LINER LTC3446

LTC3446
Monolithic Buck Regulator
with Dual VLDO Regulators
FEATURES
DESCRIPTION
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The LTC®3446 combines a synchronous buck DC/DC converter with two very low dropout (VLDO) linear regulators
to provide up to three stepped-down output voltages from
a single input voltage. The input voltage range is ideally
suited for Li-Ion battery-powered applications, as well
as powering low voltage logic from 5V or 3.3V rails. The
output voltage range extends down to 0.4V for the VLDO
regulators and 0.8V for the buck.
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High Efficiency Triple Step-Down Outputs from a
Single Input Supply
1A Synchronous Buck Regulator Provides Main
Step-Down Output and Powers Two 300mA
VLDO™ Linear Regulators
Output Voltages as Low as 400mV (VLDO Outputs)
Power Good Output
Input Voltage Range: 2.7V to 5.5V
Independent Internal Soft-Start for Each Supply
Independent Enable Pin for Each Supply
Low (140μA Typ) No-Load Quiescent Current with
all Outputs Enabled
Constant Frequency Current Mode Operation
2.25MHz Switching Frequency Uses Small
Inductors
Defeatable Automatic Burst Mode® Operation for
High Efficiency at Light Loads
±1.5% Reference Accuracy
Overtemperature Protection
Thermally Enhanced 4mm × 3mm 14-Pin DFN
Package
APPLICATIONS
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Low Power Handheld Devices
Low Voltage and Multivoltage Power for Digital
Logic, I/O, FPGAs, CPLDs, ASICs and CPUs
The 1A synchronous buck converter provides the main
output with high efficiency, typically 85%. The two 300mA
VLDO regulators can run off the main output to provide
two additional lower voltage outputs. A built-in supply
monitor provides a power good indication.
The buck converter switches at 2.25MHz, allowing the use
of small surface mount inductors and capacitors. Constant
frequency current mode operation produces controlled
output spectrum and fast transient response. A modeselect pin allows automatic Burst Mode operation to be
enabled for higher efficiency at light load, or disabled for
lower noise operation down to very light loads. The two
VLDO regulators are stable with ceramic output capacitors
as small as 1μF.
L, LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology Corporation.
VLDO is a trademark of Linear Technology Corporation. All other trademarks are the property
of their respective owners. Protected by U.S. Patents including 5481178, 6611131, 6304066,
6498466, 6580258.
TYPICAL APPLICATION
L1
1.8μH
VIN
2.9V TO 5.5V
VIN
22μF
X7R
SW
59k
22μF
X7R
VOUT
1.8V
400mA MAX
Buck Efficiency vs Buck Load Current
100
BUCKFB
LVIN
PGOOD LVOUT1
MODESEL
ENBUCK
ENLDO1 LVFB1
ENLDO2
DIGITAL
CONTROL
47.5k
110k
2.2μF
X7R
LVOUT2
1000pF
80.6k
2.2μF
X7R
VOUT
1.2V
300mA MAX
LVFB2
GND
3446 TA01
PWM
MODE
80
70
60
40.2k
3.3k
ITH
VOUT
1.5V
300mA MAX
EFFICIENCY (%)
LTC3446
Burst Mode
OPERATION
90
40.2k
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
50
40
1
10
100
LOAD CURRENT (mA)
1000
3446 TA01b
L1: TOKO A960AW-1R8M
3446fd
1
LTC3446
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
VIN, LVIN to GND ......................................... – 0.3V to 6V
MODESEL, ENBUCK, ENLDO1,
ENLDO2 to GND .......................................... –0.3V to 6V
BUCKFB to GND .......................................... –0.3V to 6V
LVFB1, LVFB2 to GND .................................... –0.3V to 6V
ITH to GND ..... –0.3V to the Lesser of (VIN + 0.3V) or 3V
SW to GND......–0.3V to the Lesser of (VIN + 0.3V) or 6V
LVOUT1, LVOUT2
to GND .......... –0.3V to the Lesser of (LVIN + 0.3V) or 6V
PGOOD to GND ............................................ –0.3V to 6V
LVOUT1, LVOUT2 Short-Circuit
to GND Duration............................................... Indefinite
Operating Temperature Range (Note 2)
LTC3446EDE ........................................ –40°C to 85°C
LTC3446IDE ....................................... –40°C to 125°C
Junction Temperature (Note 1) ........................... 125°C
Storage Temperature Range.................. –65°C to 125°C
TOP VIEW
MODESEL
1
14 SW
VIN
2
13 ENBUCK
ITH
3
PGOOD
4
LVOUT1
5
10 LVFB1
LVIN
6
9 LVFB2
LVOUT2
7
8 ENLDO2
12 BUCKFB
11 ENLDO1
15
DE PACKAGE
14-LEAD (4mm × 3mm) PLASTIC DFN
TJMAX = 125°C, θJA = 35°C/W
EXPOSED PAD (PIN 15) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC3446EDE#PBF
LTC3446EDE#TRPBF
3446
14-Lead (4mm × 3mm) Plastic DFN
–40°C to 85°C
LTC3446IDE#PBF
LTC3446IDE#TRPBF
3446
14-Lead (4mm × 3mm) Plastic DFN
–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/
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 3.6V unless otherwise specified. (Note 2)
