FEATURES
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LT3501
Monolithic Dual Tracking
3A Step-Down Switching
Regulator
DESCRIPTION
The LT
®
3501 is a dual current mode PWM step-down
DC/DC converter with two internal 3.5A switches. Inde-
pendent input voltage, feedback, soft-start and power
good pins for each channel simplify complex power
supply tracking/sequencing requirements.
Both converters are synchronized to either a common
external clock input or a resistor programmable fixed
250kHz to 1.5MHz internal oscillator. At all frequencies, a
180° phase relationship between channels is maintained,
reducing voltage ripple and component size. Programmable
frequency allows for optimization between efficiency and
external component size.
Minimum input-to-output voltage ratios are improved
by allowing the switch to stay on through multiple clock
cycles, only switching off when the boost capacitor needs
recharging, resulting in ~95% maximum duty cycle.
Each output can be independently disabled using its own
soft-start pin, or by using the SHDN pin the entire part can
be placed in a low quiescent current shutdown mode.
The LT3501 is available in a 20-lead TSSOP package with
exposed leadframe for low thermal resistance.
, LT, LTC, and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
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Wide Input Range: 3.1V to 25V
Two Switching Regulators with 3A Output Capability
Independent Supply to Each Regulator
Adjustable/Synchronizable Fixed Frequency
Operation from 250kHz to 1.5MHz
Antiphase Switching
Outputs Can be Paralleled
Independent, Sequential, Ratiometric or Absolute
Tracking Between Outputs
Independent Soft-Start and Power Good Pins
Enhanced Short-Circuit Protection
Low Dropout: 95% Maximum Duty Cycle
Low Shutdown Current: <10µA
20-Lead TSSOP Package with Exposed Leadframe
APPLICATIONS
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DSP Power Supplies
Disc Drives
DSL/Cable Modems
Wall Transformer Regulation
Distributed Power Regulation
TYPICAL APPLICATION
3.3V and 1.8V Dual 3A Step-Down Converter with Output Tracking
V
IN
12V
Efficiency
4.7µF
V
IN1
SHDN
4.7µH
BST1
0.47µF
SW1
SW2
LT3501
IND1
V
OUT1
47µF
24.9k
IND2
V
OUT2
10k
B360A
V
OUT2
1.8V
100µF 3A
B360A
V
IN2
R
T
/SYNC
BST2
0.47µF
61.9k
100
90
V
OUT1
= 5V
V
OUT1
= 3.3V
3.3µH
EFFICIENCY (%)
80
70
60
50
40
30
20
10
V
IN
= 12V
I
OUT2
= 0A
FREQUENCY = 500kHz
0
0.5
1.5
2
1
LOAD CURRENT (A)
2.5
3
V
OUT1
= 1.8V
V
OUT1
= 2.5V
PMEG4005
V
OUT1
3.3V
3A
PMEG4005
470pF
8.06k
10pF
40.2k
PG1
PG2
FB1
FB2
V
C1
V
C2
SS/TRACK1 SS/TRACK2
GND
0.1µF
470pF
40.2k
47pF
8.06k
3501 TA01a
0
3501 TA01b
3501fb
1
LT3501
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PACKAGE/ORDER INFORMATION
TOP VIEW
V
IN1
SW1
IND1
V
OUT1
PG1
PG2
V
OUT2
IND2
SW2
1
2
3
4
5
6
7
8
9
21
20 BST1
19 SS/TRACK1
18 V
C1
17 FB1
16 R
T
/SYNC
15 SHDN
14 FB2
13 V
C2
12 SS/TRACK2
11 BST2
V
IN1/2
, SHDN, PG1/2 ...................................... 25V/–0.3V
SW1/2 ....................................................................V
IN1/2
BST1/2 ........................................................... 35V/–0.3V
BST1/2 Pins Above SW1/2........................................25V
IND1/2 .....................................................................±5A
V
OUT1/2
........................................................ V
IN1/2
/–0.3V
FB1/2, SS1/2, R
T
/SYNC ............................................5.5V
V
C1/2
......................................................................±1mA
Operating Junction Temperature Range
LT3501EFE (Notes 2, 8) ..................... –40°C to 125°C
LT3501IFE (Notes 2, 8) ...................... –40°C to 125°C
Storage Temperature Range................... –65°C to 150°C
Lead Temperature (Soldering, 10 sec) .................. 300°C
V
IN2
10
FE PACKAGE
20-LEAD PLASTIC TSSOP
T
JMAX
= 125°C,
θ
JA
= 45°C/W,
θ
JC(PAD)
= 10°C/W
EXPOSED PAD (PIN 21) IS GND, MUST BE SOLDERED TO PCB
ORDER PART NUMBER
LT3501EFE
LT3501IFE
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.
