to
be pulled high. Leave open if unused.
GNDSW: Pulsed GND Output. This open-drain output is
pulled low when V2UV is being monitored, otherwise it
is released high. Connect a resistive divider between V2,
V2UV and GNDSW to set V2 undervoltage threshold.
Leave open if unused.
Exposed Pad: For best thermal performance, solder the
exposed pad to a large PCB area.
GND: Device Ground.
OUT: Output Voltage Supply. OUT is a prioritized voltage
output that is either connected to V1, V2 or is unpowered
as indicated in Table 1 of the Applications Information section. Additionally, OUT must be at least 50mV below the
input supply for a connection to that supply to be activated.
Bypass with a capacitor of 1µF or greater. See Applications Information for bypass capacitor recommendations.
V1: Primary Power Supply. OUT is internally switched to
V1 if V1 ≥ 1.55V and ADJ ≥ 1.097V. When in freshness
seal, applying V1 ≥ 1.55V and ADJ ≥ 1.097V for 32ms
disables freshness seal. Bypass with 1µF or greater. Tie
to GND if unused.
V2: Backup Power Supply. OUT is internally switched to
V2 if ADJ < 1.047V or V1 < 1.55V, provided other conditions listed in Table 1 in Applications Information are met.
Bypass with 1µF or greater. Tie to GND if unused.
V2DIS: V2 Power Path Disable Input. When driven low,
this pin disables the V2 to OUT power path if input V2UV
drops below 0.387V. Connect a resistor between V2 or
OUT and this pin to provide additional pull-up. Leave open
if unused. This pin is initialized high during power-up.
V2OK: V2OK Logic Output. V2OK is an output that is driven
high with a 5µA pull-up if V2UV > 0.387V at the end of
the V2UV monitoring period. Otherwise it is driven low.
Connect a resistor between OUT and this pin to provide
additional pull-up. As this pin is used to enable freshness
seal, do not force low or connect a pull-down resistor to
this pin. Leave open if unused.
V2TEST: V2 Undervoltage Test Enable Input. This pin sets
the duty cycle of V2 undervoltage monitoring. When V1
is valid, driving V2TEST low disables V2 monitoring while
driving it high enables V2 monitoring with a maximum
duty cycle of ~0.1%. When V1 is invalid or not present,
V2 is always monitored with V2TEST setting the duty
cycle between 0.0015% and 0.1% depending on its own
state and previously determined V2 validity. Refer to the
state diagram and waveforms in the Applications Information section for details. Leave open or connect a resistor
between V2 or OUT and this pin to provide additional
pull-up. Connect to GND if unused. This pin is initialized
high on power-up.
V2UV: V2 Undervoltage Monitor Input. V2UV is the noninverting input to a comparator whose inverting input is
internally connected to a 0.387V reference. Connect a
resistive divider between V2, V2UV and GNDSW to set V2
undervoltage threshold. See the Applications Information
section for details on V2 monitoring. Connect a pull-up
resistor to V2 if unused. Do not leave open.
4420f
6
For more information www.linear.com/LTC4420
LTC4420
Functional Diagram
CMPOUT1
3
1
12
CMP1
0.397V/
0.387V
+
–
CP1
V1
V2
OUT
FRESHNESS
SEAL
50mV
EN1
11
ADJ
1.097V/
1.047V
–
+
+
–
CUV1
–
+
CUV2
10
V2UV
7.3µs
5µA
V2OK
8
CV2UV
D
Q
E
GNDSW
EN_GNDSW
2
CREV1
64ms
1M
–
+
CREV2
9
2.5V
CADJ
V2DIS
0.397V/
0.387V
EN2
+
50mV +
–
–
+ –
+ –
CONTROL LOGIC
1.55V/
1.52V
4
6
7
V2TEST
1M
5
GND
4420 BD
4420f
For more information www.linear.com/LTC4420
7
LTC4420
Operation
The Functional Diagram shows the major blocks of the
LTC4420. The LTC4420 is a PowerPath prioritizer that
switches output OUT between primary (V1) and backup
(V2) sources depending on their validity and priority with
V1 having the highest priority. A resistive divider between
V1, ADJ and GND and comparators CUV1 and CADJ are
used to monitor V1’s voltage to establish validity. V1 is
valid if V1 ≥ 1.55V and ADJ ≥ 1.097V for 64ms after V1
rises above 1.55V. Otherwise V1 is invalid. A resistive
divider between V2, V2UV and GNDSW and comparators
CUV2 and CV2UV are used to monitor V2’s voltage to
establish validity. V2 voltage is monitored periodically in
order to minimize current consumption in the divider. V2
is valid if V2 ≥ 1.55V and V2UV ≥ 0.4V at the end of the
V2 monitoring period. Otherwise it is invalid. If neither
supply is valid, OUT stays unpowered if V2DIS is low. If
V2DIS is high and V2 > 1.55V, OUT is connected to V2.
Refer to Table 1 in the Applications Information section
for details. Switchover threshold is independent of relative
V1 and V2 voltages, permitting V1 to be lower or higher
than V2 when V1 powers OUT and vice versa.
Power connection to the output is made by enhancing backto-back internal P-channel MOSFETs. Current passed by
the MOSFETs is limited to typically 1.1A if OUT is greater
than 1V. Otherwise it is limited to 250mA. When switching
from V1 to V2, the V1 to OUT power path is first disabled
and comparator CREV2 is enabled. After the OUT voltage
drops 50mV below V2, as detected by CREV2, OUT is
then connected to V2. This break-before-make strategy
prevents OUT from backfeeding V2. Switchover back to V1
occurs in a similar manner once V1 has been revalidated.
The LTC4420 blocks reverse voltages up to –15V when
a reverse condition occurs on an inactive channel. The
LTC4420 also disables a channel if the corresponding
input supply falls below 1.52V. A small ~3µA current is
drawn from either the prioritized input supply, or the highest supply if both input supplies are below 1.55V. Very
little current (~320nA) is drawn from the unused supply.
