LINER LTC3835IGN-1-PBF Low iq synchronous step-down controller Datasheet

LTC3835
Low IQ Synchronous
Step-Down Controller
DESCRIPTION
FEATURES
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Wide Output Voltage Range: 0.8V ≤ VIN ≤ 10V
Low Operating Quiescent Current: 80μA
OPTI-LOOP® Compensation Minimizes COUT
±1% Output Voltage Accuracy
Wide VIN Range: 4V to 36V Operation
Phase-Lockable Fixed Frequency 140kHz to 650kHz
Dual N-Channel MOSFET Synchronous Drive
Very Low Dropout Operation: 99% Duty Cycle
Adjustable Output Voltage Soft-Start or Tracking
Output Current Foldback Limiting
Power Good Output Voltage Monitor
Clock Output for PolyPhase® Applications
Output Overvoltage Protection
Low Shutdown IQ: 10μA
Internal LDO Powers Gate Drive from VIN or VOUT
Selectable Continuous, Pulse-Skipping or
Burst Mode® Operation at Light Loads
Small 20-Lead TSSOP or 4mm × 5mm QFN Package
APPLICATIONS
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Automotive Systems
Telecom Systems
Battery-Operated Digital Devices
Distributed DC Power Systems
The LTC®3835 is a high performance step-down switching
regulator controller that drives an all N-channel synchronous power MOSFET stage. A constant-frequency current
mode architecture allows a phase-lockable frequency of
up to 650kHz.
The 80μA no-load quiescent current extends operating
life in battery powered systems. OPTI-LOOP compensation allows the transient response to be optimized over
a wide range of output capacitance and ESR values. The
LTC3835 features a precision 0.8V reference and a power
good output indicator. The 4V to 36V input supply range
encompasses a wide range of battery chemistries.
The TRACK/SS pin ramps the output voltage during startup. Current foldback limits MOSFET heat dissipation during
short-circuit conditions.
Comparison of LTC3835 and LTC3835-1
PART #
CLKOUT/
PHASMD
EXTVCC
PGOOD
LTC3835
YES
YES
YES
FE20/4 × 5 QFN
LTC3835-1
NO
NO
NO
GN16/3 × 5 DFN
PACKAGES
L, LT, LTC, LTM, Burst Mode, PolyPhase and OPTI-LOOP are registered trademarks of
Linear Technology Corporation. All other trademarks are the property of their respective
owners. Protected by U.S. Patents including 5408150, 5481178, 5705919, 5929620,
6304066, 6498466, 6580258, 6611131.
TYPICAL APPLICATION
Efficiency and Power Loss
vs Load Current
High Efficiency Synchronous Step-Down Converter
VIN
PLLLPF
RUN
0.01μF
33k
20k
LTC3835
62.5k
80
3.3μH
VOUT
3.3V
5A
0.012Ω
SW
100pF
SGND
INTVCC
PLLIN/MODE
EXTVCC
VFB
150μF
10000
1000
60
50
100
40
20
BG
EFFICIENCY
VIN = 12V; VOUT = 3.3V
70
30
4.7μF
POWER LOSS
10
1
10
SENSE–
SENSE+
90
POWER LOSS (mW)
330pF
BOOST
100000
100
0.22μF
PGOOD
TRACK/SS
ITH
VIN
4V TO
36V
10μF
TG
EFFICIENCY (%)
CLKOUT
0
0.001 0.01
PGND
3835 TA01
0.1
0.1
1
10 100 1000 10000
LOAD CURRENT (mA)
3835 TA01b
3835fc
1
LTC3835
ABSOLUTE MAXIMUM RATINGS
(Note 1)
Input Supply Voltage (VIN) ......................... 36V to –0.3V
Top Side Driver Voltage (BOOST) ............... 42V to –0.3V
Switch Voltage (SW) ..................................... 36V to –5V
INTVCC, (BOOST-SW), CLKOUT, PGOOD ... 8.5V to –0.3V
RUN, TRACK/SS ......................................... 7V to –0.3V
SENSE+, SENSE– Voltages ........................ 11V to –0.3V
PLLIN/MODE, PHASMD, PLLLPF ......... INTVCC to –0.3V
EXTVCC ...................................................... 10V to –0.3V
ITH, VFB Voltages ...................................... 2.7V to –0.3V
Peak Output Current <10μs (TG,BG) ...........................3A
INTVCC Peak Output Current ................................. 50mA
Operating Temperature Range (Note 2).... –40°C to 85°C
Junction Temperature (Note 3) ............................. 125°C
Storage Temperature Range
FE Package ........................................ –65°C to 150°C
Storage Temperature Range
UFD Package ..................................... –65°C to 125°C
Lead Temperature (FE Package, Soldering, 10 sec)... 300°C
PIN CONFIGURATION
18 PGOOD
TRACKS/SS
4
VFB
5
SGND
6
15 RUN
PGND
7
14 BOOST
BG
8
13 TG
INTVCC
9
12 SW
EXTVCC 10
11 VIN
17 SENSE+
21
16 SENSE–
FE PACKAGE
20-LEAD PLASTIC TSSOP
TJMAX = 125°C, θJA = 35°C/W
EXPOSED PAD (PIN 21) IS SGND MUST BE SOLDERED TO PCB
PLLIN/MODE
3
PHASMD
ITH
20 19 18 17
ITH 1
16 PGOOD
TRACK/SS 2
15 SENSE+
VFB 3
14 SENSE–
21
SGND 4
13 RUN
12 BOOST
PGND 5
11 TG
BG 6
7
8
9 10
SW
19 PLLIN/MODE
VIN
20 PHASMD
2
INTVCC
1
PLLLPF
EXTVCC
CLKOUT
PLLLPF
TOP VIEW
CLKOUT
TOP VIEW
UFD PACKAGE
20-PIN (4mm s 5mm) PLASTIC QFN
TJMAX = 125°C, θJA = 37°C/W
EXPOSED PAD (PIN 21) IS SGND MUST BE SOLDERED TO PCB
3835fc
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LTC3835
ORDER INFORMATION
(Note 2)
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC3835EFE#PBF
LTC3835EFE#TRPBF
LTC3835EFE
20-Lead Plastic TSSOP
–40°C to 85°C
LTC3835IFE#PBF
LTC3835IFE#TRPBF
LTC3835IFE
20-Lead Plastic TSSOP
–40°C to 85°C
LTC3835EUFD#PBF
LTC3835EUFD#TRPBF
3835
20-Pin (4mm × 5mm) Plastic DFN
–40°C to 85°C
LTC3835IUFD#PBF
LTC3835IUFD#TRPBF
3835
20-Pin (4mm × 5mm) Plastic DFN
–40°C to 85°C
LEAD BASED FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC3835EFE
LTC3835EFE#TR
LTC3835EFE
20-Lead Plastic TSSOP
–40°C to 85°C
LTC3835IFE
LTC3835IFE#TR
LTC3835IFE
20-Lead Plastic TSSOP
–40°C to 85°C
LTC3835EUFD
LTC3835EUFD#TR
3835
20-Pin (4mm × 5mm) Plastic DFN
–40°C to 85°C
LTC3835IUFD
LTC3835IUFD#TR
3835
20-Pin (4mm × 5mm) Plastic DFN
–40°C to 85°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/
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, VRUN = 5V unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
0.792
0.800
0.808
V
–5
–50
nA
0.002
0.02
%/V
0.1
–0.1
0.5
–0.5
%
%
Main Control Loops
VFB
Regulated Feedback Voltage
(Note 4); ITH Voltage = 1.2V
IVFB
Feedback Current
(Note 4)
VREFLNREG
Reference Voltage Line Regulation
VIN = 4V to 30V (Note 4)
VLOADREG
Output Voltage Load Regulation
(Note 4)
Measured in Servo Loop; ΔITH Voltage = 1.2V to 0.7V
Measured in Servo Loop; ΔITH Voltage = 1.2V to 2V
gm
Transconductance Amplifier gm
ITH = 1.2V; Sink/Source 5μA (Note 4)
IQ
Input DC Supply Current
Sleep Mode
Shutdown
(Note 5)
RUN = 5V, VFB = 0.83V (No Load)
VRUN = 0V
UVLO
Undervoltage Lockout
VIN Ramping Down
VOVL
Feedback Overvoltage Lockout
Measured at VFB Relative to Regulated VFB
ISENSE
Sense Pins Total Source Current
VSENSE– = VSENSE+ = 0V
DFMAX
Maximum Duty Factor
In Dropout
l
l
l
1.55
l
8
mmho
80
10
125
20
μA
μA
3.5
4
V
10
12
%
–660
μA
98
99.4
%
ITRACK/SS
Soft-Start Charge Current
VTRACK = 0V
0.75
1.0
1.35
μA
VRUN ON
RUN Pin ON Threshold
VRUN Rising
0.5
0.7
0.9
V
VSENSE(MAX)
Maximum Current Sense Threshold
VFB = 0.7V, VSENSE– = 3.3V
VFB = 0.7V, VSENSE– = 3.3V
90
80
100
100
110
115
mV
mV
TG tr
TG tf
TG Transition Time:
Rise Time
Fall Time
(Note 6)
CLOAD = 3300pF
CLOAD = 3300pF
50
50
90
90
ns
ns
BG tr
BG tf
BG Transition Time:
Rise Time
Fall Time
(Note 6)
CLOAD = 3300pF
CLOAD = 3300pF
40
40
90
80
ns
ns
l
3835fc
3
LTC3835
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, VRUN = 5V unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TG/BG t1D
Top Gate Off to Bottom Gate On Delay CLOAD = 3300pF
Synchronous Switch-On Delay Time
70
ns
BG/TG t2D
Bottom Gate Off to Top Gate On Delay CLOAD = 3300pF
Top Switch-On Delay Time
70
ns
tON(MIN)
Minimum On-Time
180
ns
(Note 7)
TYP
MAX
UNITS
INTVCC Linear Regulator
VINTVCCVIN
Internal VCC Voltage
8.5V < VIN < 30V, VEXTVCC = 0V
VLDOVIN
INTVCC Load Regulation
ICC = 0mA to 20mA, VEXTVCC = 0V
VINTVCCEXT
Internal VCC Voltage
VEXTVCC = 8.5V
VLDOEXT
INTVCC Load Regulation
ICC = 0mA to 20mA, VEXTVCC = 8.5V
VEXTVCC
EXTVCC Switchover Voltage
EXTVCC Ramping Positive
VLDOHYS
EXTVCC Hysteresis
5
7.2
4.5
5.25
5.5
V
0.2
1.0
%
7.5
7.8
V
0.2
1.0
%
4.7
V
0.2
V
Oscillator and Phase-Locked Loop
fNOM
Nominal Frequency
VPLLLPF = No Connect
360
400
440
kHz
fLOW
Lowest Frequency
VPLLLPF = 0V
220
250
280
kHz
475
530
580
kHz
115
140
kHz
fHIGH
Highest Frequency
VPLLLPF = INTVCC
fSYNCMIN
Minimum Synchronizable Frequency
PLLIN/MODE = External Clock; VPLLLPF = 0V
fSYNCMAX
Maximum Synchronizable Frequency
PLLIN/MODE = External Clock; VPLLLPF = 2V
IPLLLPF
Phase Detector Output Current
Sinking Capability
Sourcing Capability
VPGL
650
800
kHz
fPLLIN/MODE < fOSC
fPLLIN/MODE > fOSC
–5
5
μA
μA
PGOOD Voltage Low
IPGOOD = 2mA
0.1
IPGOOD
PGOOD Leakage Current
VPGOOD = 5V
VPG
PGOOD Trip Level
VFB with Respect to Set Regulated Voltage
VFB Ramping Negative
VFB Ramping Positive
PGOOD Output
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 LTC3835E is 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 LTC3835I is guaranteed to meet
performance specifications over the full –40°C to 85°C operating
temperature range.