SYMBOL
PARAMETER
CONDITIONS
VIN
Input Voltage Range
(Note 3)
●
VUVLO
VIN Undervoltage Lockout Threshold
VIN Undervoltage Lockout Hysteresis
VIN Rising
●
IQ
VIN Quiescent Current (Note 4)
Buck Enabled Only, Not Sleeping
Buck Enabled Only, Sleeping
One LDO Enabled Only
All Three Outputs Enabled, Buck Not Sleeping
All Three Outputs Enabled, Buck Sleeping
Shutdown
VBUCKFB = 0V, ISW = 0mA
VBUCKFB = 1V, ISW = 0mA
VLVIN = 1.5V, 10μA LDO Output Load
VBUCKFB = 0V, ISW = 0mA, VLVIN = 1.5V,
10μA Output Load on Each LDO
VBUCKFB = 1V, ISW = 0mA, VLVIN = 1.5V,
10μA Load on Each LDO
VENBUCK = 0V, VENLDO1 = 0V, VENLDO2 = 0V
MIN
TYP
2.7
MAX
UNITS
5.5
V
2.37
10
2.5
30
V
mV
310
50
75
400
500
75
100
600
μA
μA
μA
μA
140
210
μA
1
μA
3446fd
2
LTC3446
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 3.6V unless otherwise specified. (Note 2)
SYMBOL
PARAMETER
CONDITIONS
VPG(THRESH) PGOOD Threshold (Note 8)
TYP
MAX
●
MIN
8
10
%
87
120
Ω
1
μA
RPGOOD
PGOOD Output Resistance
PGOOD Low, Sinking 1mA
●
IPGOOD
PGOOD Hi-Z Leakage Current
VPGOOD = 6V
●
UNITS
Synchronous Buck Converter
IBUCKFB
Feedback Current
(Note 5)
●
VBUCKFB
Regulated Feedback Voltage
(Note 5)
●
ΔVBUCKFB
Feedback Voltage Line Regulation
VIN = 2.7V to 5.5V (Note 5)
●
IMAXP
Maximum Peak Inductor Current
VBUCKFB = 0V, Duty Cycle < 35%
IMAXN
NMOS Overcurrent Limit
Feedback Voltage Load Regulation
(with Respect to VITH)
0.788
1.2
VITH = 0.5V to 1V, VMODESEL = VIN (Note 5)
●
±30
nA
0.800
0.812
V
0.3
0.5
1.55
2.0
mV/V
A
1.8
A
0.5
mV/V
fOSC
Oscillator Frequency
RPFET
RDS(ON) of P-Channel FET
ISW = 500mA
0.13
Ω
RNFET
RDS(ON) of N-Channel FET
ISW = –500mA
0.14
Ω
ILSW
SW Leakage
VENBUCK = 0V, VSW = 0V or 5.5V, VIN = 5.5V
Buck Enable Pin Threshold
Buck Enable Pin Leakage Current
VMODESEL
Mode Select Pin Threshold
IMODESEL
Mode Select Pin Leakage Current
VMODESEL = 5.5V, All Other Pins Grounded
gm
Error Amplifier Transconductance
VITH = 0.6V
VENBUCK = 5.5V, All Other Pins Grounded
2.25
2.7
±1
●
VENBUCK
IENBUCK
1.8
0.3
0.65
●
●
MHz
μA
1
V
1
μA
V
0.3
0.65
1
450
700
950
μA/V
5.5
V
20
μA
●
1
μA
Each VLDO: VIN = 3.6V, VLVIN = 1.5V, VLVOUT = 1.2V, Unless Otherwise Specified
VLVIN
LVIN Pin Operating Voltage
(Note 6)
●
ILVIN
LVIN Pin Operating Current
IOUT = 10μA
●
0.9
3
LVIN Shutdown Current
VENLDO = 0V
VLVFB
Feedback Pin Regulation Voltage (Note 7)
1mA ≤ IOUT ≤ 300mA, 1.5V ≤ VLVIN ≤ 5.5V
ILVFB
Feedback Pin Input Current
ILVOUT(MAX)
Continuous Output Current
VENLDOx
LDO Enable Pin Threshold
IENLDOx
LDO Enable Pin Leakage Current
VENLDOx = 5.5V, All Other Pins Grounded
Output Voltage Load Regulation
(Referred to the LVFB Pin)
ΔIOUT = 1mA to 300mA
–1
mV/A
LVFB Line Regulation (with Respect to
the LVIN Pin)
VLVIN = 1.5V to 5.5V, VIN = 3.6V, VLVOUT = 1.2V,
IOUT = 1mA
7.5
μV/V
LVFB Line Regulation (with Respect to
the VIN Pin)
VLVIN = 1.5V, VIN = 2.7V to 5.5V, VLVOUT = 1.2V,
IOUT = 1mA
0.44
mV/V
LVIN – LVOUT Dropout Voltage
VIN = 2.8V, VLVIN = 1.5V, VLVFB = 0.37V,
IOUT = 300mA (Note 9)
VLVFB at Regulation
●
0.395
0.392
●
●
300
●
0.3
Short-Circuit Output Current
VDO
1.5
2
μA
0.400
0.400
0.405
0.408
V
V
2
±10
nA
mA
760
0.65
●
68
mA
1
V
1
μA
175
mV
3446fd
3
LTC3446
ELECTRICAL CHARACTERISTICS
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 LTC3446E 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 LTC3446I is guaranteed to
meet performance specifications over the –40°C to 125°C operating
temperature range.
Note 3: Minimum operating VIN voltage required for the VLDO regulators
to stay in regulation is:
VIN ≥ LVOUT(MAX) + 1.4V and VIN ≥ 2.7V
Note 4: Dynamic supply current is higher due to the internal gate charge
being delivered at the switching frequency.
Note 5: The LTC3446 is tested in a feedback loop that connects the
BUCKFB pin to the output of the buck converter’s error amplifier (i.e., the
ITH pin).
Note 6: Minimum operating LVIN voltage required for the VLDO regulators
to stay in regulation is:
LVIN ≥ LVOUT(MAX) + 100mV and LVIN ≥ 0.9V
Note 7: Operating conditions are limited by maximum junction
temperature. The regulated output voltage specification will not apply
for all possible combinations of input voltage and output current. When
operating at maximum input voltage, the output current range must be
limited. When operating at maximum output current, the input voltage
range must be limited.
Note 8: PGOOD assertion indicates that the feedback voltages of all
enabled supplies are within the specified percentage of their target values.
Note 9: Dropout voltage in the DFN package is assured by design,
characterization and statistical process control.