Pins V2TEST and V2DIS provide flexibility in monitoring
and disconnecting the V2 power path using the V2UV
monitor input. V2 is monitored by activating the V2-V2UVGNDSW resistive divider. V2TEST allows for adjustability
of GNDSW duty cycle to trade off V2 quiescent current with
V2 monitoring frequency. When low, V2DIS disables the
V2 to OUT power path, if V2 is found to be invalid. Refer
to the Applications Information section for details. If V2 is
valid at the end of a V2 monitoring interval, output V2OK
is latched high. Otherwise it is latched low. V2OK retains
its state until the end of the next V2 monitoring interval
when it gets updated. V2 monitoring is disabled if V2 <
1.55V or during thermal shutdown. During initial powerup V2 monitoring is disabled and V2OK is initialized low.
The LTC4420 provides an additional comparator, CP1,
whose open-drain output pulls low either when the CMP1
pin voltage falls below 0.387V or during initial power up.
This comparator can be used to monitor supplies to provide
early power failure warning and other useful information.
The LTC4420 can be put into a V2 freshness seal mode
to prevent battery discharge during storage or shipment.
The Applications Information section lists the steps to
engage and disengage V2 freshness seal.
4420f
8
For more information www.linear.com/LTC4420
LTC4420
Applications Information
The LTC4420 is a low quiescent current 2-channel prioritizer that powers both its internal circuitry and its output
OUT from a prioritized valid input supply. Unlike an ideal
diode-OR, the LTC4420 does not necessarily draw current
from the highest supply as long as one supply is greater
than 1.8V. Table 1 lists the input supply from which the
LTC4420 draws its internal quiescent current ICC and the
supply to which OUT is connected after input supplies
have been qualified.
+
2-CELL
Li-Ion
7.4V
C1
4.7µF
R3
1M
R2
150k
R1
78.7k
+
A typical battery backup application is shown in Figure 1.
V1 is powered by a 2-cell Li-Ion battery pack whose safe
discharge limit is between 5.6V and 6V. V2 is powered by
a low self discharge 7.6V Li-Thionyl Chloride (Li-SOCl2)
hold-up battery which is completely discharged when its
voltage drops to 6V. Li-SOCL2 battery life is maximized
as very little current is drawn from V2 during normal
operation due to the low duty cycle of V2 monitoring and
the LTC4420’s low V2 standby current. To protect the
2-cell Li-Ion battery on V1, switchover threshold is set
to be ~5.6V. After switchover to V2, the Li-Ion battery
primarily supplies only divider R1-R3’s current as the
LTC4420 draws only a small standby current from V1.
Monitor CMP1 is configured to provide V1 power failure
warning by driving V1UV low when V1 falls below 6V.
Monitor input V2UV is configured to set V2’s UV threshold
to 6V and V2DIS is tied low to disconnect the V2 to OUT
power path when V2 falls below 6V. V2TEST is tied high
to monitor V2 once every 132s. Relevant equations used
to calculate these component values are discussed in the
following subsections.
2-CELL
Li-SOCl2
7.4V
C2
4.7µF
R5
4.02M
R4
280k
V1
OUT
OUT
ADJ
R6
1M
V2TEST
CMPOUT1
CMP1
COUT
10µF
R8
1M
V1UV
V2OK
V2UV
SWITCHOVER
THRESHOLD: V1 < 5.6V (V1 FALLING)
V1UV: V1 < 6V (V1 FALLING)
V2UV: V2 < 6V (V2 FALLING)
LTC4420
V2
R7
1M
V2DIS
V2UV
GNDSW
GND
4420 F01
Figure 1. The LTC4420 Protecting 2-Cell Lithium Battery Packs
on V1 and V2 from Discharge Below Their Safe Minimum Voltage
Setting Switchover and V2 Undervoltage Thresholds
Several factors affect switchover voltage and should be
taken into account when calculating resistor values. These
include resistor tolerance, 1.5% ADJ comparator threshold
error, divider impedance and worst-case ADJ pin leakage.
These factors also apply to resistive dividers connected
to monitor inputs CMP1 and V2UV. Referring to Figure
1 and the Electrical Characteristics table, the typical V1
switchover threshold:
VSW1 =
VTHA
• (R1+R2+R3)
R1+R2
(1)
Table 1. OUT and LTC4420 ICC Power
INPUT VOLTAGES
V1 > 1.55V
ADJ > 1.097V
V2 > 1.55V
V2DIS > 0.9V
V2UV > 0.397*
ICC SOURCE
OUT CONNECTION
Y†
Y†
X
X
X
V1
V1
Y
N
Y
Y
X
V2
V2
Y
N
Y
N
Y
V2
V2
Y
N
Y
N
N
V1
Hi-Z
Y
N
N
X
X
V1
Hi-Z
N
X
Y
N
N
V2
Hi-Z
N
X
Y
N
Y
V2
V2
N
X
Y
Y
X
V2
V2
N
X
N
X
X
VMAX**
Hi-Z
*Note: Refers to V2UV voltage at the end of the V2 monitoring period.
**Note: VMAX = higher of V1 and V2.
† For 64ms.
4420f
For more information www.linear.com/LTC4420
9
LTC4420
Applications Information
Typical V1 undervoltage threshold is:
VV1UV =
VTHC
• (R1+R2+R3)
R1
(2)
Worst-case VOL due to current flow into the GNDSW pin
must be taken into account while calculating values for
the V2 undervoltage resistive divider:
VTHC
VV2UV =
V
R4+ OL
100µA
V


•  R4+R5+ OL 

100µA 
(3)
Equations 1-3 assume ADJ and CMP1 pin leakages are
negligible. To account for pin leakage, equations 1-3 must
be modified by an ILEAK • REQ term where equivalent
resistance REQ must be calculated on a case-by-case basis. Worst-case component values and reference voltage
tolerances must be used to calculate the maximum and
minimum threshold voltages. For example, to calculate
minimum falling switchover threshold voltage VSW1(MIN),
use VTHA(MIN), (R2+R1)(MAX), R3(MIN) in equation 1.
125
t PDA (µs)
100
75
50
tNOV •IOUT
∆VOUT
10
100
1k
10k
dV ADJ /dt (V/s)
100k
limits output droop to less than ∆VOUT.