Note 3: TJ is calculated from the ambient temperature TA and power
dissipation PD according to the following formulas:
LTC3835FE: TJ = TA + (PD • 35°C/W)
LTC3835UFD: TJ = TA + (PD • 37°C/W)
-12
8
–10
10
0.3
V
±1
μA
–8
12
%
%
Note 4: The LTC3835 is tested in a feedback loop that servos VITH to a
specified voltage and measures the resultant VFB.
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 IMAX (see Minimum On-Time
Considerations in the Applications Information section).
3835fc
4
LTC3835
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency and Power Loss
vs Output Current
Burst Mode OPERATION
FORCED CONTINUOUS MODE
PULSE SKIPPING MODE
90
1000
80
Efficiency vs Input Voltage
98
VIN = 12V
VIN = 5V
VOUT = 3.3V
96
94
100
60
50
10
40
30 VIN = 12V
VOUT = 3.3V
20
1
POWER LOSS (mW)
70
EFFICIENCY (%)
100
80
EFFICIENCY (%)
90
Efficiency vs Load Current
10000
EFFICIENCY (%)
100
TA = 25ºC, unless otherwise noted.
70
60
0.1
0.1
1
10 100 1000 10000
LOAD CURRENT (mA)
90
88
86
50
84
10
0
0.001 0.01
92
40
0.001 0.01
VOUT = 3.3V
82
0.1
1
10 100 1000 10000
LOAD CURRENT (mA)
3835 G01
0
5
10
15 20 25 30
INPUT VOLTAGE (V)
3835 G02
FIGURE 11 CIRCUIT
FIGURE 11 CIRCUIT
Load Step
(Burst Mode Operation)
Load Step
(Forced Continuous Mode)
Load Step
(Pulse-Skipping Mode)
VOUT
100mV/
DIV AC
COUPLED
VOUT
100mV/DIV
AC
COUPLED
VOUT
100mV/DIV
AC
COUPLED
IL
2A/DIV
IL
2A/DIV
IL
2A/DIV
3835 G04
3835 G06
3835 G05
20μs/DIV
20μs/DIV
FIGURE 11 CIRCUIT
VOUT = 3.3V
FIGURE 11 CIRCUIT
VOUT = 3.3V
FIGURE 11 CIRCUIT
VOUT = 3.3V
Inductor Current at Light Load
Soft Start-Up
Tracking Start-Up
FORCED
CONTINUOUS
MODE
40
3835 G03
FIGURE 11 CIRCUIT
20μs/DIV
35
VOUT2
2V/DIV (MASTER)
VOUT
1V/DIV
VOUT1
2V/DIV
(SLAVE)
2A/DIV
BURST
MODE
PULSESKIPPING
MODE
3835 G07
4μs/DIV
FIGURE 11 CIRCUIT
VOUT = 3.3V
ILOAD = 300μA
3835 G08
20ms/DIV
FIGURE 11 CIRCUIT
3835 G09
20ms/DIV
FIGURE 11 CIRCUIT
3835fc
5
LTC3835
TYPICAL PERFORMANCE CHARACTERISTICS
Total Input Supply Current
vs Input Voltage
EXTVCC Switchover and INTVCC
Voltages vs Temperature
SUPPLY CURRENT (μA)
300
250
300μA LOAD
200
150
NO LOAD
50
0
5
10
25
20
15
INPUT VOLTAGE (V)
35
30
5.50
5.8
5.45
5.40
5.6
5.4
INTVCC
5.2
5.0
EXTVCC RISING
4.8
4.6
4.4
EXTVCC FALLING
5.05
5.00
–25
35
15
–5
55
TEMPERATURE (°C)
–40
0.2
1.0
0.4 0.6 0.8
ITH PIN VOLTAGE (V)
1.2
1.4
CURRENT SENSE THRESHOLD (mV)
–100
–200
–300
–400
–600
–700
1 2 3 4 5 6 7 8 9
VSENSE COMMON MODE VOLTAGE (V)
100
40
20
0
40
20
SENSE Pins Total Input
Bias Current vs ITH
12
3835 G16
VSENSE = 3.3V
PLLIN/MODE = 0V
10
90
85
80
75
8
6
4
70
2
65
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
FEEDBACK VOLTAGE (V)
10 20 30 40 50 60 70 80 90 100
DUTY CYCLE (%)
3835 G15
INPUT CURRENT (μA)
QUIESCENT CURRENT (μA)
100
0
60
10
95
0
80
3835 G14
TRACK/SS = 1V
40
100
Quiescent Current
vs Temperature
60
35
0
0
Foldback Current Limit
80
15 20 25 30
INPUT VOLTAGE (V)
120
3835 G13
120
10
Maximum Current Sense
Threshold vs Duty Cycle
–500
–20
5
3835 G12
Sense Pins Total Input
Bias Current
INPUT CURRENT (μA)
CURRENT SENSE THRESHOLD (mV)
0
0
0
95
0
10% Duty Cycle
MAXIMUM CURRENT SENSE VOLTAGE (V)
75
100
20
5.15
5.10
200
40
5.20
3835 G11
PULSE SKIPPING
FORCED CONTINUOUS
BURST MODE (RISING)
BURST MODE (FALLING)
60
5.25
4.0
–45
Maximum Current Sense Voltage
vs ITH Voltage
80
5.35
5.30
4.2
3835 G10
100
INTVCC Line Regulation
6.0
INTVCC VOLTAGE (V)
EXTVCC AND INTVCC VOLTAGES (V)
350
100
TA = 25ºC, unless otherwise noted.
60
–45 –30 –15
0
0 15 30 45 60
TEMPERATURE (°C)
75
90
3835 G17
0
0.2
0.4
0.6 0.8 1.0
ITH VOLTAGE (V)
1.2
1.4
3835 G18
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LTC3835
TYPICAL PERFORMANCE CHARACTERISTICS
TRACK/SS Pull-Up Current
vs Temperature
Shutdown (RUN) Threshold
vs Temperature
Regulated Feedback Voltage
vs Temperature
1.00
808
REGULATED FEEDBACK VOLTAGE (mV)
1.20
0.95
1.15
0.90
1.10
RUN PIN VOLTAGE (V)
TRACK/SS CURRENT (μA)
TA = 25ºC, unless otherwise noted.
1.05
1.00
0.95
0.90
0.85
0.80
0.75
0.70
0.65
0.60
0.85
0.55
0.80
–45 –30 –15
0 15 30 45 60
TEMPERATURE (°C)
75
0.50
–45 –30 –15
90
0 15 30 45 60
TEMPERATURE (°C)
75
804
802
800
798
796
794
792
–45 –30 –15
90
0 15 30 45 60
TEMPERATURE (°C)
3835 G20
3835 G19
Sense Pins Total Input Current
vs Temperature
75
90
3835 G21
Shutdown Current
vs Input Voltage
Oscillator Frequency
vs Temperature
25
200
800
VOUT = 10V
700
VOUT = 3.3V
20
INPUT CURRENT (μA)
0
–100
–200
–300
–400
–500
–600
600
FREQUENCY (kHz)
100
INPUT CURRENT (μA)
806
15
10
0
0 15 30 45 60
TEMPERATURE (°C)
300
VPLLLPF = GND
100
–700
–800
–45 –30 –15
VPLLLPF = FLOAT
400
200
5
VOUT = OV
VPLLLPF = INTVCC
500
75
5
90
10
25
20
15
INPUT VOLTAGE (V)
30
0
–45
35
–25
35
15
–5
55
TEMPERATURE (°C)
3835 G23
3835 G22
Undervoltage Lockout Threshold
vs Temperature
95
3835 G24
Oscillator Frequency
vs Input Voltage
4.2
75
Shutdown Current
vs Temperature
404
12
402
10
3.9
3.8
FREQUENCY (kHz)
INTVCC VOLTAGE (V)
4.0
RISING
3.7
3.6
3.5
SHUTDOWN CURRENT (μA)
4.1
400
398
396
FALLING
3.4
394
8
6
4
2
3.3
3.2
–45 –30 –15
392
0 15 30 45 60
TEMPERATURE (°C)
75
90
3835 G25
5
10
25
20
15
INPUT VOLTAGE (V)
30
35
3835 G26
0
–45 –30 –15
0 15 30 45 60
TEMPERATURE (°C)
75
90
3835 G27
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LTC3835
PIN FUNCTIONS
(FE Package/UFD Package)
CLKOUT (Pin 1/Pin 19): Open-Drain Output Clock Signal
available to daisychain other controller ICs for additional
MOSFET driver stages/phases.
PLLLPF (Pin 2/Pin 20): The phase-locked loop’s lowpass
filter is tied to this pin when synchronizing to an external
clock. Alternatively, tie this pin to GND, VIN or leave floating
to select 250kHz, 530kHz or 400kHz switching frequency.
ITH (Pin 3/Pin 1): Error Amplifier Outputs and Switching
Regulator Compensation Points. The current comparator
trip point increases with this control voltage.
