FEATURES
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LTC3868-1
Low I
Q
, Dual
2-Phase Synchronous
Step-Down Controller
DESCRIPTION
The LTC
®
3868-1 is a high performance dual step-down
switching regulator controller that drives all N-channel
synchronous power MOSFET stages. A constant frequency
current mode architecture allows a phase-lockable fre-
quency of up to 850kHz. Power loss and noise due to the
input capacitor ESR are minimized by operating the two
controller outputs out of phase.
The 170μA no-load quiescent current extends operating
life in battery powered systems. OPTI-LOOP compensation
allows the transient response to be optimized over a wide
range of output capacitance and ESR values. The LTC3868-
1 features a precision 0.8V reference and a power good
output indicator. A wide 4V to 24V input supply range
encompasses a wide range of intermediate bus voltages
and battery chemistries.
Independent soft-start pins for each controller ramp the
output voltages during start-up. Current foldback limits
MOSFET heat dissipation during short-circuit conditions.
The output short-circuit latchoff feature further protects
the circuit in short-circuit conditions.
For a leadless 32-pin QFN package with the additional fea-
tures of adjustable current limit, clock out, phase modula-
tion and two PGOOD outputs, see the LTC3868 data sheet.
L,
LT, LTC, LTM, Burst Mode, OPTI-LOOP μModule, Linear Technology and the Linear logo
,
are registered trademarks and No R
SENSE
and UltraFast are trademarks of Linear Technology
Corporation. All other trademarks are the property of their respective owners. Protected by U.S.
Patents, including 5481178, 5705919, 5929620, 6100678, 6144194, 6177787, 6304066, 6580258.
Low Operating I
Q
: 170μA (One Channel On)
Wide Output Voltage Range: 0.8V ≤ V
OUT
≤ 14V
Wide V
IN
Range: 4V to 24V
R
SENSE
or DCR Current Sensing
Out-of-Phase Controllers Reduce Required Input
Capacitance and Power Supply Induced Noise
OPTI-LOOP
®
Compensation Minimizes C
OUT
Phase-Lockable Frequency (75kHz to 850kHz)
Programmable Fixed Frequency (50kHz to 900kHz)
Selectable Continuous, Pulse-Skipping or
Burst Mode
®
Operation at Light Loads
Very Low Dropout Operation: 99% Duty Cycle
Adjustable Output Voltage Soft-Start
Power Good Output Voltage Monitor
Output Overvoltage Protection
Output Latchoff Protection During Short Circuit
Low Shutdown I
Q
: 8μA
Internal LDO Powers Gate Drive from V
IN
or EXTV
CC
No Current Foldback During Start-Up
Small 4mm
×
5mm QFN and Narrow SSOP Packages
APPLICATIONS
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Notebook and Palmtop Computers
Portable Instruments
Battery Operated Digital Devices
Distributed DC Power Systems
TYPICAL APPLICATION
High Efficiency Dual 8.5V/3.3V Step-Down Converter
4.7μF
V
IN
TG1
0.1μF
3.3μH
BOOST1
SW1
BG1
LTC3868-1
SENSE1
+
0.007Ω
V
OUT1
3.3V
5A
SENSE1
–
62.5k
150μF
20k
V
FB1
I
TH1
680pF
15k
0.1μF
SS1
SGND
SENSE2
V
FB2
I
TH2
SS2
0.1μF
–
22μF
50V
V
IN
9V TO 24V
100
Efficiency and Power Loss
vs Load Current
10000
90
INTV
CC
TG2
BOOST2
SW2
EFFICIENCY (%)
BG2
PGND
SENSE2
+
0.01Ω
V
OUT2
8.5V
3.5A
150μF
0.1μF
7.2μH
80
70
EFFICIENCY
60
50
POWER LOSS
40
30
20
10
0
0.0001
0.001
V
IN
= 12V
V
OUT
= 3.3V
FIGURE 12 CIRCUIT
0.01
0.1
1
OUTPUT CURRENT (A)
10
38681 TA01b
1000
POWER LOSS (mW)
100
10
1
193k
680pF
15k
20k
0.1
38681 TA01
38681fd
1
LTC3868-1
ABSOLUTE MAXIMUM RATINGS
(Note 1)
Input Supply Voltage (V
IN
) ......................... –0.3V to 28V
Topside Driver Voltages
BOOST1, BOOST2 ................................. –0.3V to 34V
Switch Voltage (SW1, SW2) ........................ –5V to 28V
PLLIN/MODE,
(BOOST1-SW1), (BOOST2-SW2) ................ –0.3V to 6V
RUN1, RUN2 ............................................... –0.3V to 8V
Maximum Current Sourced into Pin from