In order to estimate tNOV and IOUT, first consider a scenario
where power supplies are present on V1 and V2, and their
voltages are changing slowly compared to the ADJ comparator propagation delay tPDA. For such cases, IOUT is
ILOAD and tNOV is tSWITCH. COUT can be sized according to
equation 5 with IOUT = ILOAD(MAX) and tNOV = tSWITCH(MAX)
to limit maximum output droop when switching to a higher
supply. When switching to a lower supply, switchover is
initiated only after OUT falls VREV below the supply that
is being switched in. In such cases, total output droop is
∆VOUT + VREV.
Next consider a scenario where the input power source
powering OUT is unplugged. OUT backfeeds circuitry
connected to the input supply pin. Both input and output
droop at the same rate. Referring to Figure 1, assume
the battery on V1 is unplugged when OUT is connected
to V1. IOUT is the sum of ILOAD and the back fed current
IBACK, which in this example is IR3. As OUT and V1, since
the two are connected, droop below the ADJ threshold,
switchover occurs to V2 with a dead time
IOUT = (IBACK + ILOAD)
4420 F02
(6)
(7)
in equation 5 to size COUT:
Selecting Output Capacitor COUT
COUT ≥
COUT can be selected to control either output voltage droop
during switchover or output rising slew rate during initial
power-up or when switching to a higher supply.
In general, output droop, ΔVOUT, can be calculated by:
tNOV •IOUT
COUT
(5)
where tPDA is an overdrive dependent ADJ comparator
delay. As an approximation, use tPDA from the Electrical
Characteristics table to estimate tNOV. Use this tNOV and:
Figure 2. ADJ Comparator Propagation Delay as a
Function of Slew Rate; tPDA vs dVADJ/dt
VOUT =
COUT ≥
tNOV = tPDA + tSWITCH
25
0
where IOUT is the current supplied by COUT during nonoverlap or dead time, tNOV. Choosing:
( tPDA + tSWITCH ) •IOUT
∆VOUT
(8)
Refer to Figure 2 for a more accurate estimate of tPDA vs
dVOUT/dt. If ADJ is filtered with capacitor CADJ, its discharge
time via divider R1 – R3 increases tPDA. This results in a
higher output droop than estimated by equation (8).
(4)
4420f
10
For more information www.linear.com/LTC4420
LTC4420
Applications Information
In order to limit output rising slew rate dVOUT/dt, size:
COUT ≥
ILIM
dVOUT
dt
(9)
as the LTC4420 limits OUT charging current to ILIM until
OUT approaches the input supply to within ILIM • RON,
where RON is the channel switch resistance. Refer to the
Thermal Protection and Maximum COUT section to determine maximum allowed COUT.
Inductive Effects
Parasitic inductance and resistance can impact circuit
performance by causing overshoot and undershoot of input
and output voltages when the LTC4420 turns off. Parasitic
inductance in the power path causes positive-going
overshoot on the input and a negative-going undershoot
on the output. Another cause of positive input overshoot is
R-L-C tank ringing during hot plug of an input supply. Input
overshoot is most pronounced when the total resistance
of the input tank is low. Care must be taken to ensure over
voltage transients do not exceed the Absolute Maximum
ratings of the LTC4420. Additionally, parasitic resistance
and inductance can cause input undershoot (droop)
during power path turn on. If severe enough, undershoot
can temporarily invalidate a supply and cause repeated
power up cycles (motorboating) or unwanted switchover
between sources.
The first step to avoid these issues is to minimize parasitic
inductance and resistance in the power path. Guidelines
are given in the layout section for minimizing parasitic
inductance on the printed circuit board (PCB). External
to the PCB, twist the power and ground wires together to
minimize inductance.
V1
Second, use a bypass capacitor at the input to limit input
voltage overshoot during LTC4420 power path turn off. A
few micro farads is sufficient for most applications. When
hot plugging supplies with large parasitic inductances, it
is possible for the R-L-C tank to ring to more than twice
the nominal supply voltage. Wall adapters and batteries
typically have enough loss (i.e. series resistance) to prevent
ringing of this magnitude. However, if this is a problem,
snub input capacitor CSN1 with resistor RSN1, typically
0.5Ω. Place this network close to the supply pin.
Third, if an input capacitor is not permissible, use a TVS
(such as SMAJ16CA) in applications when supply pin
transients can exceed 24V. Use a bidirectional TVS in
applications requiring reverse input protection. Note that
a TVS does not address droop and motorboating, which
are solved only by input bypassing.
During normal operation, the LTC4420 limits power path
current to < 1.6A and internal circuitry prevents OUT from
ringing below ground during power path turn off. This is
also true for output shorts when the short is close to the
LTC4420’s OUT pin. However, if the output is shorted
through a long wire, current in the wire inductance (LPAR2
in Figure 3) builds up due to the discharge of COUT1 and
can be much higher than 1.6A. This current causes the
OUT pin to ring below its −0.3V absolute maximum rating
once COUT1 has been fully discharged. For this special
case, split the output capacitor between COUT1 and COUT2
and make COUT1 small. Snub COUT1 with resister RSN2 to
damp R-L-C ringing if required. Size COUT2 to obtain the
required total output capacitance. Also add a diode between
OUT and ground close to the LTC4420 to clamp negative
ringing if the OUT pin rings below –0.3V.
LPAR2
LTC4420
V1
OUT
LPAR1
RSN1
0.5Ω
CSN1
5µF
OPTIONAL
OUT
RSN2
1Ω
COUT2
10µF
OPTIONAL
COUT1
1µF
D1
1N5818
4420 F03
Figure 3. Recommended Inductive Transient Suppression Circuitry
4420f
For more information www.linear.com/LTC4420
11
LTC4420
Applications Information
V2 Monitoring and Control
V2 monitoring duration and time between monitoring
events are set by input V2TEST, V1 validity and V2 validity
as determined previously. Complete behavior is described
by the state diagram shown in Figure 4. This implementation was chosen for the following reasons,
The LTC4420 monitors V2 voltage through an external
resistive divider connected between V2, V2UV and GNDSW.
When V2 is being monitored, open-drain output GNDSW
is pulled low to activate the resistive divider, otherwise
it is released high. V2UV is monitored by comparator
CV2UV, whose output is latched at the end of the monitoring period. This latched output establishes V2 validity
and is used in Table 1.