TRACK/SS (Pin 4/Pin 2): External Tracking and Soft-Start
Input. The LTC3835 regulates the VFB voltage to the smaller
of 0.8V or the voltage on the TRACK/SS pin. A internal 1μA
pull-up current source is connected to this pin. A capacitor
to ground at this pin sets the ramp time to final regulated
output voltage. Alternatively, a resistor divider on another
voltage supply connected to this pin allows the LTC3835
output to track the other supply during startup.
VFB (Pin 5/Pin 3): Receives the remotely sensed feedback voltage from an external resistive divider across the output.
SGND (Pin 6/Pin 4): Small Signal Ground. Must be routed
separately from high current grounds to the common (–)
terminals of the input capacitor.
PGND (Pin 7/Pin 5): Driver Power Ground. Connects to the
source of bottom (synchronous) N-channel MOSFET, anode
of the Schottky rectifier and the (–) terminal of CIN.
BG (Pin 8/Pin 6): High Current Gate Drive for Bottom
(Synchronous) N-Channel MOSFET. Voltage swing at this
pin is from ground to INTVCC.
INTVCC (Pin 9/Pin 7): Output of the Internal Linear Low
Dropout Regulator. The driver and control circuit are
powered from this voltage source. Must be decoupled to
power ground with a minimum of 4.7μF tantalum or other
low ESR capacitor.
EXTVCC (Pin 10/Pin 8): External Power Input to an Internal LDO
Connected to INTVCC. This LDO supplies VCC power, bypassing the internal LDO powered from VIN whenever EXTVCC is
higher than 4.7V. See EXTVCC Connection in the Applications
Information section. Do not exceed 10V on this pin.
VIN (Pin 11/Pin 9): Main Supply Pin. A bypass capacitor should
be tied between this pin and the signal ground pin.
SW (Pin 12/Pin 10): Switch Node Connections to Inductor.
Voltage swing at this pin is from a Schottky diode (external)
voltage drop below ground to VIN.
TG (Pin 13/Pin 11): High Current Gate Drive for Top
N-Channel MOSFET. These are the outputs of floating drivers
with a voltage swing equal to INTVCC – 0.5V superimposed
on the switch node voltage SW.
BOOST (Pin 14/Pin 12): Bootstrapped Supply to the Top
Side Floating Driver. A capacitor is connected between the
BOOST and SW pins and a Schottky diode is tied between
the BOOST and INTVCC pins. Voltage swing at the BOOST
pin is from INTVCC to (VIN + INTVCC).
RUN (Pin 15/Pin 13): Digital Run Control Input for
Controller. Forcing this pin below 0.7V shuts down all
controller functions, reducing the quiescent current that
the LTC3835 draws to approximately 10μA.
SENSE– (Pin 16/Pin 14): The (–) Input to the Differential
Current Comparator.
SENSE+ (Pin 17/Pin 15): The (+) Input to the Differential
Current Comparator. The ITH pin voltage and controlled
offsets between the SENSE– and SENSE+ pins in conjunction with RSENSE set the current trip threshold.
PGOOD (Pin 18/Pin 16): Open-Drain Logic Output. PGOOD
is pulled to ground when the voltage on the VFB pin is not
within ±10% of its set point.
PLLIN/MODE (Pin 19/Pin 17): External Synchronization
Input to Phase Detector and Forced Continuous Control
Input. When an external clock is applied to this pin, the
phase-locked loop will force the rising TG signal to be
synchronized with the rising edge of the external clock. In
this case, an R-C filter must be connected to the PLLLPF
pin. When not synchronizing to an external clock, this
input determines how the LTC3835 operates at light loads.
Pulling this pin below 0.7V selects Burst Mode operation.
Tying this pin to INTVCC forces continuous inductor current
operation. Tying this pin to a voltage greater than 0.9V and
less than INTVCC selects pulse-skipping operation.
3835fc
8
LTC3835
PIN FUNCTIONS
(FE Package/UFD Package)
PHASMD (Pin 20/Pin 18): Control Input to Phase Selector
which determines the phase relationships between TG and
the CLKOUT signal.
Exposed Pad (Pin 21/Pin 21): SGND. Must be soldered
to the PCB.
FUNCTIONAL DIAGRAM
PLLIN/
MODE
FIN
PHASE DET
VIN
INTVCC
PHASMD
BOOST
DB
RLP PLLLPF
CLP
DROP
OUT
DET
CLK
OSCILLATOR
INTVCC
10k CLKOUT
–
0.88V
S
Q
R
Q
BOT
SWITCH
LOGIC
BURSTEN
0.72V
0.4V
+
B
VOUT
SHDN
RSENSE
L
FC
ICMP
–
0.8V
COUT
PGND
–
+
PLLIN/MODE
BG
SLEEP
–
INTVCC-0.5V
INTVCC
BOT
VFB1
+
CIN
D
SW
TOP ON
–
CB
FC
+
PGOOD
TG
TOP
+
+
–
BURSTEN
0.45V
2(VFB)
–
++
–
–
IR
SENSE+
+
6mV
SENSE–
SLOPE
COMP
–
EA
+
VIN
VIN
OV
4.7V
+
–
EXTVCC
5.25V/
7.5V
LDO
VFB
TRACK/SS
0.80V
RB
RA
+
–
0.88V
ITH
0.5μA
CC
CC2
6V
INTVCC
VFB
RC
1μA
TRACK/SS
+
SGND
INTERNAL
SUPPLY
RUN
CSS
SHDN
3835 FD
3835fc
9
LTC3835
OPERATION
(Refer to Functional Diagram)
Main Control Loop
The LTC3835 uses a constant-frequency, current mode
step-down architecture. During normal operation, the
external top MOSFET is turned on when the clock sets the
RS latch, and is turned off when the main current comparator, ICMP, resets the RS latch. The peak inductor current
at which ICMP trips and resets the latch is controlled by
the voltage on the ITH pin, which is the output of the error
amplifier EA. The error amplifier compares the output voltage feedback signal at the VFB pin, (which is generated with
an external resistor divider connected across the output
voltage, VOUT, to ground) to the internal 0.800V reference
voltage. When the load current increases, it causes a slight
decrease in VFB relative to the reference, which cause the
EA to increase the ITH voltage until the average inductor
current matches the new load current.
After the top MOSFET is turned off each cycle, the bottom
MOSFET is turned on until either the inductor current starts
to reverse, as indicated by the current comparator IR, or
the beginning of the next clock cycle.
INTVCC/EXTVCC Power
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTVCC pin.
When the EXTVCC pin is left open or tied to a voltage less
than 4.7V, an internal 5.25V low dropout linear regulator
supplies INTVCC power from VIN. If EXTVCC is taken above
4.7V, the 5.25V regulator is turned off and a 7.5V low
dropout linear regulator is enabled that supplies INTVCC
power from EXTVCC. If EXTVCC is less than 7.5V (but
greater than 4.7V), the 7.5V regulator is in dropout and
INTVCC is approximately equal to EXTVCC. When EXTVCC
is greater than 7.5V (up to an absolute maximum rating
of 10V), INTVCC is regulated to 7.5V. Using the EXTVCC
pin allows the INTVCC power to be derived from a high
efficiency external source such as one of the LTC3835
switching regulator outputs.
The top MOSFET driver is biased from the floating bootstrap
capacitor CB, which normally recharges during each off
cycle through an external diode when the top MOSFET
turns off. If the input voltage VIN decreases to a voltage
close to VOUT, the loop may enter dropout and attempt
to turn on the top MOSFET continuously. The dropout
detector detects this and forces the top MOSFET off for
about one twelfth of the clock period every tenth cycle to
allow CB to recharge.
Shutdown and Start-Up (RUN and TRACK/SS Pins)
The LTC3835 can be shut down using the RUN pin. Pulling
this pin below 0.7V shuts down the main control loop of the
controller. A low disables the controller and most internal
circuits, including the INTVCC regulator, at which time the
LTC3835 draws only 10μA of quiescent current.
Releasing the RUN pin allows an internal 0.5μA current
to pull up the pin and enable that controller. Alternatively,
the RUN pin may be externally pulled up or driven directly
by logic. Be careful not to exceed the Absolute Maximum
rating of 7V on this pin.
The start-up of the output voltage VOUT is controlled by
the voltage on the TRACK/SS pin. When the voltage on
the TRACK/SS pin is less than the 0.8V internal reference,
the LTC3835 regulates the VFB voltage to the TRACK/SS
pin voltage instead of the 0.8V reference. This allows
the TRACK/SS pin to be used to program a soft start by
connecting an external capacitor from the TRACK/SS pin
to SGND. An internal 1μA pull-up current charges this
capacitor creating a voltage ramp on the TRACK/SS pin.
As the TRACK/SS voltage rises linearly from 0V to 0.8V
(and beyond), the output voltage VOUT rises smoothly
from zero to its final value.
Alternatively the TRACK/SS pin can be used to cause the
start-up of VOUT to “track” that of another supply. Typically,
this requires connecting to the TRACK/SS pin an external
resistor divider from the other supply to ground (see
Applications Information section).
When the RUN pin is pulled low to disable the LTC3835, or
when VIN drops below its undervoltage lockout threshold
of 3.5V, the TRACK/SS pin is pulled low by an internal
MOSFET. When in undervoltage lockout, the controller is
disabled and the external MOSFETs are held off.
3835fc
10
LTC3835
OPERATION
(Refer to Functional Diagram)
Light Load Current Operation (Burst Mode Operation,
Pulse-Skipping, or Continuous Conduction)
(PLLIN/MODE Pin)
advantages of lower output ripple and less interference
to audio circuitry. In forced continuous mode, the output
ripple is independent of load current.
The LTC3835 can be enabled to enter high efficiency Burst
Mode operation, constant-frequency pulse-skipping mode,
or forced continuous conduction mode at low load currents.
To select Burst Mode operation, tie the PLLIN/MODE pin
to a DC voltage below 0.8V (e.g., SGND). To select forced
continuous operation, tie the PLLIN/MODE pin to INTVCC. To
select pulse-skipping mode, tie the PLLIN/MODE pin to a DC
voltage greater than 0.8V and less than INTVCC – 0.5V.
When the PLLIN/MODE pin is connected for pulse-skipping
mode or clocked by an external clock source to use the phaselocked loop (see Frequency Selection and Phase-Locked
Loop section), the LTC3835 operates in PWM pulse-skipping
mode at light loads. In this mode, constant-frequency operation is maintained down to approximately 1% of designed
maximum output current. At very light loads, the current
comparator ICMP may remain tripped for several cycles and
force the external top MOSFET to stay off for the same number
of cycles (i.e., skipping pulses). The inductor current is not
allowed to reverse (discontinuous operation). This mode,
like forced continuous operation, exhibits low output ripple
as well as low audio noise and reduced RF interference as
compared to Burst Mode operation. It provides higher low
current efficiency than forced continuous mode, but not
nearly as high as Burst Mode operation.