ELECTRICAL CHARACTERISTICS
PARAMETER
SHDN Threshold
SHDN Input Current
Minimum Input Voltage Ch 1 (Note 3)
Minimum Input Voltage Ch 2
Supply Shutdown Current Ch 1
Supply Shutdown Current Ch 2
Supply Quiescent Current Ch 1
Supply Quiescent Current Ch 2
Feedback Voltage Ch 1/2
Feedback Voltage Line Regulation
Feedback Voltage Offset Ch 1 to Ch 2
Feedback Bias Current Ch 1/2
Error Amplifier g
m
Ch 1/2
Error Amplifier Gain Ch 1/2
Error Amplifier to Switch Gain Ch 1/2
CONDITIONS
The
●
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
J
= 25°C. V
VIN1/2
= 15V, V
BST1/2
= open, V
RT/SYNC
= 2V, V
VOUT1/2
= open,
unless otherwise specified.
MIN
●
TYP
1.28
10
3
2.8
2.8
MAX
1.37
13
5
3
3
30
5
5
500
0.816
1
16
250
450
UNITS
V
µA
µA
V
V
µA
µA
mA
µA
V
%
mV
nA
µmho
V/V
A/V
3501fb
V
OUT1/2
= 0V, R
T
/SYNC = 133k
V
SHDN
= 1.375V
V
SHDN
= 1.225V
V
FB1/2
= 0V, V
VOUT1/2
= 0V, V
IND1/2
= 0V, R
T
/SYNC = 133k
V
FB1/2
= 0V, V
VOUT1/2
= 0V, V
IND1/2
= 0V
V
SHDN
= 0V
V
SHDN
= 0V
V
FB1/2
= 0.9V
V
FB1/2
= 0.9V
V
VC1/2
= 1V
V
VIN1/2
= 3V to 25V
V
VC1/2
= 1V
V
FB1/2
= 0.8V, V
VC1/2
= 1V
V
VC1/2
= 1V, I
VC1/2
= ±5µA
1.23
7
2
●
9
0
3.5
200
●
●
●
●
●
0.784
–1
–16
–250
150
0.8
0
0
75
275
1000
3.3
2
LT3501
ELECTRICAL CHARACTERISTICS
PARAMETER
Error Amplifier Source Current Ch 1/2
Error Amplifier Sink Current Ch 1/2
Error Amplifier High Clamp Ch 1/2
Error Amplifier Switching Threshold Ch 1/2
Soft-Start Source Current Ch 1/2
Soft-Start V
OH
Ch 1/2
Soft-Start Sink Current Ch 1/2
Soft-Start V
OL
Ch 1/2
Soft-Start to Feedback Offset Ch 1/2
Soft-Start Sink Current Ch 1/2 POR
Soft-Start POR Threshold Ch 1/2
Soft-Start Switching Threshold Ch 1/2
Power Good Leakage Ch 1/2
Power Good Threshold Ch 1/2
Power Good Hysteresis Ch 1/2
Power Good Sink Current Ch 1/2
Power Good Shutdown Sink Current Ch 1/2
R
T
/SYNC Reference Voltage
Switching Frequency
Switching Phase Angle Ch A to Ch B
Minimum Boost for 100% Duty Cycle Ch 1/2
SYNC Frequency Range
SYNC Switching Phase Angle Ch A to Ch B
IND + V
OUT
Current Ch 1/2
IND to V
OUT
Maximum Current Ch 1/2
Switch Leakage Current Ch 1/2
Switch Saturation Voltage Ch 1/2
Boost Current Ch 1/2
Minimum Boost Voltage Ch 1/2
CONDITIONS
V
FB1/2
= 0.6V, V
VC1/2
= 1V
V
FB1/2
= 1V, V
VC1/2
= 1V
V
FB1/2
= 0.7V
V
OUT1/2
= 5V, R
T
/SYNC = 133k
V
FB1/2
= 0.6V, V
SS1/2
= 0.4V
V
FB1/2
= 0.9V
V
FB1/2
= 0.6V, V
SS1/2
= 1V
V
FB1/2
= 0V
V
VC1/2
= 1V, V
SS1/2
= 0.4V
V
SS1/2
= 0.4V (Note 4), V
VC
= 1V
V
FB1/2
= 0V (Note 4)
V
FB1/2
= 0V
V
FB1/2
= 0.9V, V
PG1/2
= 25V, V
VIN1/2
= 25V
V
FB1/2
Rising, PG1/2 = 20k to 5V
V
FB1/2
Falling, PG1/2 = 20k to 5V
V
FB1/2
= 0.65V, V
PG1/2
= 0.4V
V
VIN1/2
= 2V, V
FB1/2
= 0V, V
PG1/2
= 0.4V
V
FB1/2
= 0.9V, I
RT/SYNC
= –40µA
R
T
/SYNC = 133k, V
FB1/2
= 0.6V, V
BST1/2
= V
SW
+ 3V
R
T
/SYNC = 15.4k, V
FB1/2
= 0.6V, V
BST1/2
= V
SW
