FEATURES
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LTC3850/LTC3850-1
Dual, 2-Phase
Synchronous Step-Down
Switching Controller
DESCRIPTION
The LTC
®
3850 is a high performance dual synchronous
step-down switching regulator controller that drives all
N-channel power MOSFET stages. A constant-frequency
current mode architecture allows a phase-lockable fre-
quency of up to 780kHz. Power loss and supply noise are
minimized by operating the two controller output stages
out of phase.
OPTI-LOOP
®
compensation allows the transient response
to be optimized over a wide range of output capacitance
and ESR values. The LTC3850 features a precision 0.8V
reference and a power good output indicator. A wide 4V to
24V (28V maximum/30V for LTC3850I) input supply range
encompasses most battery chemistries and intermediate
bus voltages.
Independent TK/SS pins for each controller ramp the
output voltages during start-up. Current foldback limits
MOSFET heat dissipation during short-circuit condi-
tions. The MODE/PLLIN pin selects among Burst Mode
®
operation, pulse-skipping mode, or continuous inductor
current mode and allows the IC to be synchronized to an
external clock.
The LTC3850 is available in low profile 28-pin 4mm
×
4mm,
4mm
×
5mm QFN and narrow SSOP packages.
Dual, 180° Phased Controllers Reduce Required
Input Capacitance and Power Supply Induced Noise
High Efficiency: Up to 95%
R
SENSE
or DCR Current Sensing
±1% 0.8V Output Voltage Accuracy
Phase-Lockable Fixed Frequency 250kHz to 780kHz
Supports Pre-Biased Output
Dual N-Channel MOSFET Synchronous Drive
Wide V
IN
Range: 4V to 24V (30V for LTC3850I)
Operation
Adjustable Soft-Start Current Ramping or Tracking
Foldback Output Current Limiting
Output Overvoltage Protection
Power Good Output Voltage Monitor
28-Pin 4mm
×
4mm, 4mm
×
5mm QFN and Narrow
SSOP Packages
APPLICATIONS
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Notebook and Palmtop Computers
Portable Instruments
Battery-Operated Digital Devices
DC Power Distribution Systems
L,
LT, LTC, LTM, OPTI-LOOP and BURST MODE are registered 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.
TYPICAL APPLICATION
High Efficiency Dual 3.3V/2.5V Step-Down Converter
4.7μF
V
IN
PGOOD INTV
CC
0.1μF
2.2μH
TG1
BOOST1
SW1
BG1
2.2k
500kHz
LTC3850
TG2
BOOST2
SW2
BG2
PGND
FREQ/PLLFLTR
SENSE2
+
RUN2
SENSE2
–
V
FB2
I
TH2
SGND
TK/SS2
0.1μF
10nF
10k
220pF
15k
20k
38501 TA01
Efficiency