3446fd
4
LTC3446
TYPICAL PERFORMANCE CHARACTERISTICS
LDO1 Regulated Feedback
Voltage vs Temperature
808
408
806
406
804
404
802
402
VLVFB1 (mV)
VBUCKFB (mV)
Buck Regulated Feedback Voltage
vs Temperature
800
798
796
400
398
396
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
VIN = 5.5V
794
394
792
–50 –30 –10 10 30 50 70 90 110 130
TEMPERATURE (°C)
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
VIN = 5.5V
392
–50 –30 –10 10 30 50 70 90 110 130
TEMPERATURE (°C)
3446 G01
3446 G02
LDO2 Regulated Feedback
Voltage vs Temperature
Undervoltage Lockout Threshold
vs Temperature
2.50
408
2.45
406
2.40
2.35
402
2.30
UVLO RISING
UVLO FALLING
VIN (V)
VLVFB2 (mV)
404
2.25
400
2.20
398
2.15
396
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
VIN = 5.5V
394
2.10
2.05
2.00
–50 –30 –10 10 30 50 70 90 110 130
TEMPERATURE (°C)
392
–50 –30 –10 10 30 50 70 90 110 130
TEMPERATURE (°C)
3446 G03
3446 G04
Oscillator Frequency
vs Temperature
2.5
2.4
2.3
2.2
2.1
2.0
1.9
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1.0
–50 –30
2.50
2.45
OSCILLATOR FREQUENCY (MHz)
CURRENT (A)
Maximum Peak Inductor Current
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
VIN = 5.5V
2.40
2.35
2.30
2.25
2.20
2.15
2.10
2.05
–10 10 30 50 70 90 110 130
TEMPERATURE (°C)
3446 G05
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
VIN = 5.5V
2.00
–50 –30 –10 10 30 50 70 90 110 130
TEMPERATURE (°C)
3446 G06
3446fd
5
LTC3446
TYPICAL PERFORMANCE CHARACTERISTICS
Peak Inductor Current
vs ITH Voltage
VLDO Current Limit
vs LVIN Voltage
1800
VIN = 3.6V
1.4 VMODESEL = 3.6V
1.2
1.0
0.8
0.6
0.4
1400
1200
1000
800
PULSED MEASUREMENT,
TJ ≈ TA ≈ 27°C
600
400
0.2
0
VIN = 3.6V
LVOUT = 0V
1600
VLDO CURRENT LIMIT (mA)
PEAK INDUCTOR CURRENT (A)
1.6
200
0
0.25 0.5 0.75 1 1.25 1.5 1.75
ITH PIN VOLTAGE (V)
0
0.00
2
1.00
2.00 3.00 4.00
LVIN PIN VOLTAGE (V)
3446 G07
3446 G08
VIN Bias Current
vs VLDO Load Current
VLDO Dropout Voltage
vs Load Current
450
VIN = 2.8V
90 LVOUT = 1.2V
–45°C
80
27°C
90°C
70
130°C
60
400
350
VIN CURRENT (μA)
DROPOUT VOLTAGE (mV)
100
50
40
30
300
250
150
100
10
50
0
50
150
200
100
LOAD CURRENT (mA)
250
VIN = 3.6V
LVIN = 1.5V
LVOUT = 1.2V
ONLY ONE VLDO
ENABLED
–45°C
27°C
90°C
130°C
200
20
0
0
300
0
100
150
50
250
200
VLDO OUTPUT CURRENT (mA)
3446 G09
300
3446 G10
LVIN No-Load Operating Current
Enable/MODESEL Thresholds
1000
20
16
ENABLE/MODESEL THRESHOLD (mV)
VIN = 5V
BOTH VLDOS ON AND
REGULATING 0.8V
18
LVIN CURRENT (μA)
6.00
5.00
14
12
10
8
6
–45°C
27°C
90°C
130°C
4
2
0
0
1
3
4
LVIN PIN VOLTAGE (V)
2
5
900
800
700
600
500
400
300
–45°C
27°C
90°C
130°C
200
100
0
6
3446 G11
2.5
3
3.5
4
VIN (V)
4.5
5
5.5
3446 G12
3446fd
6
LTC3446
TYPICAL PERFORMANCE CHARACTERISTICS
Buck NMOS Switch On-Resistance
200
180
180
160
140
120
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
VIN = 5.5V
100
80
–45 –20
55
30
80
5
TEMPERATURE (°C)
105
130
NMOS RESISTANCE (mΩ)
PMOS RESISTANCE (mΩ)
Buck PMOS Switch On-Resistance
200
160
140
120
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
VIN = 5.5V
100
80
–45 –20
55
30
80
5
TEMPERATURE (°C)
105
3446 G17
130
3446 G18
Buck Transient Response with
Burst Mode Defeated
Buck Transient Response with
Burst Mode Enabled
BUCK OUTPUT
VOLTAGE
50mV/DIV
AC-COUPLED
BUCK OUTPUT
VOLTAGE
50mV/DIV
AC-COUPLED
500mA
500mA
50mA
LOAD
CURRENT
50mA
LOAD
CURRENT
100μs/DIV
FRONT PAGE APPLICATION CIRCUIT
3446 G13
100μs/DIV
FRONT PAGE APPLICATION CIRCUIT
3446 G14
VLDO Rejection of Buck DC/DC
Burst Mode Ripple
VLDO Transient Response
VLVIN =
BUCK VOUT
20mV/DIV
AC-COUPLED
VLDO OUTPUT
VOLTAGE
20mV/DIV
AC-COUPLED
VLVOUT
10mV/DIV
AC-COUPLED
300mA
LOAD
CURRENT
30mA
100μs/DIV
LVOUT = 1.5V OR 1.2V
FRONT PAGE APPLICATION CIRCUIT
3446 G15
2μs/DIV
VLVIN = BUCK VOUT = 1.8V
VIN = 4.2V
VLVOUT = 1.5V
ILVOUT = 50mA
CLVOUT = 2.2μF
3446 G16
3446fd
7
LTC3446
PIN FUNCTIONS
MODESEL (Pin 1): Chooses Between Burst Mode Operation
and Pulse Skipping Operation at Light Loads. Forcing this
pin below 0.3V allows the buck converter to automatically
enter Burst Mode operation at light loads. Forcing this pin
above 1V disallows entering Burst Mode operation; the
buck converter will cycle skip at light loads. Do not leave
this pin floating. This is a MOS gate input.
VIN (Pin 2): Input Supply to the LTC3446. Must be
closely decoupled to GND with a 10μF or greater ceramic
capacitor.
ITH (Pin 3): Buck Error Amplifier Output and Servo-Loop
Compensation Point.
PGOOD (Pin 4): Supply Monitor Output, Open-Drain
NMOS.
LVOUT1 (Pin 5): Output of the First VLDO Regulator.
LVIN (Pin 6): Input Supply to the LTC3446’s VLDO Circuits.
Bypass LVIN to GND with at least a 1μF low ESR ceramic
capacitor. Typical LTC3446 application circuits will connect
this pin to the output of the buck converter but this is not
required. The VLDO regulators may be used independently
of the buck converter.
LVOUT2 (Pin 7): Output of the Second VLDO Regulator.
LVFB2 (Pin 9): Feedback Pin for the Second VLDO Regulator.
An output divider should be connected from LVOUT2 to LVFB2
to set the desired LVOUT2 regulated output voltage.
LVFB1 (Pin 10): Feedback Pin for the First VLDO Regulator.
An output divider should be connected from LVOUT1 to LVFB1
to set the desired LVOUT1 regulated output voltage.
ENLDO1/ENLDO2 (Pin 11/Pin 8): Enable Pin for the First
and Second VLDO Regulators, Respectively. Forcing this
pin above 1V enables the corresponding VLDO regulator
and forcing this pin below 0.3V shuts it down. Each VLDO
regulator draws <1μA of supply current in shutdown. Do
not leave this pin floating. This is a MOS gate input.
BUCKFB (Pin 12): Buck Converter’s Feedback Pin. Receives
the feedback voltage from an external resistive divider
across the output. External resistance from this pin to
ground should be equal to or less than 50k.
ENBUCK (Pin 13): Enable Pin for the LTC3446’s Buck
Converter Circuit. Forcing this pin above 1V enables the
buck converter and forcing this pin below 0.3V shuts down
the converter. In shutdown, the buck converter draws <1μA
of supply current. Do not leave this pin floating. This is a
MOS gate input.