When the LTC3835 is enabled for Burst Mode operation,
the peak current in the inductor is set to approximately
one-tenth of the maximum sense voltage even though the
voltage on the ITH pin indicates a lower value. If the average inductor current is lower than the load current, the
error amplifier EA will decrease the voltage on the ITH pin.
When the ITH voltage drops below 0.4V, the internal sleep
signal goes high (enabling “sleep” mode) and both external
MOSFETs are turned off. The ITH pin is then disconnected
from the output of the EA and “parked” at 0.425V.
In sleep mode, much of the internal circuitry is turned off,
reducing the quiescent current that the LTC3835 draws to
only 80μA. In sleep mode, the load current is supplied by
the output capacitor. As the output voltage decreases, the
EA’s output begins to rise. When the output voltage drops
enough, the ITH pin is reconnected to the output of the
EA, the sleep signal goes low, and the controller resumes
normal operation by turning on the top external MOSFET
on the next cycle of the internal oscillator.
When the LTC3835 is enabled for Burst Mode operation,
the inductor current is not allowed to reverse. The reverse
current comparator (RICMP) turns off the bottom external
MOSFET just before the inductor current reaches zero,
preventing it from reversing and going negative, thus
operating in discontinuous operation.
In forced continuous operation, the inductor current is
allowed to reverse at light loads or under large transient
conditions. The peak inductor current is determined by the
voltage on the ITH pin, just as in normal operation. In this
mode, the efficiency at light loads is lower than in Burst
Mode operation. However, continuous operation has the
Frequency Selection and Phase-Locked Loop
(PLLLPF and PLLIN/MODE Pins)
The selection of switching frequency is a tradeoff between
efficiency and component size. Low frequency operation increases efficiency by reducing MOSFET switching
losses, but requires larger inductance and/or capacitance
to maintain low output ripple voltage.
The switching frequency of the LTC3835’s controllers can
be selected using the PLLLPF pin.
If the PLLIN/MODE pin is not being driven by an external
clock source, the PLLLPF pin can be floated, tied to INTVCC,
or tied to SGND to select 400kHz, 530kHz, or 250kHz,
respectively.
A phase-locked loop (PLL) is available on the LTC3835
to synchronize the internal oscillator to an external clock
source that is connected to the PLLIN/MODE pin. In this
case, a series R-C should be connected between the PLLLPF
pin and SGND to serve as the PLL’s loop filter. The LTC3835
phase detector adjusts the voltage on the PLLLPF pin to
align the turn-on of the external top MOSFET to the rising
edge of the synchronizing signal.
3835fc
11
LTC3835
OPERATION
(Refer to Functional Diagram)
The typical capture range of the LTC3835’s phase-locked
loop is from approximately 115kHz to 800kHz, with a
guarantee to be between 140kHz and 650kHz. In other
words, the LTC3835’s PLL is guaranteed to lock to an
external clock source whose frequency is between 140kHz
and 650kHz.
The typical input clock thresholds on the PLLIN/MODE
pin are 1.6V (rising) and 1.2V (falling).
PolyPhase Applications (CLKOUT and PHASMD Pins)
The LTC3835 features two pins (CLKOUT and PHASMD)
that allow other controller ICs to be daisy-chained with
the LTC3835 in PolyPhase applications. The clock output
signal on the CLKOUT pin can be used to synchronize
additional power stages in a multiphase power supply
solution feeding a single, high current output or multiple
separate outputs. The PHASMD pin is used to adjust the
phase of the CLKOUT signal, as summarized in Table 1.
The phases are calculated relative to the zero degrees
phase being defined as the rising edge of the top gate
driver output (TG).
The CLKOUT pin has an open-drain output device. Normally,
a 10k to 100k resistor can be connected from this pin to a
voltage supply that is less than or equal to 8.5V.
Table 1
VPHASMD
CLKOUT PHASE
GND
90°
Floating
120°
INTVCC
180°
Output Overvoltage Protection
An overvoltage comparator guards against transient overshoots as well as other more serious conditions that may
overvoltage the output. When the VFB pin rises to more
than 10% higher than its regulation point of 0.800V, the top
MOSFET is turned off and the bottom MOSFET is turned
on until the overvoltage condition is cleared.
Power Good (PGOOD) Pin
The PGOOD pin is connected to an open drain of an internal
N-channel MOSFET. The MOSFET turns on and pulls the
PGOOD pin low when the VFB pin voltage is not within
±10% of the 0.8V reference voltage. The PGOOD pin is also
pulled low when the RUN pin is low (shut down). When
the VFB pin voltage is within the ±10% requirement, the
MOSFET is turned off and the pin is allowed to be pulled
up by an external resistor to a source of up to 8.5V.
3835fc
12
LTC3835
APPLICATIONS INFORMATION
RSENSE Selection For Output Current
RSENSE is chosen based on the required output current.
The current comparator has a maximum threshold of
100mV/RSENSE and an input common mode range of
SGND to 10V. The current comparator threshold sets the
peak of the inductor current, yielding a maximum average
output current IMAX equal to the peak value less half the
peak-to-peak ripple current, ΔIL.
Allowing a margin for variations in the IC and external
component values yields:
80mV
RSENSE =
IMAX
When using the controller in very low dropout conditions,
the maximum output current level will be reduced due to the
internal compensation required to meet stability criterion for
buck regulators operating at greater than 50% duty factor. A
curve is provided to estimate this reduction in peak output
current level depending upon the operating duty factor.
Operating Frequency and Synchronization
The choice of operating frequency, is a trade-off between
efficiency and component size. Low frequency operation
improves efficiency by reducing MOSFET switching losses,
both gate charge loss and transition loss. However, lower
frequency operation requires more inductance for a given
amount of ripple current.
The internal oscillator of the LTC3835 runs at a nominal
400kHz frequency when the PLLLPF pin is left floating
and the PLLIN/MODE pin is a DC low or high. Pulling the
PLLLPF to INTVCC selects 530kHz operation; pulling the
PLLLPF to SGND selects 250kHz operation.
Alternatively, the LTC3835 will phase-lock to a clock
signal applied to the PLLIN/MODE pin with a frequency
between 140kHz and 650kHz (see Phase-Locked Loop
and Frequency Synchronization).
Inductor Value Calculation
The operating frequency and inductor selection are interrelated in that higher operating frequencies allow the use
of smaller inductor and capacitor values. So why would
anyone ever choose to operate at lower frequencies with
larger components? The answer is efficiency. A higher
frequency generally results in lower efficiency because
of MOSFET gate charge losses. In addition to this basic
trade-off, the effect of inductor value on ripple current and
low current operation must also be considered.
The inductor value has a direct effect on ripple current.
The inductor ripple current ΔIL decreases with higher
inductance or frequency and increases with higher VIN:
ΔIL =
⎛ V ⎞
1
VOUT ⎜ 1 – OUT ⎟
( f)(L)
VIN ⎠
⎝
Accepting larger values of ΔIL allows the use of low inductances, but results in higher output voltage ripple and
greater core losses. A reasonable starting point for setting
ripple current is ΔIL=0.3(IMAX). The maximum ΔIL occurs
at the maximum input voltage.
The inductor value also has secondary effects. The transition to Burst Mode operation begins when the average
inductor current required results in a peak current below
10% of the current limit determined by RSENSE. Lower
inductor values (higher ΔIL) will cause this to occur at
lower load currents, which can cause a dip in efficiency in
the upper range of low current operation. In Burst Mode
operation, lower inductance values will cause the burst
frequency to decrease.
Inductor Core Selection
Once the value for L is known, the type of inductor must
be selected. High efficiency converters generally cannot
afford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite or molypermalloy
cores. Actual core loss is independent of core size for a
fixed inductor value, but it is very dependent on inductance
selected. As inductance increases, core losses go down.
Unfortunately, increased inductance requires more turns
of wire and therefore copper losses will increase.
Ferrite designs have very low core loss and are preferred
at high switching frequencies, so design goals can
concentrate on copper loss and preventing saturation.
Ferrite core material saturates “hard,” which means that
3835fc
13
LTC3835
APPLICATIONS INFORMATION
inductance collapses abruptly when the peak design current
is exceeded. This results in an abrupt increase in inductor
ripple current and consequent output voltage ripple. Do
not allow the core to saturate!
Power MOSFET and Schottky Diode (Optional)
Selection
Two external power MOSFETs must be selected for the
LTC3835: One N-channel MOSFET for the top (main)
switch, and one N-channel MOSFET for the bottom (synchronous) switch.
The peak-to-peak drive levels are set by the INTVCC voltage.
This voltage is typically 5V during start-up (see EXTVCC Pin
Connection). Consequently, logic-level threshold MOSFETs
must be used in most applications. The only exception
is if low input voltage is expected (VIN < 5V); then, sublogic level threshold MOSFETs (VGS(TH) < 3V) should be
used. Pay close attention to the BVDSS specification for
the MOSFETs as well; most of the logic level MOSFETs are
limited to 30V or less.
Selection criteria for the power MOSFETs include the “ON”
resistance RDS(ON), Miller capacitance CMILLER, input
voltage and maximum output current. Miller capacitance,
CMILLER, can be approximated from the gate charge curve
usually provided on the MOSFET manufacturers’ data
sheet. CMILLER is equal to the increase in gate charge
along the horizontal axis while the curve is approximately
flat divided by the specified change in VDS. This result is
then multiplied by the ratio of the application applied VDS
to the Gate charge curve specified VDS. When the IC is
operating in continuous mode the duty cycles for the top
and bottom MOSFETs are given by:
Main Switch Duty Cycle =
VOUT
VIN
Synchronous Switch Duty Cycle =
VIN – VOUT
VIN
The MOSFET power dissipations at maximum output
current are given by:
PMAIN =
VOUT
2
IMAX ) (1+ δ )RDS(ON) +
(
VIN
⎞
( VIN )2 ⎛⎜⎝ IMAX
(R )(C
)•
2 ⎟⎠ DR MILLER
⎡
1
1 ⎤
+
⎢
⎥ ( f)
–
V
V
V
THMIN ⎦
⎣ INTVCC THMIN
VIN – VOUT
2
PSYNC =
IMAX ) (1+ δ )RDS(ON)
(
VIN
where δ is the temperature dependency of RDS(ON) and
RDR (approximately 2Ω) is the effective driver resistance
at the MOSFET’s Miller threshold voltage. VTHMIN is the
typical MOSFET minimum threshold voltage.