1. To provide flexibility in monitoring and disconnecting the backup battery as required by the application,
while minimizing current draw through the V2 resistive
divider. V2TEST and V2DIS need to be actively driven
to achieve this.
POWER
OR
UP
SWITCHOVER
EXIT THERMAL
OR SHUTDOWN
TO V2
V2 < 1.55V OR
ST
TE H
V2 HIG
DISABLE
V2 MONITORING
OT
V2 N ID
L
A
D
V
N
T
ES A
V2T H
HIG
V2 ID
AL
D V
AN
T
V2
THERMAL
SHUTDOWN
NO
UV
V2 NOT
V2TEST
AND
AND V1 VALID
VALID
HIGH
2ms
V2 MONITORING
WHEN V1 IS
VALID
V2
V2TEST
AND
VALID
HIGH
)
V1
V2TEST
AND
VALID
LOW
V2
V2TEST
AND
VALID
HIGH
ND
S
ID
L
VA
V1
ND
A
D
W
O
TL
AN
ID
V1
L
VA
S
TE
V2
TE
(V
NO
A
GH
I
TH
V2
(
V1
V2TEST
V2
NOT VALID AND LOW OR NOT VALID
EST
V2T
ND LOW
V1 A
ID
VAL
WAIT FOR RISING
V2TEST OR
OVERRIDE CONDITION
V1
V2TEST
V2 NOT
AND
AND
VALID
HIGH
VALID
ID
AL
TV
V2 NOT VALID AND V1 NOT VALID
128ms
V2 MONITORING
)
V1 VALID AND V2TEST LOW
NO
2ms
V2 MONITORING
WHEN V1 IS
INVALID
T
V2
VA
LI
D AN
D
NO
T
V1
VA
LI
D
V1 NOT VALID AND (V2TEST LOW AND V2 NOT VALID)
V2 VALID OR V2TEST HIGH
4420 F04
Figure 4. State Diagram Describing V2 Monitoring
4420f
12
For more information www.linear.com/LTC4420
LTC4420
Applications Information
2. To provide default battery backup monitoring and disconnect in systems where V2TEST and V2DIS are not
actively driven. V2TEST and V2DIS are either tied high
or low in these applications.
3. To allow a system powered by OUT to shut itself down
if there is no valid input supply.
4. To support backup battery charging without having to
disconnect the battery from the system.
5. Handling exceptions such as initial power up, recovery from thermal shutdown and switchover after long
intervals when V2 was not being monitored.
Configuring V2TEST and V2DIS
V2TEST controls the duration of and the time between
V2 monitoring events. It can either be tied high, low or
actively driven based on the application. The following
section explores common scenarios.
In applications where primary supply V1 is going to be
valid for long periods of time and where V2TEST can be
actively driven, V2TEST should generally be driven low and
only pulsed high when V2 status is needed. This minimizes
V2-V2UV-GNDSW divider current. This scenario also
applies when V2 is a battery that slowly discharges over
time, making a V2 status update every 132s superfluous.
When operating off V2, V2TEST may be pulsed at intervals
shorter than 131s to check V2’s validity especially after
large load current spikes.
If V2TEST cannot be actively driven, it should be tied to
either V2 or OUT through a pull-up resistor. If V2 can be
reversed, tie V2TEST to OUT. Tying V2TEST high ensures
that V2 is monitored every 132s as long as V2 > 1.55V.
V2 monitoring duration is 128ms when V2 is valid and
reduces to 2ms if V2 becomes invalid. Use smaller resistors in the V2-V2UV-GNDSW divider if V2 is a battery that
can develop a passivation layer when it is not being used.
Larger V2 current helps break the passivation whenever
the V2 divider is active.
In special cases where V2 needs to be monitored only
when V1 goes invalid and when battery passivation is not
an issue, tie V2TEST low.
If automatic V2 disconnect is desired when a V2 UV event
occurs, tie V2DIS low. Otherwise leave open or tie to either
OUT or V2 through a pull up resistor. If V2 can be reversed,
tie V2DIS to OUT. If V2DIS can be actively driven, driving
it low some time after a V2 UV event (output V2OK goes
low) allows systems powered by OUT to finish active tasks,
backup data and initiate shutdown proceedings.
Actively Driving V2TEST
In Figure 5, V2TEST is actively driven. When V1 powers up
above switchover threshold VSW1, it is qualified for 64ms
after which the V1 to OUT power path is activated. When
V2 rises above 1.55V, GNDSW is pulsed low for 128ms
and V2UV is monitored, even though V2TEST is low. V2
is found to be valid resulting in V2OK being driven high.
As long as V1 remains valid, V2 is monitored only when
V2TEST is driven high with V2 monitoring time being the
lower of either the V2TEST high time or 128ms. Figure 5
shows two such monitoring events of durations t1 and
t2 where t1 and t2 are less than 128ms. When V1 drops
below VSW1, OUT is switched to V2 and V2 validity is
refreshed by monitoring it once for 128ms independent
of the state of V2TEST. Following this, since V2 is the
only valid supply, V2 is monitored for 2ms every 132s
if V2TEST is low or for 128ms every 132s if V2TEST is
high. If V2 becomes invalid and V2DIS is low, the V2 to
OUT power path gets disabled.
V2TEST Tied Low
Figure 6 shows voltage waveforms for the case where
V2TEST is tied low. When V2 powers up above 1.55V,
GNDSW is pulsed low and V2 is monitored once for 128ms.
Simultaneously, the V2 to OUT power path is activated in
order to allow a system powered by OUT to power itself
up and drive V2DIS to a desired state. V2 is determined
to be valid causing V2OK to be driven high and the V2
power path to remain activated. If V2 was determined to
be invalid and V2DIS was low, V2’s power path would have
been disabled and V2OK pulled low after 128ms. Since
both V1 and V2TEST are low, V2 is monitored for 2ms
every 132s. When V1 becomes valid, OUT is switched to
V1 and V2 monitoring is halted until V1 becomes invalid.