22μF
50V
V
IN
7V TO
24V
100
V
IN
= 12V
95 V
OUT
= 3.3V
90
85
80
75
70
65
60
100
POWER LOSS
10000
EFFICIENCY
0.1μF
2.2μH
2.2k
EFFICIENCY (%)
POWER LOSS (mW)
1000
MODE/PLLIN
I
LIM
SENSE1
+
RUN1
SENSE1
–
V
FB1
I
TH1
TK/SS1
0.1μF
V
OUT1
3.3V
5A
100μF
6V
0.1μF
V
OUT2
2.5V
5A
100μF
6V
55
50
10
100
1000
LOAD CURRENT (mA)
10
10000
38501 TA01b
63.4k
220pF
20k
15k
43.2k
0.1μF
38501fb
1
LTC3850/LTC3850-1
ABSOLUTE MAXIMUM RATINGS
(Note 1)
Input Supply Voltage (V
IN
) ......................... 28V to –0.3V
Input Supply Voltage (V
IN
),
LTC3850I Only ....................................... 30V to –0.3V
Input Supply Transient Voltage (V
IN
) < 500ms, with
INTV
CC
≥ 5V, LTC3850I Only................... 34V to –0.3V
Top Side Driver Voltages
BOOST1, BOOST2.................................. 34V to –0.3V
Switch Voltage (SW1, SW2) ......................... 28V to –5V
Switch Voltage (SW1, SW2),
LTC3850I Only ....................................... 30V to –0.3V
INTV
CC
, RUN1, RUN2, PGOOD, EXTV
CC
,
(BOOST1-SW1), (BOOST2-SW2) ................. 6V to –0.3V
SENSE1
+
, SENSE2
+
, SENSE1
–
,
SENSE2
–
Voltages..................................... 5.5V to –0.3V
MODE/PLLIN, I
LIM
,TK/SS1,TK/SS2, FREQ/PLLFLTR
Voltages ................................................ INTV
CC
to –0.3V
I
TH1
, I
TH2
, V
FB1
, V
FB2
Voltages .................. 2.7V to –0.3V
INTV
CC
Peak Output Current ................................100mA
Operating Temperature Range (Note 2)....–40°C to 85°C
Junction Temperature (Note 3) ............................. 125°C
Storage Temperature Range...................–65°C to 125°C
Lead Temperature (Soldering, 10 sec)
(GN Package) .................................................... 300°C
PIN CONFIGURATION
TOP VIEW
RUN1
SENSE1
+
SENSE1
–
TK/SS1
I
TH1
V
FB1
SGND
V
FB2
I
TH2
1
2
3
4
5
6
7
8
9
28 FREQ/PLLFLTR
SENSE1
–
27 MODE/PLLIN
26 SW1
25 TG1
24 BOOST1
23 BG1
22 V
IN
21 INTV
CC
20 BG2
19 PGND
18 BOOST2
17 TG2
16 SW2
15 PGOOD
TK/SS1 1
I
TH1
2
V
FB1
3
V
FB2
4
I
TH2
5
TK/SS2 6
SENSE2
–
7
8
SENSE2
+
9 10 11 12 13 14
EXTV
CC
I
LIM
SW2
RUN2
PGOOD
TG2
29
SENSE1
+
RUN1
TOP VIEW
FREQ/PLLFLTR
MODE/PLLIN
SW1
28 27 26 25 24 23 22
21 BOOST1
20 BG1
19 V
IN
18 INTV
CC
17 BG2
16 PGND
15 BOOST2
TK/SS2 10
SENSE2
–
11
SENSE2
+
12
RUN2 13
I
LIM
(EXTV
CC
)* 14
GN PACKAGE
28-LEAD NARROW PLASTIC SSOP
UF PACKAGE
28-LEAD (4mm × 4mm) PLASTIC QFN
T
JMAX
= 125°C,
θ
JA
= 95°C/W
*PIN 14 = I
LIM
FOR LTC3850GN, EXTV