Both MOSFETs have I2R losses while the topside N-channel
equation includes an additional term for transition losses,
which are highest at high input voltages. For VIN < 20V
the high current efficiency generally improves with larger
MOSFETs, while for VIN > 20V the transition losses rapidly
increase to the point that the use of a higher RDS(ON) device
with lower CMILLER actually provides higher efficiency. The
synchronous MOSFET losses are greatest at high input
voltage when the top switch duty factor is low or during
a short-circuit when the synchronous switch is on close
to 100% of the period.
The term (1+δ) is generally given for a MOSFET in the
form of a normalized RDS(ON) vs Temperature curve, but
δ = 0.005/°C can be used as an approximation for low
voltage MOSFETs.
The optional Schottky diode D1 shown in Figure 6 conducts
during the dead-time between the conduction of the two
power MOSFETs. This prevents the body diode of the
bottom MOSFET from turning on, storing charge during
the dead-time and requiring a reverse recovery period that
could cost as much as 3% in efficiency at high VIN. A 1A
to 3A Schottky is generally a good compromise for both
regions of operation due to the relatively small average
current. Larger diodes result in additional transition losses
due to their larger junction capacitance.
3835fc
14
LTC3835
APPLICATIONS INFORMATION
CIN and COUT Selection
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle (VOUT)/(VIN). To prevent
large voltage transients, a low ESR capacitor sized for the
maximum RMS current must be used. The maximum RMS
capacitor current is given by:
1/ 2
I
CIN Required IRMS ≈ MAX VOUT VIN – VOUT
VIN
LTC3835
)]
This formula has a maximum at VIN = 2VOUT, where IRMS
= IOUT/2. This simple worst-case condition is commonly
used for design because even significant deviations do not
offer much relief. Note that capacitor manufacturers’ ripple
current ratings are often based on only 2000 hours of life.
This makes it advisable to further derate the capacitor, or
to choose a capacitor rated at a higher temperature than
required. Several capacitors may be paralleled to meet
size or height requirements in the design. Due to the high
operating frequency of the LTC3835, ceramic capacitors
can also be used for CIN. Always consult the manufacturer
if there is any question.
The selection of COUT is driven by the effective series
resistance (ESR). Typically, once the ESR requirement
is satisfied, the capacitance is adequate for filtering. The
output ripple (ΔVOUT) is approximated by:
⎛
1 ⎞
ΔVOUT ≈ IRIPPLE ⎜ ESR +
⎟
8 fCOUT ⎠
⎝
where f is the operating frequency, COUT is the output
capacitance and IRIPPLE is the ripple current in the inductor. The output ripple is highest at maximum input voltage
since IRIPPLE increases with input voltage.
Setting Output Voltage
The LTC3835 output voltage is set by an external feedback resistor divider carefully placed across the output,
as shown in Figure 1. The regulated output voltage is
determined by:
⎛ R ⎞
VOUT = 0.8V • ⎜ 1+ B ⎟
⎝ RA ⎠
VOUT
RB
CFF
VFB
RA
3835 F01
Figure 1. Setting Output Voltage
200
100
0
INPUT CURRENT (μA)
[( )(
To improve the frequency response, a feed-forward capacitor, CFF, may be used. Great care should be taken to
route the VFB line away from noise sources, such as the
inductor and the SW line.
–100
–200
–300
–400
–500
–600
–700
0
1 2 3 4 5 6 7 8 9
VSENSE COMMON MODE VOLTAGE (V)
10
3835 F02
Figure 2. SENSE Pins Input Bias Current
vs Common Mode Voltage
SENSE+ and SENSE– Pins
The common mode input range of the current comparator
is from 0V to 10V. Continuous linear operation is provided
throughout this range allowing output voltages from 0.8V
to 10V. The input stage of the current comparator requires
that current either be sourced or sunk from the SENSE pins
depending on the output voltage, as shown in the curve in
Figure 2. If the output voltage is below 1.5V, current will
flow out of both SENSE pins to the main output. In these
cases, the output can be easily pre-loaded by the VOUT
resistor divider to compensate for the current comparator’s
negative input bias current. Since VFB is servoed to the
0.8V reference voltage, RA in Figure 1 should be chosen
to be less than 0.8V/ISENSE, with ISENSE determined from
Figure 2 at the specified output voltage.
3835fc
15
LTC3835
APPLICATIONS INFORMATION
Tracking and Soft-Start (TRACK/SS Pin)
The start-up of VOUT is controlled by the voltage on the
TRACK/SS pin. When the voltage on the TRACK/SS pin is
less than the internal 0.8V reference, the LTC3835 regulates
the VFB pin voltage to the voltage on the TRACK/SS pin
instead of 0.8V. The TRACK/SS pin can be used to program
an external soft-start function or to allow VOUT to “track”
another supply during start-up.
LTC3835
TRACK/SS
CSS
SGND
3835 F03
Figure 3. Using the TRACK/SS Pin to Program Soft-Start
Soft-start is enabled by simply connecting a capacitor
from the TRACK/SS pin to ground, as shown in Figure 3.
An internal 1μA current source charges up the capacitor,
providing a linear ramping voltage at the TRACK/SS pin.
The LTC3835 will regulate the VFB pin (and hence VOUT)
according to the voltage on the TRACK/SS pin, allowing
VOUT to rise smoothly from 0V to its final regulated value.
The total soft-start time will be approximately:
0 . 8V
t SS = C SS •
1μ A
Alternatively, the TRACK/SS pin can be used to track two
(or more) supplies during start-up, as shown qualitatively
in Figures 4a and 4b. To do this, a resistor divider should
be connected from the master supply (VX) to the TRACK/
SS pin of the slave supply (VOUT), as shown in Figure 5.
During start-up VOUT will track VX according to the ratio
set by the resistor divider:
VX
RA
R
+ R TRACKB
=
• TRACKA
VOUT R TRACKA
R A + RB
For coincident tracking (VOUT = VX during start-up),
RA = RTRACKA
RB = RTRACKB
VX (MASTER)
OUTPUT VOLTAGE
OUTPUT VOLTAGE
VX (MASTER)
VOUT (SLAVE)
TIME
VOUT (SLAVE)
TIME
3835 F04A
(4a) Coincident Tracking
3835 F04B
(4b) Ratiometric Tracking
Figure 4. Two Different Modes of Output Voltage Tracking
Vx
VOUT
RB
LTC3835
VFB
RA
RTRACKB
TRACK/SS
RTRACKA
3835 F05
Figure 5. Using the TRACK/SS Pin for Tracking
3835fc
16
LTC3835
APPLICATIONS INFORMATION
INTVCC Regulators
The LTC3835 features two separate internal P-channel low
dropout linear regulators (LDO) that supply power at the
INTVCC pin from either the VIN supply pin or the EXTVCC
pin, respectively, depending on the connection of the
EXTVCC pin. INTVCC powers the gate drivers and much of
the LTC3835’s internal circuitry. The VIN LDO regulates
the voltage at the INTVCC pin to 5.25V and the EXTVCC
LDO regulates it to 7.5V. Each of these can supply a peak
current of 50mA and must be bypassed to ground with
a minimum of 4.7μF tantalum, 10μF special polymer, or
low ESR electrolytic capacitor. A ceramic capacitor with a
minimum value of 4.7μF can also be used if a 1Ω resistor
is added in series with the capacitor. No matter what type of
bulk capacitor is used, an additional 1μF ceramic capacitor
placed directly adjacent to the INTVCC and PGND IC pins is
highly recommended. Good bypassing is needed to supply
the high transient currents required by the MOSFET gate
drivers and to prevent interaction between the channels.
High input voltage applications in which large MOSFETs are
being driven at high frequencies may cause the maximum
junction temperature rating for the LTC3835 to be exceeded.
The INTVCC current, which is dominated by the gate charge
current, may be supplied by either the 5V VIN LDO or the
7.5V EXTVCC LDO. When the voltage on the EXTVCC pin is
less than 4.7V, the VIN LDO is enabled. Power dissipation
for the IC in this case is highest and is equal to VIN • IINTVCC.
The gate charge current is dependent on operating frequency
as discussed in the Efficiency Considerations section.
The junction temperature can be estimated by using the
equations given in Note 2 of the Electrical Characteristics.
For example, the LTC3835 INTVCC current is limited to less
than 41mA from a 24V supply when in the G package and
not using the EXTVCC supply:
TJ = 70°C + (41mA)(36V)(95°C/W) = 125°C
To prevent the maximum junction temperature from being
exceeded, the input supply current must be checked while
operating in continuous conduction mode (PLLIN/MODE
= INTVCC) at maximum VIN.
When the voltage applied to EXTVCC rises above 4.7V, the
VIN LDO is turned off and the EXTVCC LDO is enabled. The
EXTVCC LDO remains on as long as the voltage applied to
EXTVCC remains above 4.5V. The EXTVCC LDO attempts
to regulate the INTVCC voltage to 7.5V, so while EXTVCC
is less than 7.5V, the LDO is in dropout and the INTVCC
voltage is approximately equal to EXTVCC. When EXTVCC
is greater than 7.5V up to an absolute maximum of 10V,
INTVCC is regulated to 7.5V.
Using the EXTVCC LDO allows the MOSFET driver and
control power to be derived from the LTC3835 switching
regulator output (4.7V ≤ VOUT ≤ 10V) during normal
operation and from the VIN LDO when the output is out
of regulation (e.g., startup, short-circuit). If more cur-rent
is required through the EXTVCC LDO than is specified, an
external Schottky diode can be added between the EXTVCC
and INTVCC pins. Do not apply more than 10V to the EXTVCC
pin and make sure than EXTVCC ≤ VIN.
Significant efficiency and thermal gains can be realized
by powering INTVCC from the output, since the VIN current resulting from the driver and control currents will be
scaled by a factor of (Duty Cycle)/(Switcher Efficiency). For
4.7V to 10V regulator outputs, this means connecting the
EXTVCC pin directly to VOUT. Tying the EXTVCC pin to a 5V
supply reduces the junction temperature in the previous
example from 125°C to:
TJ = 70°C + (24mA)(5V)(95°C/W) = 81°C
However, for 3.3V and other low voltage outputs, additional circuitry is required to derive INTVCC power from
the output.
