regulator controller optimized for high switching frequency
and fast transient response. The constant on-time valley
current mode architecture allows for a wide input range,
including very low duty factor operation. No external sense
resistor or slope compensation is required. System flex-
ibility is offered through a user-programmable soft-start pin
and independent RUN pin. The voltage ramping soft-start
function can be programmed with a capacitor or made to
track an external reference source.
Operating frequency is set by an external resistor and
compensated for variations in V
IN
to offer excellent line
stability. Discontinuous mode operation provides high
efficiency during light load conditions. A forced continu-
ous control pin allows the user to reduce noise and RF
interference. Safety features include output overvoltage
protection and programmable current limit with foldback.
The current limit is user programmable.
The LTC3879 allows operation from 4V to 38V at the input
and from 0.6V to 90% V
IN
at the output. The LTC3879 is
available in a small 16-pin thermally enhanced MSOP or
3mm
×
3mm QFN packages.
Wide V
IN
Range: 4V to 38V
±1% 0.6V Voltage Reference
Extremely Fast Transient Response
t
ON(MIN)
: 43ns
No R
SENSE
™ Valley Current Mode Control
Stable with Low ESR Ceramic C
OUT
Supports Smooth Start-Up in Pre-Biased Output
Optimized for High Step-Down Ratios
Adjustable Output Voltage Soft-Start or Tracking
Power Good Output Voltage Monitor
Dual N-Channel MOSFET Synchronous Drive
Adjustable Switching Frequency
Programmable Current Limit with Foldback
Output Overvoltage Protection
16-Pin MSOP and 3mm
×
3mm QFN Packages
APPLICATIONS
n
n
n
Distributed Power Systems
Embedded Computing
Communications Infrastructure
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.
Protected by U.S. Patents, including 5481178, 6100678, 6580258, 5847554, 6304066.
TYPICAL APPLICATION
High Efficiency Step-Down Converter
0.1μF
432k
I
ON
RUN
TRACK/SS V
IN
220pF
TG
LTC3879
SW
I
TH
SGND BOOST
MODE
INTV
CC
BG
4.7μF
PGOOD PGND
10k
V
RNG
V
FB
10k
3879 TA01a
Efficiency vs Load Current
100
V
IN
4.5V TO 28V
90
80
EFFICIENCY (%)
70
60
50
40
30
20
10
0
0.01
0.1
V
IN
= 12V
V
OUT
= 1.2V
SW FREQ = 400kHz
FIGURE 10 CIRCUIT
1
10
LOAD CURRENT (A)
100
3879 TA01b
DISCONTINUOUS
MODE
27k
RJK0305
0.56μH
0.22μF
RJK0330
10μF
V
OUT
1.2V
15A
330μF
2
CONTINUOUS
MODE
3879f
1
LTC3879
ABSOLUTE MAXIMUM RATINGS
(Note 1)
Input Supply Voltage (V
IN
) ......................... –0.3V to 40V
I
ON
Voltage ................................................. –0.3V to 40V
BOOST Voltage .......................................... –0.3V to 46V
SW Voltage ................................................... –5V to 40V
INTV
CC
, (BOOST-SW), TRACK/SS, RUN,
PGOOD Voltages .......................................... –0.3V to 6V
MODE, V
RNG
Voltages ................–0.3V to INTV
CC
+ 0.3V
V
FB
, I
TH
Voltages ....................................... –0.3V to 2.7V
Operating Temperature Range (Note 4)....–40°C to 85°C
Junction Temperature (Note 2) ............................. 125°C
Storage Temperature Range...................–65°C to 150°C
Lead Temperature (Soldering, 10 sec)
MSOP Only ....................................................... 300°C
PIN CONFIGURATION
TOP VIEW
BOOST
PGND
12 BG
17
11 INTV
CC
10 V
IN
9
5
I
TH
6
SGND
7
I
ON
8
V
FB
RUN
SW
TG
TOP VIEW
TRACK/SS
PGOOD
V
RNG
MODE
I
TH
SGND
I
ON
V
FB
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
BOOST
TG
SW
PGND
BG
INTV
CC
V
IN
RUN
TRACK/SS 1
PGOOD 2
V
RNG
3
MODE 4
16 15 14 13
17
MSE PACKAGE
16-LEAD PLASTIC MSOP
T
JMAX
= 125°C,
θ
JA
= 40°C/W
EXPOSED PAD (PIN 17) IS SGND, MUST BE SOLDERED TO PCB
