FEATURES
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LTC3850-2
Dual, 2-Phase
Synchronous Step-Down
Switching Controller
DESCRIPTION
The LTC
®
3850-2 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
frequency 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 LTC3850-2 features a precision 0.8V
reference and a power good output indicator. A wide 4V
to 30V 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 LTC3850-2 is identical to the LTC3850-1, except they
have different pin assignments.
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 30V Operation
Adjustable Soft-Start Current Ramping or Tracking
Foldback Output Current Limiting
Output Overvoltage Protection
Power Good Output Voltage Monitor
28-Pin Narrow SSOP Package
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
TG2
0.1μF
2.2μH
2.2k
EFFICIENCY (%)
22μF
50V
V
IN
7V TO
26V
100
V
IN
= 12V
95 V
OUT
= 3.3V
90
85
80
75
70
65
60
0.1μF
V
OUT2
2.5V
5A
100μF
6V
38502 TA01
Efficiency
10000
EFFICIENCY
POWER LOSS (mW)
BOOST1
BOOST2
SW1
SW2
LTC3850-2
BG2
BG1
500kHz
MODE/PLLIN
EXT
VCC
0.1μF
SENSE1
+
RUN1
SENSE1
–
V
FB1
I
TH1
220pF
TK/SS1
0.1μF
SGND
20k
15k
PGND
FREQ/PLLFLTR
SENSE2
+
RUN2
SENSE2
–
V
FB2
I
TH2
TK/SS2
0.1μF
10nF
10k
220pF
15k
20k
1000
2.2k
100
POWER LOSS
55
50
10
100
1000
LOAD CURRENT (mA)
10
10000
38502 TA01b
V
OUT1
3.3V
5A
100μF
6V
63.4k
43.2k
38502f
1
LTC3850-2
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW
RUN1
SENSE1
+
SENSE1
–
V
FB1
TK/SS1
I
TH1
SGND
I
TH2
TK/SS2
1
2
3
4
5
6
7
8
9
28 FREQ/PLLFLTR
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
Input Supply Voltage (V
IN
) ......................... 30V to –0.3V
Input Supply Transient Voltage (V
IN
) < 500ms, with
INTV
CC
≥ 5V ........................................... 34V to –0.3V
Top Side Driver Voltages
BOOST1, BOOST2.................................. 34V to –0.3V
Switch Voltage (SW1, SW2) ......................... 30V to –5V
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, 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
V
FB2
10
SENSE2
–
11
SENSE2
+
12
RUN2 13
EXTV
CC
14
GN PACKAGE
28-LEAD NARROW PLASTIC SSOP
T
JMAX
= 125°C,
θ
JA
= 95°C/W
ORDER INFORMATION
LEAD FREE FINISH
LTC3850IGN-2#PBF
TAPE AND REEL
LTC3850IGN-2#TRPBF
PART MARKING
LTC3850GN-2
PACKAGE DESCRIPTION
28-Lead Narrow Plastic SSOP
TEMPERATURE RANGE
–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/
38502f
2
LTC3850-2
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
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
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
40
97.2
0.86
±1
1.3
1.22
80
50
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
60
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
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
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
38502f
3
LTC3850-2
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 LTC3850I-2 is 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:
LTC3850IGN-2: T
J
= T
A
+ (P
D
• 95°C/W)
Note 4:
The LTC3850I-2 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).
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
38502 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
38502 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
38502 G03
CIRCUIT OF FIGURE 14
CIRCUIT OF FIGURE 14
CIRCUIT OF FIGURE 14
38502f
4
LTC3850-2
TYPICAL PERFORMANCE CHARACTERISTICS
Load Step
(Burst Mode Operation)
I
LOAD
2A/DIV
200mA TO 2.5A
I
L
2A/DIV
V
OUT
100mV/DIV
AC COUPLED
38502 G04
Load Step
(Forced Continuous Mode)
I
LOAD
2A/DIV
200mA TO 2.5A
I
L
2A/DIV
V
OUT
100mV/DIV
AC COUPLED
40μs/DIV
CIRCUIT OF FIGURE 14
V
IN
= 12V, V
OUT
= 1.8V
40μs/DIV
CIRCUIT OF FIGURE 14
V
IN
= 12V, V
OUT
= 1.8V
38502 G05
Load Step
(Pulse-Skipping Mode)
I
LOAD
2A/DIV
200mA TO 2.5A
I
L
2A/DIV
V
OUT
100mV/DIV
AC COUPLED
FORCED
CONTINUOUS
MODE
2A/DIV
Burst Mode
OPERATION
2A/DIV
PULSE-SKIPPING
MODE
2A/DIV
40μs/DIV
CIRCUIT OF FIGURE 14
V
IN
= 12V, V
OUT
= 1.8V
38502 G06
Inductor Current at Light Load
1μs/DIV
CIRCUIT OF FIGURE 14
V
IN
= 12V, V
OUT
= 1.8V
I
LOAD
= 100μA
38502 G07
Prebiased Output at 2V
RUN1
2V/DIV
V
TK/SS
500mV/DIV
Coincident Tracking
V
OUT
2V/DIV
V
OUT1
, 3.3V
3Ω LOAD, 1V/DIV
V
FB
500mV/DIV
2.5ms/DIV
38502 G08
V
OUT2
, 1.8V
1.5Ω LOAD
1V/DIV
1ms/DIV
38502 G09
38502f
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