LTC3026
1.5A Low Input Voltage
VLDO Linear Regulator
Features
Description
Input Voltage Range:
1.14V to 3.5V (with Boost Enabled)
1.14V to 5.5V (with External 5V Boost)
n Low Dropout Voltage: 100mV at I
OUT = 1.5A
n Adjustable Output Range: 0.4V to 2.6V
n Output Current: Up to 1.5A
n Excellent Supply Rejection Even Near Dropout
n Shutdown Disconnects Load from V and V
IN
BST
n Low Operating Current: I = 950µA at V = 1.5V
IN
IN
n Low Shutdown Current:
IIN < 1µA (Typ), IBST = 0.1µA (Typ)
n Stable with 10µF or Greater Ceramic Capacitors
n Short-Circuit, Reverse Current Protected
n Overtemperature Protected
n Available in 10-Lead MSOP and 10-Lead
(3mm × 3mm) DFN Packages
The LTC®3026 is a very low dropout (VLDO™) linear regulator that can operate at input voltages down to 1.14V. The
device is capable of supplying 1.5A of output current with
a typical dropout voltage of only 100mV. To allow operation at low input voltages the LTC3026 includes a boost
converter that provides the necessary headroom for the
internal LDO circuitry.
n
Output current comes directly from the input supply to
maximize efficiency. The boost converter requires only a
small chip inductor and ceramic capacitor for operation.
Additionally, the boosted output voltage of one LTC3026
can supply the boost voltage for other LTC3026s, thus
requiring a single inductor for multiple LDOs. A user supplied boost voltage can be used eliminating the need for
an inductor altogether.
Applications
High Efficiency Linear Regulator
Post Regulator for Switching Supplies
n Microprocessor Supply
n
n
L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks
and ThinSOT, VLDO are trademarks of Linear Technology Corporation. All other trademarks are
the property of their respective owners.
The LTC3026 regulator is stable with 10µF or greater
ceramic output capacitors. The device has a low 0.4V
reference voltage which is used to program the output
voltage via two external resistors. The device also has
internal current limit, overtemperature shutdown, and
reverse output current protection. The LTC3026 is available in a small 10-lead MSOP or low profile (0.75mm)
10-lead 3mm × 3mm DFN package.
Typical Application
1.2V Output Voltage from 1.5V Input Supply
Dropout Voltage vs Output Current
150
SW
5V BOOST BST
CONVERTER
4.7µF
IN
VIN = 1.5V
4.7µF
OFF ON
0.4V
+
–
8.06k
ADJ
SHDN
LTC3026
GND
VOUT = 1.2V,
1.5A
OUT
100k
DROPOUT (mV)
L1
10µH
100
1.2V
1.5V
2.0V
2.6V
50
COUT
10µF
0
4.02k
PG
3026 TA01a
0
1.0
0.5
1.5
IOUT (A)
3026 TA01b
L1: MURATA LQH2MCN100K02
3026ff
1
LTC3026
Absolute Maximum Ratings
(Note 1)
VBST to GND.................................................. –0.3V to 6V
VIN to GND.................................................... –0.3V to 6V
PG to GND.................................................... –0.3V to 6V
SHDN to GND............................................. –0.3V to 6.3V
ADJ to GND................................... –0.3V to (VIN + 0.3V)
Output Short-Circuit Duration........................... Indefinite
Operating Junction Temperature Range
(Note 8).............................................. –40°C to 125°C
Storage Temperature Range................... –65°C to 125°C
Lead Temperature (MSE, Soldering, 10 sec).......... 300°C
Pin Configuration
TOP VIEW
IN
1
IN
2
GND
3
SW
4
BST
5
TOP VIEW
10 OUT
11
GND
IN
IN
GND
SW
BST
9 OUT
8 ADJ
7 PG
6 SHDN
DD PACKAGE