SW (Pin 14): Switch Node Connection to Inductor. This
pin connects to the drains of the internal main and synchronous power MOSFET switches.
Exposed Pad (Pin 15): Ground. The Exposed Pad is the
only ground and must connect to the PCB ground for
electrical contact and rated thermal performance.
3446fd
8
LTC3446
BLOCK DIAGRAM
3
2
ITH
VIN
ENBUCK
0.4V
0.8V
SOFT-START
VOLTAGE
REFERENCE
ITH
PARK
ERROR
AMPLIFIER
ITH
LIMIT
+
12
BUCKFB
–
–
VB
+
B
PMOS CURRENT
COMPARATOR
BCLAMP
+
–
+
BURST
COMPARATOR
SLOPE
COMPENSATION
OSCILLATOR
SW
14
+
4
PGOOD
LOGIC
POWER
GOOD
NMOS
OVERCURRENT
COMPARATOR
–
–
1
+
13
ENBUCK
REVERSE
CURRENT
COMPARATOR
MODESEL
VIN
SOFT-START
GND
LVIN
+
VLDO1
–
6μA
LVOUT1
LVFB1
11
15
EXPOSED
PAD
ENLDO1
6
5
10
VIN
SOFT-START
+
VLDO2
–
6μA
LVOUT2
LVFB2
8
ENLDO2
7
9
3446 BD
3446fd
9
LTC3446
OPERATION
The LTC3446 combines a constant frequency, current mode
synchronous buck converter with two very low dropout
(VLDO) linear DC regulators to provide up to three high
efficiency, low voltage outputs from a single higher voltage
input source. Each output can be independently enabled
and disabled, and has its own independent soft-start circuit to help reduce inrush current. A power good circuit
monitors all three supplies. The LTC3446 incorporates
an undervoltage lockout circuit that shuts down the IC
when the input voltage drops below about 2.4V to prevent
unstable operation.
SYNCHRONOUS BUCK OPERATION
A buck converter takes power from a high input voltage,
VIN, and delivers it at a lower output voltage, VOUT. The buck
converter inside the LTC3446 achieves over 80% efficient
power conversion under a wide range of VIN, VOUT and load
conditions, whereas a linear regulator is limited by physics
to a maximum efficiency of (VOUT/VIN) × 100%.
Main Control Loop
During normal operation, the internal oscillator produces a
constant frequency 2.25MHz clock. The top power switch
(P-channel MOSFET) turns on at the beginning of a clock
cycle. Inductor current increases to a peak value which is
set by the voltage on the ITH pin. Then the top switch turns
off and the energy stored in the inductor flows through
the bottom switch (N-channel MOSFET) into the load until
the next clock cycle.
The peak inductor current is controlled by the voltage on
the ITH pin, which is the output of the error amplifier. This
amplifier compares the BUCKFB pin to the 0.8V reference.
When the load current increases, the BUCKFB voltage decreases slightly below the reference. This decrease causes
the error amplifier to increase the ITH voltage until the
average inductor current matches the new load current.
The main control loop is shut down by pulling the ENBUCK
pin to ground. A soft-start is enabled whenever ENBUCK
is brought high. Soft-start limits the peak inductor current
from reaching maximum for the first millisecond after
ENBUCK is brought high.
Overcurrent Protection
To help avert inductor current runaway in case the buck
output is accidentally shorted to ground, the LTC3446
features a bottom switch NMOS overcurrent limit, which
works as follows.
When the buck output is shorted to ground, inductor
current will rise to its maximum peak level, IMAXP, such
that on every oscillator cycle the PMOS top switch will
turn on for only its minimum duty cycle, and the bottom
switch NMOS turns on for the remainder of the cycle.
Temporarily ignoring inductor, switch and parasitic resistance drops, which in most applications are designed to
be small in order to maximize buck converter efficiency,
it is to first order true that when the PMOS is on, the VIN
supply voltage is placed across the inductor, increasing
the inductor current, but when the NMOS is on, there is no
output voltage to be placed across the inductor to reduce
its current. Inductor current ratchets up each cycle and
could lead to the destruction of the buck IC.
The NMOS overcurrent limit helps prevent this by sensing
the current through the NMOS bottom switch, and for as
long as this current exceeds the overcurrent limit level,
IMAXN, it:
1. Keeps the NMOS on, allowing the tiny voltage drops from
parasitic resistances to reduce the inductor current.
2. Refuses to allow the PMOS to turn on, preventing any
additional energy from being fed into the system.
Low Current Operation
The MODESEL pin controls the buck converter’s behavior at
light load currents to help optimize efficiency, output ripple
and noise. When the load is relatively light and MODESEL
is grounded, the buck converter automatically switches
into Burst Mode operation, which operates the PMOS
switch intermittently based on load demand rather than
at a constant frequency. Every switch cycle during Burst
Mode operation delivers more energy than would occur
in constant frequency operation, minimizing the switching loss per unit of energy delivered. Since the dominant
power loss at light loads is gate charge switching loss in
the power MOSFETs, operating in Burst Mode operation
3446fd
10
LTC3446
OPERATION
can dramatically improve light load efficiency. The tradeoff
is higher output ripple than in constant frequency operation, as well as the presence of noise below the 2.25MHz
clock frequency.
If MODESEL were instead tied to VIN, pulse skipping mode
is selected. In this mode, the buck converter continues to
switch at a constant frequency down to very light loads
where it will eventually begin skipping pulses. Because
constant frequency operation is extended down to light
loads, low output ripple is maintained and any coupled
or radiated noise is at or higher than the clock frequency.
The tradeoff is lower efficiency compared to Burst Mode
operation.
Dropout Operation
When the input supply voltage decreases toward the
output voltage, the duty cycle increases to 100%, which
is known as the dropout condition. In dropout, the PMOS
switch is turned on continuously with the output voltage
equal to the input voltage minus any voltage drop across
the PMOS switch and the external inductor.
VLDO LINEAR REGULATOR OPERATION
The two micropower, VLDO (very low dropout) linear
regulators in the LTC3446 operate from input voltages as
low as 0.9V. Each VLDO regulator provides a high accuracy
output that is capable of supplying 300mA of output current with a typical dropout voltage of only 70mV. A single
ceramic capacitor as small as 1μF is all that is required
for output bypassing. A low reference voltage of 400mV
allows the VLDO regulators to be programmed to much
lower voltages than available in common LDOs.
As shown in the Block Diagram, the VIN input supplies
the internal reference and biases the VLDO circuitry while
all output current comes directly from the LVIN input for
high efficiency regulation. The low per-VLDO quiescent
supply currents ILVIN = 4μA, IVIN = 80μA drop to ILVIN <
2μA, IVIN < 1μA in shutdown, are well-suited to batterypowered systems.