+ 3V
R
T
/SYNC = 133k, V
FB1/2
= 0.6V, V
BST1/2
= V
SW
+ 3V
V
FB1/2
= 0.7V, I
RT/SYNC
= –35µA (Note 5), V
OUT
= 0V
V
BST1/2
= V
SW
+ 3V
SYNC Frequency = 250kHz, V
BST1/2
= V
SW
+ 3V
V
VOUT1/2
= 0V, V
FB1/2
= 0.9V
V
VOUT1/2
= 5V
V
VOUT1/2
= 0.5V (Note 6), V
FB1/2
= 0.7V, V
BST1/2
= 20V
V
VOUT1/2
= 5V (Note 6), R
T
/SYNC = 133k, V
BST1/2
= 20V
V
SW1/2
= 0V, V
VIN1/2
= 25V
I
SW1/2
= 3A, V
BST1/2
= 20V, V
FB1/2
= 0.7V
I
SW1/2
= 3A, V
BST1/2
= 20V, V
FB1/2
= 0.7V
I
SW1/2
= 3A, V
BST1/2
= 20V, V
FB1/2
= 0.7V
●
●
●
●
●
The
●
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
J
= 25°C. V
VIN1/2
= 15V, V
BST1/2
= open, V
RT/SYNC
= 2V, V
VOUT1/2
= open,
unless otherwise specified.
MIN
10
15
1.75
0.5
2.5
1.9
200
50
–16
0.5
55
30
87
20
400
10
0.93
200
1.2
120
250
120
40
3.25
3.5
180
70
0
4
4
0
250
25
60
1.4
TYP
15
20
2.0
0.7
3.25
2
600
80
0
1.5
80
50
0
90
30
800
50
0.975
250
1.5
180
1.7
MAX
25
30
2.25
1.0
4
2.4
1000
125
16
2
105
70
1
93
50
1200
100
1
300
1.8
210
2
1500
210
100
1
5
5
50
600
100
2.5
UNITS
µA
µA
V
V
µA
V
µA
mV
mV
mA
mV
mV
µA
%
mV
µA
µA
V
kHz
MHz
Deg
V
kHz
Deg
µA
µA
A
A
µA
mV
mA
V
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 LT3501EFE 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
LT3501IFE 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 internal
bias lines are regulated so that the reference voltage and oscillator remain
constant. Actual minimum input voltage to maintain a regulated output
will depend upon output voltage and load current. See Applications
Information.
Note 4:
An internal power-on reset (POR) latch is set on the positive
transition of the SHDN pin through its threshold. The output of the latch
activates current sources on each SS pin which typically sink 1.5mA,
discharging the SS capacitor. The latch is reset when both SS pins are
driven below the soft-start POR threshold or the SHDN pin is taken below
its threshold.
Note 5:
To enhance dropout operation, the output switch will be turned off
for the minimum off time only when the voltage across the boost capacitor
drops below the minimum boost for 100% duty cycle threshold.
3501fb
3
LT3501
ELECTRICAL CHARACTERISTICS
Note 6:
The IND to V
OUT
maximum current is defined as the value of
current flowing from the IND pin to the V
OUT
pin which resets the switch
latch when the V
C
pin is at its high clamp.
Note 7:
This is the minimum voltage across the boost capacitor needed to
guarantee full saturation of the internal power switch.
Note 8:
This IC includes overtemperature protection that is intended
to protect the device during momentary overload conditions. Junction
temperature will exceed 125°C when overtemperature protection is active.
Continuous operation above the specified maximum operating junction
temperature may impair device reliability.
