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 ex-
ternal 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, LTM, Linear Technology, the Linear logo and PolyPhase are registered trademarks
and 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
10
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
+
BG1
SENSE1
–
PGND1
LTC3708
SW2
SENSE2
+
BG2
SENSE2
–
PGND2
f
IN
M3
19.1k
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
25k
1M
EFFICIENCY (%)
180pF
0.1µF
L1: PANASONIC ETQP3HIR4BF
L2: PANASONIC ETQP2HIR2BF
For more information
www.linear.com/LTC3708
+
M2
0.22µF
L2
1.2µH
B340A
M4
12.1k
+
V
OUT2
1.8V
POSCAP 15A
470µF
2.5V
×2
2ms/DIV
100
95
90
85
80
75
70
0.01
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
POWER LOSS (W)
V
IN
1k
100k
5V
33k
0.1µF
180pF
3708 TA01
6.04k
0.01µF
M1, M2: RENESAS HAT2168
M3, M4: RENESAS HAT2165
1
0.1
LOAD CURRENT (A)
10
3708 TA01c
3708fc
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
EXTLPF
INTLPF
BOOST2
SW2
I
ON2
TG2
SENSE2
+
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
BOOST1
PGOOD
V
RNG1
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
http://www.linear.com/product/LTC3708#orderinfo
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/
.
Some packages are available in 500 unit reels through
designated sales channels with #TRMPBF suffix.
SW1
I
ON1
TG1
FCB
3708fc
2
For more information
www.linear.com/LTC3708
LTC3708
ELECTRICAL CHARACTERISTICS
SYMBOL
I
Q
I
FB1,2
V
REF
V
FB1,2
∆V
FB(LINEREG)1,2
g
m(EA)1,2
t
ON1,2
t
ON(MIN)1,2
t
OFF(MIN)1,2
V
SENSE(MAX)1,2
V
SENSE(MIN)1,2
∆V
FB(OV)1,2
∆V
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)
TRACK Pin Input Current
Feedback Voltage at Tracking
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)
–10
190
390
–100
0
200
400
–150
–10
210
410
nA
mV
mV
mV
PARAMETER
Input DC Supply Current
Normal
Shutdown
Feedback Pin Input Current
Internal Reference Voltage
Feedback Voltage
Feedback Voltage Line Regulation
Error Amplifier Transconductance
On-Time
Minimum On-Time
Minimum Off-Time
Maximum Current Sense Threshold
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)
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
l
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
Main Control Loop
2.4
250
–50
0.594
0.591
0.594
0.600
0.600
0.600
0.02
–0.05
1.2
94
186
1.45
116
233
50
270
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
11.5
–460
1.8
3.3
2.8
–2
3
3.6
3.8
–0.2
1.7
138
280
85
350
160
110
220
3
400
–100
0.606
0.609
0.606
mA
µA
nA
V
V
V
%/V
%
mS
ns
ns
ns
ns
mV
mV
mV
mV
mV
mV
%
mV
V
V
V
µA
µA
V
V
Ω
Ω
Ω
Ω
∆V
FB(LOADREG)1,2
Feedback Voltage Load Regulation
Minimum Current Sense Threshold
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
RUN/SS Pin Rising
RUN/SS Pin Falling
V
RUN/SS
= 0V
V
RUN/SS
= V
RUN/SS(LE)
, V
FB1
or V
FB2
= 0V
V
CC
Falling
V
CC
Rising
TG High (Note 6)
TG Low (Note 6)
BG High (Note 6)
BG Low (Note 6)
0.8
2.6
2.2
–0.5
0.8
3708fc
For more information
www.linear.com/LTC3708
3
LTC3708
ELECTRICAL CHARACTERISTICS
SYMBOL
PGOOD Output
∆V
FBH1,2
∆V
FBL1,2
∆V
FB(HYS)1,2
V
PGL
I
PGOOD
PG Delay
V
FCB(DC)
V
FCB(AC)
I
EXTLPF
I
INTLPF
t
ON(PLL)1
t
ON(PLL)2
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
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
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
= 2.4V
f
SW1
< f
SW2
, V
INTLPF
= 0V
f
SW1
> f
SW2
, V
INTLPF
= 2.4V
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
100
1.9
1
2.1
1.5
20
–20
20
–20
233
58
233
58
2.3
2
8.5
–8.5
10
–10
3
0.1
11.5
–11.5
5
0.4
±1
%
%
%
V
µA
µs
V
V
µA
µA
µA
µA
ns
ns
ns
ns
PARAMETER
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
Phase-Locked Loops
80
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.
3708fc
4
For more information
www.linear.com/LTC3708
LTC3708
TYPICAL PERFORMANCE CHARACTERISTICS
Load Transient on Channel 1
I
OUT1
10A/DIV
V
OUT1
100mV/DIV
I
OUT2
10A/DIV
V
OUT1
100mV/DIV
Load Transient on Channel 2
V
OUT2
100mV/DIV
20µs/DIV
3708 G01
V
OUT2
100mV/DIV
20µs/DIV
3708 G02
Coincident Tracking
V
OUT1
0.5V/DIV
V
OUT2
0.5V/DIV
Ratiometric Tracking
V
OUT1
0.5V/DIV
V
OUT2
0.5V/DIV
2ms/DIV
3708 G03
2ms/DIV
3708 G04
Soft-Start
V
OUT1
2V/DIV
V
OUT2
2V/DIV
POWER LOSS (W)
RUN/SS
5V/DIV
6
5
4
3
2
1
0
Power Loss vs Input Voltage
I
OUT
= 15A
V
OUT
= 2.5V
I
L1
5A/DIV
V
OUT
= 1.8V
50ms/DIV
3708 G05
5
10
15
20
INPUT VOLTAGE (V)
25
3708 G06
3708fc
For more information
www.linear.com/LTC3708
5
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