LTC3355
20V 1A Buck DC/DC with
Integrated SCAP Charger
and Backup Regulator
FEATURES
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DESCRIPTION
The
LTC
®
3355
is a complete input power interrupt ride-
through DC/DC system. The part charges a supercapacitor
while delivering load current to V
OUT
, and uses energy from
the supercapacitor to provide continuous V
OUT
backup
power when V
IN
power is lost. The LTC3355 contains a
nonsynchronous constant frequency current mode mono-
lithic 1A buck switching regulator to provide a 2.7V to 5V
regulated output voltage from an input supply of up to 20V.
A 1A programmable CC/CV linear charger charges the
supercapacitor from V
OUT
. When the V
IN
supply drops
below the PFI threshold, the devices’s constant frequency
nonsynchronous current mode 5A boost switching
regulator delivers power from the supercapacitor to V
OUT
.
A thermal regulation loop maximizes charge current while
limiting the die temperature to 110°C. The IC has boost,
charger and V
IN
programmable current limits. The LTC3355
is available in a 20-lead 4mm
×
4mm QFN surface mount
package.
L,
LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their respective
owners.
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V
IN
Voltage Range: 3V to 20V
V
OUT
Voltage Range: 2.7V to 5V
1A Current Mode Buck Main Regulator
5A Boost Backup Regulator Powered from Single
Supercapacitor
Boost Regulator Operates Down to 0.5V for
Maximum Utilization of Supercapacitor Energy
Programmable Supercapacitor Charge Current to
1A with Overvoltage Protection
Charger Supports Single Cell CC/CV Battery Charging
Programmable V
IN
Current Limit
Programmable Boost Current Limit
V
IN
Power Fail Indicator
V
CAP
Power Good Indicator
V
OUT
Power On Reset Output
Compact 20-Lead 4mm
×
4mm QFN Package
APPLICATIONS
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Ride-Through “Dying Gasp” Supplies
Power Meters/Industrial Alarms/Solid State Drives
TYPICAL APPLICATION
Supercapacitor Charger and Ride-Through Power Supply
V
IN
12V
0.091
10µF
10µF
1µF
5
V
IN
SW1
6.8µH
7
V
OUT
4V
47µF 1A (MAX)
VOLTAGE (V)
Backup Operation
14
12
10
8
6
4
2
0
332k
0
5
V
CAP
V
OUT
V
IN
CAPACITOR = 3F
I
VOUT
= 0.125A
6
V
INM5
V
INS
4
1
PFI
PFO
BUCK V
OUT
1A
15
4.7pF
402k
100k
2.49M
200k
10
PFOB
13
RSTB
9
CPGOOD
8
EN_CHG
3
MODE
INTV
CC
18
1µF
I
CHG
12
60.4k
BOOST
FB
2
V
CAP
SW2
LTC3355
14
16
17
3.3µH
665k
2.4V
SCAP
1F TO 50F
CFB
11
I
BSTPK
20
200k
V
CBST
19
154k
220pF
3355 TA01a
10
TIME (SECONDS)
15
20
3355 TA01b
3355fb
For more information
www.linear.com/LTC3355
1
LTC3355
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW
INTV
CC
I
BSTPK
V
CBST
SW2
SW2
15 V
OUT
21
GND
14 V
CAP
13 RSTB
12 I
CHG
11 CFB
6
V
INM5
7
SW1
8
EN_CHG
9 10
CPGOOD
PFOB
V
IN
, V
INS
, V
INM5
........................................................22V
V
IN
±V
INS
.................................................................. 0.1V
V
SW1
.......................................................... –0.4V to 22V
V
SW2
............................................................ –0.4V to 6V
V
OUT
, INTV
CC
, PFOB, RSTB,
CPGOOD, V
CAP
............................................. –0.3V to 6V
PFI, EN_CHG, MODE, FB .............................. –0.3V to 6V
CFB ............................................–0.3V to INTV
CC
+ 0.3V
I
CPGOOD
, I
PFOB
, I
RSTB
................................................1mA
Operating Junction Temperature Range
(Notes 2, 3) ............................................ –40°C to 125°C
Storage Temperature Range .................. –65°C to 150°C
20 19 18 17 16
PFI 1
FB 2
MODE 3
V
INS
4
V
IN
5
UF PACKAGE
20-LEAD (4mm
×
4mm) PLASTIC QFN
T
JMAX
= 125°C,
θ
JA
= 47°C/W
EXPOSED PAD (PIN 21) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
LTC3355EUF#PBF
LTC3355IUF#PBF
TAPE AND REEL
LTC3355EUF#TRPBF
LTC3355IUF#TRPBF
PART MARKING
3355
3355
PACKAGE DESCRIPTION
20-Lead (4mm
×
4mm) Plastic QFN
20-Lead (4mm
×
4mm) Plastic QFN
TEMPERATURE RANGE
–40°C to 125°C
–40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
Consult LTC Marketing for information on nonstandard lead based finish parts.