CC
FOR LTC3850GN-1
TOP VIEW
FREQ/PLLFLTR
MODE/PLLIN
T
JMAX
= 125°C,
θ
JA
= 37°C/W,
θ
JC
= 2.6°C/W
EXPOSED PAD (PIN 29) IS GND, MUST BE SOLDERED TO PCB
SENSE1
+
RUN1
SW1
28 27 26 25 24 23
SENSE1
–
1
TK/SS1 2
ITH1 3
VFB1 4
VFB2 5
ITH2 6
TK/SS2 7
SENSE2
–
8
9 10 11 12 13 14
SENSE2
+
ILIM
EXTV
CC
RUN2
PGOOD
SW2
29
22 BOOST1
21 BG1
20 VIN
19 INTV
CC
18 BG2
17 PGND
16 BOOST2
15 TG2
UFD PACKAGE
28-LEAD (4mm × 5mm) PLASTIC QFN
T
JMAX
= 125°C,
θ
JA
= 34°C/W, EXPOSED PAD (PIN 29) IS GND, MUST BE SOLDERED TO PCB
38501fb
2
TG1
TG1
LTC3850/LTC3850-1
ORDER INFORMATION
LEAD FREE FINISH
LTC3850EGN#PBF
LTC3850EGN-1#PBF
LTC3850IGN#PBF
LTC3850IGN-1#PBF
LTC3850EUF#PBF
LTC3850EUFD#PBF
LTC3850IUF#PBF
LTC3850IUFD#PBF
TAPE AND REEL
LTC3850EGN#TRPBF
LTC3850EGN-1#TRPBF
LTC3850IGN#TRPBF
LTC3850IGN-1#TRPBF
LTC3850EUF#TRPBF
LTC3850EUFD#TRPBF
LTC3850IUF#TRPBF
LTC3850IUFD#TRPBF
PART MARKING
LTC3850GN
LTC3850GN-1
LTC3850GN
LTC3850GN-1
3850
3850
3850
3850
PACKAGE DESCRIPTION
28-Lead Narrow Plastic SSOP
28-Lead Narrow Plastic SSOP
28-Lead Narrow Plastic SSOP
28-Lead Narrow Plastic SSOP
28-Lead (4mm
×
4mm) Plastic QFN
28-Lead (4mm
×
5mm) Plastic QFN
28-Lead (4mm
×
4mm) Plastic QFN
28-Lead (4mm
×
5mm) Plastic QFN
TEMPERATURE RANGE
–40°C to 85°C
–40°C to 85°C
–40°C to 85°C
–40°C to 85°C
–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.
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/
38501fb
3
LTC3850/LTC3850-1
ELECTRICAL CHARACTERISTICS
SYMBOL
V
FB1,2
I
FB1,2
V
REFLNREG
V
LOADREG
PARAMETER
Regulated Feedback Voltage
Feedback Current
Reference Voltage Line Regulation
Output Voltage Load Regulation
Main Control Loops
I
TH1,2
Voltage = 1.2V; (Note 4)
(Note 4)
V
IN
= 6V to 24V (Note 4)
(Note 4)
Measured in Servo Loop;
ΔI
TH
Voltage = 1.2V to 0.7V
Measured in Servo Loop;
ΔI
TH
Voltage = 1.2V to 1.6V
I
TH1,2
= 1.2V; Sink/Source 5μA; (Note 4)
(Note 5)
V
IN
= 15V; EXTV
CC
Tied to V
OUT1
; V
OUT1
= 5V
V
RUN1,2
= 0V
V
INTVCC
Ramping Down
In Dropout
Measured at V
FB1,2
(Each Channel) V
SENSE1,2
= 3.3V
V
TK/SS1,2
= 0V
V
RUN1
, V
RUN2
Rising
V
FB1,2
= 0.7V, V
SENSE1,2
= 3.3V, I
LIM
= 0V
V
FB1,2
= 0.7V, V
SENSE1,2
= 3.3V, I
LIM
= Float
V
FB1,2
= 0.7V, V
SENSE1,2
= 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)
7V < V
IN
< 24V
I
CC
= 0mA to 50mA
EXTV
CC
Ramping Positive
I
CC
= 20mA, V
EXTVCC
= 5V
l
l
l
l
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, V
RUN1,2
= 5V, unless otherwise noted.