Source >8V ......................................................100μA
SENSE1
+
, SENSE2
+
, SENSE1
–
SENSE2
–
Voltages...................................... –0.3V to 16V
FREQ Voltages ..................................... –0.3V to INTV
CC
EXTV
CC
...................................................... –0.3V to 14V
I
TH1
, I
TH2
, V
FB1
, V
FB2
Voltages ..................... –0.3V to 6V
PGOOD1 Voltage ......................................... –0.3V to 6V
SS1, SS2, INTV
CC
Voltages ......................... –0.3V to 6V
Operating Temperature Range (Note 2).... –40°C to 85°C
Junction Temperature (Note 3) ............................. 125°C
Storage Temperature Range................... –65°C to 150°C
Lead Temperature (Soldering, 10 sec)
SSOP ................................................................ 300°C
PIN CONFIGURATION
TOP VIEW
PGOOD1
I
TH1
SW1
TG1
V
FB1
SENSE1
+
22 BOOST1
21 BG1
20 V
IN
29
SGND
19 PGND
18 EXTV
CC
17 INTV
CC
16 BG2
15 BOOST2
9 10 11 12 13 14
SENSE2
+
V
FB2
I
TH2
SS2
TG2
SW2
SENSE1
–
FREQ
PLLIN/MODE
SGND
RUN1
RUN2
1
2
3
4
5
6
7
8
9
V
FB1
I
TH1
SS1
TOP VIEW
28 SS1
27 PGOOD1
26 TG1
25 SW1
24 BOOST1
23 BG1
22 V
IN
21 PGND
20 EXTV
CC
19 INTV
CC
18 BG2
17 BOOST2
16 SW2
15 TG2
28 27 26 25 24 23
SENSE1
+
1
SENSE1
–
2
FREQ 3
PLLIN/MODE 4
SGND 5
RUN1 6
RUN2 7
SENSE2
–
8
SENSE2
–
10
SENSE2
+
11
V
FB2
12
I
TH2
13
SS2 14
UFD PACKAGE
28-LEAD (4mm 5mm) PLASTIC QFN
T
JMAX
= 125°C,
θ
JA
= 43°C/W
EXPOSED PAD (PIN 29) IS SGND, MUST BE SOLDERED TO PCB
GN PACKAGE
28-LEAD PLASTIC SSOP
T
JMAX
= 125°C,
θ
JA
= 90°C/W
ORDER INFORMATION
LEAD FREE FINISH
LTC3868EUFD-1#PBF
LTC3868IUFD-1#PBF
LTC3868EGN-1#PBF
LTC3868IGN-1#PBF
TAPE AND REEL
LTC3868EUFD-1#TRPBF
LTC3868IUFD-1#TRPBF
LTC3868EGN-1#TRPBF
LTC3868IGN-1#TRPBF
PART MARKING*
38681
38681
LTC3868GN-1
LTC3868GN-1
PACKAGE DESCRIPTION
28-Lead (4mm
×
5mm) Plastic QFN
28-Lead (4mm
×
5mm) Plastic QFN
28-Lead Plastic SSOP
28-Lead Plastic SSOP
TEMPERATURE RANGE
–40°C to 85°C
–40°C to 85°C
–40°C to 85°C
–40°C to 85°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping
container.Consult LTC Marketing for information on non-standard 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/
38681fd
2
LTC3868-1
ELECTRICAL CHARACTERISTICS
SYMBOL
V
IN
V
FB1,2
I
FB1,2
V
REFLNREG
V
LOADREG
PARAMETER
Input Supply Operating Voltage Range
Regulated Feedback Voltage
Feedback Current
Reference Voltage Line Regulation
Output Voltage Load Regulation
(Note 4) I
TH1,2
Voltage = 1.2V
(Note 4)
(Note 4) V
IN
= 4.5V to 24V
(Note4)
Measured in Servo Loop,
∆I
TH
Voltage = 1.2V to 0.7V
(Note4)
Measured in Servo Loop,
∆I
TH
Voltage = 1.2V to 2V
g
m1,2
I
Q
Transconductance Amplifier g
m
Input DC Supply Current
Pulse-Skipping or Forced Continuous
Mode (One Channel On)
Pulse-Skipping or Forced Continuous
Mode (Both Channels On)
Sleep Mode (One Channel On)
Sleep Mode (Both Channels On)
Shutdown
UVLO
V
OVL
I
SENSE+
I
SENSE –
Undervoltage Lockout
Feedback Overvoltage Protection
SENSE
+
Pin Current
SENSE
–
Pin Current
(Note 4) I
TH1,2
= 1.2V, Sink/Source = 5μA
(Note 5)
RUN1 = 5V and RUN2 = 0V, V
FB1
= 0.83V (No Load) or
RUN1 = 0V and RUN2 = 5V, V
FB2
= 0.83V (No Load)
RUN1,2 = 5V, V
FB1,2
= 0.83V (No Load)
RUN1 = 5V and RUN2 = 0V, V
FB1
= 0.83V (No Load) or
RUN1 = 0V and RUN2 = 5V, V
FB2
= 0.83V (No Load)
RUN1,2 = 5V, V
FB1,2
= 0.83V (No Load)
RUN1,2 = 0V
INTV
CC
Ramping Up
INTV
CC
Ramping Down
Measured at V
FB1,2
, Relative to Regulated
V
FB1,2
Each Channel
Each Channel
V
OUT1,2
< INTV
CC
– 0.5V
V
OUT1,2
> INTV
CC
+ 0.5V
In Dropout, FREQ = 0V
V
SS1,2
= 0V
V
RUN1
, V
RUN2
Rising
V
SS1
, V
SS2
Rising from 1V
V
SS1
, V
SS2
Rising from 2V
Short-Circuit Condition V
FB1,2
= 0V
V
SS1,2
= 5V
V
FB1,2
= 0.7V, V
SENSE1
–,
2
– = 3.3V
l
l
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
IN
= 12V, V
RUN1,2
= 5V, EXTV
CC
= 0V unless otherwise noted.