Each VLDO includes current limit protection. The fast
transient response of the follower output stage overcomes
the traditional tradeoff between dropout voltage, quiescent
current and load transient response inherent in most LDO
regulator architectures. Overshoot detection circuitry is
included to bring the output back into regulation when
going from heavy to light output loads (“load-dump”
handling).
POWER GOOD CIRCUIT OPERATION
The LTC3446 has a built-in supply monitor. The feedback
voltage of each enabled supply is monitored by a window
comparator to determine whether it is within 8% of its
target value. If they all are, then the PGOOD pin becomes
high impedance. If no supply is enabled, or if any enabled
supply is more than 8% away from its target, then the
PGOOD pin is driven to ground by an internal open-drain
NMOS.
The PGOOD pin may be connected through a pull-up
resistor to a supply voltage of up to 5.5V, independent of
the VIN pin voltage.
3446fd
11
LTC3446
APPLICATIONS INFORMATION
A general LTC3446 application circuit is shown in Figure 1.
External component selection is driven by output voltage
and load requirements. The following text is divided into
two sections: the first covers Buck regulator design and
the second covers use of the linear VLDO regulators.
BUCK REGULATOR DESIGN
Buck regulator design begins with the selection of the
L1 inductor based on desired ripple current. Once L1 is
chosen, CIN and COUTB can be selected based on output
voltage ripple requirements. Output voltage is programmed
through R1 and R2, and loop response can be optimized
by choice of RITH and CITH.
Inductor Selection
Although the inductor does not influence the operating frequency, the inductor value has a direct effect on
ripple current. The inductor ripple current ΔIL decreases
with higher inductance and increases with higher VIN or
VOUTB:
IL =
VOUTB
f O• L
V
• 1 OUTB V IN VIN
2.7V TO 5.5V
Accepting larger values of ΔIL allows the use of low inductances, but results in higher output voltage ripple, greater
core losses, and lower output current capability.
A reasonable starting point for setting ripple current is
ΔIL = 0.3 • IMAXP, where IMAXP is the peak switch current
limit. The largest ripple current ΔIL occurs at the maximum
input voltage. To guarantee that the ripple current stays
below a specified maximum, the inductor value should be
chosen according to the following equation:
V
V
L = OUTB • 1 OUTB f O• IL V IN(MAX) The inductor value will also have an effect on Burst Mode
operation. The transition from low current operation
begins when the peak inductor current falls below a level
set by the burst clamp. Lower inductor values result in
higher ripple current which causes this to occur at lower
load currents. This causes 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 increase.
L1
VIN
SW
D1
OPT R2
CIN
CF
OPT
VOUTB
COUTB
BUCKFB
LTC3446
LVIN
PGOOD LVOUT1
MODESEL
ENBUCK
ENLDO1 LVFB1
ENLDO2
DIGITAL
CONTROL
R1
VOUT1
R4
COUT1
R3
RTH
ITH
VOUT2
LVOUT2
CITH
R6
COUT2
LVFB2
GND
R5
3446 F01
Figure 1. General LTC3446 Application Circuit
3446fd
12
LTC3446
APPLICATIONS INFORMATION
Inductor Core Selection
Different core materials and shapes will change the
size/current and price/current relationship of an inductor. Toroid or shielded pot cores in ferrite or permalloy
materials are small and don’t radiate much energy, but
generally cost more than powdered iron core inductors
with similar electrical characteristics. The choice of which
style inductor to use often depends more on the price vs
size requirements and any radiated field/EMI requirements
than on what the LTC3446 requires to operate. Table 1
shows some typical surface mount inductors that work
well in LTC3446 applications.
Table 1. Representative Surface Mount Inductors
MANUFACTURER PART NUMBER
MAX DC
VALUE CURRENT
Toko
A914BYW-2R2MD52LC
2.2μH
Toko
A915AY-2ROMD53LC
Coilcraft
DCR
HEIGHT
2.05A
49mΩ
2mm
2μH
3.3A
22mΩ
3mm
D01608C-222
2.2μH
2.3A
70mΩ
3mm
Coilcraft
LP01704-222M
2.2μH
2.4A
120mΩ
1mm
Sumida
CDRH4D282R2
2.2μH
2.04A
23mΩ
3mm
Sumida
CDC5D232R2
2.2μH
2.16A
30mΩ
2.5mm
Taiyo
Yuden
N06DB2R2M
2.2μH
3.2A
29mΩ
3.2mm
Taiyo
Yuden
N05DB2R2M
2.2μH
2.9A
32mΩ
2.8mm
Murata
LQN6C2R2M04
2.2μH
3.2A
24mΩ
5mm
Würth
744042001
1μH
2.6A
20mΩ
2mm
Catch Diode Selection
Although unnecessary in most applications, a small
improvement in efficiency can be obtained in a few applications by including the optional diode D1 shown in
Figure 1, which conducts when the synchronous switch
is off. When using Burst Mode operation or pulse skip
mode, the synchronous switch is turned off at a low current
and the remaining current will be carried by the optional
diode. It is important to adequately specify the diode peak
current and average power dissipation so as not to exceed
the diode ratings. The main problem with Schottky diodes
is that their parasitic capacitance reduces the efficiency,
usually negating the possible benefits for LTC3446 circuits.
Another problem that a Schottky diode can introduce is
higher leakage current at high temperatures, which could
reduce the low current efficiency.
Remember to keep lead lengths short and observe proper
grounding to avoid ringing and increased dissipation when
using a catch diode.
Input Capacitor (CIN) Selection
In continuous mode, the input current of the converter is a
square wave with a duty cycle of approximately VOUTB /VIN.
To prevent large voltage transients, a low equivalent series
resistance (ESR) input capacitor sized for the maximum
RMS current must be used. The maximum RMS capacitor
current is given by:
IRMS IMAX
VOUTB (VIN VOUTB )
VIN
where the maximum average output current IMAX equals
the peak current minus half the peak-to-peak ripple current, IMAX = IMAXP – ΔIL /2.
This formula has a maximum at VIN = 2VOUTB, where IRMS
= IOUT/2. This simple worst case is commonly used to
design because even significant deviations do not offer
much relief. Note that capacitor manufacturer’s ripple current ratings are often based on only 2000 hours lifetime.
This makes it advisable to further derate the capacitor,
or choose a capacitor rated at a higher temperature than
required. Several capacitors may also be paralleled to meet
the size or height requirements of the design. An additional
0.1μF to 1μF ceramic capacitor is also recommended on
VIN for high frequency decoupling, when not using an all
ceramic capacitor solution.
