FEATURES
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
LTC3851
Synchronous
Step-Down Switching
Regulator Controller
DESCRIPTION
The LTC
®
3851 is a high performance synchronous
step-down switching regulator controller that drives
an all N-channel synchronous power MOSFET stage. A
constant frequency current mode architecture allows a
phase-lockable frequency of up to 750kHz.
OPTI-LOOP compensation allows the transient response
to be optimized over a wide range of output capacitance
and ESR values. The LTC3851 features a precision 0.8V
reference that is compatible with a wide 4V to 38V input
supply range.
The TK/SS pin ramps the output voltage during start-up.
Current foldback limits MOSFET heat dissipation during
short-circuit conditions. The MODE/PLLIN pin selects
among Burst Mode operation, pulse skipping mode or
continuous inductor current mode at light loads and allows
the IC to be synchronized to an external clock.
, LT, LTC, LTM, Burst Mode and OPTI-LOOP are registered trademarks of Linear
Technology Corporation. All other trademarks are the property of their respective owners.
Protected by U.S. Patents, including 5408150, 5481178, 5705919, 5929620, 6304066,
6498466, 6580258, 6611131.
Wide V
IN
Range: 4V to 38V Operation
R
SENSE
or DCR Current Sensing
±1% Output Voltage Accuracy
Phase-Lockable Fixed Frequency: 250kHz to 750kHz
Dual N-Channel MOSFET Synchronous Drive
Very Low Dropout Operation: 99% Duty Cycle
Adjustable Output Voltage Soft-Start or Tracking
Output Current Foldback Limiting
Output Overvoltage Protection
5V Internal Regulator
OPTI-LOOP
®
Compensation Minimizes C
OUT
Selectable Continuous, Pulse-Skipping or
Burst Mode
®
Operation at Light Loads
Low Shutdown I
Q
: 20μA
V
OUT
Range: 0.8V to 5.5V
16-Lead Narrow SSOP or 3mm
×
3mm QFN Package
APPLICATIONS
n
n
n
n
Automotive Systems
Telecom Systems
Industrial Equipment
Distributed DC Power Systems
TYPICAL APPLICATION
High Efficiency Synchronous Step-Down Converter
I
LIM
V
IN
0.68μH
0.22μF
10k
INTV
CC
10nF
I
TH
1k
100pF
PLLIN/MODE
SENSE
–
V
FB
BG
GND
SENSE
+
0.22μF
13k
62.5k
150μF
V
IN
4V TO 38V
V
OUT
3.3V
10A
330μF
EFFICIENCY (%)
100
Efficiency and Power Loss
vs Load Current
V
IN
= 12V
95 V
OUT
= 3.3V
90
85
80
75
70
65
60
55
50
10
100
1000
LOAD CURRENT (mA)
10
10000
3851TA01b
10000
FREQ/PLLFLTR TG
RUN
SW
LTC3851
BOOST
TK/SS
0.1μF
EFFICIENCY
1000
POWER LOSS (mW)
POWER LOSS
100
20k
3851 TA01a
3851f
1
LTC3851
ABSOLUTE MAXIMUM RATINGS
(Note 1)
Input Supply Voltage (V
IN
) ......................... 40V to –0.3V
Topside Driver Voltage (BOOST) ................ 46V to –0.3V
Switch Voltage (SW) ..................................... 40V to –5V
INTV
CC
, (BOOST – SW), RUN ...................... 6V to –0.3V
TK/SS, I
LIM
............................................ INTV
CC
to –0.3V
SENSE
+
, SENSE
–
.......................................... 6V to –0.3V
MODE/PLLIN, FREQ/PLLFLTR ............... INTV
CC
to –0.3V
I
TH
, V
FB
Voltages .......................................... 3V to –0.3V
INTV
CC
Peak Output Current ..................................50mA
Operating Temperature Range (Note 2).... –40°C to 85°C
Junction Temperature (Note 3) ............................. 125°C
Storage Temperature Range
GN ..................................................... –65°C to 150°C
UD ......................................................... –65 to 125°C
Lead Temperature (Soldering, 10 sec)
GN .................................................................... 300°C