The following list summarizes the four possible connections for EXTVCC:
1. EXTVCC Left Open (or Grounded). This will cause
INTVCC to be powered from the internal 5.25V regulator
resulting in an efficiency penalty of up to 10% at high
input voltages.
2. EXTVCC Connected Directly to VOUT. This is the normal
connection for a 5V regulator and provides the highest
efficiency.
3. EXTVCC Connected to an External supply. If an external
supply is available in the 5V to 7V range, it may be used
to power EXTVCC providing it is compatible with the
MOSFET gate drive requirements.
3835fc
17
LTC3835
APPLICATIONS INFORMATION
4. EXTVCC Connected to an Output-Derived Boost Network.
For 3.3V and other low voltage regulators, efficiency
gains can still be realized by connecting EXTVCC to an
output-derived voltage that has been boosted to greater
than 4.7V. This can be done with the capacitive charge
pump shown in Figure 6.
VIN
CIN
1μF
+
BAT85
VIN
0.22μF
BAT85
LTC3835
RSENSE
N-CH
EXTVCC
BAT85
VN2222LL
TG1
VOUT
L1
SW
+
COUT
BG1
N-CH
Fault Conditions: Current Limit and Current Foldback
The LTC3835 includes current foldback to help limit load
current when the output is shorted to ground. If the output
falls below 70% of its nominal output level, then the
maximum sense voltage is progressively lowered from
100mV to 30mV. Under short-circuit conditions with very
low duty cycles, the LTC3835 will begin cycle skipping in
order to limit the short-circuit current. In this situation the
bottom MOSFET will be dissipating most of the power but
less than in normal operation. The short-circuit ripple current is determined by the minimum on-time tON(MIN) of the
LTC3835 (≈180ns), the input voltage and inductor value:
ΔIL(SC) = tON(MIN) (VIN/L)
The resulting short-circuit current is:
10mV 1
ISC =
– Δ IL(SC)
R SENSE 2
PGND
3835 F06
Figure 6. Capacitive Charge Pump for EXTVCC
Topside MOSFET Driver Supply (CB, DB)
External bootstrap capacitors CB connected to the BOOST
pins supply the gate drive voltages for the topside MOSFET.
Capacitor CB in the Functional Diagram is charged
though external diode DB from INTVCC when the SW pin
is low. When the topside MOSFET is to be turned on,
the driver places the CB voltage across the gate-source
of the desired MOSFET. This enhances the MOSFET and
turns on the topside switch. The switch node voltage, SW,
rises to VIN and the BOOST pin follows. With the topside
MOSFET on, the boost voltage is above the input supply:
VBOOST = VIN + VINTVCC. The value of the boost capacitor
CB needs to be 100 times that of the total input capacitance
of the topside MOSFET. The reverse breakdown of the
external Schottky diode must be greater than VIN(MAX).
When adjusting the gate drive level, the final arbiter is the
total input current for the regulator. If a change is made
and the input current decreases, then the efficiency has
improved. If there is no change in input current, then there
is no change in efficiency.
Fault Conditions: Overvoltage Protection (Crowbar)
The overvoltage crowbar is designed to blow a system
input fuse when the output voltage of the regulator rises
much higher than nominal levels. The crowbar causes huge
currents to flow, that blow the fuse to protect against a
shorted top MOSFET if the short occurs while the controller
is operating.
A comparator monitors the output for overvoltage
conditions. The comparator (OV) detects overvoltage faults
greater than 10% above the nominal output voltage. When
this condition is sensed, the top MOSFET is turned off and
the bottom MOSFET is turned on until the overvoltage
condition is cleared. The bottom MOSFET remains on
continuously for as long as the overvoltage condition
persists; if VOUT returns to a safe level, normal operation
automatically resumes. A shorted top MOSFET will result in
a high current condition which will open the system fuse.
The switching regulator will regulate properly with a leaky
top MOSFET by altering the duty cycle to accommodate
the leakage.
3835fc
18
LTC3835
APPLICATIONS INFORMATION
Phase-Locked Loop and Frequency Synchronization
The LTC3835 has a phase-locked loop (PLL) comprised of
an internal voltage-controlled oscillator (VCO) and a phase
detector. This allows the turn-on of the top MOSFET (TG)
to be locked to the rising edge of an external clock signal
applied to the PLLIN/MODE pin. The phase detector is
an edge sensitive digital type that provides zero degrees
phase shift between the external and internal oscillators.
This type of phase detector does not exhibit false lock to
harmonics of the external clock.
The output of the phase detector is a pair of complementary current sources that charge or discharge the
external filter network connected to the PLLLPF pin. The
relationship between the voltage on the PLLLPF pin and
operating frequency, when there is a clock signal applied
to PLLIN/MODE, is shown in Figure 7 and specified in the
Electrical Characteristics table. Note that the LTC3835 can
only be synchronized to an external clock whose frequency
is within range of the LTC3835’s internal VCO, which is
nominally 115kHz to 800kHz. This is guaranteed to be
between 140kHz and 650kHz. A simplified block diagram
is shown in Figure 8.
If the external clock frequency is greater than the internal
oscillator’s frequency, fOSC, then current is sourced continuously from the phase detector output, pulling up the
PLLLPF pin. When the external clock frequency is less
than fOSC, current is sunk continuously, pulling down
the PLLLPF pin. If the external and internal frequencies
are the same but exhibit a phase difference, the current
sources turn on for an amount of time corresponding to
the phase difference. The voltage on the PLLLPF pin is
adjusted until the phase and frequency of the internal and
external oscillators are identical. At the stable operating
point, the phase detector output is high impedance and
the filter capacitor CLP holds the voltage.
The loop filter components, CLP and RLP, smooth out the
current pulses from the phase detector and provide a stable
input to the voltage-controlled oscillator. The filter components CLP and RLP determine how fast the loop acquires
lock. Typically RLP = 10k and CLP is 2200pF to 0.01μF.
Typically, the external clock (on PLLIN/MODE pin) input high
threshold is 1.6V, while the input low threshold is 1.2V.
Table 2 summarizes the different states in which the
PLLLPF pin can be used.
Table 2
PLLLPF PIN
PLLIN/MODE PIN
FREQUENCY
0V
DC Voltage
250kHz
Floating
DC Voltage
400kHz
INTVCC
DC Voltage
530kHz
RC Loop Filter
Clock Signal
Phase-Locked to External Clock
2.4V
900
RLP
800
CLP
FREQUENCY (kHz)
700
600
PLLIN/
MODE
500
EXTERNAL
OSCILLATOR
400
PLLLPF
DIGITAL
PHASE/
FREQUENCY
DETECTOR
OSCILLATOR
300
200
100
0
0
0.5
1
1.5
2
PLLLPF PIN VOLTAGE (V)
3835 F08
2.5
Figure 8. Phase-Locked Loop Block Diagram
3835 F07
Figure 7. Relationship Between Oscillator Frequency and Voltage
at the PLLLPF Pin When Synchronizing to an External Clock
3835fc
19
LTC3835
APPLICATIONS INFORMATION
Minimum On-Time Considerations
Minimum on-time tON(MIN) is the smallest time duration
that the LTC3835 is capable of turning on the top MOSFET.
It is determined by internal timing delays and the gate
charge required to turn on the top MOSFET. Low duty
cycle applications may approach this minimum on-time
limit and care should be taken to ensure that
V
t ON(MIN) < OUT
VIN( f)
If the duty cycle falls below what can be accommodated
by the minimum on-time, the controller will begin to skip
cycles. The output voltage will continue to be regulated,
but the ripple voltage and current will increase.
The minimum on-time for the LTC3835 is approximately
180ns. However, as the peak sense voltage decreases
the minimum on-time gradually increases up to about
200ns. This is of particular concern in forced continuous
applications with low ripple current at light loads. If the
duty cycle drops below the minimum on-time limit in this
situation, a significant amount of cycle skipping can occur
with correspondingly larger current and voltage ripple.
Efficiency Considerations
The percent efficiency of a switching regulator is equal to
the output power divided by the input power times 100%.
It is often useful to analyze individual losses to determine
what is limiting the efficiency and which change would
produce the most improvement. Percent efficiency can
be expressed as:
%Efficiency = 100% – (L1 + L2 + L3 + ...)
where L1, L2, etc. are the individual losses as a percentage of input power.
Although all dissipative elements in the circuit produce
losses, four main sources usually account for most of the
losses in LTC3835 circuits: 1) IC VIN current, 2) INTVCC
regulator current, 3) I2R losses, 4) Topside MOSFET
transition losses.
1. The VIN current has two components: the first is the
DC supply current given in the Electrical Characteristics
table, which excludes MOSFET driver and control currents; the second is the current drawn from the 3.3V
linear regulator output. VIN current typically results in
a small (< 0.1%) loss.
2. INTVCC current is the sum of the MOSFET driver and
control currents. The MOSFET driver current results
from switching the gate capacitance of the power
MOSFETs. Each time a MOSFET gate is switched from
low to high to low again, a packet of charge dQ
moves from INTVCC to ground. The resulting dQ/dt is
a current out of INTVCC that is typically much larger
than the control circuit current. In continuous mode,
IGATECHG = f(QT+QB), where QT and QB are the gate
charges of the topside and bottom side MOSFETs.
Supplying INTVCC power through the EXTVCC switch
input from an output-derived source will scale the VIN
current required for the driver and control circuits by
a factor of (Duty Cycle)/(Efficiency). For example, in a
20V to 5V application, 10mA of INTVCC current results
in approximately 2.5mA of VIN current. This reduces the
mid-current loss from 10% or more (if the driver was
powered directly from VIN) to only a few percent.
3. I2R losses are predicted from the DC resistances of the
fuse (if used), MOSFET, inductor, current sense resistor,
and input and output capacitor ESR. In continuous
mode the average output current flows through L and
RSENSE, but is “chopped” between the topside MOSFET
and the synchronous MOSFET. If the two MOSFETs have
approximately the same RDS(ON), then the resistance
of one MOSFET can simply be summed with the
resistances of L, RSENSE and ESR to obtain I2R losses.
For example, if each RDS(ON) = 30mΩ, RL = 50mΩ,
RSENSE = 10mΩ and RESR = 40mΩ (sum of both input
and output capacitance losses), then the total resistance
is 130mΩ. This results in losses ranging from 3% to
13% as the output current increases from 1A to 5A for
3835fc
20
LTC3835
APPLICATIONS INFORMATION
a 5V output, or a 4% to 20% loss for a 3.3V output.
Efficiency varies as the inverse square of VOUT for the
same external components and output power level. The
combined effects of increasingly lower output voltages
and higher currents required by high performance digital
systems is not doubling but quadrupling the importance
of loss terms in the switching regulator system!