3446fd
13
LTC3446
APPLICATIONS INFORMATION
Output Capacitor (COUTB) Selection
The selection of COUTB is driven by the required ESR to
minimize voltage ripple and load step transients. Typically,
once the ESR requirement is satisfied, the capacitance
is adequate for filtering. The output ripple (ΔVOUTB) is
determined by:
1
VOUTB IL ESR +
8fO COUTB where f = 2.25MHz, COUTB = output capacitance and ΔIL =
ripple current in the inductor. The output ripple is highest
at maximum input voltage since ΔIL increases with input
voltage.
Once the ESR requirements for COUTB have been met, the
RMS current rating generally far exceeds the IRIPPLE(P-P)
requirement, except for an all ceramic solution.
In surface mount applications, multiple capacitors may
have to be paralleled to meet the capacitance, ESR or RMS
current handling requirement of the application. Aluminum
electrolytic, special polymer, ceramic and dry tantulum
capacitors are all available in surface mount packages. The
OS-CON semiconductor dielectric capacitor available from
Sanyo has the lowest ESR(size) product of any aluminum
electrolytic at a somewhat higher price. Special polymer
capacitors, such as Sanyo POSCAP, offer very low ESR,
but have a lower capacitance density than other types.
Tantalum capacitors have the highest capacitance density,
but have a larger ESR and it is critical that the capacitors
are surge tested for use in switching power supplies.
An excellent choice is the AVX TPS series of surface
mount tantalums, avalable in case heights ranging from
2mm to 4mm. Aluminum electrolytic capacitors have a
significantly larger ESR, and are often used in extremely
cost-sensitive applications provided that consideration is
given to ripple current ratings and long-term reliability.
Ceramic capacitors have the lowest ESR and cost but also
have the lowest capacitance density, a high voltage and
temperature coefficient and exhibit audible piezoelectric
effects. In addition, the high Q of ceramic capacitors along
with trace inductance can lead to significant ringing. Other
capacitor types include the Panasonic specialty polymer
(SP) capacitors.
In most cases, 0.1μF to 1μF of ceramic capacitors should
also be placed close to the LTC3446 in parallel with the
main capacitors for high frequency decoupling.
Ceramic Input and Output Capacitors
Higher value, lower cost ceramic capacitors are now becoming available in smaller case sizes. These are tempting
for switching regulator use because of their very low ESR.
Unfortunately, the ESR is so low that it can cause loop
stability problems. Solid tantalum capacitor ESR generates a loop “zero” at 5kHz to 50kHz that is instrumental in
giving acceptable loop phase margin. Ceramic capacitors
remain capacitive to beyond 300kHz and usually resonate
with their ESL before ESR becomes effective. Also, ceramic
caps are prone to temperature effects which requires the
designer to check loop stability over the operating temperature range. To minimize their large temperature and
voltage coefficients, only X5R or X7R ceramic capacitors
should be used. A good selection of ceramic capacitors
is available from Taiyo Yuden, TDK and Murata.
Great care must be taken when using only ceramic input
and output capacitors. When a ceramic capacitor is used
at the input and the power is being supplied through long
wires, such as from a wall adapter, a load step at the output
can induce ringing at the VIN pin. At best, this ringing can
couple to the output and be mistaken as loop instability.
At worst, the ringing at the input can be large enough to
damage the part.
Since the ESR of a ceramic capacitor is so low, the input
and output capacitor must instead fulfill a charge storage
requirement. During a load step, the output capacitor must
instantaneously supply the current to support the load
until the feedback loop raises the switch current enough
to support the load. The time required for the feedback
loop to respond is dependent on the compensation components and the output capacitor size. Typically, 3 to 4
cycles are required to respond to a load step, but only in
3446fd
14
LTC3446
APPLICATIONS INFORMATION
the first cycle does the output drop linearly. The output
droop, VDROOP, is usually about 2 to 3 times the linear
drop of the first cycle. Thus, a good place to start is with
the output capacitor size of approximately:
COUTB 2.5
IOUT
fO • VDROOP
More capacitance may be required depending on the duty
cycle and load step requirements.
In most applications, the input capacitor is merely required
to supply high frequency bypassing, since the impedance
to the supply is very low. A 10μF ceramic capacitor is
usually enough for these conditions.
Setting the Buck Converter’s Output Voltage
The buck develops a 0.8V reference voltage between the
feedback pin, BUCKFB, and the signal ground as shown
in Figure 1. The output voltage is set by a resistive divider
according to the following formula:
R2 VOUTB 0.8V 1+ R1
Keep R1 at or less than 50k. Great care should be taken
to route the BUCKFB line away from noise sources, such
as the inductor or the SW line.
To improve high frequency loop response, a feed forward
capacitor, CF, can be added as shown in Figure 1. Capacitor
CF provides phase lead by creating a high frequency zero
with R2, improving phase margin.
Buck Converter Shutdown and Soft-Start
The ENBUCK pin enables and shuts down the LTC3446’s
buck converter. Do not leave this pin floating! Tying
ENBUCK to ground disables the buck converter. Bringing
ENBUCK more than 1V above ground enables the buck.
When the buck is brought out of shutdown, an internal
soft-start clamp reduces surge currents from VIN by
gradually increasing the internal peak inductor current
limit over the course of a millisecond.
Checking Buck Converter Transient Response
The OPTI-LOOP compensation allows the transient response to be optimized for a wide range of loads and
output capacitors. The availability of the ITH pin not only
allows optimization of the 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 front page
Typical Application circuit will provide an adequate starting
point for most applications. The series R-C 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 feedback factor gain and phase. An output current
pulse of 20% to 100% 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.
Switching regulators take several cycles to respond to a
step in load current. When a load step occurs, VOUTB immediately shifts by an amount equal to ΔILOAD • ESR, where
ESR is the effective series resistance of COUT. ΔILOAD also
begins to charge or discharge COUTB generating a feedback
error signal used by the regulator to return VOUTB to its
steady-state value. During this recovery time, VOUTB can
be monitored for overshoot or ringing that would indicate
a stability problem.
The initial output voltage step may not be within the
bandwidth of the feedback loop, so the standard second
order overshoot/DC ratio cannot be used to determine
phase margin. The gain of the loop increases with R and
the bandwidth of the loop increases with decreasing C.
3446fd
15
LTC3446
APPLICATIONS INFORMATION
If R is increased by the same factor that C is decreased,
the zero frequency will be kept the same, thereby keeping
the phase the same in the most critical frequency range
of the feedback loop.
Although all dissipative elements in the circuit produce
losses, four main sources usually account for most of
the losses in LTC3446 circuits: 1) LTC3446 VIN current,
2) switching losses, 3) I2R losses, 4) other losses.
The output voltage settling behavior is related to the stability
of the closed-loop system and will demonstrate the actual
overall supply performance. For a detailed explanation of
optimizing the compensation components, including a
review of control loop theory, refer to Linear Technology
Application Note 76.
