step-down DC/DC converter that can deliver up to 8A output
current from a 4V to 18V (20V maximum) input supply. It
uses a valley current control architecture to deliver very
low duty cycle operation at high frequency with excellent
transient response. The operating frequency is selected
by an external resistor and is compensated for variations
in V
IN
and V
OUT
.
The LTC3608 can be configured for discontinuous or
forced continuous operation at light load. Forced continu-
ous operation reduces noise and RF interference while
discontinuous mode provides high efficiency by reducing
switching losses at light loads.
Fault protection is provided by internal foldback current
limiting, an output overvoltage comparator and an optional
short-circuit shutdown timer. Soft-start capability for sup-
ply sequencing is accomplished using an external timing
capacitor. The regulator current limit is user programmable.
A power good output voltage monitor indicates when
the output is in regulation. The LTC3608 is available in a
compact 7mm
×
8mm QFN package.
8A Output Current
Wide V
IN
Range = 4V to 18V
Internal N-Channel MOSFETs
True Current Mode Control
Optimized for High Step-Down Ratios
t
ON(MIN)
≤ 100nsec
Extremely Fast Transient Response
Stable with Ceramic C
OUT
±1% 0.6V Voltage Reference
Power Good Output Voltage Monitor
Adjustable On-Time/Switching Frequency
Adjustable Current Limit
Programmable Soft-Start
Output Overvoltage Protection
Optional Short-Circuit Shutdown Timer
Low Shutdown I
Q
: 15μA
Available in a 7mm
×
8mm 52-Lead QFN Package
APPLICATIONS
n
n
Point of Load Regulation
Distributed Power Systems
L,
LT, LTC and LTM are registered trademarks 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 V
OUT
187k
V
ON
RUN/SS
100pF
LTC3608
0.8μH
SW
1500pF
11.3k
I
TH
SGND
BOOST
INTV
CC
FCB
V
RNG
PGOOD
EXTV
CC
PGND
V
FB
3608 TA01a
Efficiency and Power Loss
vs Load Current
100
10000
EFFICIENCY
1000
POWER LOSS (mW)
V
IN
4V TO 18V
95
90
EFFICIENCY (%)
V
OUT
2.5V
8A
85
80
75
70
65
60
V
IN
= 12V
V
OUT
= 2.5V
EXTV
CC
= 5V
0.1
1
LOAD CURRENT (A)
POWER LOSS
10
100
I
ON
V
IN
10μF
×3
0.22μF
100μF
×2
30.1k
4.7μF
55
50
0.01
9.53k
1
10
3608 TA01b
3608fa
1
LTC3608
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW
52 PV
IN
51 PV
IN
50 PV
IN
49 PV
IN
48 PV
IN
47 SW
46 SW
45 SW
44 SW
43 SW
42 SW
41 SW
Input Supply Voltage (SV
IN
, PV
IN
, I
ON
)....... 20V to –0.3V
Boosted Topside Driver Supply Voltage
(BOOST) ................................................ 26V to –0.3V
SW Voltage ............................................... 20V to –5V
INTV
CC
, EXTV
CC
, (BOOST – SW), RUN/SS,
PGOOD Voltages ...................................... 7V to –0.3V
FCB, V
ON
, V
RNG
Voltages............ INTV
CC
+ 0.3V to –0.3V
I
TH
, V
FB
Voltages ....................................... 2.7V to –0.3V
Operating Junction Temperature Range
(Notes 2, 4)........................................ –40°C to 125°C
Storage Temperature Range................... –55°C to 125°C
PV
IN
1
PV
IN
2
PV
IN
3
PV
IN
4
PV
IN
5
PV
IN
6
PV
IN
7
SW 8
NC 9
SGND 10
BOOST 11
RUN/SS 12
V
ON
13
SGND 14
54
SGND
53
PV
IN
40 PGND
39 PGND
38 PGND
55
SW
37 PGND
36 PGND
35 PGND
34 PGND
33 SW
32 INTV
CC
31 INTV
CC
30 SV
IN
29 EXTV
CC
28 NC
27 SGND
PGOOD 16
SGND 20
SGND 15
WKG PACKAGE
52-LEAD (7mm
×
8mm) QFN MULTIPAD
T
JMAX
= 125°C,
θ
JA
= 29°C/W
ORDER INFORMATION
LEAD FREE FINISH
LTC3608EWKG#PBF
LTC3608IWKG#PBF
TAPE AND REEL
LTC3608EWKG#TRPBF
LTC3608IWKG#TRPBF
PART MARKING*
LTC3608WKG
LTC3608WKG
PACKAGE DESCRIPTION
52-Lead (7mm
×
8mm) Plastic QFN
52-Lead (7mm
×
8mm) Plastic QFN
TEMPERATURE RANGE
–40°C to 125°C
–40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
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/
SGND 26
FCB 19
NC 21
NC 24
V
RNG
17
I
TH
18
I
ON
22
V
FB
23
NC 25
3608fa
2
LTC3608
ELECTRICAL CHARACTERISTICS
SYMBOL
Main Control Loop
SV
IN
I
Q
Operating Input Voltage Range
Input DC Supply Current
Normal
Shutdown Supply Current
Feedback Reference Voltage
Feedback Voltage Line Regulation
Feedback Voltage Load Regulation
Feedback Input Current
Error Amplifier Transconductance
Forced Continuous Threshold
Forced Continuous Pin Current
On-Time
Minimum On-Time
Minimum Off-Time
Maximum Valley Current
Maximum Reverse Valley Current
Output Overvoltage 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
Top Switch On-Resistance
Bottom Switch On-Resistance
Internal V
CC
Voltage
Internal V
CC
Load Regulation
EXTV
CC
Switchover Voltage
EXTV
CC
Switch Drop Voltage
EXTV
CC
Switchover Hysteresis
PGOOD Upper Threshold
PGOOD Lower Threshold
PGOOD Hysteresis
PGOOD Low Voltage
V
FB
Rising
V
FB
Falling
V
FB
Returning
I
PGOOD
= 5mA
7
–7
6V < V
IN
< 18V, V
EXTVCC
= 4V
I
CC
= 0mA to 20mA, V
EXTVCC
= 4V
I
CC
= 20mA, V
EXTVCC
Rising
I
CC
= 20mA, V
EXTVCC
= 5V
l
l
l
The
l
denotes the specifications which apply over the full operating
junction temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 15V unless otherwise noted.