4. Transition losses apply only to the topside MOSFET, and
become significant only when operating at high input
voltages (typically 15V or greater). Transition losses
can be estimated from:
Transition Loss = (1.7) VIN2 IO(MAX) CRSS f
Other “hidden” losses such as copper trace and internal
battery resistances can account for an additional 5% to
10% efficiency degradation in portable systems. It is
very important to include these “system” level losses
during the design phase. The internal battery and fuse
resistance losses can be minimized by making sure that
CIN has adequate charge storage and very low ESR at
the switching frequency. A 25W supply will typically
require a minimum of 20μF to 40μF of capacitance having a maximum of 20mΩ to 50mΩ of ESR. Other losses
including Schottky conduction losses during dead-time
and inductor core losses generally account for less than
2% total additional loss.
Checking Transient Response
The regulator loop response can be checked by looking at
the load current transient response. Switching regulators
take several cycles to respond to a step in DC (resistive)
load current. When a load step occurs, VOUT shifts by
an amount equal to ΔILOAD (ESR), where ESR is the effective series resistance of COUT. ΔILOAD also begins to
charge or discharge COUT generating the feedback error
signal that forces the regulator to adapt to the current
change and return VOUT to its steady-state value. During
this recovery time VOUT can be monitored for excessive
overshoot or ringing, which would indicate a stability
problem. OPTI-LOOP compensation allows the transient
response to be optimized over a wide range of output
capacitance and ESR values. The availability of the ITH pin
not only allows optimization of control loop behavior but
also provides a DC coupled and AC filtered closed loop
response test point. The DC step, rise time and settling
at this test point truly reflects the closed loop response.
Assuming a predominantly second order system, phase
margin and/or damping factor can be estimated using the
percentage of overshoot seen at this pin. The bandwidth
can also be estimated by examining the rise time at the
pin. The ITH external components shown in the Typical
Application circuit will provide an adequate starting point
for most applications.
The ITH series RC-CC filter sets the dominant pole-zero
loop compensation. The values can be modified slightly
(from 0.5 to 2 times their suggested values) to optimize
transient response once the final PC layout is done and
the particular output capacitor type and value have been
determined. The output capacitors need to be selected
because the various types and values determine the loop
gain and phase. An output current pulse of 20% to 80%
of full-load current having a rise time of 1μs to 10μs will
produce output voltage and ITH pin waveforms that will
give a sense of the overall loop stability without breaking
the feedback loop. Placing a power MOSFET directly
across the output capacitor and driving the gate with an
appropriate signal generator is a practical way to produce
a realistic load step condition. The initial output voltage
step resulting from the step change in output current may
not be within the bandwidth of the feedback loop, so this
signal cannot be used to determine phase margin. This is
why it is better to look at the ITH pin signal which is in the
feedback loop and is the filtered and compensated control
loop response. The gain of the loop will be increased
by increasing RC and the bandwidth of the loop will be
increased by decreasing CC. If RC is increased by the same
factor that CC is decreased, the zero frequency will be kept
the same, thereby keeping the phase shift the same in the
3835fc
21
LTC3835
APPLICATIONS INFORMATION
most critical frequency range of the feedback loop. The
output voltage settling behavior is related to the stability
of the closed-loop system and will demonstrate the actual
overall supply performance.
A second, more severe transient is caused by switching
in loads with large (>1μF) supply bypass capacitors. The
discharged bypass capacitors are effectively put in parallel
with COUT, causing a rapid drop in VOUT. No regulator can
alter its delivery of current quickly enough to prevent this
sudden step change in output voltage if the load switch
resistance is low and it is driven quickly. If the ratio of
CLOAD to COUT is greater than 1:50, the switch rise time
should be controlled so that the load rise time is limited
to approximately 25 • CLOAD. Thus a 10μF capacitor would
require a 250μs rise time, limiting the charging current
to about 200mA.
Design Example
As a design example, assume VIN = 12V(nominal), VIN =
22V(max), VOUT = 1.8V, IMAX = 5A, and f = 250kHz.
The inductance value is chosen first based on a 30% ripple
current assumption. The highest value of ripple current
occurs at the maximum input voltage. Tie the PLLLPF
pin to GND, generating 250kHz operation. The minimum
inductance for 30% ripple current is:
⎛
⎞
V
V
ΔIL = OUT ⎜ 1 – OUT ⎟
( f)(L) ⎝
VIN ⎠
A 4.7μH inductor will produce 23% ripple current and a
3.3μH will result in 33%. The peak inductor current will be
the maximum DC value plus one half the ripple current, or
5.84A, for the 3.3μH value. Increasing the ripple current will
also help ensure that the minimum on-time of 180ns is not
violated. The minimum on-time occurs at maxi-mum VIN:
VOUT
1 . 8V
t ON(MIN) =
=
= 327n s
VIN(MAX )f 22V(250kHz)
The RSENSE resistor value can be calculated by using the
maximum current sense voltage specification with some
accommodation for tolerances:
R SENSE ≤
80mV
≈ 0 . 012Ω
5 . 84A
Choosing 1% resistors: R1 = 25.5k and R2 = 32.4k yields
an output voltage of 1.816V.
The power dissipation on the top side MOSFET can be easily
estimated. Choosing a Fairchild FDS6982S dual MOSFET
results in: RDS(ON) = 0.035Ω/0.022Ω, CMILLER = 215pF. At
maximum input voltage with T(estimated) = 50°C:
1 . 8V 2
(5) [1+ (0 . 005)(50 °C – 25 °C)] •
22V
5A
(0 . 0 3 5Ω) + (22V )2 ⎛⎜⎝ ⎞⎟⎠ ( 4Ω)(215pF ) •
2
PMAIN =
1 ⎤
⎡ 1
⎢ 5 – 2 . 3 + 2 . 3 ⎥ ( 300kHz ) = 332mW
⎣
⎦
A short-circuit to ground will result in a folded back
current of:
25mV 1 ⎛ 120ns(22V) ⎞
–
= 2 . 1A
ISC =
0 . 01Ω 2 ⎜⎝ 3 . 3μ H ⎟⎠
with a typical value of RDS(ON) and δ = (0.005/°C)(20) = 0.1.
The resulting power dissipated in the bottom MOSFET is:
22V – 1 . 8 V
(2 . 1A )2 (1 . 125)(0 . 022Ω)
22V
= 100mW
PSYNC =
which is less than under full-load conditions.
CIN is chosen for an RMS current rating of at least 3A at
temperature assuming only this channel is on. COUT is
chosen with an ESR of 0.02Ω for low output ripple. The
output ripple in continuous mode will be highest at the
maximum input voltage. The output voltage ripple due to
ESR is approximately:
VORIPPLE = RESR (ΔIL) = 0.02Ω(1.67A) = 33mVP–P
3835fc
22
LTC3835
APPLICATIONS INFORMATION
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of
the IC. These items are also illustrated graphically in the
layout diagram of Figure 9. The Figure 10 illustrates the
current waveforms present in the various branches of the
synchronous regulator operating in the continuous mode.
Check the following in your layout:
1. Is the top N-channel MOSFET M1 located within 1cm
of CIN?
2. Are the signal and power grounds kept separate? The
combined IC signal ground pin and the ground return
of CINTVCC must return to the combined COUT (–) terminals. The path formed by the top N-channel MOSFET,
Schottky diode and the CIN capacitor should have short
leads and PC trace lengths. The output capacitor (–)
terminals should be connected as close as possible
to the (–) terminals of the input capacitor by placing
the capacitors next to each other and away from the
Schottky loop described above.
3. Does the LTC3835 VFB pin resistive divider connect to the
(+) terminals of COUT? The resistive divider must be connected between the (+) terminal of COUT and signal ground.
The feedback resistor connections should not be along the
high current input feeds from the input capacitor(s).
4. Are the SENSE– and SENSE+ leads routed together with
minimum PC trace spacing? The filter capacitor between
SENSE+ and SENSE– should be as close as possible
to the IC. Ensure accurate current sensing with Kelvin
connections at the SENSE resistor.
5. Is the INTVCC decoupling capacitor connected close to
the IC, between the INTVCC and the power ground pins?
This capacitor carries the MOSFET drivers current peaks.
An additional 1μF ceramic capacitor placed immediately
next to the INTVCC and PGND pins can help improve
noise performance substantially.
6. Keep the switching node (SW), top gate node (TG), and
boost node (BOOST) away from sensitive small-signal
nodes. All of these nodes have very large and fast moving
signals and therefore should be kept on the “output side”
of the LTC3835 and occupy minimum PC trace area.
7. Use a modified “star ground” technique: a low impedance, large copper area central grounding point on
the same side of the PC board as the input and output
capacitors with tie-ins for the bottom of the INTVCC
decoupling capacitor, the bottom of the voltage feedback
resistive divider and the SGND pin of the IC.
PC Board Layout Debugging
It is helpful to use a DC-50MHz current probe to monitor
the current in the inductor while testing the circuit. Monitor
the output switching node (SW pin) to synchronize the
oscilloscope to the internal oscillator and probe the actual
output voltage as well. Check for proper performance
over the operating voltage and current range expected
in the application. The frequency of operation should be
maintained over the input voltage range down to dropout
and until the output load drops below the low current
operation threshold—typically 10% of the maximum
designed current level in Burst Mode operation.
The duty cycle percentage should be maintained from cycle
to cycle in a well-designed, low noise PCB implementation.
Variation in the duty cycle at a subharmonic rate can suggest noise pickup at the current or voltage sensing inputs
or inadequate loop compensation. Overcompensation of
the loop can be used to tame a poor PC layout if regulator
bandwidth optimization is not required.
Reduce VIN from its nominal level to verify operation
of the regulator in dropout. Check the operation of the
undervoltage lockout circuit by further lowering VIN while
monitoring the outputs to verify operation.
Investigate whether any problems exist only at higher output currents or only at higher input voltages. If problems
coincide with high input voltages and low output currents,
look for capacitive coupling between the BOOST, SW, TG,
and possibly BG connections and the sensitive voltage
and current pins. The capacitor placed across the current
sensing pins needs to be placed immediately adjacent to
the pins of the IC. This capacitor helps to minimize the
effects of differential noise injection due to high frequency
capacitive coupling. If problems are encountered with
high current output loading at lower input voltages, look
for inductive coupling between CIN, Schottky and the top
3835fc
23
LTC3835
APPLICATIONS INFORMATION
MOSFET components to the sensitive current and voltage
sensing traces. In addition, investigate common ground
path voltage pickup between these components and the
SGND pin of the IC.
