FEATURES
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LTC3708
Fast 2-Phase, No R
SENSE
Buck Controller with
Output Tracking
DESCRIPTION
The LTC
®
3708 is a dual, 2-phase synchronous step-down
switching regulator with output voltage up/down tracking
capability. The IC allows either coincident or ratiometric
tracking. Multiple LTC3708s can be daisy-chained in ap-
plications requiring more than two voltages to be tracked.
Power supply sequencing is accomplished using an
external soft-start timing capacitor.
The LTC3708 uses a constant on-time, valley current mode
control architecture to deliver very low duty factors without
requiring a sense resistor. Operating frequency is selected
by an external resistor and is compensated for variations in
input supply voltage. An internal phase-locked loop allows
the IC to be synchronized to an external clock.
Fault protection is provided by an output overvoltage
comparator and an optional short-circuit shutdown timer.
The regulator current limit level is user programmable. A
wide supply range allows voltages as high as 36V to be
stepped down to 0.6V output.
L,
LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
No R
SENSE
is a trademark of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
Very Low Duty Factor Operation (t
ON(MIN)
< 85ns)
No R
SENSE
™
Option for Maximum Efficiency
Very Fast Transient Response
Programmable Output Voltage Up/Down Tracking
2-Phase Operation Reduces Input Capacitance
0.6V ±1% Output Voltage Reference
External Frequency Synchronization
Monotonic Soft-Start
Onboard High Current MOSFET Drivers
Wide V
IN
Range: Up to 36V
Adjustable Cycle-by-Cycle Current Limit
Instant Output Overvoltage Protection
Optional Short-Circuit Shutdown Timer
Power Good Output with 100μs Masking
Available in 5mm
×
5mm QFN Package
APPLICATIONS
n
n
Digital Signal Processors
Network Servers
TYPICAL APPLICATION
High Efficiency Dual Output Step-Down Converter
5V
10Ω
1μF
4.7μF
100k
10μF
50V
4
V
IN
3.3V TO 28V
V
OUT1
(0.5V/DIV)
V
OUT2
(0.5V/DIV)
V
OUT1
2.5V
15A POSCAP
+
330μF
4V
2
12.1k
L1
1.4μH
B340A
M1
0.22μF
DRV
CC
PGOOD
V
CC
TG1
TG2
BOOST2
BOOST1
SW1
SENSE1
+
SW2
SENSE2
+
LTC3708
BG2
SENSE2
–
PGND2
f
IN
M3
19.1k
BG1
SENSE1
–
PGND1
V
IN
1.5M
6.04k
6.04k
0.01μF
33k
10k
0.01μF
V
FB1
V
FB2
TRACK2
FCB
I
ON1
I
ON2
I
TH1
I
TH2
INTLPF
EXTLPF
RUN/SS
TRACK1
V
RNG2
SGND
V
RNG1
1M
V
IN
EFFICIENCY (%)
180pF
0.1μF
L1: PANASONIC ETQP3HIR4BF
L2: PANASONIC ETQP2HIR2BF
+
M2
0.22μF
L2
1.2μH
B340A
M4
12.1k
POWER LOSS (W)
90
85
80
75
70
0.01
3708 TA01
+
V
OUT2
1.8V
POSCAP 15A
470μF
2.5V
2
2ms/DIV
100
95
3708 TA01b
9.0
7.5
6.0
4.5
20V
IN
TO 2.5VOUT
5V
IN
TO 2.5VOUT
20V
IN
TO 1.8VOUT
5V
IN
TO 1.8VOUT
3.0
1.5
0
1
0.1
LOAD CURRENT (A)
10
3708 TA01c
1k
100k
33k
0.1μF
0.01μF
6.04k
25k
5V
180pF
M1, M2: RENESAS HAT2168
M3, M4: RENESAS HAT2165
3708fb
1
LTC3708
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW
SENSE1
+
24 SENSE1
–
23 PGND1
22 BG1
33
21 DRV
CC
20 BG2
19 PGND2
18 SENSE2
–
17 V
CC
9 10 11 12 13 14 15 16
V
RNG2
SENSE2
+
