switching regulator controller that drives all N-channel
synchronous power MOSFET stages. A constant frequency
current mode architecture allows a phase-lockable fre-
quency of up to 850kHz. Power loss and noise due to the
ESR of the input capacitor ESR are minimized by operating
the two controller output stages out of phase.
The 50μA no-load quiescent current extends operating life
in battery-powered systems. The LTC3857-1 features a
precision 0.8V reference and a power good output indicator.
A wide 4V to 38V input supply range encompasses a wide
range of intermediate bus voltages and battery chemistries.
Independent TRACK/SS pins for each controller ramp the
output voltages during start-up. Current foldback limits
MOSFET heat dissipation during short-circuit conditions.
The PLLIN/MODE pin selects among Burst Mode opera-
tion, pulse-skipping mode, or continuous inductor current
mode at light loads.
For a leadless 32-pin QFN package with the additional fea-
tures of adjustable current limit, clock out, phase modula-
tion and two PGOOD outputs, see the LTC3857 data sheet.
L,
LT, LTC, LTM, Burst Mode, OPTI-LOOP μModule, Linear Technology and the Linear logo
,
are registered trademarks and No R
SENSE
and UltraFast are trademarks of Linear Technology
Corporation. All other trademarks are the property of their respective owners. Protected by
U.S. Patents, including 5481178, 5929620, 6177787, 6144194, 5408150, 6580258, 5705919,
6100678.
Low Operating I
Q
: 50μA (One Channel On)
Wide Output Voltage Range: 0.8V ≤ V
OUT
≤ 24V
Wide V
IN
Range: 4V to 38V
R
SENSE
or DCR Current Sensing
Out-of-Phase Controllers Reduce Required Input
Capacitance and Power Supply Induced Noise
OPTI-LOOP
®
Compensation Minimizes C
OUT
Phase-Lockable Frequency (75kHz-850kHz)
Programmable Fixed Frequency (50kHz-900kHz)
Selectable Continuous, Pulse-Skipping or Low Ripple
Burst Mode
®
Operation at Light Loads
Very Low Dropout Operation: 99% Duty Cycle
Adjustable Output Voltage Soft-Start or Tracking
Power Good Output Voltage Monitor
Output Overvoltage Protection
Low Shutdown I
Q
: <8μA
Internal LDO Powers Gate Drive from V
IN
or EXTV
CC
No Current Foldback During Start-Up
Narrow SSOP Package
APPLICATIONS
n
n
n
Automotive Always-On Systems
Battery Operated Digital Devices
Distributed DC Power Systems
TYPICAL APPLICATION
High Efficiency Dual 3.3V/8.5V Step-Down Converter
1μF
V
IN
TG1
0.1μF
3.3μH
BOOST1
SW1
BG1
LTC3857-1
SENSE1
+
0.007Ω
V
OUT1
3.3V
5A
SENSE1
–
V
FB1
I
TH1
680pF
20k
15k
SENSE2
–
V
FB2
I
TH2
680pF
15k
20k
INTV
CC
TG2
BOOST2
SW2
BG2
PGND
SENSE2
+
0.010Ω
V
OUT2
8.5V
3.5A
150μF
EFFICIENCY (%)
0.1μF
7.2μH
22μF
50V
V
IN
9V TO 38V
100
Efficiency and Power Loss
vs Output Current
V
IN
= 12V
90 V
OUT
= 3.3V
FIGURE 13 CIRCUIT
80
70
60
50
40
30
20
10
0
0.00001 0.0001 0.001 0.01
0.1
OUTPUT CURRENT (A)
0.1
1
10
3857 TA01b
10000
1000
POWER LOSS (mW)
100
10
1
62.5k
150μF
193k
TRACK/SS1 SGND TRACK/SS2
0.1μF
0.1μF
38571 TA01
38571fc
1
LTC3857-1
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW
I
TH1
V
FB1
SENSE1
+
SENSE1
–
FREQ
PLLIN/MODE
SGND
RUN1
RUN2
1
2
3
4
5
6
7
8
9
28 TRACK/SS1
27 PGOOD1
26 TG1
25 SW1
24 BOOST1
23 BG1
22 V
IN
21 PGND
20 EXTV
CC
19 INTV
CC
18 BG2
17 BOOST2
16 SW2
15 TG2
Input Supply Voltage (V
IN
) ......................... –0.3V to 40V
Topside Driver Voltages
BOOST1, BOOST2 ................................. –0.3V to 46V
Switch Voltage (SW1, SW2) ........................ –5V to 40V
(BOOST1-SW1), (BOOST2-SW2), INTV
CC
... –0.3V to 6V
RUN1, RUN2 ................................................ –0.3V to 8V
Maximum Current Sourced into Pin
from Source >8V...............................................100μA
SENSE1
+
, SENSE2
+
, SENSE1
–
SENSE2
–
Voltages...................................... –0.3V to 28V
PLLIN/MODE, FREQ Voltages .............. –0.3V to INTV
CC
EXTV
CC
...................................................... –0.3V to 14V
I
TH1
, I
TH2
,V
FB1
, V
FB2
Voltages ...................... –0.3V to 6V
PGOOD1 Voltage ......................................... –0.3V to 6V
TRACK/SS1, TRACK/SS2 Voltages .............. –0.3V to 6V
Operating Junction Temperature Range
(Note 2).................................................. –40°C to 125°C
Maximum Junction Temperature (Note 3) ............ 125°C
Storage Temperature Range................... –65°C to 150°C
SENSE2
–
10
SENSE2
+
11
V
FB2
12
I
TH2
13
TRACK/SS2 14
GN PACKAGE
28-LEAD PLASTIC SSOP
T
JMAX
= 125°C,
θ
JA
= 90°C/W
ORDER INFORMATION
LEAD FREE FINISH
LTC3857EGN-1#PBF
LTC3857IGN-1#PBF
TAPE AND REEL
LTC3857EGN-1#TRPBF
LTC3857IGN-1#TRPBF
PART MARKING*
LTC3857GN-1
LTC3857GN-1
PACKAGE DESCRIPTION
28-Lead Plastic SSOP
28-Lead Plastic SSOP
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.