PIN CONFIGURATION
TOP VIEW
FREQ/PLLFLTR
TOP VIEW
MODE/PLLIN
FREQ/PLLFLTR
RUN
TK/SS
I
TH
FB
SENSE
–
SENSE
+
1
2
3
4
5
6
7
8
16 SW
15 TG
14 BOOST
13 V
IN
12 INTV
CC
11 BG
10 GND
9
I
LIM
RUN 1
TK/SS 2
I
TH
3
FB 4
5
SENSE
–
6
+
MODE/PLLIN
SW
16 15 14 13
12 BOOST
17
11 V
IN
10 INTV
CC
9
7
I
LIM
8
GND
BG
GN PACKAGE
16-LEAD PLASTIC SSOP NARROW
T
JMAX
= 125°C,
θ
JA
= 90°C/W
UD PACKAGE
16-LEAD (3mm 3mm) PLASTIC QFN
T
JMAX
= 125°C,
θ
JA
= 68°C/W,
θ
JC
= 4.2°C/W
EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
LTC3851EGN#PBF
LTC3851EUD#PBF
TAPE AND REEL
LTC3851EGN#TRPBF
LTC3851EUD#TRPBF
PART MARKING
3851
LCXN
PACKAGE DESCRIPTION
16-Lead Plastic SSOP
16-Lead (3mm
×
3mm) Plastic QFN
TEMPERATURE RANGE
–40°C to 85°C
–40°C to 85°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
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/
SENSE
TG
3851f
2
LTC3851
ELECTRICAL CHARACTERISTICS
SYMBOL
V
IN
V
FB
I
FB
V
REFLNREG
V
LOADREG
PARAMETER
Operating Input Voltage Range
Regulated Feedback Voltage
Feedback Current
Reference Voltage Line Regulation
Output Voltage Load Regulation
I
TH
= 1.2V (Note 4)
(Note 4)
V
IN
= 6V to 38V (Note 4)
(Note 4)
Measured in Servo Loop,
ΔI
TH
= 1.2V to 0.7V
(Note 4)
Measured in Servo Loop,
ΔI
TH
= 1.2V to 1.6V
g
m
g
m
GBW
I
Q
Transconductance Amplifier g
m
Transconductance Amp Gain Bandwidth
Input DC Supply Current
Normal Mode
Shutdown
Undervoltage Lockout on INTV
CC
UVLO Hysteresis
Feedback Overvoltage Lockout
SENSE Pins Total Current
Soft-Start Charge Current
RUN Pin On Threshold
RUN Pin On Hysteresis
Maximum Current Sense Threshold
V
FB
= 0.7V, V
SENSE
= 3.3V, I
LIM
= 0V
V
FB
= 0.7V, V
SENSE
= 3.3V, I
LIM
= Float
V
FB
= 0.7V, V
SENSE
= 3.3V, I
LIM
= INTV
CC
TG High
TG Low
BG High
BG Low
(Note 6)
C
LOAD
= 3300pF
C
LOAD
= 3300pF
(Note 6)
C
LOAD
= 3300pF
C
LOAD
= 3300pF
C
LOAD
= 3300pF Each Driver
C
LOAD
= 3300pF Each Driver
(Note 7)
6V < V
IN
< 38V
I
CC
= 0mA to 50mA
4.8
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
= 15V, V
RUN
= 5V unless otherwise noted.
CONDITIONS
l
l
MIN
4
0.792
TYP
MAX
38
UNITS
V
V
nA
%/V
%
Main Control Loops
0.800
–10
0.002
0.01
0.808
–50
0.02
0.1
l
–0.01
2
3
1.2
20
3.25
0.4
–0.1
%
mmho
MHz
mA
μA
V
V
I
TH
= 1.2V, Sink/Source = 5μA (Note 4)
I
TH
= 1.2V
(Note 5)
V
OUT
= 5V
V
RUN
= 0V
V
INTVCC
Ramping Down
Measured at V
FB
V
TK/SS
= 0V
V
RUN
Rising
l
l
35
UVLO
UVLO Hys
V
OVL
I
SENSE
I
TK/SS
V
RUN
V
RUNHYS
V
SENSE(MAX)
0.86
0.6
1.10
20
40
65
0.88
±1
1
1.22
130
30
50
75
2.6
1.5
2.4
1.1
25
25
25
25
30
30
90
5
0.5
0.90
±2
2
1.35
40
65
90
V
μA
μA
V
mV
mV
mV
mV
Ω
Ω
Ω
Ω
ns
ns
ns
ns
ns
ns
ns
TG R
UP
TG R
DOWN
BG R
UP
BG R
DOWN
TG t
r
TG t
f
BG tr
BG tf
TG/BG t
1D
BG/TG t
2D
t
ON(MIN)
V
INTVCC
V
LDO
INT
TG Driver Pull-Up On-Resistance
TG Driver Pull-Down On-Resistance
BG Driver Pull-Up On-Resistance
BG Driver Pull-Down On-Resistance
TG Transition Time
Rise Time
Fall Time
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
INTV
CC
Linear Regulator
5.2
2
V
%
3851f
3
LTC3851
ELECTRICAL CHARACTERISTICS
SYMBOL
f
NOM
f
LOW
f
HIGH
R
MODE/PLLIN
I
FREQ
PARAMETER
Nominal Frequency
Lowest Frequency
Highest Frequency
MODE/PLLIN Input Resistance
Phase Detector Output Current
Sinking Capability
Sourcing Capability
f
MODE
> f
OSC
f
MODE
< f
OSC
Oscillator and Phase-Locked Loop
R
FREQ
= 60k
R
FREQ
= 160k
R
FREQ
= 36k
480
220
710
500
250
750
100
–10
10
530
280
790
kHz
kHz
kHz
kΩ
μA
μA
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 15V, V
RUN
= 5V unless otherwise noted.