CONDITIONS
MIN
4
0.788
0.8
±5
0.002
0.01
TYP
MAX
24
0.812
±50
0.02
0.1
UNITS
V
V
nA
%/V
%
l
–0.01
2
1.3
2
170
300
8
3.6
7
4
3.8
10
–0.1
%
mmho
mA
mA
250
450
25
4.2
4
13
±1
±1
950
1.4
1.31
2.1
1.7
13
57
μA
μA
μA
V
V
%
μA
μA
μA
%
μA
V
mV
V
V
μA
mV
Ω
Ω
Ω
Ω
ns
ns
550
98
0.7
1.21
1.9
1.3
7
43
99.4
1
1.26
50
2
1.5
10
50
2.5
1.5
2.4
1.1
DF
MAX
I
SS1,2
V
RUN1,2
On
V
SS1,2
LA
V
SS1,2
LT
I
DSC1,2
LT
Maximum Duty Factor
Soft-Start Charge Current
RUN Pin On Threshold
SS Pin Latchoff Arming Threshold
SS Pin Latchoff Threshold
SS Discharge Current
V
RUN1,2
Hyst RUN Pin Hysteresis
V
SENSE(MAX)
Maximum Current Sense Threshold
Gate Driver
TG1,2
BG1,2
Pull-Up On-Resistance
Pull-Down On-Resistance
Pull-Up On-Resistance
Pull-Down On-Resistance
TG Transition Time:
Rise Time
Fall Time
TG1,2 t
r
TG1,2 t
f
(Note 6)
C
LOAD
= 3300pF
C
LOAD
= 3300pF
25
16
38681fd
3
LTC3868-1
ELECTRICAL CHARACTERISTICS
SYMBOL
BG1,2 t
r
BG1,2 t
f
TG/BG t
1D
BG/TG t
1D
t
ON(MIN)
V
INTVCCVIN
V
LDOVIN
V
INTVCCEXT
V
LDOEXT
V
EXTVCC
V
LDOHYS
f
25kΩ
f
65kΩ
f
105kΩ
f
LOW
f
HIGH
f
SYNC
V
PGL
I
PGOOD
V
PG
PARAMETER
BG Transition Time:
Rise Time
Fall Time
Top Gate Off to Bottom Gate On Delay
Synchronous Switch-On Delay Time
Bottom Gate Off to Top Gate On Delay
Top Switch-On Delay Time
Minimum On-Time
Internal V
CC
Voltage
INTV
CC
Load Regulation
Internal V
CC
Voltage
INTV
CC
Load Regulation
EXTV
CC
Switchover Voltage
EXTV
CC
Hysteresis
Programmable Frequency
Programmable Frequency
Programmable Frequency
Low Fixed Frequency
High Fixed Frequency
Synchronizable Frequency
PGOOD1 Voltage Low
PGOOD1 Leakage Current
PGOOD1 Trip Level
R
FREQ
= 25k, PLLIN/MODE = DC Voltage
R
FREQ
= 65k, PLLIN/MODE = DC Voltage
R
FREQ
= 105k, PLLIN/MODE = DC Voltage
V
FREQ
= 0V, PLLIN/MODE = DC Voltage
V
FREQ
= INTV
CC
, PLLIN/MODE = DC Voltage
PLLIN/MODE = External Clock
I
PGOOD
= 2mA
V
PGOOD
= 5V
V
FB
with Respect to Set Regulated Voltage
V
FB
Ramping Negative
Hysteresis
V
FB
with Respect to Set Regulated Voltage
V
FB
Ramping Positive
Hysteresis
t
PG
Delay for Reporting a Fault (PGOOD Low)
Note 1:
Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Ratings for extended periods may affect device reliability and
lifetime.