PARAMETER
CONDITIONS
MIN
4
900
15
I
TH
= 1.2V, –40°C to 85°C (Note 3)
I
TH
= 1.2V, –40°C to 125°C (Note 3)
V
IN
= 4V to 18V, 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)
V
FCB
= 0.6V
I
ON
= 60μA, V
ON
= 1.5V
I
ON
= 60μA, V
ON
= 0V
I
ON
= 180μA, V
ON
= 0V
I
ON
= 30μA, V
ON
= 1.5V
V
RNG
= 0.5V, V
FB
= 0.56V, FCB = 0V
V
RNG
= 0V, V
FB
= 0.56V, FCB = 0V
V
RNG
= 0.5V, V
FB
= 0.64V, FCB = 0V
V
RNG
= 0V, V
FB
= 0.64V, FCB = 0V
l
l
l
l
l
TYP
MAX
18
2000
30
0.606
0.610
–0.3
±50
2
0.66
–2
340
100
500
UNITS
V
μA
μA
V
V
%/V
%
nA
mS
V
μA
ns
ns
ns
ns
A
A
V
FB
ΔV
FB(LINEREG)
ΔV
FB(LOADREG)
I
FB
g
m(EA)
V
FCB
I
FCB
t
ON
t
ON(MIN)
t
OFF(MIN)
I
VALLEY(MAX)
I
VALLEY(MIN)
ΔV
FB(OV)
V
RUN/SS(ON)
V
RUN/SS(LE)
V
RUN/SS(LT)
I
RUN/SS(C)
I
RUN/SS(D)
V
IN(UVLO)
V
IN(UVLOR)
R
DS(ON)
0.594
0.590
0.600
0.600
0.002
–0.05
–5
1.4
0.54
220
1.7
0.6
–1
280
110
60
320
5
8
3.5
5.5
7
0.8
11
16
5.5
7.5
10
1.5
4
3.5
7.5
9.5
13
2
4.5
4.2
–3
3
3.9
4
19
14
5.5
±2
300
A
A
%
V
V
V
μA
μA
V
V
mΩ
mΩ
V
%
V
mV
mV
RUN/SS Pin Rising
RUN/SS Pin Falling
V
RUN/SS
= 0V
V
RUN/SS
= 4.5V, V
FB
= 0V
INTV
CC
Falling
INTV
CC
Rising
l
l
–0.5
0.8
–1.2
1.8
3.4
3.5
10
8
Internal V
CC
Regulator
V
INTVCC
ΔV
LDO(LOADREG)
V
EXTVCC
ΔV
EXTVCC
ΔV
EXTVCC(HYS)
PGOOD Output
ΔV
FBH
ΔV
FBL
ΔV
FB(HYS)
V
PGL
10
–10
1
0.15
13
–13
2.5
0.4
%
%
%
V
3608fa
4.7
4.5
5
–0.1
4.7
150
500
3
LTC3608
ELECTRICAL CHARACTERISTICS
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:
LTC3608: T
J
= T
A
+ (P
D
• 29°C/W)(θ
JA
is simulated per JESD51-7 high
effective thermal conductivity test board)
θ
JC
= 1°C/W (θ
JC
is simulated when heat sink is applied at the bottom
of the package.)
Note 3:
The LTC3608 is tested in a feedback loop that adjusts V
FB
to
achieve a specified error amplifier output voltage (I
TH
). The specification at
85°C is not tested in production. This specification is assured by design,
characterization, and correlation to testing at 125°C.
Note 4:
The LTC3608E is guaranteed to meet performance specifications
from 0°C to 125°C. Specifications over the –40°C to 125°C operating
junction temperature range are assured by design, characterization and
correlation with statistical process controls. The LTC3608I is guaranteed
over the full –40°C to 125°C operating junction temperature range.
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