An embarrassing problem, which can be missed in an
otherwise properly working switching regulator, results
when the current sensing leads are hooked up backwards.
C1
1nF
The output voltage under this improper hookup will still
be maintained but the advantages of current mode control
will not be realized. Compensation of the voltage loop will
be much more sensitive to component selection. This
behavior can be investigated by temporarily shorting out
the current sensing resistor—don’t worry, the regulator
will still maintain control of the output voltage.
VIN
CB
CIN
M1
L1
VIN
SW
TG
BOOST
RUN
SENSE–
D1
OPTIONAL
M2
COUT
EXTVCC
INTVCC
BG
PGND
SGND
DB
VFB
SENSE+
TRACK/SS
PGOOD
ITH
PLLIN/MODE
PLLLPF
PHASMD
CLKOUT
LTC3835EFE
VOUT
3835 F09
Figure 9. LTC3835 Recommended Printed Circuit Layout Diagram
SW
VIN
L1
RSENSE
VOUT
RIN
CIN
D1
COUT
RL1
3835 F10
BOLD LINES INDICATE HIGH SWITCHING
CURRENT. KEEP LINES TO A MINIMUM LENGTH.
Figure 10. Branch Current Waveforms
3835fc
24
LTC3835
TYPICAL APPLICATIONS
High Efficiency 9.5V, 3A Step-Down Converter
INTVCC
100k
CLKOUT
VIN
PLLLPF
0.01μF
TG
RUN
100k
PGOOD
TRACK/SS
ITH
560pF
35k
LTC3835
BOOST
CIN
10μF
M1
7.2μH
0.012Ω
SW
DB
CMDSH-3
100pF
39.2k
CB
0.22μF
SGND
INTVCC
PLLIN/MODE
EXTVCC
VFB
VIN
4V TO 36V
VOUT
9.5V
3A
COUT
150μF
4.7μF
M2
BG
SENSE–
432k
SENSE+
PGND
3835 TA02
High Efficiency 12V to 1.8V, 2A Step-Down Converter
CLKOUT
VIN
PLLLPF
TG
RUN
0.01μF
PGOOD
TRACK/SS
ITH
3300pF
20mΩ
DB
CMDSH-3
SGND
INTVCC
PLLIN/MODE
EXTVCC
VFB
CIN
10μF
SW
100pF
169k
M1
L1
3.3μH
BOOST
LTC3835
2.49k
CB
0.22μF
BG
VIN
12V
VOUT
1.8V
2A
COUT
100μF
CERAMIC
4.7μF
M2
SENSE–
215k
100pF
M1, M2: Si4840DY
L1: TOKO DS3LC A915AY-3R3M
SENSE+
PGND
3835 TA03
3835fc
25
LTC3835
TYPICAL APPLICATIONS
High Efficiency 5V, 5A Step-Down Converter
CLKOUT
VIN
PLLLPF
TG
RUN
0.01μF
PGOOD
TRACK/SS
ITH
470pF
0.012Ω
DB
CMDSH-3
SGND
INTVCC
PLLIN/MODE
EXTVCC
VFB
3.3μH
SW
LTC3835
69.8k
CIN
10μF
M1
BOOST
100pF
10k
CB
0.22μF
VIN
4V TO
36V
VOUT
5V
5A
COUT
150μF
4.7μF
M2
BG
SENSE–
365k
SENSE+
PGND
3835 TA04
High Efficiency 1.2V, 5A Step-Down Converter
INTVCC
CLKOUT
10k
GND
PLLLPF
TG
RUN
0.01μF
VIN
PGOOD
TRACK/SS
ITH
2.2nF
BOOST
118k
59k
0.012Ω
DB
CMDSH-3
100pF
SGND
INTVCC
PLLIN/MODE
EXTVCC
VFB
2.2μH
CIN
10μF
SW
LTC3835
10k
CB
0.22μF
M1
BG
VIN
4V TO
36V
VOUT
1.2V
5A
COUT
150μF
4.7μF
M2
SENSE–
SENSE+
PGND
3835 TA05
3835fc
26
LTC3835
PACKAGE DESCRIPTION
FE Package
20-Lead Plastic TSSOP (4.4mm)
(Reference LTC DWG # 05-08-1663)
Exposed Pad Variation CB
6.40 – 6.60*
(.252 – .260)
3.86
(.152)
3.86
(.152)
20 1918 17 16 15 14 13 12 11
6.60 p 0.10
2.74
(.108)
4.50 p 0.10
6.40
2.74 (.252)
(.108) BSC
SEE NOTE 4
0.45 p 0.05
1.05 p 0.10
0.65 BSC
1 2 3 4 5 6 7 8 9 10
RECOMMENDED SOLDER PAD LAYOUT
4.30 – 4.50*
(.169 – .177)
0.09 – 0.20
(.0035 – .0079)
0.25
REF
1.20
(.047)
MAX
0o – 8o
0.65
(.0256)
BSC
0.50 – 0.75
(.020 – .030)
NOTE:
1. CONTROLLING DIMENSION: MILLIMETERS
MILLIMETERS
2. DIMENSIONS ARE IN
(INCHES)
3. DRAWING NOT TO SCALE
0.05 – 0.15
(.002 – .006)
0.195 – 0.30
(.0077 – .0118)
TYP
FE20 (CB) TSSOP 0204
4. RECOMMENDED MINIMUM PCB METAL SIZE
FOR EXPOSED PAD ATTACHMENT
*DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.150mm (.006") PER SIDE
UFD Package
20-Lead Plastic QFN (4mm × 5mm)
(Reference LTC DWG # 05-08-1711 Rev B)
4.00 p 0.10
(2 SIDES)
0.75 p 0.05
PIN 1 NOTCH
R = 0.20 OR
C = 0.35
1.50 REF
R = 0.05 TYP
19
20
0.70 p0.05
0.40 p 0.10
PIN 1
TOP MARK
(NOTE 6)
1
4.50 p 0.05
1.50 REF
3.10 p 0.05
2.65 p 0.05
2
3.65 p 0.05
5.00 p 0.10
(2 SIDES)
2.50 REF
3.65 p 0.10
PACKAGE
OUTLINE
0.25 p0.05
0.50 BSC
2.50 REF
4.10 p 0.05
5.50 p 0.05
2.65 p 0.10
(UFD20) QFN 0506 REV B
0.200 REF
0.00 – 0.05
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
R = 0.115
TYP
0.25 p 0.05
0.50 BSC
BOTTOM VIEW—EXPOSED PAD
NOTE:
1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WXXX-X).
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION
ON THE TOP AND BOTTOM OF PACKAGE
3835fc
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
27
LTC3835
TYPICAL APPLICATION
CLKOUT
PLLLPF
TG
RUN
0.01μF
PGOOD
TRACK/SS
ITH
M1
L1
3.3μH
CIN
10μF
VOUT
3.3V
5A
0.012Ω
SW
LTC3835
DB
CMDSH-3
100pF
68.1k
CB
0.22μF
BOOST
1200pF
10k
VIN
4V TO
36V
VIN
SGND
INTVCC
PLLIN/MODE
EXTVCC
VFB
COUT
150μF
4.7μF
M2
BG
SENSE–
215k
SENSE+
39pF
PGND
3835 TA06
M1, M2: Si7848DP
L1: CDEP105-3R2M
COUT: SANYO 10TPD150M
Figure 11. High Efficiency Step-Down Converter
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LTC1628/
LTC1628-PG/
LTC1628-SYNC
LTC1629/
LTC1629-PG
LTC1708-PG
2-Phase, Dual Output Synchronous Step-Down
DC/DC Controller
Reduces CIN and COUT, Power Good Output Signal, Synchronizable,
3.5V ≤ VIN ≤ 36V, IOUT Up to 20A, 0.8V ≤ VOUT ≤ 5
20A to 200A PolyPhase Synchronous Controllers
2-Phase, Dual Synchronous Controller with Mobile VID
Expandable from 2-Phase to 12-Phase, Uses All Surface Mount
Components, No Heatsink, VIN Up to 36V
3.5V ≤ VIN ≤ 36V, VID Sets VOUT1, PGOOD
LT1709/
LT1709-8
LTC1735
High Efficiency, 2-Phase Synchronous Step-Down Switching
Regulators with 5-Bit VID
High Efficiency Synchronous Step-Down Switching Regulator
1.3V ≤ VOUT ≤ 3.5V, Current Mode Ensures Accurate Current
Sharing, 3.5V ≤ VIN ≤ 36V
Output Fault Protection, 16-Pin SSOP
LTC1736
High Efficiency Synchronous Controller with 5-Bit Mobile
VID Control
No RSENSE Current Mode Synchronous Step-Down Controllers
Output Fault Protection, 24-Pin SSOP, 3.5V ≤ VIN ≤ 36V
LTC1778/
LTC1778-1
LTC3708
Dual, 2-Phase, DC/DC Controller with Output Tracking
LTC3728
No RSENSE Current Mode Synchronous Step-Down Controller
with Digital 5-Bit Interface
Dual, 550kHz, 2-Phase Synchronous Step-Down Controller
LTC3729
20A to 200A, 550kHz PolyPhase Synchronous Controller
LTC3731
3- to 12-Phase Step-Down Synchronous Controller
LTC3711
LTC3827/
Low IQ Dual Synchronous Controller
LTC3827-1
No RSENSE is a trademark of Linear Technology Corporation.
Up to 97% Efficiency, 4V ≤ VIN ≤ 36V, 0.8V ≤ VOUT ≤ (0.9)(VIN),
IOUT Up to 20A
Current Mode, No RSENSE, Up/Down Tracking, Synchronizable
Up to 97% Efficiency, Ideal for Pentium® III Processors,
0.925V ≤ VOUT ≤ 2V, 4V ≤ VIN ≤ 36V, IOUT Up to 20A
Dual 180° Phased Controllers, VIN 3.5V to 35V, 99% Duty Cycle,
5x5QFN, SSOP-28
Expandable from 2-Phase to 12-Phase, Uses All Surface Mount
Components, VIN Up to 36V
60A to 240A Output Current, 0.6V ≤ VOUT ≤ 6V, 4.5V ≤ VIN ≤ 32V
2-Phase Operation; 115μA Total No Load IQ, 4V ≤ VIN ≤ 36V
80μA No Load IQ with One Channel On
3835fc
28 Linear Technology Corporation
LT 0508 REV C • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2008
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