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/
2
3355fb
For more information
www.linear.com/LTC3355
LTC3355
The
l
denotes the specifications which apply over the specified operating
junction temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 12V unless otherwise noted. (Note 2)
SYMBOL
V
IN
I
Q
V
FB
I
FB
V
VOUT
PARAMETER
V
IN
Operating Voltage Range
V
IN
Quiescent Current
V
OUT
Quiescent Current
FB Reference Voltage
FB Line Regulation
FB Input Bias Current
V
OUT
Voltage Range
V
OUT
Overvoltage Limit
V
OUT
Undervoltage Lockout Threshold
V
INM5
V
INTVCC
V
VCAP
I
VCAP
V
IN
-V
INM5
INTV
CC
Internal Voltage Power Supply
V
CAP
Voltage Range
V
CAP
Current Accuracy
V
CAP
Programmable Current Range
V
ICHG
R
ICHG
I
CFB
V
CFB
I
CHG
Reference Voltage
I
CHG
Set Resistor Range
CFB Input Bias Current
CFB Reference Voltage
CFB Hysteresis
CFB Overvoltage Hysteretic Comparator
Switch Point
I
ICL
V
INS(CMI)
f
SW
V
PFI
I
PFI
I
SW1
t
SS
DC Max
R
PMOS
I
LEAKP
V
IN
Input Current Limit
V
INS
Common Mode Range
Switching Frequency
Foldback Frequency (Buck Only)
PFI Falling Threshold
PFI Hysteresis
PFI Leakage Current
SW1 Peak Current
Soft-Start Time
Maximum Duty Cycle
PMOS On-Resistance
PMOS Leakage Current
Buck Disabled
–2
FB = 0V
100
0.5
1
2
PWM Mode (Note 5)
Burst Mode
®
(Note 5)
–20
1.3
1.65
0.5
1000
1A Buck Regulator
2
A
A
µs
%
Ω
µA
FB ≥ 0.5V
FB ≤ 0.3V
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ELECTRICAL CHARACTERISTICS
CONDITIONS
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MIN
3
60
110
0.775
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TYP
120
265
0.1
MAX
20
215
420
0.825
20
5
UNITS
V
µA
µA
V
%/V
nA
V
V
V
V
V
V
%
A
V
kΩ
nA
V
mV
V
V
Charger Off, Not Switching, V
OUT
= 3.3V, No
Load, In Regulation, Supercapacitor Charged
V
OUT
= 2.7V to 5V
–20
2.7
5.4
1.8
2
0
V
CAP
= 2V, V
OUT
= 3.3V, I
VCAP
= 1A
EN_CHG = High
EN_CHG = High
EN_CHG = High
–10
0.1
0.78
60.4
–20
EN_CHG = High
EN_CHG = High
CFB Rising
CFB Falling
V
INS
-V
IN
to Disable Charger
V
INS
-V
IN
to Disable Buck
37
42
3.0
0.75
0.775
1
100
0.8
17
0.78
0.8
30
V
CFB
+0.035
V
CFB
5.65
2
4.65
Buck or Boost Enabled
Boost Enabled
V
IN
> 7V
5.95
2.2
5
5
10
1
0.82
604
20
0.82
43
50
20
1.25
0.825
20
mV
mV
V
MHz
kHz
V
mV
nA
3355fb
For more information
www.linear.com/LTC3355
3
LTC3355
The
l
denotes the specifications which apply over the specified operating
junction temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 12V unless otherwise noted. (Note 2)
SYMBOL
I
VOUT
I
SW2
R
NMOS
I
LEAKN
DC Max
V
SBOOST
A
V
g
m
V
IBSTPK
R
IBSTPK
V
IL
V
IH
I
IL
, I
IH
V
OL
I
OH
PARAMETER
V
OUT
Quiescent Current
SW2 Peak Current
NMOS On-Resistance
NMOS Leakage Current
Boost Maximum Duty Cycle
Boost Input Supply Voltage Range
Boost Minimum Input Supply
Boost Error Amplifier Voltage Gain
Boost Error Amplifier Transconductance
I
BSTPK
Reference Voltage
I
BSTPK
Set Resistor Range
Input Low Logic Voltage
Input High Logic Voltage
Input Low/High Current
Output Logic Low Voltage
Logic High Leakage Current
CPGOOD Rising Threshold
CPGOOD Hysteresis
RSTB Falling Threshold
RSTB Hysteresis
RSTB Delay
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:
The LTC3355 is tested under pulsed load conditions such that
T
J
≈ T
A
. The LTC3355E is guaranteed to meet specifications from
0°C to 85°C junction temperature. Specifications over the –40°C to
125°C operating junction temperature range are assured by design,
characterization and correlation with statistical process controls. The
LTC3355I is guaranteed over the –40°C to 125°C operating junction
temperature range. Note that the maximum ambient temperature
consistent with these specifications is determined by specific operating
conditions in conjunction with board layout, the rated package thermal
impedance and other environmental factors. The junction temperature
(T
J
, in °C) is calculated from the ambient temperature (T
A
, in °C) and
power dissipation (P
D
, in Watts) according to the formula:
T
J
= T
A
+ (P
D
•
θ
JA
)
where
θ
JA
= 47°C/W for the UF package.