1) The VIN current is the DC supply current given in the
electrical characteristics which excludes MOSFET driver
and control currents. VIN current results in a small
(<0.1%) loss that increases with VIN, even at no load.
Although a buck regulator is capable of providing the full
output current in dropout, it should be noted that as the
input voltage VIN drops toward VOUT, the load step capability
does decrease due to the decreasing voltage across the
inductor. Applications that require large load step capability near dropout should use a different topology such as
SEPIC, Zeta or single inductor, positive buck/boost.
In some applications, a more severe transient can be caused
by switching in loads with large (>1μF) input capacitors.
The discharged input capacitors are effectively put in parallel with COUTB, causing a rapid drop in VOUT. No regulator
can deliver enough current to prevent this problem, if the
switch connecting the load has low resistance and is driven
quickly. The solution is to limit the turn-on speed of the load
switch driver. A Hot Swap controller is designed specifically
for this purpose and usually incorporates current limiting,
short-circuit protection, and soft-starting.
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.
2) The switching 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 VIN to ground. The resulting dQ/dt is a current
out of VIN that is typically much larger than the DC bias
current. In continuous mode, IGATECHG = fO(QT + QB),
where QT and QB are the gate charges of the internal
top and bottom MOSFET switches. The gate charge
losses are proportional to VIN and thus their effects
will be more pronounced at higher supply voltages.
3) I2R Losses are calculated from the DC resistances of
the internal switches, RSW, and external inductor, RL. In
continuous mode, the average output current flowing
through inductor L is “chopped” between the internal
top and bottom switches. Thus, the series resistance
looking into the SW pin is a function of both top and
bottom MOSFET RDS(ON) and the duty cycle (DC) as
follows:
RSW = (RDS(ON)TOP)(DC) + (RDS(ON)BOT)(1 – DC)
4) Other “hidden” losses such as copper trace and internal
battery resistances can account for additional efficiency
degradations in portable systems. It is very important
to include these “system” level losses in the design of a
system. 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. Other losses including diode conduction losses
during dead-time and inductor core losses generally
account for less than 2% total additional loss.
Hot Swap is a trademark of Linear Technology Corporation.
3446fd
16
LTC3446
APPLICATIONS INFORMATION
VLDO LINEAR REGULATOR DESIGN
connect each LVFB pin to its resistor divider with a short
PCB trace and minimize all other stray capacitance to the
LVFB pin.
Adjustable Output Voltage
Each VLDO regulator’s output voltage is set by the ratio
of two external resistors as shown in Figure 2. The VLDO
regulator servos the output to maintain the LVFB pin voltage
at 0.4V (referenced to ground). Thus the current in R1 is
equal to 0.4V/R1. For good transient response, stability and
accuracy, the current in R1 should be at least 8μA, thus
the value of R1 should be no greater than 50k. The current
in R2 is the current in R1 plus the LVFB pin bias current.
Since the LVFB pin bias current is typically <10nA, it can
be ignored in the output voltage calculation. The output
voltage can be calculated using the formula in Figure 2.
Note that in shutdown, the output is turned off and the
divider current will be zero once COUT is discharged.
Each VLDO regulator operates at a relatively high gain of
–0.7μV/mA referred to its LVFB input. Thus a load current
change of 1mA to 300mA produces a –0.2mV drop at the
LVFB input. To calculate the change referred to the output,
simply multiply by the gain of the feedback network (i.e.,
1 + R2/R1). For example, to program the output for 1.2V,
choose R2/R1 = 2. In this example, an output current
change of 1mA to 300mA produces –0.2mV • (1 + 2) =
0.6mV drop at the output.
Because the LVFB pins are relatively high impedance (depending on the resistor dividers used), stray capacitance
at these pins should be minimized (<10pF) to prevent
phase shift in the error amplifier loop. Additionally, special
attention should be given to any stray capacitances that
can couple external signals onto the LVFB pins producing
undesirable output ripple. For optimum performance,
LVOUT
LTC3446
R2
LVFB
VLDO Regulator Output Capacitance and Transient
Response
The VLDO regulators are designed to be stable with a
wide range of ceramic output capacitors. The ESR of the
output capacitor affects stability, most notably with small
capacitors. A minimum output capacitor of 1μF with an
ESR of 0.05Ω or less is recommended to ensure stability.
The VLDO regulators are micropower devices and output
transient response will be a function of output capacitance.
Larger values of output capacitance decrease the peak
deviations and provide improved transient response for
larger load current changes. Note that bypass capacitors
used to decouple individual components powered by a
VLDO regulator will increase the effective output capacitor value. High ESR tantalum and electrolytic capacitors
may be used, but a low ESR ceramic capacitor must be
in parallel at the output. There is no minimum ESR or
maximum capacitor size requirements.
Extra consideration must be given to the use of ceramic
capacitors. Ceramic capacitors are manufactured with a
variety of dielectrics, each with different behavior across
temperature and applied voltage. The most common dielectrics used are Z5U, Y5V, X5R and X7R. The Z5U and
Y5V dielectrics are good for providing high capacitances
in a small package, but exhibit large voltage and temperature coefficients as shown in Figures 3 and 4. When
used with a 2V regulator, a 1μF Y5V capacitor can lose as
much as 75% of its intial capacitance over the operating
temperature range. The X5R and X7R dielectrics result
( )
VOUT = 0.4V 1+ R2
R1
COUT
R1
GND
3446 F02
Figure 2. Programming a VLDO Regulator’s Output Voltage
3446fd
17
LTC3446
APPLICATIONS INFORMATION
20
20
BOTH CAPACITORS ARE 1μF,
10V, 0603 CASE SIZE
0
CHANGE IN VALUE (%)
CHANGE IN VALUE (%)
0
X5R
–20
–40
Y5V
–60
Y5V
–40
–60
–80
–80
–100
X5R
–20
BOTH CAPACITORS ARE 1μF,
10V, 0603 CASE SIZE
0
2
6
4
DC BIAS VOLTAGE (V)
8
–100
–50
10
–25
0
25
50
TEMPERATURE (°C)
3446 F03
3446 F04
Figure 3. Ceramic Capacitor DC Bias Characteristics
in more stable characteristics and are usually more suitable for use as the output capacitor. The X7R type has
better stability across temperature, while the X5R is less
expensive and is available in higher values. In all cases,
the output capacitance should never drop below 0.4μF, or
instability or degraded performance may occur.
VLDO Output Short-Circuit Protection
The VLDO regulators have built-in short-circuit limiting.
During short-circuit conditions, internal circuitry automatically limits the output current to approximately 760mA.
