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. The LTC3878
is pin compatible with the LTC1778 in applications that do
not use EXTV
CC
while offering better efficiency. Consult
with the factory to verify compatibility.
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.
Soft-start capability for supply sequencing is accomplished
through an external timing capacitor. The current limit is
user programmable.
The LTC3878 allows operation from 4V to 38V at the input
and from 0.8V to 90% V
IN
at the output. The LTC3878 is
available in a small 16-pin narrow SSOP package.
Wide V
IN
Range: 4V to 38V
±1% 0.8V Voltage Reference
Extremely Fast Transient Response
t
ON(MIN)
: 43ns
Valley Current Mode Control
Stable for Low ESR Ceramic C
OUT
No Sense Resistor Required
Optimized for High Step-Down Ratios
Pin Compatible with the LTC1778 (No EXTV
CC
Pin)
Power Good Output Voltage Monitor
Dual N-Channel MOSFET Synchronous Drive
Adjustable Switching Frequency
Programmable Current Limit with Foldback
Programmable Soft-Start
Output Overvoltage Protection
Small 16-Pin Narrow SSOP Package
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
100
0.1μF
RUN/SS
20k
120pF
432k
I
ON
V
IN
TG
SW
I
TH
SGND BOOST
LTC3878
FCB
INTV
CC
BG
4.7μF
PGOOD PGND
5.11k
V
RNG
V
FB
10k
3878 TA01a
Efficiency vs Load Current
90
V
IN
4.5V TO 28V
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 7 CIRCUIT
1
10
LOAD CURRENT (A)
100
3878 G07
DISCONTINUOUS
MODE
RJK0305
0.56μH
0.22μF
RJK0330
10μF
V
OUT
1.2V
15A
750μF
CONTINUOUS
MODE
3878f
1
LTC3878
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW
RUN/SS
PGOOD
V
RNG
FCB
I
TH
SGND
I
ON
V
FB
1
2
3
4
5
6
7
8
16 BOOST
15 TG
14 SW
13 PGND
12 BG
11 INTV
CC
10 V
IN
9
NC
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), RUN/SS,
PGOOD Voltages .......................................... –0.3V to 6V
FCB, 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) .................. 300°C
GN PACKAGE
16-LEAD PLASTIC SSOP NARROW
T
JMAX
= 125°C,
θ
JA
= 110°C/W
ORDER INFORMATION
LEAD FREE FINISH
LTC3878EGN#PBF
LTC3878IGN#PBF
TAPE AND REEL
LTC3878EGN#TRPBF
LTC3878IGN#TRPBF
PART MARKING*
3878
3878
PACKAGE DESCRIPTION
16-Lead Plastic SSOP
16-Lead Plastic SSOP
TEMPERATURE RANGE
–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/
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
FCB
t
ON
t
ON(MIN)
Feedback Input Current
Error Amplifier Transconductance
FCB Threshold
FCB Pin Current
On-Time
Minimum On-Time
PARAMETER
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
4
1500
18
I
TH
= 1.2V (Note 3)
V
IN
= 4V to 38V, I
TH
= 1.2V (Note 3)
I
TH
= 0.5V to 1.9V (Note 3)
V
FB
= 0.8V
I
TH
= 1.2V (Note 3)
V
FCB
= 0.8V
I
ON
= 30μA
I
ON
= 15μA
I
ON
= 180μA
198
370
1.4
0.76
l
l
TYP
MAX
38
2000
35
0.808
–0.3
±50
2
0.84
±1
268
510
75
UNITS
V
μA
μA
V
%/V
%
nA
mS
V
μA
ns
ns
ns
3878f
V
FBREF
0.792
0.8
0.002
–0.05
–5
1.7
0.8
0
233
440
43
2
LTC3878
ELECTRICAL CHARACTERISTICS
SYMBOL
t
OFF(MIN)
V
SENSE(MAX)
PARAMETER
Minimum Off-Time
Valley Current Sense Threshold
V
PGND
– V
SW
Peak Current = Valley + Ripple
Minimum Current Sense Threshold
V
PGND
– V
SW
Forced Continuous Operation
RUN/SS Pin On Threshold
Soft-Start Charging Current
INTV
CC(UVLO)
INTV
CC(UVLOR)
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
TG/BG t
1D
TG/BG t
2D
Top Gate Off to Bottom Gate On Delay
Synchronous Switch-On Delay Time
Bottom Gate Off to Top Gate On Delay
Synchronous Switch-On Delay Time
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
• 110°C/W)
Note 3:
The LTC3878 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
5.5
–5.5
7.5
–7.5
2
0.15
12
9.5
–9.5
3.5
0.4
%
%
%
V
μs
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
I
ON
= 30μA
V
RNG
= 1V, V
FB
= 0.76V
V
RNG
= 0V, V
FB
= 0.76V
V
RNG
= INTV
CC
, V
FB
= 0.76V
V
RNG
= 1V, V
FB
= 0.84V
V
RNG
= 0V, V
FB
= 0.84V
V
RNG
= INTV
CC
, V
FB
= 0.84V
V
RUN/SS
Rising
V
RUN/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
MIN
108
74
152
TYP
220
133
93
186
–67
–47
–93
MAX
300
165
119
224
UNITS
ns
mV
mV
mV
mV
mV
mV
V
SENSE(MIN)
V
RUN/SS
1.4
1.5
–1.2
3.3
3.6
2.5
1.2
2.5
0.7
20
20
20
20
15
15
1.6
3.9
4
V
μA
V
V
Ω
Ω
Ω
Ω
ns
ns
ns
ns
ns
ns
Internal V
CC
Regulator
6V < V
IN
< 38V
I
CC
= 0mA to 20mA
5.15
5.3
–0.1
5.45
±2
V
%
Note 4:
The LTC3878E 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 LTC3878I 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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