MODE, EN_CHG
MODE, EN_CHG
MODE, EN_CHG
PFOB, CPGOOD, RSTB; Sink 100µA
PFOB, CPGOOD, RSTB; 5V
V
CAP
as a % of Final Target
∆V
CAP
as a % of Final Value
V
OUT
as a % of Final Target
∆V
OUT
as a % of Final Value
90
90
92.5
2.5
92.5
2.5
250
95
1.2
-1
1
50
1
95
0.775
200
V
OUT(MAX)
= 4V
(Note 5)
Boost Disabled
–5
88
0.75
0.5
850
27
0.825
1000
0.4
92
CONDITIONS
V
OUT
= 3.3V, No Load, In Regulation, No
Switching, Burst Mode
R
IBSTPK
= 200k, PWM Mode
R
IBSTPK
= 200k, Burst Mode
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ELECTRICAL CHARACTERISTICS
MIN
80
4.5
TYP
160
5
1.5
70
MAX
280
5.5
UNITS
µA
A
A
mΩ
µA
%
V
V
V/V
μS
V
kΩ
V
V
µA
mV
µA
%
%
%
%
ms
5A Boost Regulator
5
98
5
Logic (MODE, EN_CHG, CPGOOD, RSTB, PFOB)
Note 3:
The LTC3355 has a thermal regulation loop that limits the
maximum junction temperature to 110°C by limiting the charger current.
Note 4:
The current limit features of this part are intended to protect the
IC from short-term or intermittent fault conditions. Continuous operation
above the maximum specified pin current may result in device degradation
or failure.
Note 5:
Guaranteed by design and/or correlation to static test.
Note 6:
The LTC3355 has a thermal shutdown that will shut down the part
when the die temperature reaches 155°C.
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3355fb
For more information
www.linear.com/LTC3355
LTC3355
TYPICAL PERFORMANCE CHARACTERISTICS
Buck Efficiency
100
90
80
EFFICIENCY (%)
EFFICIENCY (%)
70
60
50
40
30
20
10
0
0
V
OUT
= 4V
MODE = HIGH
V
IN
= 18V
V
IN
= 12V
V
IN
= 6V
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
LOAD CURRENT (A)
3355 G01
T
A
= 25°C unless otherwise noted
Boost Efficiency
100
90
80
LOAD CURRENT (A)
70
60
50
40
30
20
10
0
0
V
CAP
= 2.4V
MODE = HIGH
V
OUT
= 3.3V
V
OUT
= 4V
V
OUT
= 5V
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
LOAD CURRENT (A)
3355 G02
Maximum Buck Load Current
1.20
1.00
0.80
0.60
0.40
0.20
0
V
OUT
= 4V
L = 6.8µH
INPUT CURRENT SET RESISTOR = 0
4
6
8
10
12 14
V
IN
(V)
16
18
20
3355 G03
Maximum Boost Load Current
1.2
1.0
SW VOLTAGE DROP (mV)
LOAD CURRENT (A)
0.8
0.6
0.4
0.2
0
0.75
V
OUT
= 4V
700
600
Buck Switch Voltage Drop
1200
1150
1100
FREQUENCY (kHz)
1050
1000
950
900
850
0 100 200 300 400 500 600 700 800 900 1000
SWITCH CURRENT (mA)
3355 G05
Oscillator Frequency
vs Temperature
500
400
300
200
100
1.25
1.75
2.25
2.75
3.25
3.75
3355 G04
0
800
–50 –25
0
V
CAP
(V)
75
50
25
TEMPERATURE (°C)
100
125
3355 G06
Buck Frequency
vs Feedback Voltage
1000
900
800
INPUT VOLTAGE (V)
FREQUENCY (kHz)
700
600
500
400
300
200
100
0
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
FB (V)
3355 G07
4.6
4.4
4.2
4.0
3.8
3.6
3.4
3.2
3.0
Typical Minimum Buck Input
Voltage (V
OUT
= 3.3V)
V
OUT
= 3.3V
L = 6.8µH
INPUT CURRENT SET RESISTOR = 0.05
6.3
6.1
5.9
5.7
V
IN
(V)
5.5
5.3
5.1
4.9
4.7
Typical Minimum Buck Input
Voltage (V
OUT
= 5V)
V
OUT
= 5V
L = 6.8µH
INPUT CURRENT SET RESISTOR = 0.05
1
10
100
V
OUT
LOAD CURRENT (mA)
1000
3355 G08
4.5
1
10
100
V
OUT
LOAD CURRENT (mA)
1000
3355 G09
3355fb
For more information
www.linear.com/LTC3355
5
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