Note 2:
The LTC3868E-1 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. The LTC3868I-1 is guaranteed over the
full –40°C to 85°C operating temperature range.
Note 3:
T
J
is calculated from the ambient temperature T
A
and power
dissipation P
D
according to the following formula:
T
J
= T
A
+ (P
D
•
θ
JA
)
–13
–10
2.5
10
2.5
25
l
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 12V, V
RUN1,2
= 5V, EXTV
CC
= 0V unless otherwise noted.
CONDITIONS
(Note 6)
C
LOAD
= 3300pF
C
LOAD
= 3300pF
C
LOAD
= 3300pF Each Driver
C
LOAD
= 3300pF Each Driver
(Note 7)
6V < V
IN
< 24V, V
EXTVCC
= 0V
I
CC
= 0mA to 50mA, V
EXTVCC
= 0V
6V < V
EXTVCC
< 13V
I
CC
= 0mA to 50mA, V
EXTVCC
= 8.5V
EXTV
CC
Ramping Positive
4.5
4.85
4.85
MIN
TYP
28
13
30
30
95
5.1
0.7
5.1
0.6
4.7
250
105
375
320
485
75
0.2
440
835
350
535
380
585
850
0.4
±1
–7
505
5.35
1.1
5.35
1.1
4.9
MAX
UNITS
ns
ns
ns
ns
ns
V
%
V
%
V
mV
kHz
kHz
kHz
kHz
kHz
kHz
V
μA
%
%
%
%
μs
INTV
CC
Linear Regulator
Oscillator and Phase-Locked Loop
PGOOD1 Output
7
13
where
θ
JA
= 43°C for the QFN package and
θ
JA
= 90°C for the SSOP
package.
Note 4:
The LTC3868-1 is tested in a feedback loop that servos V
ITH1,2
to a
specified voltage and measures the resultant V
FB1,2
.
Note 5:
Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency. See Applications information.
Note 6:
Rise and fall times are measured using 10% and 90% levels. Delay
times are measured using 50% levels.
Note 7:
The minimum on-time condition is specified for an inductor
peak-to-peak ripple current ≥ 40% of I
MAX
(See Minimum On-Time
Considerations in the Applications Information section).
38681fd
4
LTC3868-1
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency and Power Loss
vs Output Current
100
FIGURE 12 CIRCUIT
90 V
IN
= 12V
V
OUT
= 3.3V
80
70
60
50
40
30
20
10
0
0.0001
0.001
Burst Mode
OPERATION
PULSE-
SKIPPING
FCM
0.01
0.1
1
OUTPUT CURRENT (A)
10
38681 G01
Efficiency vs Load Current
10000
100
90
1000
POWER LOSS (mW)
EFFICIENCY (%)
80
70
60
50
40
30
20
10
0.1
0
0.0001
0.001
V
OUT
= 3.3V
FIGURE 12 CIRCUIT
0.01
0.1
1
OUTPUT CURRENT (A)
10
38681 G02
V
IN
= 5V
EFFICIENCY (%)
V
IN
= 12V
100
10
1
Efficiency vs Input Voltage
98
96
94
EFFICIENCY (%)
92
90
88
86
84
82
80
0
5
20
10
15
INPUT VOLTAGE (V)
25
28
I
L
2A/DIV
FIGURE 12 CIRCUIT
V
OUT
= 3.3V
I
OUT
= 4A
V
OUT
100mV/DIV
AC-
COUPLED
Load Step (Burst Mode Operation)
Load Step
(Forced Continuous Mode)
V
OUT
100mV/DIV
AC-
COUPLED
I
L
2A/DIV
V
OUT
= 3.3V
20μs/DIV
FIGURE 12 CIRCUIT
38681 G04
20μs/DIV
V
OUT
= 3.3V
FIGURE 12 CIRCUIT
38681 G05
38681 G03
Load Step (Pulse-Skipping Mode)
V
OUT
100mV/DIV
AC-
COUPLED
Inductor Current at Light Load
Soft Start-Up
FORCED
CONTINUOUS
MODE
V
OUT2
2V/DIV
I
L
2A/DIV
Burst Mode
OPERATION
2A/DIV
PULSE-
SKIPPING
MODE
V
OUT
= 3.3V
20μs/DIV
FIGURE 12 CIRCUIT
38681 G06
V
OUT1
2V/DIV
V
OUT
= 3.3V
2μs/DIV
I
LOAD
= 200μA
FIGURE 12 CIRCUIT
38681 G07
20ms/DIV
FIGURE 12 CIRCUIT
38681 G08
38681fd
5
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