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/
38571fc
2
LTC3857-1
ELECTRICAL CHARACTERISTICS
SYMBOL
V
IN
V
FB1,2
PARAMETER
Input Supply Operating Voltage Range
Regulated Feedback Voltage
(Note 4) I
TH1,2
Voltage = 1.2V
–40°C to 125°C
–40°C to 85°C
(Note 4)
(Note 4) V
IN
= 4.5V to 38V
(Note 4)
Measured in Servo Loop,
ΔI
TH
Voltage = 1.2V to 0.7V
(Note 4)
Measured in Servo Loop,
ΔI
TH
Voltage = 1.2V to 2V
g
m1,2
I
Q
Transconductance Amplifier g
m
Input DC Supply Current
Pulse-Skipping or Forced Continuous
Mode (One Channel On)
Pulse-Skipping or Forced Continuous
Mode (Both Channels On)
Sleep Mode (One Channel On)
Sleep Mode (Both Channels On)
Shutdown
UVLO
V
OVL
I
SENSE+
I
SENSE–
Undervoltage Lockout
Feedback Overvoltage Protection
SENSE
+
Pin Current
SENSE
–
Pins Current
(Note 4) I
TH1,2
= 1.2V, Sink/Source = 5μA
(Note 5)
RUN1 = 5V and RUN2 = 0V, V
FB1
= 0.83V (No Load) or
RUN1 = 0V and RUN2 = 5V, V
FB2
= 0.83V (No Load)
RUN1,2 = 5V, V
FB1,2
= 0.83V (No Load)
RUN1 = 5V and RUN2 = 0V, V
FB1
= 0.83V (No Load) or
RUN1 = 0V and RUN2 = 5V, V
FB2
= 0.83V (No Load)
RUN1,2 = 5V, V
FB1,2
= 0.83V (No Load)
RUN1,2 = 0V
INTV
CC
Ramping Up
INTV
CC
Ramping Down
Measured at V
FB1,2
, Relative to Regulated V
FB1,2
Each Channel
Each Channel
V
SENSE–
< INTV
CC
– 0.5V
V
SENSE–
> INTV
CC
+ 0.5V
In Dropout, FREQ = 0V
V
TRACK1,2
= 0V
V
RUN1
, V
RUN2
Rising
V
FB1,2
= 0.7V, V
SENSE1
–,
2
– = 3.3V
l
l
l
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 (Note 2). V
IN
= 12V, V
RUN1,2
= 5V, EXTV
CC
= 0V unless otherwise noted.