TYPICAL PERFORMANCE CHARACTERISTICS
Feedback Voltage vs Temperature
0.816
0.811
0.806
0.801
0.796
0.791
0.786
–50 –25
0.97
1.05
R
T
/SYNC Voltage vs Temperature
3.0
2.5
2.0
1.5
1.0
0.5
Shutdown Threshold and Minimum
Input Voltage vs Temperature
1.03
VOLTAGE (V)
MINIMUM INPUT
VOLTAGE
SHUTDOWN
THRESHOLD
VOLTAGE
VOLTAGE (V)
1.01
0.99
50
25
75
0
TEMPERATURE (°C)
100
125
0.95
–50
VOLTAGE (V)
–25
50
25
0
75
TEMPERATURE (°C)
100
125
0
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
3501 G02
3501 G03
3501 G04
Shutdown Quiescent Current
vs Temperature
16
14
12
CURRENT (µA)
10
8
6
4
2
V
VIN2
0
–50 –25
0
50
75
25
TEMPERATURE (°C)
100
125
V
VIN1
CURRENT (µA)
4.0
3.8
3.6
3.4
3.2
3.0
2.8
2.6
2.4
2.2
Soft-Start Source Current
vs Temperature
5.0
4.8
4.6
4.4
CURRENT (A)
4.2
4.0
3.8
3.6
3.4
3.2
50
25
0
75
TEMPERATURE (°C)
100
125
IND to V
OUT
Maximum Current
vs Temperature
V
OUT
= 5V
V
OUT
= 0V
2.0
–50 –25
3.0
–50 –25
50
25
0
75
TEMPERATURE (°C)
100
125
3501 G05
3501 G07
3501 G30
3501fb
4
LT3501
TYPICAL PERFORMANCE CHARACTERISTICS
Soft-Start to Feedback Offset
Voltage vs Temperature
4
3
2
VOLTAGE (mV)
V
OUT
= 5V
VOLTAGE (V)
1
0
–1
–2
–3
–4
–50
–25
0
50
75
25
TEMPERATURE (°C)
100
125
500
400
–50 –25
VOLTAGE (V)
800
700
V
OUT
= 0V
600
740
720
700
680
660
640
620
50
25
75
0
TEMPERATURE (°C)
100
125
600
–50 –25
50
25
0
75
TEMPERATURE (°C)
100
125
RISING
FALLING
1000
900
V
C
Switching Threshold Voltage
vs Temperature
800
780
760
Power Good Threshold Voltage
vs Temperature
3501 G08
3501 G09
3501 G10
Power Good Sink Current
vs Temperature
1000
950
900
850
CURRENT (µA)
TIME (ns)
800
750
700
650
600
550
500
–50 –25
50
25
0
75
TEMPERATURE (°C)
100
125
250
230
210
Minimum Switching Times
vs Temperature
300
290
280
MINIMUM ON TIME
FREQUENCY (kHz)
270
260
250
240
230
220
210
50
25
0
75
TEMPERATURE (°C)
100
125
Switching Frequency and Channel
Phase vs Temperature
R
T
/SYNC = 133kΩ
200
190
180
PHASE
170
PHASE (DEG)
160
150
FREQUENCY
140
130
120
110
50
25
0
75
TEMPERATURE (°C)
100
100
125
190
170
150
130
110
90
70
MINIMUM OFF TIME
50
–50 –25
200
–50 –25
3501 G11
3501 G12
3501 G13
Switching Frequency and Channel
Phase vs Temperature
1650
1600
FREQUENCY (kHz)
1550
FREQUENCY
1500
1450
1400
1350
–50 –25
R
RT
/SYNC = 15.4k
PHASE
200
195
190
FREQUENCY (kHz)
185
PHASE (DEG)
180
175
170
165
160
155
50
25
75
0
TEMPERATURE (°C)
100
150
125
2000
2500
Syncronization Clock Frequency
Range vs Temperature
188
186
164
PHASE (DEG)
MAXIMUM
SYNCHRONIZATION
FREQUENCY
182
180
178
176
174
500
MINIMUM
SYNCHRONIZATION
FREQUENCY
–25
50
25
0
75
TEMPERATURE (°C)
100
125
172
170
Channel Phase vs Temperature
with External Synchronization
1500
SYNCHRONIZATION
FREQUENCY = 250kHz
1000
SYNCHRONIZATION
FREQUENCY = 1500kHz
50
25
0
75
TEMPERATURE (°C)
100
125
0
–50
168
–50 –25
3501 G14
3501 G15
3501 G16
3501fb
5
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