CONDITIONS
MIN
0.792
TYP
0.800
–10
0.002
0.01
–0.01
2.2
850
30
3
0.5
96
0.84
0.9
1.1
20
40
60
97.2
0.86
±1
1.3
1.22
80
30
50
75
2.6
1.5
2.4
1.1
25
25
25
25
30
30
90
4.8
4.5
5
0.5
4.7
50
200
100
5.2
2
40
60
90
0.88
±2
1.7
1.35
MAX
0.808
–50
0.02
0.1
–0.1
UNITS
V
nA
%/V
%
%
mmho
μA
μA
V
V
%
V
μA
μA
V
mV
mV
mV
mV
Ω
Ω
Ω
Ω
ns
ns
ns
ns
ns
ns
ns
V
%
V
mV
mV
g
m1,2
I
Q
Transconductance Amplifier g
m
Input DC Supply Current
Normal Mode
Shutdown
Undervoltage Lockout on INTV
CC
UVLO Hysteresis
Maximum Duty Factor
Feedback Overvoltage Lockout
Sense Pin Bias Current
Soft-Start Charge Current
RUN Pin ON Threshold
RUN Pin ON Hysteresis
Maximum Current Sense Threshold
(Note 8)
TG Driver Pull-Up On-Resistance
TG Driver Pulldown On-Resistance
BG Driver Pull-Up On-Resistance
BG Driver Pulldown 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
EXTV
CC
Switchover Voltage
EXTV
CC
Voltage Drop
EXTV
CC
Hysteresis
50
UVLO
UVLO
HYS
DF
MAX
V
OVL
I
SENSE
I
TK/SS1,2
V
RUN1,2
V
RUN1,2HYS
V
SENSE(MAX)
TG R
UP
TG R
DOWN
BG R
UP
BG R
DOWN
TG1,2 t
r
TG1,2 t
f
BG1,2 t
r
BG1,2 t
f
TG/BG t
1D
BG/TG t
2D
t
ON(MIN)
V
INTVCC
V
LDO
INT
V
EXTVCC
V
LDO
EXT
V
LDOHYS
INTV
CC
Linear Regulator
38501fb
4
LTC3850/LTC3850-1
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
PGOOD Voltage Low
PGOOD Leakage Current
PGOOD Trip Level
f
MODE
< f
OSC
f
MODE
> f
OSC
I
PGOOD
= 2mA
V
PGOOD
= 5V
V
FB
with Respect to Set Regulated Voltage
V
FB
Ramping Negative
V
FB
Ramping Positive
–5
5
– 7.5
7.5
Oscillator and Phase-Locked Loop
V
FREQ
= 1.2V
V
FREQ
= 0V
V
FREQ
≥ 2.4V
450
210
700
500
250
780
250
–13
13
0.1
0.3
±2
–10
10
550
290
860
kHz
kHz
kHz
kΩ
μA
μA
V
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
RUN1,2
= 5V, unless otherwise noted.
CONDITIONS
MIN
TYP
MAX
UNITS
PGOOD Output
V
PGL
I
PGOOD
V
PG
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 LTC3850E/LTC3850E-1 are 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 LTC3850I/LTC3850I-1 are
guaranteed to meet performance specifications over the –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 formulas:
LTC3850GN: T
J
= T
A
+ (P
D
• 95°C/W)
LTC3850UF: T
J
= T
A
+ (P
D
• 37°C/W)
LTC3850UFD: T
J
= T
A
+ (P
D
• 43°C/W)
Note 4:
The LTC3850 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).
Note 8:
V
SENSE(MAX)
defaults to 50mV typical for the LTC3850-1.
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency vs Output Current
and Mode
100
V
IN
= 12V
90 V
OUT
= 1.8V
80
EFFICIENCY (%)
70
60
50
40
30
20
10
0
10
100
1000
LOAD CURRENT (mA)
10000
38501 G01
Efficiency vs Output Current
and Mode
100
90
BURST
EFFICIENCY (%)
80
DCM
EFFICIENCY (%)
70
60
50
40
30
20
10
0
10
V
IN
= 12V
V
OUT
= 3.3V
100
1000
LOAD CURRENT (mA)
10000
38501 G02
Efficiency and Power Loss
vs Input Voltage
100
V
OUT
= 3.3V
I
OUT
= 2A
EFFICIENCY
2000
BURST
95
1500
POWER LOSS (mW)
DCM
90
POWER LOSS
85
1000
CCM
CCM
500
80
5
10
15
20
INPUT VOLTAGE (V)
0
25
38501 G03
CIRCUIT OF FIGURE 14
CIRCUIT OF FIGURE 14
CIRCUIT OF FIGURE 14
38501fb
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