CONDITIONS
MIN
4
0.788
0.792
0.800
0.800
±5
0.002
0.01
TYP
MAX
38
0.812
0.808
±50
0.02
0.1
UNITS
V
V
V
nA
%/V
%
I
FB1,2
V
REFLNREG
V
LOADREG
Feedback Current
Reference Voltage Line Regulation
Output Voltage Load Regulation
l
–0.01
2
2
2
50
65
8
3.6
7
4.0
3.8
10
–0.1
%
mmho
mA
mA
75
120
20
4.2
4
13
±1
±1
μA
μA
μA
V
V
%
μA
μA
μA
%
μA
V
mV
mV
Ω
Ω
Ω
Ω
ns
ns
ns
ns
550
98
0.7
1.21
43
99.4
1.0
1.26
50
50
2.5
1.5
2.4
1.1
57
1.4
1.31
DF
MAX
I
TRACK/SS1,2
V
RUN1,2
On
V
SENSE(MAX)
Gate Driver
TG1,2
BG1,2
Maximum Duty Factor
Soft-Start Charge Current
RUN Pin On Threshold
Maximum Current Sense Threshold
Pull-Up On-Resistance
Pull-Down On-Resistance
Pull-Up On-Resistance
Pull-Down On-Resistance
TG Transition Time:
Rise Time
Fall Time
BG Transition Time:
Rise Time
Fall Time
V
RUN1,2
Hyst RUN Pin Hysteresis
TG1,2 t
r
TG1,2 t
f
BG1,2 t
r
BG1,2 t
f
(Note 6)
C
LOAD
= 3300pF
C
LOAD
= 3300pF
(Note 6)
C
LOAD
= 3300pF
C
LOAD
= 3300pF
25
16
28
13
38571fc
3
LTC3857-1
ELECTRICAL CHARACTERISTICS
SYMBOL
TG/BG t
1D
BG/TG t
1D
t
ON(MIN)
V
INTVCCVIN
V
LDOVIN
V
INTVCCEXT
V
LDOEXT
V
EXTVCC
V
LDOHYS
f
25kΩ
f
65kΩ
f
105kΩ
f
LOW
f
HIGH
f
SYNC
V
PGL
I
PGOOD
V
PG
PARAMETER
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
Internal V
CC
Voltage
INTV
CC
Load Regulation
EXTV
CC
Switchover Voltage
EXTV
CC
Hysteresis
Programmable Frequency
Programmable Frequency
Programmable Frequency
Low Fixed Frequency
High Fixed Frequency
Synchronizable Frequency
PGOOD1 Voltage Low
PGOOD1 Leakage Current
PGOOD1 Trip Level
R
FREQ
= 25k, PLLIN/MODE = DC Voltage
R
FREQ
= 65k, PLLIN/MODE = DC Voltage
R
FREQ
= 105k, PLLIN/MODE = DC Voltage
V
FREQ
= 0V, PLLIN/MODE = DC Voltage
V
FREQ
= INTV
CC
, PLLIN/MODE = DC Voltage
PLLIN/MODE = External Clock
I
PGOOD
= 2mA
V
PGOOD
= 5V
V
FB1
with Respect to Set Regulated Voltage
V
FB1
Ramping Negative
Hysteresis
V
FB1
with Respect to Set Regulated Voltage
V
FB1
Ramping Positive
Hysteresis
t
PG
Delay for Reporting a Fault
–13
–10
2.5
10
2.5
25
l
The
l
denotes the specifications which apply over the full operating junction
temperature range, otherwise specifications are at T
A
= 25°C (Note 2). V
IN
= 12V, V
RUN1,2
= 5V, EXTV
CC
= 0V unless otherwise noted.
CONDITIONS
C
LOAD
= 3300pF Each Driver
C
LOAD
= 3300pF Each Driver
(Note 7)
6V < V
IN
< 38V, V
EXTVCC
= 0V
I
CC
= 0mA to 50mA, V
EXTVCC
= 0V
6V < V
EXTVCC
< 13V
I
CC
= 0mA to 50mA, V
EXTVCC
= 8.5V
EXTV
CC
Ramping Positive
4.5
4.85
4.85
MIN
TYP
30
30
95
5.1
0.7
5.1
0.6
4.7
250
105
375
320
485
75
0.2
440
835
350
535
380
585
850
0.4
±1
–7
505
5.35
1.1
5.35
1.1
4.9
MAX
UNITS
ns
ns
ns
V
%
V
%
V
mV
kHz
kHz
kHz
kHz
kHz
kHz
V
μA
%
%
%
%
μs
INTV
CC
Linear Regulator
Oscillator and Phase-Locked Loop
PGOOD1 Output
7
13
Note 1:
Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Ratings for extended periods may affect device reliability and
lifetime.
Note 2:
The LTC3857-1 is tested under pulsed conditions such that
T
J
≈
T
A
. The LTC3857E-1 is guaranteed to meet performance specifications
from 0°C to 85°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 LTC3857I-1 is guaranteed
over the full –40°C to 125°C operating junction temperature range. Note
that the maximum ambient temperature is determined by specific operating
conditions in conjunction with board layout, the rated package thermal
resistance and other environmental factors.
Note 3:
T
J
is calculated from the ambient temperature T
A
and power
dissipation P
D
according to the following formula:
T
J
= T
A
+ (P
D
• 90°C/W)
Note 4:
The LTC3857-1 is tested in a feedback loop that servos V
ITH1,2
to
a specified voltage and measures the resultant V
FB1,2
. The specification at
85°C is not tested in production. This specification is assured by design,
characterization and correlation to production testing at 125°C.
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).
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