4420f
For more information www.linear.com/LTC4420
13
LTC4420
Applications Information
VSW1
VSW1
V1
1.55V
V2
VUV2
t1
t2
t1
t2
V2TEST
128ms
128ms
131s
2ms
GNDSW
V2OK
~64ms
OUT
V2 POWER PATH ACTIVE
V1 POWER PATH ACTIVE
TIME
4420 F05
Figure 5. V2 Monitoring by Actively Driving VTEST. Note That t1 and t2 are < 128ms
VSW1
V1
V2
1.55V
VUV2
128ms
131s
2ms
131s
2ms
GNDSW
V2OK
OUT
~64ms
V1 POWER
PATH ACTIVE
V2 POWER PATH ACTIVE
TIME
4420 F06
Figure 6. V2 Monitoring When V2TEST Is Low
4420f
14
For more information www.linear.com/LTC4420
LTC4420
Applications Information
INCREASING CMP1 HYSTERESIS
In some applications, built in CMP1 hysteresis may be
insufficient. In such cases, CMP1 hysteresis can be increased as shown in Figure 7. Hysteresis at the monitored
input VMON with R8 present and assuming R9 << R8, is
given by:
R3
R3
VHYST = VHYSTC
+ VPU
(10)
R1||R3||R8
R8
where VHYSTC is found in the electrical table and is typically 10mV. Account for supply VPU and resistor R8 when
calculating rising and falling thresholds of monitored
input VMON.
VMON
R3
VPU
CMP1
R9
LTC4420
R1
4420 F07
Figure 7. Increasing COMP1 Hysteresis
Supply Impedance and ADJ Comparator Hysteresis
In some applications, V1 could be supplied by a battery
pack with high ESR or through a long cable with appreciable
series resistance. Load current, IOUT, flowing through this
resistance reduces the monitored V1 voltage by:
(11)
The drop can be as high as:
ΔV1 = ILIM • RESR


∆V
VSW1 •COUT •  1–
 2 •ILIM •RESR 
C1>
∆V
(13)
to ensure no switchover occurs when COUT is initially being charged. If the resulting C1 value causes large inrush
current, is physically too big or requires a large snubber
resistor when V1 is plugged (refer to the Typical Applications
section), select C1 to be as high a value as the application
can tolerate. A filter capacitor CADJ can also be added to
ADJ, to ride through the initial output charge up time. CADJ
should be minimized as it slows ADJ response, resulting
in a larger output droop when the input supply powering
V1 is either unplugged or drops quickly.
Input Shorts and Supply Brownout
CMPOUT1
R8
∆V1 = IOUT • RESR
Referring to Figure 1, in order to prevent switchover when
COUT is being initially charged add input capacitor C1.
Ideally, if V1 is greater than switchover threshold VSW1
by ∆V, size:
(12)
when COUT is initially being charged. Voltage droop at the
V1 pin can result in repeated switchover between V1 and
V2 if built-in V1 (ADJ) hysteresis is insufficient.
The LTC4420 temporarily turns off its active power path
during input shorts or brownout conditions if the input
supply falls below OUT by 0.7V. If the primary input supply
becomes invalid, switchover to the backup supply occurs.
The power path is reactivated when the input recovers to
within 0.7V of the output.
Figure 8 shows the response of the LTC4420 to a brownout and recovery on V1 where switchover to V2 does not
occur as V1 stays above 1.8V. When V1 falls, OUT gets
disconnected from V1 and is slowly discharged by load
resistance ROUT. When V1 recovers, the power path is
reactivated and OUT tracks V1. In Figure 9, when V1 falls,
OUT gets disconnected from V1 as V1 drops below the
switchover threshold. When V1 recovers, it needs to be
qualified for 64ms before it is reconnected to OUT. OUT
gets discharged by ROUT and is connected to V2 once its
voltage is 50mV less than V2.
4420f
For more information www.linear.com/LTC4420
15
LTC4420
Applications Information
2. Once V2OK is asserted high, drive it below 50mV.
V1
5V/DIV
3. Power up V1 and ADJ for at least 94ms. Complete steps
2 and 3 within 80s of V2OK asserting high. Freshness
seal is enabled.
V2
5V/DIV
1
OUT
5V/DIV
COUT = 10µF
ROUT = 100Ω
100µs/DIV
2
3
1.8V
V2
4420 F08
Figure 8. Voltage Waveforms During a Brownout on V1
That Does Not Result in Switchover to V2. Switchover
Threshold = 1.8V
0.48V
V2UV
DRIVEN LOW
EXTERNALLY
V2OK
1.8V
V1
5V/DIV
V1
1.116V
94ms
V2
5V/DIV
ADJ
OUT
5V/DIV
COUT = 10µF
ROUT = 100Ω
100µs/DIV
4420 F10
< 80s
Figure 10. Freshness Seal Engage Procedure
4420 F09
Figure 9. Voltage Waveforms When a Brownout on V1
Results in Switchover to V2. Switchover Threshold = 3V
Reverse Voltage Blocking
The LTC4420 blocks reverse voltages on supply pins V1
and V2 up to –15V relative to GND and up to –39V relative
to OUT. Transient voltage suppressors (TVS) connected to
V1 and V2 must be bidirectional and capacitors connected
to these pins must be rated to handle reverse voltages. A
reverse voltage on V2 does not disrupt V1 operation and
vice-versa.
Freshness Seal Mode
Freshness seal mode prevents V2 battery discharge by
keeping V2 disconnected from OUT even if V1 is absent
or invalid. Very little current is drawn from V2—typically
just 120nA. The following sequence (refer to Figure 10)
puts the LTC4420 in freshness seal mode:
1. Power up V2 and V2UV.
FSEAL
ENABLED
Engage this mode if V2 is a backup battery either during
storage or during shipment. Once freshness seal has been
engaged, if V1 is disconnected, V2 stays disconnected
from OUT. Freshness seal is automatically disabled the
next time V1 is revalidated. Limit V2OK pin capacitance
to less than 10nF in order to prevent freshness seal mode
from accidentally being engaged.
