LT3435
High Voltage 3A, 500kHz
Step-Down Switching Regulator
with 100µA Quiescent Current
DESCRIPTIO
The LT
®
3435 is a 500kHz monolithic buck switching
regulator that accepts input voltages up to 60V. A high
efficiency 3A, 0.1Ω switch is included on the die along with
all the necessary oscillator, control and logic circuitry.
Current mode topology is used for fast transient response
and good loop stability.
Innovative design techniques along with a new high volt-
age process achieve high efficiency over a wide input
range. Efficiency is maintained over a wide output current
range by employing Burst Mode operation at low currents,
utilizing the output to bias the internal circuitry, and by
using a supply boost capacitor to fully saturate the power
switch. Patented circuitry maintains peak switch current
over the full duty cycle range.* Shutdown reduces input
supply current to less than 1µA. External synchronization
can be implemented by driving the SYNC pin with logic-level
inputs. A single capacitor from the C
SS
pin to the output
provides a controlled output voltage ramp (soft-start). The
device also has a power good flag with a programmable
threshold and time-out and thermal shutdown protection.
The LT3435 is available in a 16-pin TSSOP package with an
exposed pad leadframe for low thermal resistance.
FEATURES
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■
■
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■
Wide Input Range: 3.3V to 60V
3A Peak Switch Current
Burst Mode
®
Operation: 100µA Quiescent Current**
Low Shutdown Current: I
Q
< 1µA
Power Good Flag with Programmable Threshold
Load Dump Protection to 60V
500kHz Switching Frequency
Saturating Switch Design: 0.1Ω On-Resistance
Peak Switch Current Maintained Over
Full Duty Cycle Range*
1.25V Feedback Reference Voltage
Easily Synchronizable
Soft-Start Capability
Small 16-Pin Thermally Enhanced TSSOP Package
APPLICATIO S
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High Voltage Power Conversion
14V and 42V Automotive Systems
Industrial Power Systems
Distributed Power Systems
Battery-Powered Systems
USB Powered Systems
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a registered trademark of Linear Technology Corporation. All other trademarks
are the property of their respective owners. *Protected by U.S. Patents including 6498466
**See Burst Mode Operation section for conditions.
TYPICAL APPLICATIO
14V to 3.3V Step-Down Converter with
100µA No Load Quiescent Current
V
IN
SHDN
LT3435
V
C
4700pF
10k
1µF
C
T
SYNC
GND
470pF
C
SS
V
BIAS
FB
PGFB
PG
27pF
165k
1%
100k
1%
4148
Supply Current vs Input Voltage
150
V
OUT
3.3V
2A
V
OUT
= 3.3V
T
A
= 25°C
SUPPLY CURRENT (µA)
0.1µF
EFFICIENCY (%)
4.7µF
100V
CER
V
IN
BOOST
0.33µF 15µH
SW
B360A
125
100
75
50
25
0
+
100µF
10V
TANT
3435 TA01
0
10
30
40
20
INPUT VOLTAGE (V)
50
60
3435 TA03
U
U
U
Efficiency and Power Loss
vs Load Current
100
90
80
70
60
50
40
30
20
10
0
0.0001
0.001
0.1
0.01
LOAD CURRENT (A)
1
0.001
10
3435 TA02
V
IN
= 12V
10
V
OUT
= 5V
EFFICIENCY
V
OUT
= 3.3V
1
POWER LOSS (W)
0.1
TYPICAL
POWER LOSS
0.01
3435fa
1
LT3435
ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
NC
SW
V
IN
V
IN
SW
BOOST
C
T
GND
1
2
3
4
5
6
7
8
17
16 PGOOD
15 SHDN
14 SYNC
13 PGFB
12 FB
11 V
C
10 BIAS
9
C
SS
V
IN
, SHDN, BIAS, PGOOD, SW ............................... 60V
BOOST Pin Above SW ............................................ 35V
BOOST Pin Voltage ................................................. 68V
SYNC, C
SS
, PGFB, FB ................................................ 6V
Operating JunctionTemperature Range
(Note 2) ........................................... – 40°C to 125°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
FE PACKAGE
16-LEAD PLASTIC TSSOP
T
JMAX
= 125°C,
θ
JA
= 45°C/W,
θ
JC(PAD)
= 10°C/W
EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO GND (PIN 8)
ORDER PART NUMBER
LT3435EFE
LT3435IFE
FE PART MARKING
3435EFE
3435IFE
Order Options
Tape and Reel: Add #TR
Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF
Lead Free Part Marking:
http://www.linear.com/leadfree/
Consult LTC Marketing for parts specified with wider operating temperature ranges.
The
●
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
J
= 25°C. V
IN
= 12V, SHDN = 12V, BIAS = 5V, FB/PGFB = 1.25V, C
SS
/SYNC = 0V
unless otherwise noted.