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 SGND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
LTC3879EMSE#PBF
LTC3879IMSE#PBF
LTC3879EUD#PBF
LTC3879IUD#PBF
TAPE AND REEL
LTC3879EMSE#TRPBF
LTC3879IMSE#TRPBF
LTC3879EUD#TRPBF
LTC3879IUD#TRPBF
PART MARKING*
3879
3879
LDTM
LDTM
PACKAGE DESCRIPTION
16-Lead Plastic MSOP
16-Lead Plastic MSOP
16-Lead (3mm
×
3mm) Plastic QFN
16-Lead (3mm
×
3mm) Plastic QFN
TEMPERATURE RANGE
–40°C to 85°C (Note 4)
–40°C to 85°C (Note 4)
–40°C to 85°C (Note 4)
–40°C to 85°C (Note 4)
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/
3879f
2
LTC3879
ELECTRICAL CHARACTERISTICS
SYMBOL
Main Control Loop
Input Operating Voltage Range
I
Q
Input DC Supply Current
Normal
Shutdown Supply Current
Feedback Reference Voltage
Feedback Voltage Line Regulation
Feedback Voltage Load Regulation
I
FB
g
m(EA)
V
MODE
t
ON
t
ON(MIN)
t
OFF(MIN)
V
SENSE(MAX)
Feedback Input Current
Error Amplifier Transconductance
MODE Threshold
MODE Pin Current
On-Time
Minimum On-Time
Minimum Off-Time
Valley Current Sense Threshold
V
PGND
– V
SW
Peak Current = Valley + Ripple
Minimum Current Sense Threshold
V
PGND
– V
SW
Force Continuous Operation
RUN On Threshold
RUN Pull-Up Current
Soft-Start Charging Current
INTV
CC
Undervoltage Lockout
INTV
CC
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
TG Rise Time
TG Fall Time
BG Rise Time
BG 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
V
MODE
= 0.8V
I
ON
= 30μA
I
ON
= 15μA
I
ON
= 180μA
I
ON
= 30μA
V
RNG
= 1V, V
FB
= 0.56V
V
RNG
= 0V, V
FB
= 0.56V
V
RNG
= INTV
CC
, V
FB
= 0.56V
V
RNG
= 1V, V
FB
= 0.64V
V
RNG
= 0V, V
FB
= 0.64V
V
RNG
= INTV
CC
, V
FB
= 0.64V
V
RUN
Rising
V
RUN
= 0V
V
TRACK/SS
= 0V
Falling
Rising
TG High
TG Low
BG High
BG Low
C
LOAD
= 3300pF (Note 5)
C
LOAD
= 3300pF (Note 5)
C
LOAD
= 3300pF (Note 5)
C
LOAD
= 3300pF (Note 5)
C
LOAD
= 3300pF Each Driver (Note 5)
C
LOAD
= 3300pF Each Driver (Note 5)
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
= 15V, unless otherwise noted.
PARAMETER
CONDITIONS
MIN
4
1350
18
I
TH
= 1.2V (Note 3)
V
IN
= 4V to 30V, I
TH
= 1.2V (Note 3)
I
TH
= 0.5V to 1.9V (Note 3)
V
FB
= 0.6V
I
TH
= 1.2V (Note 3)
1.4
0.76
198
370
l
l
TYP
MAX
38
2000
35
0.606
–0.3
±50
2
0.84
±1
268
510
75
300
165
48
98
UNITS
V
μA
μA
V
%/V
%
nA
mS
V
μA
ns
ns
ns
ns
mV
mV
mV
mV
mV
mV
V
FBREF
0.594
0.6
0.005
–0.05
–5
1.7
0.8
0
233
440
43
220
108
22
62
133
30
75
–61
–12
–33
V
SENSE(MIN)
V
RUN
I
RUN
I
TRACK/SS
1.3
–0.45
1.5
–1.2
–1
3.3
3.6
2.5
1.2
2.5
0.7
20
20
20
20
15
15
1.7
–2
3.9
4
V
μA
μA
V
V
Ω
Ω
Ω
Ω
ns
ns
ns
ns
ns
ns
3879f
3
LTC3879
ELECTRICAL CHARACTERISTICS
SYMBOL
PARAMETER
Internal V
CC
Voltage
Internal V
CC
Load Regulation
PGOOD Output
PGOOD Upper Threshold
PGOOD Lower Threshold
PGOOD Hysteresis
PGOOD Low Voltage
PGOOD Turn-On Delay
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:
T
J
is calculated from the ambient temperature T
A
and power
dissipation P
D
, as follows:
T
J
= T
A
+ (P
D
•
θ
JA
)
Note 3:
The LTC3879 is tested in a feedback loop that adjusts V
FB
to
achieve a specified error amplifier output voltage (I
TH
).
V
FB
Rising
V
FB
Falling
V
FB
Returning
I
PGOOD
= 5mA
7.8
–7.8
10
–10
2
0.15
12
12.2
–12.2
3.8
0.4
%
%
%
V
μs
Internal V
CC
Regulator
6V < V
IN
< 38V
I
CC
= 0mA to 20mA
5.15
5.3
–0.1
5.45
±2
V
%
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 15V, unless otherwise noted.
CONDITIONS
MIN
TYP
MAX
UNITS
Note 4:
The LTC3879E is guaranteed to meet 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 LTC3879I is guaranteed to meet
specifications over the full –40°C to 85°C operating temperature range.
Note 5:
Rise and fall time are measured using 10% and 90% levels. Delay
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