CONDITIONS
MIN
TYP
MAX
UNITS
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 LTC3851E 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 3:
T
J
is calculated from the ambient temperature T
A
and power
dissipation P
D
according to the following formulas:
LTC3851EGN: T
J
= T
A
+ (P
D
• 90°C/W)
LTC3851EUD: T
J
= T
A
+ (P
D
• 68°C/W)
Note 4:
The LTC3851 is tested in a feedback loop that servos V
ITH
to a
specified voltage and measures the resultant V
FB
.
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).
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency vs Output Current and
Mode (Figure TBD)
100
100
90
80
BURST
PULSE
SKIP
CCM
EFFICIENCY (%)
70
EFFICIENCY(%)
60
50
40
30
20
10
0
10
100
1000
LOAD CURRENT (mA)
10000
3851 G01
Efficiency vs Output Current
and Mode
100
Efficiency vs Output Current and
Mode (Figure TBD)
V
IN
= 12V
90 V
OUT
= 5V
80
BURST
PULSE
SKIP
CCM
V
IN
= 12V
90 V
OUT
= 1.2V
80
70
BURST
PULSE
SKIP
CCM
70
60
50
40
30
EFFICIENCY(%)
60
50
40
30
20
10
0
V
IN
= 12V
V
OUT
= 3.3V
FIGURE 11 CIRCUIT
10
100
1000
LOAD CURRENT (mA)
10000
3851 G02
20
10
0
10
100
1000
LOAD CURRENT (mA)
10000
3851 G03
3851f
4
LTC3851
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency and Power Loss vs
Input Voltage
100
V
IN
= 12V
V
OUT
= 3.3V
95 FIGURE 11 CIRCUIT
10000
POWER LOSS,
I
OUT
= 5A
POWER LOSS (mW)
EFFICIENCY,
I
OUT
= 5A
1000
POWER LOSS,
I
OUT
= 0.5A
EFFICIENCY,
I
OUT
= 0.5A
4
8
12
16 20 24 28 32
INPUT VOLTAGE (V)
100
36
40
I
LOAD
5A/DIV
0.2A TO 7.5A
I
L
5A/DIV
V
OUT
100mV/DIV
AC COUPLED
V
OUT
= 1.5V
100μs/DIV
V
IN
= 12V
FIGURE 11 CIRCUIT
3851 G05
Load Step
(Burst Mode Operation)
I
LOAD
5A/DIV
0.2A TO 7.5A
I
L
5A/DIV
V
OUT
100mV/DIV
AC COUPLED
Load Step
(Forced Continuous Mode)
EFFICIENCY (%)
90
85
80
75
70
V
OUT
= 1.5V
100μs/DIV
V
IN
= 12V
FIGURE 11 CIRCUIT
3851 G06
3851 G04
Load Step
(Pulse-Skip Mode)
I
LOAD
5A/DIV
0.2A TO 7.5A
I
L
5A/DIV
V
OUT
100mV/DIV
AC COUPLED
V
OUT
= 1.5V
100μs/DIV
V
IN
= 12V
FIGURE 11 CIRCUIT
3851 G07
Inductor Current at Light Load
FORCED
CONTINOUS
MODE
5A/DIV
Burst Mode
OPERATION
5A/DIV
PULSE SKIP
MODE
5A/DIV
V
OUT
= 1.5V
1μs/DIV
V
IN
= 12V
I
LOAD
= 1mA
FIGURE 11 CIRCUIT
3851 G08
Start-Up with Prebiased Output
at 2V
V
OUT
2V/DIV
TK/SS
0.5V/DIV
V
FB
0.5V/DIV
20ms/DIV
3851 G09
Coincident Tracking with Master
Supply
V
MASTER
0.5V/DIV
Ratiometric Tracking with Master
Supply
3.0
V
MASTER
0.5V/DIV
2.5
SUPPLY CURRENT (mA)
10ms/DIV
3851 G11
Input DC Supply Current vs Input
Voltage
V
OUT
2A LOAD
0.5V/DIV
2.0
1.5
1.0
0.5
0
4
8
12
16 20 24 28 32
INPUT VOLTAGE (V)
36
40
V
OUT
2A LOAD
0.5V/DIV
10ms/DIV
3851 G10
3851 G12
3851f
5
京公网安备 11010802033920号
EEWORLD