SYMBOL
V
SHDN
I
SHDN
I
VINS
PARAMETER
SHDN Threshold
SHDN Input Current
Minimum Input Voltage (Note 3)
Supply Shutdown Current
Supply Sleep Current (Note 4)
I
VIN
Supply Quiescent Current
Minimum BIAS Voltage (Note 5)
I
BIASS
I
BIAS
BIAS Sleep Current (Note 4)
BIAS Quiescent Current
Minimum Boost Voltage (Note 6)
Input Boost Current (Note 7)
V
REF
I
FB
Reference Voltage (V
REF
)
FB Input Bias Current
EA Voltage Gain (Note 8)
EA Voltage g
m
dI(V
C
)=
±10µA
400
SYNC = 3.3V
I
SW
= 1.5A
I
SW
= 3A
3.3V < V
VIN
< 60V
●
ELECTRICAL CHARACTERISTICS
CONDITIONS
SHDN = 12V
SHDN = 0V, BOOST = 0V, FB/PGFB = 0V
BIAS = 0V, FB = 1.35V
FB = 1.35V
BIAS = 0V, FB = 1.15V
BIAS = 5V, FB = 1.15V
●
●
●
●
●
●
MIN
1.15
TYP
1.3
5
2.4
0.1
170
45
3.3
2.6
2.7
125
700
1.8
65
MAX
1.45
20
3
2
250
75
7
6
3.1
180
900
85
1.275
200
900
UNITS
V
µA
V
µA
µA
µA
mA
mA
V
µA
µA
V
mA
V
nA
V/V
µMho
3435fa
1.225
1.25
75
900
650
2
U
W
U
U
W W
W
LT3435
The
●
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
J
= 25°C. V
IN
= 12V, SHDN = 12V, BIAS = 5V, FB/PGFB = 1.25V, C
SS
/SYNC = 0V
unless otherwise noted.
SYMBOL
PARAMETER
EA Source Current
EA Sink Current
V
C
to SW g
m
V
C
High Clamp
I
PK
SW Current Limit
Switch On Resistance (Note 9)
Switching Frequency
Maximum Duty Cycle
Minimum SYNC Amplitude
SYNC Frequency Range
SYNC Input Impedance
I
CSS
I
PGFB
V
PGFB
I
CT
V
CT
C
SS
Current Threshold (Note 10)
PGFB Input Current
PGFB Voltage Threshold (Note 11)
C
T
Source Current (Note 11)
C
T
Sink Current (Note 11)
C
T
Voltage Threshold (Note 11)
PG Leakage (Note 11)
PG Sink Current (Note 11)
PGFB = 1V, PG = 400mV
100
●
ELECTRICAL CHARACTERISTICS
CONDITIONS
FB = 1.15V
FB = 1.35V
FB = 1.15V
●
●
●
MIN
20
15
2.1
3
425
86
575
TYP
40
30
6
2.2
5.2
0.1
500
92
1.5
45
MAX
55
40
2.4
6.5
0.25
575
2.0
700
UNITS
µA
µA
A/V
V
A
Ω
kHz
%
V
kHz
kΩ
µA
nA
%
µA
mA
V
µA
µA
FB = 0V
7
88
2
1
1.16
13
25
90
3.6
2
1.2
0.1
200
20
100
92
5.5
1.26
1
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 LT3435EFE is guaranteed to meet performance specifications
from 0°C to 125°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
LT3435IFE is guaranteed and tested over the full –40°C to 125°C
operating junction temperature range.
Note 3:
Minimum input voltage is defined as the voltage where switching
starts. Actual minimum input voltage to maintain a regulated output will
depend upon output voltage and load current. See Applications
Information.
Note 4:
Supply input current is the quiescent current drawn by the input
pin. Its typical value depends on the voltage on the BIAS pin and operating
state of the LT3435. With the BIAS pin at 0V, all of the quiescent current
required to operate the LT3435 will be provided by the V
IN
pin. With the
BIAS voltage above its minimum input voltage, a portion of the total
quiescent current will be supplied by the BIAS pin. Supply sleep current is
defined as the quiescent current during the “sleep” portion of Burst Mode
operation. See Applications Information for determining application supply
currents.
Note 5:
Minimum BIAS voltage is the voltage on the BIAS pin when I
BIAS
is
sourced into the pin.
Note 6:
This is the minimum voltage across the boost capacitor needed to
guarantee full saturation of the internal power switch.
Note 7:
Boost current is the current flowing into the BOOST pin with the
pin held 3.3V above input voltage. It flows only during switch on time.
Note 8:
Gain is measured with a V
C
swing from 1.15V to 750mV.
Note 9:
Switch on resistance is calculated by dividing V
IN
to SW voltage by
the forced current (3A). See Typical Performance Characteristics for the
graph of switch voltage at other currents.
Note 10:
The C
SS
threshold is defined as the value of current sourced into
the C
SS
pin which results in an increase in sink current from the V
C
pin.
See the Soft-Start section in Applications Information.
Note 11:
The PGFB threshold is defined as the percentage of V
REF
voltage
which causes the current source output of the C
T
pin to change from
sinking (below threshold) to sourcing current (above threshold). When
sourcing current, the voltage on the C
T
pin rises until it is clamped
internally. When the clamp is activated, the output of the PG pin will be set
to a high impedance state. When the C
T
clamp is inactive the PG pin will
be set active low with a current sink capability of 200µA.