Design Example
In Figure 11, the LTC4420 prioritizes between a 5V supply
connected to V1 and a 7.4V 2-cell Li-Ion battery connected to V2. The system is designed to switch OUT to
V2 when V1 drops below 4V, provide early power failure
warning when V1 drops below 4.5V and disconnect the
backup battery voltage when it drops below 6V. Maximum
anticipated load current is 100mA and maximum allowed
output droop is 100mV. Output rising slew rate is limited to
<0.1V/µs and V1 and V2 input capacitances are limited to
10µF to avoid large inrush current. 1% tolerance resistors
are used ADJ, CMP1 and V2UV pin leakages and GNDSW
VOL are ignored as their design impact is small.
4420f
16
For more information www.linear.com/LTC4420
LTC4420
Applications Information
5V
INPUT
V1
RSN1
0.5Ω
R3
365k
C1
2.2µF
OUT
COUT
15µF
LTC4420
ADJ
R6
1M
V2TEST
OUT
R7
1M
R2
88.7k
R1
44.2k
+
CMP1
V2DIS
CMPOUT1
PFV1
V2OK
V2UV
V2
2-CELL
Li-Ion
7.4V
R5
1.37M
R4
95.3k
C2
2.2µF
First choose total resistive divider current to be ~10µA for
V1 and 5µA for V2. Since the V2 divider is pulsed with a
maximum duty cycle of 0.1%, average V2 divider current
is negligible. For the 5V supply, this results in:
5V
= 500kΩ
10µA
(14)
Since desired switchover threshold, VSW1, and total divider
impedance are known, use equation 1 to first calculate R3.
Using R3 and equation 2, calculate R1 and R2. Rewriting
equation 1 results in:
(R1+R2) =
VTHA • (R1+R2+R3)
VSW1
(15)
Using (R1+R2+R3) = 500kΩ from equation 14, results in:
(R1+R2) =
1.047V • 500kΩ
= 130.9kΩ
4V
(16)
R3 ~ (500kΩ – 130.9kΩ) = 369.1kΩ
(17)
Using the nearest 1% resistor value yields R3 = 365kΩ.
Rearranging equation 2, results in:
(18)
R1=
0.387V
• ( 500kΩ )
4.5V
(19)
A similar procedure is used to calculate R4 and R5 using
equation 3 and total divider current. Resistance of the
GNDSW pull-down, typically 120Ω, is neglected as it is
small compared to R4 and R5. The design equations are
shown below.
4420 F11
Figure 11. Design Example
R1+R2+R3 =
VTHC • (R1+R2+R3)
VPFV1
Solving equations 16 and 19 results in R1= 43.3kΩ and
R2 = 87.6kΩ. Using the nearest 1% resistors results in
R2 = 88.7kΩ. Recalculating equation 1 using calculated
R2 and R3 values and using standard 1% resistor values
close to 43.3kΩ for R1 results in R1= 44.2kΩ.
V2UV
GNDSW
GND
R1=
R4+R5 =
7.4V
= 1.48MΩ
5µA
(20)
as desired current in the divider is 5µA.
Rewriting equation 3 neglecting pin leakage and assuming
R5>>R4 results in:
R4 =
VTHC • (R4+R5)
VV2UV
(21)
R4 =
0.387V •1.48MΩ
6V
(22)
Solving equations 20 and 22 results in R4 = 96.2kΩ and
R5 = 1.38MΩ. Choosing the nearest 1% resistor results
in R4 = 95.3kΩ and R5 = 1.37MΩ.
COUT affects both OUT droop during switchover as determined by equation 4 and OUT rising slew rate as determined by equation 9. Calculate minimum COUT required
to meet desired output droop and slew rate specifications
using equations 8 and 9 and size COUT to be the larger of
the two values.
4420f
For more information www.linear.com/LTC4420
17
LTC4420
Applications Information
COUT required to limit OUT droop to < 100mV is given by
equation 8,
COUT ≥
COUT
( tPDA + tSWITCH ) •ILOAD
100mV
(23)
(7.3µs + 2.5µs) • 0.1A = 9.8µF
≥
100mV
(24)
COUT required to limit OUT slew rate to < 0.1V/µs is given
by equation 9,
COUT ≥
ILIM
= 11µF
0.1V/µs
(25)
Choose a COUT capacitor whose minimum value is 11µF
accounting for voltage and temperature coefficients. Do
this for other capacitors as well. Assuming correct PCB
layout, choose C1 to be 2.2µF, which is ~1/5th of COUT to
suppress inductive transients. Also snub C1 with a 0.5Ω
resistor to prevent ringing.
Layout Consideration
Make power and ground traces as wide as possible. Place
bypass capacitors, snubbers and TVS devices as close to
the pin as possible to reduce power path resistance and
parasitic inductance. These result in smaller overvoltage
transients and improved overvoltage protection. Place
resistive dividers close to the pins to improve noise immunity. Use a 4-layer board if possible with layer 2 as
dedicated GND and solder the exposed pad to a large PCB
GND trace for better heat dissipation. A partial layout for
a 2-layer PCB is shown in Figure 12.
GND
GND
C1
V1
C2
V2
LTC4420
OUT
GND
COUT
4419 F12
Figure 12. Recommended 12-Lead MSE Layout for a 2-Layer PCB
4420f
18
For more information www.linear.com/LTC4420
LTC4420
Applications Information
Depending on the difference between input and output
voltages, the LTC4420’s internal power dissipation can be
high when operating in current limit mode. This usually
occurs when a large COUT is being charged either during
initial power up or when OUT switches over to a higher
supply. The situation is worsened if there is a DC load on
OUT, as this reduces the current available to charge COUT.
In such cases, self heating can cause power path turn-off
due to activation of the thermal protection circuitry. The
power path is reactivated when die temperature drops to
a safe value. This process can repeat indefinitely if COUT is
discharged fully by load current IOUT in the interval when
the power path is off.
COUT < 500µF if possible. If a larger COUT is necessary,
use Figure 13 to choose COUT. Also follow PCB layout
guidelines to improve heat dissipation.