EXTLPF
INTLPF
BOOST2
SW2
I
ON2
TG2
BOOST1
PGOOD
V
RNG1
SW1
I
ON1
TG1
FCB
Input Supply Voltage (V
CC
, DRV
CC
) ............. 7V to –0.3V
Boosted Topside Driver Supply Voltage
BOOST1, 2 ............................................ 42V to –0.3V
Switch Voltage (SW1, 2) .............................. 36V to –5V
SENSE1
+
, SENSE2
+
Voltages ....................... 36V to –5V
SENSE1
–
, SENSE2
–
Voltages .................... 10V to –0.3V
I
ON1
, I
ON2
Voltages .................................... 21V to –0.3V
(BOOST – SW) Voltages .............................. 7V to –0.3V
RUN/SS, PGOOD Voltages .......................... 7V to –0.3V
PGOOD DC Current ................................................. 5mA
TRACK1, TRACK2 Voltages ..............V
CC
+ 0.3V to –0.3V
V
RNG1
, V
RNG2
Voltages .................... V
CC
+ 0.3V to –0.3V
I
TH1
, I
TH2
Voltages.................................... 2.7V to –0.3V
V
FB1
, V
FB2
Voltages .................................. 2.7V to –0.3V
INTLPF EXTLPF Voltages ......................... 2.7V to –0.3V
,
FCB Voltages ............................................... 7V to –0.3V
Operating Temperature Range (Note 5).... –40°C to 85°C
Junction Temperature (Note 2) ........................... 125°C
Storage Temperature Range................... –65°C to 125°C
Reflow Peak Body Temperature ........................... 260°C
32 31 30 29 28 27 26 25
RUN/SS 1
I
TH1
2
V
FB1
3
TRACK1 4
SGND 5
TRACK2 6
V
FB2
7
I
TH2
8
UH PACKAGE
32-LEAD (5mm 5mm) PLASTIC QFN
T
JMAX
= 125°C,
θ
JA
= 34°C/W
EXPOSED PAD (PIN 33) IS SGND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
LTC3708EUH#PBF
LEAD BASED FINISH
LTC3708EUH
TAPE AND REEL
LTC3708EUH#TRPBF
TAPE AND REEL
LTC3708EUH#TR
PART MARKING
3708
PART MARKING
3708
PACKAGE DESCRIPTION
32-Lead (5mm
×
5mm) Plastic QFN
PACKAGE DESCRIPTION
32-Lead (5mm
×
5mm) Plastic QFN
TEMPERATURE RANGE
–40°C to 85°C
TEMPERATURE RANGE
–40°C to 85°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
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/
ELECTRICAL CHARACTERISTICS
SYMBOL
I
Q
PARAMETER
Input DC Supply Current
Normal
Shutdown
Feedback Pin Input Current
Internal Reference Voltage
Feedback Voltage
Feedback Voltage Line Regulation
Main Control Loop
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
CC
= 5V, DRV
CC
= 5V, unless otherwise noted.
CONDITIONS
MIN
TYP
MAX
UNITS
2.4
250
I
TH
= 1.2V (Notes 3, 4)
I
TH
= 1.2V, 0°C to 85°C (Notes 3, 4)
I
TH
= 1.2V (Notes 3, 4)
I
TH
= 1.2V (Note 3)
V
CC
= 4.5V to 6.5V (Note 3)
l
3
400
–100
0.606
0.609
0.606
mA
μA
nA
V
V
V
%/V
3708fb
I
FB1,2
V
REF
V
FB1,2
ΔV
FB(LINEREG)1,2
–50
0.594
0.591
0.594
0.600
0.600
0.600
0.02
2
LTC3708
ELECTRICAL CHARACTERISTICS
SYMBOL
g
m(EA)1,2
t
ON1,2
t
ON(MIN)1,2
t
OFF(MIN)1,2
V
SENSE(MAX)1,2
PARAMETER
Error Amplifier Transconductance
On-Time
Minimum On-Time
Minimum Off-Time
Maximum Current Sense Threshold
ΔV
FB(LOADREG)1,2
Feedback Voltage Load Regulation
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
CC
= 5V, DRV
CC
= 5V, unless otherwise noted.