3435fa
3
LT3435
TYPICAL PERFOR A CE CHARACTERISTICS
FB Voltage
1.30
1.29
1.28
FREQUENCY (kHz)
1.27
VOLTAGE (V)
1.26
1.25
1.24
1.23
1.22
1.21
1.20
–50
–25
50
25
0
75
TEMPERATURE (°C)
100
125
510
500
490
480
470
460
450
–50 –25
50
25
0
75
TEMPERATURE (°C)
100
125
VOLTAGE (V)
SHDN Pin Current
5.5
5.0
4.5
4.0
T
J
= 25°C
CURRENT (µA)
CURRENT (µA)
3.5
3.0
2.5
2.0
1.5
1.0
0
0
10
30
40
20
SHDN VOLTAGE (V)
50
60
3435 G04
15
CURRENT (µA)
Bias Sleep Current
200
180
160
140
CURRENT (µA)
CURRENT (µA)
VOLTAGE (V)
120
100
80
60
40
20
0
–50 –25
50
25
0
75
TEMPERATURE (°C)
100
125
4
U W
3435 G01
3435 G07
Oscillator Frequency
550
540
530
520
1.25
1.20
0.15
1.10
1.05
1.40
1.35
1.30
SHDN Threshold
1.00
–50 –25
50
25
0
75
TEMPERATURE (°C)
100
125
3435 G02
3435 G03
Shutdown Supply Current
25
200
180
20
V
IN
= 60V
160
140
120
100
80
60
5
V
IN
= 42V
0
–50 –25
V
IN
= 12V
100
125
40
20
Sleep Mode Supply Current
V
BIAS
= 0V
10
V
BIAS
= 5V
50
25
0
75
TEMPERATURE (°C)
0
–50 –25
50
25
0
75
TEMPERATURE (°C)
100
125
3435 G05
3435 G06
PGFB Threshold
1.20
1.18
1.16
1.14
1.12
1.10
1.08
1.06
1.04
1.02
1.00
–50 –25
50
25
0
75
TEMPERATURE (°C)
100
125
50
150
200
250
PG Sink Current
100
0
–50 –25
50
25
0
75
TEMPERATURE (°C)
100
125
3435 G08
3435 G09
3435fa
LT3435
TYPICAL PERFOR A CE CHARACTERISTICS
Switch Peak Current Limit
6.0
5.5
5.0
4.5
4.0
3.5
3.0
–50 –25
50
45
PEAK SWITCH CURRENT (A)
NORMALIZED FREQUENCY (%)
CURRENT (µA)
–0
25
50
75
TEMPERATURE (°C)
500
450
400
350
VOLTAGE (mV)
300
250
200
150
100
50
0
Switch On Voltage (V
CESAT
)
8.0
7.5
INPUT VOLTAGE (V)
6.5
6.0
5.5
5.0
4.5
4.0
3.5
START-UP
V
OUT
= 5V
RUNNING
START-UP
V
OUT
= 3.3V
RUNNING
0
0.5
1.0
1.5
2.0
LOAD CURRENT (A)
3.0
T
J
= 25°C
LOAD CURRENT (mA)
T
J
= 125°C
T
J
= –40°C
0.5
1.0
2.0
1.5
LOAD CURRENT (A)
Minimum On-Time for Continuous
Mode Operation
600
550
500
LOAD CURRENT = 1A
1000
950
900
BOOST CURRENT (mA)
FREQUENCY (kHz)
450
ON TIME (ns)
400
350
300
250
200
150
100
–50 –25
50
25
0
75
TEMPERATURE (°C)
100
125
U W
100
3435 G10
Soft-Start Current Threshold
vs FB Voltage
T
J
= 25°C
100
90
80
70
60
50
40
30
20
10
0
0
0.2
0.6
0.8
0.4
FB VOLTAGE (V)
1.0
1.2
3435 G11
Oscillator Frequency
vs FB Voltage
40
35
30
25
20
15
10
5
0
125
SOFT-START
DEFEATED
0
0.25
0.50
0.75
FB VOLTAGE (V)
1.00
1.25
3435 G12
Minimum Input Voltage
1000
Burst Mode Threshold vs Input
Voltage
V
OUT
= 3.3V
900 L = 15µH
C
OUT
= 100µF
800 T = 25°C
A
700
600
500
400
300
200
100
0
2.5
BURST MODE EXIT
(INCREASING LOAD)
7.0
BURST MODE ENTER
(DECREASING LOAD)
2.5
3.0
3435 G13
3.0
5
10
15
INPUT VOLTAGE (V)
20
3435 G16
3435 G15
Maximum Synchronization
Frequency vs Temperature
70
60
50
40
30
20
10
50
25
0
75
TEMPERATURE (°C)
125
Boost Current vs Switch Current
850
800
750
700
650
600
550
500
–50 –25
100
0
0
500
1000 1500 2000 2500
SWITCH CURRENT (mA)
3000
3435 G17
3435 G14
3435 G18
3435fa
5
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