75
Figure 4. Ceramic Capacitor Temperature Characteristics
VLDO Regulator Soft-Start
Each VLDO regulator includes a soft-start feature to
prevent excessive current flow during start-up. When
the VLDO regulator is enabled, the soft-start circuitry
gradually increases the VLDO regulator reference voltage
from 0V to 0.4V over a period of about 600μs. There is a
short 700μs delay from the time the part is enabled until
the LDO output starts to rise. Figure 5 shows the start-up
output waveform.
1.5V VLDO OUTPUT
1.2V VLDO OUTPUT
0.5V/DIV
BOTH VLDO
ENABLES
5V/DIV
VLVIN = 1.8V
200μs/DIV
VIN = 3.6V
FRONT PAGE APPLICATION CIRCUIT
WITH 10mA RESISTOR LOADS ON
EACH VLDO OUTPUT
3446 F05
Figure 5. VLDO Regulator Output Start-Up
3446fd
18
LTC3446
PACKAGE DESCRIPTION
DE Package
14-Lead Plastic DFN (4mm × 3mm)
(Reference LTC DWG # 05-08-1708 Rev B)
0.70 ±0.05
3.30 ±0.05
3.60 ±0.05
2.20 ±0.05
1.70 ± 0.05
PACKAGE
OUTLINE
0.25 ± 0.05
0.50 BSC
3.00 REF
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
R = 0.115
TYP
4.00 ±0.10
(2 SIDES)
R = 0.05
TYP
3.00 ±0.10
(2 SIDES)
8
0.40 ± 0.10
14
3.30 ±0.10
1.70 ± 0.10
PIN 1 NOTCH
R = 0.20 OR
0.35 × 45°
CHAMFER
PIN 1
TOP MARK
(SEE NOTE 6)
(DE14) DFN 0806 REV B
7
0.200 REF
1
0.25 ± 0.05
0.50 BSC
0.75 ±0.05
3.00 REF
0.00 – 0.05
BOTTOM VIEW—EXPOSED PAD
NOTE:
1. DRAWING PROPOSED TO BE MADE VARIATION OF VERSION (WGED-3) IN JEDEC
PACKAGE OUTLINE MO-229
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
3446fd
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.
19
LTC3446
TYPICAL APPLICATION
L1
1.8μH
VIN
2.9V TO 5.5V
SW
VIN
22μF
X7R
59k
22μF
X7R
VOUT
1.8V
400mA MAX
BUCKFB
LTC3446
LVIN
PGOOD LVOUT1
MODESEL
ENBUCK
ENLDO1 LVFB1
ENLDO2
DIGITAL
CONTROL
47.5k
110k
2.2μF
X7R
40.2k
3.3k
ITH
LVOUT2
1000pF
VOUT
1.5V
300mA MAX
80.6k
2.2μF
X7R
VOUT
1.2V
300mA MAX
LVFB2
GND
3446 TA01
40.2k
L1: TOKO A960AW-1R8M
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LT3023
Dual, 2x100mA,
Low Noise Micropower LDO
VIN: 1.8V to 20V, VOUT(MIN) = 1.22V, VDO = 0.30V, IQ = 40μA, ISD < 1μA,
VOUT = ADJ, DFN, MS Packages, Low Noise < 20μVRMS(P-P), Stable with
1μF Ceramic Capacitors
LT3024
Dual, 100mA/500mA,
Low Noise Micropower LDO
VIN: 1.8V to 20V, VOUT(MIN) = 1.22V, VDO = 0.30V, IQ = 60μA, ISD < 1μA,
VOUT = ADJ, DFN, TSSOP Packages, Low Noise < 20μVRMS(P-P), Stable with
1μF Ceramic Capacitors
LTC3025
300mA, Micropower VLDO
Linear Regulator
VIN: 0.9V to 5.5V, VOUT(MIN) = 0.4V, 2.7V to 5.5V Bias Voltage Required,
VDO = 45mV, IQ = 50μA, ISD < 1μA, VOUT = ADJ, DFN Packages, Stable with
1μF Ceramic Capacitors
LTC3407
Dual Synchronous 600mA
Synchronous Step-Down DC/DC Regulator
1.5MHz Constant Frequency Current Mode Operation, VIN from 2.5V to
5.5V, VOUT Down to 0.6V, DFN, MS Packages
LTC3407-2
Dual Synchronous 800mA Synchronous
Step-Down DC/DC Regulator, 2.25MHz
2.25MHz Constant Frequency Current Mode Operation, VIN from 2.5V to
5.5V, VOUT Down to 0.6V, DFN, MS Packages
LTC3445
I2C Controllable Buck Regulator
with Two LDOs and and
Backup Battery Input
600mA, 1.5MHz Current Mode Buck Regulator, I2C Programmable VOUT
from 0.85V to 1.55V, two 50mA LDOs, Backup Battery Input with
PowerPath™ Control, QFN Package
LTC3555
High Efficiency USB Power Manager Plus
Triple Step-Down DC/DC
Maximizes Available Power from USB Port, Bat-Track™, “Instant On” Operation,
1.5A Maximum Charge Current, 180mΩ Ideal Diode with <50Ω Option, 3.3V/25mA
Always-On LDO, Three Synchronous Buck Regulators
(400mA/400mA/1A), 4mm × 5mm QFN28 Package
LTC3557
USB Power Manager with Li-Ion/Polymer
Charger and Triple Synchrounous
Buck Converter
Complete Multifunction ASSP: Linear Power Manager and Three Buck Regulators,
Charge Current Programmable Up to 1.5A from Wall Adapter Input,
Thermal Regulation, Synchronous Buck Efficiency: >95%, ADJ Outputs: 0.8V to 3.6V
at 400mA/400mA/600mA, Bat-Track Adaptive Output Control,
200mΩ Ideal Diode, 4mm × 4mm QFN28 Package
LTC3559
Linear USB Li-Ion Battery Charger with
Dual Buck Regulators
Charge Current Programmable Up to 950mA, USB Compatible, Dual Synchronous
400mA Buck Regulators, Efficiency >90%, 3mm × 3mm QFN16 Package
LTC3672B-1/
LTC3672B-2
Fixed-Output Monolithic 400mA Buck
Regulator with Dual 150mA LDOs in
a 2mm × 2mm DFN
>90% Efficiency, VIN: 2.9V to 5.5V, IQ = 260μA,
LTC3672B-1: Buck = 1.8V, LDO1 = 1.2V, LDO2 = 2.8V
LTC3672B-2: Buck = 1.2V, LDO1 = 2.8V, LDO2 = 1.8V
Consult LTC Factory for Other Voltage Combinations
LTC3700
Step-Down DC/DC Controller with
LDO Regulator
VIN from 2.65V to 9.8V, Constant Frequency 550kHz Operation
PowerPath and Bat-Track is a trademark of Linear Technology Corporation.
20 Linear Technology Corporation
3446fd
LT 0108 REV D • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
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© LINEAR TECHNOLOGY CORPORATION 2006