CONDITIONS
I
TH
= 0.5V to 1.9V (Note 3)
I
TH
= 1.2V (Note 3)
I
ON
= 60μA, V
FCB
= 0V
I
ON
= 30μA, V
FCB
= 0V
I
ON
= 180μA
I
ON
= 30μA
V
RNG
= 1V, V
FB
= 0.565V
V
RNG
= 0V, V
FB
= 0.565V
V
RNG
= VCC, V
FB
= 0.565V
V
RNG
= 1V, V
FB
= 0.635V
V
RNG
= 0V, V
FB
= 0.635V
V
RNG
= V
CC
, V
FB
= 0.635V
8.5
–380
l
l
MIN
1.2
94
186
TYP
–0.05
1.45
116
233
50
270
MAX
–0.2
1.7
138
280
85
350
160
110
220
UNITS
%
mS
ns
ns
ns
ns
mV
mV
mV
mV
mV
mV
125
90
180
143
100
200
–62
–42
–88
10
–420
1.3
3
2.5
–1.2
2
3.2
3.5
2
2
3
1
–100
V
SENSE(MIN)1,2
Minimum Current Sense Threshold
ΔV
FB(OV)1,2
ΔV
FB(UV)1,2
V
RUN/SS(ON)
V
RUN/SS(LE)
V
RUN/SS(LT)
I
RUN/SS(C)
I
RUN/SS(D)
V
CC(UVLO)
V
CC(UVLOR)
TG R
UP1,2
TG R
DOWN1,2
BG R
UP1,2
BG R
DOWN1,2
Tracking
I
TRACK1,2
V
FB(TRACK1,2)
Overvoltage Fault Threshold
Undervoltage Fault Threshold
RUN Pin Start Threshold
RUN Pin Latchoff Enable Threshold
RUN Pin Latchoff Threshold
Soft-Start Charge Current
Soft-Start Discharge Current
Undervoltage Lockout
Undervoltage Lockout Release
TG Driver Pull-Up On-Resistance
TG Driver Pull-Down On-Resistance
BG Driver Pull-Up On-Resistance
BG Driver Pull-Down On-Resistance
TRACK Pin Input Current
Feedback Voltage at Tracking
RUN/SS Pin Rising
RUN/SS Pin Falling
V
RUN/SS
= 0V
V
RUN/SS
= V
RUN/SS(LE)
, V
FB1,2
= 0V
V
CC
Falling
V
CC
Rising
TG High (Note 6)
TG Low (Note 6)
BG High (Note 6)
BG Low (Note 6)
I
TH
= 1.2V, V
TRACK
= 0.2V (Note 3)
V
TRACK
= 0V, I
TH
= 1.2V (Note 3)
V
TRACK
= 0.2V, I
TH
= 1.2V (Note 3)
V
TRACK
= 0.4V, I
TH
= 1.2V (Note 3)
Either V
FB
Rising
Either V
FB
Falling
V
FB
Returning
I
PGOOD
= 5mA
V
PGOOD
= 7V
V
FB
Falling
Measured with a DC Voltage at FCB Pin
Measured with a AC Pulse at FCB Pin
f
FCB
< f
SW1
, V
EXTLPF
= 0V
f
FCB
> f
SW1
, V
EXTLPF
= 2.4
11.5
–460
1.8
3.3
2.8
–2
3
3.6
3.8
%
mV
V
V
V
μA
μA
V
V
Ω
Ω
Ω
Ω
0.8
2.6
2.2
–0.5
0.8
–150
–10
210
410
11.5
–11.5
5
0.4
±1
nA
mV
mV
mV
%
%
%
V
μA
μs
–10
190
390
8.5
–8.5
0
200
400
10
–10
3
0.1
PGOOD Output
ΔV
FBH1,2
ΔV
FBL1,2
ΔV
FB(HYS)1,2
V
PGL
I
PGOOD
PG Delay
V
FCB(DC)
V
FCB(AC)
I
EXTLPF
PGOOD Upper Threshold
PGOOD Lower Threshold
PGOOD Hysteresis
PGOOD Low Voltage
PGOOD Leakage Current
PGOOD Delay
Forced Continuous Threshold
Clock Input Threshold
External Phase Detector Output Current
Sourcing Capability
Sinking Capability
100
1.9
1
2.1
1.5
20
–20
2.3
2
Phase-Locked Loops
V
V
μA
μA
3708fb
3
LTC3708
ELECTRICAL CHARACTERISTICS
SYMBOL
I
INTLPF
PARAMETER
Internal Phase Detector Output Current
Sourcing Capability
Sinking Capability
t
ON1
Modulation Range by External PLL
Up Modulation
Down Modulation
t
ON2
Modulation Range by Internal PLL
Up Modulation
Down Modulation
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
CC
= 5V, DRV
CC
= 5V, unless otherwise noted.
