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 run
time in battery-powered systems. The LTC3857 features a
precision 0.8V reference and power good output indicators.
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 leaded 28-lead SSOP package with a fixed current
limit and one PGOOD output, without phase modulation
or a clock output, see the LTC3857-1 data sheet.
L,
LT, LTC, LTM, Burst Mode, OPTI-LOOP PolyPhase, 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 (40V Abs Max)
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
Selectable Current Limit
Very Low Dropout Operation: 99% Duty Cycle
Adjustable Output Voltage Soft-Start or Tracking
Power Good Output Voltage Monitors
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
5mm
×
5mm QFN 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
4.7μF
V
IN
TG1
0.1μF
3.3μH
BOOST1
SW1
BG1
LTC3857
SENSE1
+
0.007Ω
V
OUT1
3.3V
5A
SENSE1
–
62.5k
150μF
20k
V
FB1
I
TH1
680pF
15k
SENSE2
V
FB2
I
TH2
–
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
10
100
10000
INTV
CC
TG2
BOOST2
SW2
BG2
PGND
SENSE2
+
0.010Ω
V
OUT2
8.5V
3.5A
150μF
0.1μF
7.2μH
EFFICIENCY (%)
1000
POWER LOSS (mW)
20
10
0
0.00001 0.0001 0.001 0.01
0.1
OUTPUT CURRENT (A)
1
193k
680pF
15k
20k
0.1
1
10
3857 TA01b
TRACK/SS1 SGND TRACK/SS2
0.1μF
0.1μF
3857 TA01
3857fd
1
LTC3857
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW
TRACK/SS1
SENSE1
+
PGOOD1
SW1
24 BOOST1
23 BG1
22 V
IN
33
SGND
21 PGND
20 EXTV
CC
19 INTV
CC
18 BG2
17 BOOST2
9 10 11 12 13 14 15 16
SENSE2
–
SENSE2
+
V
FB2
TRACK/SS2
PGOOD2
SW2
I
TH2
TG2
V
FB1
I
TH1
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
PLLIN/MODE, (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
FREQ Voltages ..................................... –0.3V to INTV
CC
I
LIM
, PHASMD 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, PGOOD2 Voltages ....................... –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
I
LIM
32 31 30 29 28 27 26 25
SENSE1
–
1
FREQ 2
PHASMD 3
CLKOUT 4
PLLIN/MODE 5
SGND 6
RUN1 7
RUN2 8
UH PACKAGE
32-LEAD (5mm 5mm) PLASTIC QFN
T
JMAX
= 125°C,
θ
JA
= 34°C/W
EXPOSED PAD (PIN 33) IS SGND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
LTC3857EUH#PBF
LTC3857IUH#PBF
TAPE AND REEL
LTC3857EUH#TRPBF
LTC3857IUH#TRPBF
PART MARKING*
3857
3857
PACKAGE DESCRIPTION
32-Lead (5mm
×
5mm) Plastic QFN
32-Lead (5mm
×
5mm) 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.
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/
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.
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
l
ELECTRICAL CHARACTERISTICS
CONDITIONS
MIN
4
0.788
0.792
TG1
TYP
MAX
38
UNITS
V
V
V
nA
%/V
0.800
0.800
±5
0.002
0.812
0.808
±50
0.02
I
FB1,2
V
REFLNREG
Feedback Current
Reference Voltage Line Regulation
3857fd
2
LTC3857
ELECTRICAL CHARACTERISTICS
SYMBOL
V
LOADREG
PARAMETER
Output Voltage Load Regulation
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
(Note 4)
Measured in Servo Loop,
∆I
TH
Voltage = 1.2V to 0.7V
(Note 4)
Measured in Servo Loop,
∆I
TH
Voltage = 1.2V to 2V
l
MIN
TYP
0.01
MAX
0.1
UNITS
%
l
–0.01
2
1.3
2
50
65
8
–0.1
%
mmho
mA
mA
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
(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, I
LIM
= 0
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
l
l
l
l
l
l
75
120
20
4.2
4
13
±1
±1
950
1.4
1.31
36
57
85
μA
μA
μA
V
V
%
μA
μA
μA
%
μA
V
mV
mV
mV
mV
Ω
Ω
Ω
Ω
ns
ns
ns
ns
ns
ns
ns
UVLO
V
OVL
I
SENSE+
I
SENSE–
Undervoltage Lockout
Feedback Overvoltage Protection
SENSE
+
Pin Current
SENSE
–
Pins Current
3.6
7
4.0
3.8
10
550
98
0.7
1.21
22
43
64
99.4
1
1.26
50
30
50
75
2.5
1.5
2.4
1.1
DF
MAX
I
TRACK/SS1,2
V
RUN1,2
On
V
SENSE(MAX)
Maximum Duty Factor
Soft-Start Charge Current
RUN Pin On Threshold
Maximum Current Sense Threshold
V
RUN1,2
Hyst RUN Pin Hysteresis
Gate Driver
TG1,2
BG1,2
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
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
(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)
TG1,2 t
r
TG1,2 t
f
BG1,2 t
r
BG1,2 t
f
TG/BG t
1D
BG/TG t
1D
t
ON(MIN)
25
16
28
13
30
30
95
3857fd
3
LTC3857
ELECTRICAL CHARACTERISTICS
SYMBOL
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
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
PGOOD Voltage Low
PGOOD Leakage Current
PGOOD 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
FB
with Respect to Set Regulated Voltage
V
FB
Ramping Negative
Hysteresis
V
FB
with Respect to Set Regulated Voltage
V
FB
Ramping Positive
Hysteresis
t
PG
Delay for Reporting a Fault
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 is tested under pulsed conditions such that
T
J
≈
T
A
. The LTC3857E 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 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.
–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
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
MIN
4.85
TYP
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
MAX
5.35
1.1
5.35
1.1
4.9
UNITS
V
%
V
%
V
mV
kHz
kHz
kHz
kHz
kHz
kHz
V
μA
%
%
%
%
μs
INTV
CC
Linear Regulator
Oscillator and Phase-Locked Loop
PGOOD1 and PGOOD2 Outputs
7
13
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
• 34°C/W)
Note 4:
The LTC3857 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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