60k
ILOAD = 0
10k
COUT (µF)
THERMAL PROTECTION AND MAXIMUM COUT
1k
100
–40°C
25°C
85°C
5
10
15
VIN (V)
Maximum allowed COUT to prevent activation of the thermal
protection circuit depends on several factors such as input
supply and output voltages, starting ambient temperature,
heat dissipation in the PCB and DC output current. Choose
20
4420 F13
Figure 13. Maximum Allowed COUT vs Input
Voltage for Different TA
Typical Applications
Battery Backup with Interface to Low Voltage Logic
5V TO 18V
WALL ADAPTER
RSN1
0.5Ω
C1
10µF
R3
365k
R2
88.7k
R1
44.2k
3.6V TO 18V
BACKUP
RSN2
0.5Ω
C2
10µF
R5
1M
R4
150k
V1
OUT
ADJ
COUT
10µF
IN
OUT
LTC1763-3.3V
SHDN GND
LTC4420
CMP1
C3
10µF
R6
1M
R7
1M
R8
1M
V2TEST
CMPOUT1
V2
V2OK
V2UV
3.3V
V1UV
SYSTEM
V2OK
V2DIS
GNDSW
GND
SWITCHOVER THRESHOLD: V1 < 4V (V1 FALLING)
V1UV THRESHOLD: V1 < 4.5V (V1 FALLING)
V2OK THRESHOLD: V2 < 3V (V2 FALLING)
4420 TA02
4420f
For more information www.linear.com/LTC4420
19
LTC4420
Typical Applications
SuperCap Backup with SuperCap Charging
L1 3.3µH
C1
10µF
1.7V TO 5.5V
INPUT
C3
120pF
R13
12.1k
R14
127k
SW1
RSENP
RSENS
4.2V
SW2
VOUT
C2
940mF
MID
LTC3128
IN
PROG
MAXV
GND
940mF
FB
RUN
V1
V2
R5
1M
R3
1M
R11
1.87M
R2
237k
R12
301k
R1
121k
OUT
LTC4420
R4
127k
V2UV
COUT
10µF
R6
1M
R7
1M
R8
1M
OUT
V2TEST
V2DIS
GNDSW
ADJ
V2OK
V2OK
CMP1 CMPOUT1
V1UV
GND
4420 TA03
SWITCHOVER THRESHOLD: V1 < 4V (V1 FALLING)
V1UV THRESHOLD: V1 < 4.4V (V1 FALLING)
V2OK THRESHOLD: V2 < 3.5V (V2 FALLING)
C2: MURATA DMF325R5H474M3DTA0
Triple Voltage Monitor
+
+
14.8V
Li-Ion
9V
ALKALINE
C1
10µF
C2
10µF
R3
2M
R1
191k
R5
1M
R4
59k
V1
OUT
ADJ
CMP1
LTC4420
V2
R10
2M
R7
1M
R8
1M
OUT
R9
113k
CMPOUT1
V2TEST
COUT
10µF
OUTUV
V2DIS
V2UV
V2OK
V2OK
GNDSW
GND
SWITCHOVER THRESHOLD: V1 < 12V (V1 FALLING)
V2OK THRESHOLD: V2 < 7V (V2 FALLING)
OUTUV THRESHOLD: OUT < 7.5V (OUT FALLING)
4420 TA04
4420f
20
For more information www.linear.com/LTC4420
LTC4420
Typical Applications
Early Power Failure Warning with Low Battery Indication
L1, 10µH
C4
0.1µF
SW1
BST1
5V TO 15V
INPUT
C5
0.1µF
SW2
VIN
TO OTHER
CIRCUITS
BST2
LTC3111
VOUT
COMP
C8
10µF
R13
1M
R14
137k
FB
SNSGND
V1
R8
20k
C9
18pF
C6
39pF
RUN
12V
C7, 1nF R12, 44.2k
VCC
R6
1M
ADJ
R2
75k
R11
158k
C3
1µF
PWM
R10
2.21M
CMP1
4-CELL
14.8V
Li-ION
R7
1M
COUT
10µF
R8
1M
LTC4420
R1
41.2k
+
OUT
OUT
C1
22µF
R3
1M
C2
10µF
R5
2M
R4
66.5k
PFV1, V1 POWER FAILURE THRESHOLD: V1 < 10.6V (V1 FALLING)
SWITCHOVER THRESHOLD: V1 < 10V (V1 FALLING)
V2OK THRESHOLD: V2 < 12V (V2 FALLING)
V2
V2UV
V2TEST
V2DIS
CMPOUT1
PFV1
V2OK
V2OK
GNDSW GND
4420 TA05
Prioritization with Failsafe Backup Supply
DS
BAT54
COIN
CELL
3V
12V
WALL
ADAPTER
RSN1
0.5Ω
C1
10µF
R3
1M
R2
75k
R1
41.2k
+
2-CELL
Li-Ion
7.4V
C2
10µF
R5
1.37M
R4
95.3k
V1
OUT
ADJ
OUT
R7
1M
V2TEST
R8
1M
CMPOUT1
PFV1
V2OK
V2UV
CMP1
COUT
10µF
LTC4420
V2
V2DIS
V2UV
GNDSW
GND
PFV1 THRESHOLD: V1 < 10.6V (V1 FALLING)
SWITCHOVER THRESHOLD: V1 < 10V (V1 FALLING)
V2UV THRESHOLD: V2 < 6V (V2 FALLING)
4420 TA06
4420f
For more information www.linear.com/LTC4420
21
LTC4420
Package Description
Please refer to http://www.linear.com/product/LTC4420#packaging for the most recent package drawings.
DD Package
12-Lead Plastic DFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1725 Rev A)
R = 0.115
TYP
7
0.40 ±0.10
12
0.70 ±0.05
3.50 ±0.05
2.10 ±0.05
2.38 ±0.05
1.65 ±0.05
3.00 ±0.10
(4 SIDES)
2.38 ±0.10
1.65 ±0.10
6
0.25 ±0.05
PIN 1 NOTCH
R = 0.20 OR
0.25 × 45°
CHAMFER
PIN 1
TOP MARK
(SEE NOTE 6)
PACKAGE
OUTLINE
0.200 REF
0.45 BSC
1
0.23 ±0.05
0.45 BSC
0.75 ±0.05
2.25 REF
2.25 REF
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
0.00 – 0.05
(DD12) DFN 0106 REV A
BOTTOM VIEW—EXPOSED PAD
NOTE:
5. EXPOSED PAD AND TIE BARS SHALL BE SOLDER PLATED
1. DRAWING IS NOT A JEDEC PACKAGE OUTLINE
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE
2. DRAWING NOT TO SCALE
TOP AND BOTTOM OF PACKAGE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
4420f
22
For more information www.linear.com/LTC4420
LTC4420
Package Description
Please refer to http://www.linear.com/product/LTC4420#packaging for the most recent package drawings.