CONDITIONS
f
SW1
< f
SW2
, V
INTLPF
= 0V
f
SW1
> f
SW2
, V
INTLPF
= 2.4
I
ON1
= 60μA, V
EXTLPF
= 1.8V
I
ON1
= 60μA, V
EXTLPF
= 0.6V
I
ON1
= 60μA, V
EXTLPF
= 1.8V
I
ON1
= 60μA, V
EXTLPF
= 0.6V
186
MIN
TYP
20
–20
233
58
233
58
MAX
UNITS
μA
μA
ns
ns
ns
ns
t
ON(PLL)1
80
t
ON(PLL)2
186
80
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
reliabilty and lifetime.
Note 2:
T
J
is calculated from the ambient temperature T
A
and power
dissipation P
D
as follows:
T
J
= T
A
+ (P
D
• 34°C/W)
Note 3:
The LTC3708 is tested in a feedback loop that adjusts V
FB
to
achieve a specified error amplifier output voltage (I
TH
).
Note 4:
Internal reference voltage is tested indirectly by extracting error
amplifier offset from the feedback voltage.
Note 5:
The LTC3708E is guaranteed to meet performance specifications
from 0°C to 85°C. Specifications over the –40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls.
Note 6:
R
DS(ON)
limit is guaranteed by design and/or correlation to static
test.
TYPICAL PERFORMANCE CHARACTERISTICS
Load Transient on Channel 1
I
OUT1
10A/DIV
I
OUT2
10A/DIV
Load Transient on Channel 2
V
OUT1
100mV/DIV
V
OUT1
100mV/DIV
V
OUT2
100mV/DIV
20μs/DIV
3708 G01
V
OUT2
100mV/DIV
20μs/DIV
3708 G02
Coincident Tracking
Ratiometric Tracking
V
OUT1
0.5V/DIV
V
OUT2
0.5V/DIV
V
OUT1
0.5V/DIV
V
OUT2
0.5V/DIV
2ms/DIV
3708 G03
2ms/DIV
3708 G04
3708fb
4
LTC3708
TYPICAL PERFORMANCE CHARACTERISTICS
Soft-Start
V
OUT1
2V/DIV
V
OUT2
2V/DIV
RUN/SS
5V/DIV
POWER LOSS (W)
6
5
V
OUT
= 2.5V
4
3
2
1
0
5
10
15
20
INPUT VOLTAGE (V)
25
3708 G06
Power Loss vs Input Voltage
I
OUT
= 15A
I
L1
5A/DIV
V
OUT
= 1.8V
50ms/DIV
3708 G05
Power Loss vs Load Current
3.5
3.0
POWER LOSS (W)
2.5
V
OUT
= 1.8V
2.0
1.5
1.0
0.5
0
10
100
1000
LOAD (mA)
3707 G07
Frequency vs Input Voltage
260
250
I
OUT
= 15A
FREQUENCY (kHz)
Frequency vs Load Current
V
IN
= 5V
V
OUT
= 2.5V
FREQUENCY (kHz)
240
200
220
EXTERNAL SYNCHRONIZATION (ANY I
OUT
)
I
OUT
= 0A
150
200
100
FORCED CONTINUOUS MODE
EXTERNAL
SYNCHRONIZATION
DISCONTINUOUS MODE
0
10
5
LOAD CURRENT (A)
15
3708 G09
180
50
10000
100000
160
5
10
15
INPUT VOLTAGE (V)
20
0
25
3708 G08
On-Time vs I
ON
Current
10000
300
On-Time vs Temperature
300
CURRENT SENSE THRESHOLD (mV)
I
ON
= 30μA
250
250
200
150
100
50
0
–50
–100
–150
–200
Current Sense Threshold
vs I
TH
Voltage
V
RNG
=
2V
1.4V
1V
0.7V
0.5V
ON-TIME (ns)
ON-TIME (ns)
1000
200
150
100
50
I
ON
= 60μA
100
10
1
10
100
I
ON
CURRENT (μA)
1000
3708 G10
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3708 G11
0
0.5
1.5
1
I
TH
VOLTAGE (V)
2
2.5
3708 G12
3708fb
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