MSE Package
12-Lead Plastic MSOP, Exposed Die Pad
(Reference LTC DWG # 05-08-1666 Rev G)
BOTTOM VIEW OF
EXPOSED PAD OPTION
2.845 ±0.102
(.112 ±.004)
5.10
(.201)
MIN
2.845 ±0.102
(.112 ±.004)
0.889 ±0.127
(.035 ±.005)
6
1
1.651 ±0.102
(.065 ±.004)
1.651 ±0.102 3.20 – 3.45
(.065 ±.004) (.126 – .136)
12
0.65
0.42 ±0.038
(.0256)
(.0165 ±.0015)
BSC
TYP
RECOMMENDED SOLDER PAD LAYOUT
0.254
(.010)
0.35
REF
4.039 ±0.102
(.159 ±.004)
(NOTE 3)
0.12 REF
DETAIL “B”
CORNER TAIL IS PART OF
DETAIL “B” THE LEADFRAME FEATURE.
FOR REFERENCE ONLY
7
NO MEASUREMENT PURPOSE
0.406 ±0.076
(.016 ±.003)
REF
12 11 10 9 8 7
DETAIL “A”
0° – 6° TYP
3.00 ±0.102
(.118 ±.004)
(NOTE 4)
4.90 ±0.152
(.193 ±.006)
GAUGE PLANE
0.53 ±0.152
(.021 ±.006)
DETAIL “A”
1.10
(.043)
MAX
0.18
(.007)
SEATING
PLANE
0.22 – 0.38
(.009 – .015)
TYP
1 2 3 4 5 6
0.650
(.0256)
BSC
NOTE:
1. DIMENSIONS IN MILLIMETER/(INCH)
2. DRAWING NOT TO SCALE
3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.
MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.
INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
6. EXPOSED PAD DIMENSION DOES INCLUDE MOLD FLASH. MOLD FLASH ON E-PAD SHALL
NOT EXCEED 0.254mm (.010") PER SIDE.
0.86
(.034)
REF
0.1016 ±0.0508
(.004 ±.002)
MSOP (MSE12) 0213 REV G
4420f
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.
For more
information
www.linear.com/LTC4420
23
LTC4420
Typical Application
High Efficiency Backup
C4
22nF
RSN1
0.5Ω
C1
10µF
R3
1M
R2
237k
R1
121k
V1
OUT
ADJ
COUT
10µF
V2TEST
R12
1.1M
BST1 SW1
VIN
RUN
R11
1.05M
LTC4420
CMP1
RUN
MPPC
VS1
VS2
VCC
SW2 BST2
OUT
5V
C6
10µF
R7
1M
R8
1M
LTC3129-1
SYSTEM
5V
WALL
ADAPTER
C5
22nF
L1
3.3µH
PWM GND PGND VS3
C3
2.2µF
+
2-CELL
7.4V
Li-Ion
C2
10µF
R5
1.37M
R4
95.3k
V2
V1UV
CMPOUT1
V2UV
V2OK
V2UV
V2DIS
GNDSW
GND
R13
1M
SWITCHOVER THRESHOLD: V1 < 4V (V1 FALLING)
V1UV THRESHOLD: V1 < 4.4V (V1 FALLING)
V2UV THRESHOLD: V2 < 6V (V2 FALLING)
4420 TA07
Related Parts
PART NUMBER
DESCRIPTION
COMMENTS
LT 1763
500mA, Low Noise Micropower LDO Regulators
VIN: 1.8V to 20V, 12-DFN, 8-SO Packages
LTC2952
Pushbutton PowerPath Controller with Supervisor
VIN: 2.7V to 28V, On/Off Timers, ±8kV HBM ESD, TSSOP-20
and QFN-20 Packages
LTC3103
15V, 300mA Synchronous Step-Down DC/DC Converter
VIN: 2.5V to 15V, DFN-10 and MSE-10 Packages
LTC3129/LTC3129-1
15V, 200mA Synchronous Buck-Boost DC/DC Converter
with 1.3µA Quiescent Current
VIN: 1.92V to 15V, QFN-16 and MSE-16 Packages
®
LTC3388-1/LTC3388-3 20V, 50mA High Efficiency Nanopower Step-Down
Regulator
VIN: 2.7V to 20V, DFN-10 and MSE-10 Packages
LTC4411
2.6A Low Loss Ideal Diode in ThinSOT™
Internal 2.6A P-Channel, 2.6V to 5.5V, 40μA IQ, SOT-23 Package
LTC4412
36V Low Loss PowerPath Controller in ThinSOT
2.5V to 36V, P-Channel, 11μA IQ, SOT-23 Package
LTC4415
Dual 4A Ideal Diodes with Adjustable Current Limit
Dual Internal P-Channel, 1.7V to 5.5V, MSOP-16 and DFN-16
Packages
LTC4416
36V Low Loss Dual PowerPath Controller for Large PFETs
3.6V to 36V, 35μA IQ per Supply, MSOP-10 Package
LTC4417
3-Channel Prioritized PowerPath Controller
Triple P-Channel Controller, 2.5V to 36V, SSOP-24 and QFN-24
Packages
LTC4355
Positive High Voltage Ideal Diode-OR with Supply and
Fuse Monitors
Dual N-channel, 9V to 80V, SO-16, MSOP-16 and DFN-14 Packages
LTC4359
Ideal Diode Controller with Reverse Input Protection
N-channel, 4V to 80V, MSOP-8 and DFN-6 Packages
4420f
24 Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
For more information www.linear.com/LTC4420
(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com/LTC4420
LT 0816 • PRINTED IN USA
 LINEAR TECHNOLOGY CORPORATION 2016