10-LEAD (3mm × 3mm) PLASTIC DFN
TJMAX = 125°C, θJA = 40°C/W
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
1
2
3
4
5
11
GND
10
9
8
7
6
OUT
OUT
ADJ
PG
SHDN
MSE PACKAGE
10-LEAD PLASTIC MSOP
TJMAX = 125°C, θJA = 40°C/W
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
Order Information
LEAD FREE FINISH
TAPE AND REEL
PART MARKING
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC3026EDD#PBF
LTC3026EDD#TRPBF
LBHW
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 125°C
LTC3026IDD#PBF
LTC3026IDD#TRPBF
LBHW
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 125°C
LTC3026EMSE#PBF
LTC3026EMSE#TRPBF
LTBJB
10-Lead Plastic MSOP
–40°C to 125°C
LTC3026IMSE#PBF
LTC3026IMSE#TRPBF
LTBJB
10-Lead Plastic MSOP
–40°C to 125°C
LEAD BASED FINISH
TAPE AND REEL
PART MARKING
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC3026EDD
LTC3026EDD#TR
LBHW
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 125°C
LTC3026IDD
LTC3026IDD#TR
LBHW
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 125°C
LTC3026EMSE
LTC3026EMSE#TR
LTBJB
10-Lead Plastic MSOP
–40°C to 125°C
LTC3026IMSE
LTC3026IMSE#TR
LTBJB
10-Lead Plastic MSOP
–40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
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/
3026ff
2
LTC3026
Electrical Characteristics
(BOOST ENABLED, LSW = 10µH)
The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at
TJ = 25°C. VIN = 1.5V, VOUT = 1.2V, CIN = CBST = 4.7µF, COUT = 10µF (all capacitors ceramic) unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
VIN
Operating Voltage
(Note 2)
IIN
Operating Current
IOUT = 0mA, VOUT = 0.8V, VSHDN = VIN, VIN = 1.2V
IOUT = 0mA, VOUT = 1.2V, VSHDN = VIN, VIN = 1.5V
IOUT = 0mA, VOUT = 1.2V, VSHDN = VIN, VIN = 2.5V
IOUT = 0mA, VOUT = 1.2V, VSHDN = VIN, VIN = 3.5V
Shutdown Current
VSHDN = 0V, VIN = 3.5V
IINSHDN
MIN
l
VBST
Boost Output Voltage Range
Boost Undervoltage Lockout
Boost Output Drive (Note 3)
1.14
MAX
3.5
1160
950
640
400
l
V
µA
µA
µA
µA
20
µA
10
40
µH
mA
4.8
5
5.2
V
4.0
4.2
4.4
V
4.7
150
VSHDN = VIN
UNITS
0.6
l
Inductor Size Requirement
Inductor Peak Current Requirement
VBSTUVLO
TYP
VIN < 1.4V
VIN ≥ 1.4V
7
10
mA
mA
(BOOST DISABLED, VSW = 0V or Floating)
The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at
TJ = 25°C. VIN = 1.5V, VOUT = 1.2V, VBST = 5V, CIN = CBST = 1µF, COUT = 10µF (all capacitors ceramic) unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
VIN
Operating Voltage
(Note 2)
l
IIN
Operating Current
IOUT = 100µA, VSHDN = VIN, 1.2V ≤ VIN ≤ 5V
l
IINSHDN
Shutdown Current
VSHDN = 0V, VIN = 3.5V
l
VSHDN = VIN
l
4.5
5
5.5
V
l
4.0
4.25
4.4
V
175
275
µA
1
5
µA
VBST
Boost Operating Voltage (Note 7)
VBSTUVLO
Undervoltage Lockout
IBST
Boost Operating Current
IOUT = 100µA, VSHDN = VIN
IBSTSHDN
Boost Shutdown Current
VSHDN = 0V
MIN
TYP
1.14
l
MAX
UNITS
5.5
V
95
200
µA
0.6
20
µA
(BOOST ENABLED or DISABLED)
The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at
TJ = 25°C. VIN = 1.5V, VOUT = 1.2V, CIN = CBST = 1µF, COUT = 10µF (all capacitors ceramic) unless otherwise noted.
SYMBOL PARAMETER
VADJ
OUT
Regulation Voltage (Note 5)
CONDITIONS
1mA ≤ IOUT ≤ 1.5A, 1.14V ≤ VIN ≤ 3.5V, VBST = 5V, VOUT = 0.8V
1mA ≤ IOUT ≤ 1.5A, 1.14V ≤ VIN ≤ 3.5V, VBST = 5V, VOUT = 0.8V
Programming Range
MIN
TYP
MAX
UNITS
l
0.397
0.395
0.4
0.4
0.403
0.405
V
V
l
0.4
2.6
V
250
mV
100
nA
Dropout Voltage (Note 6)
VIN = 1.5V, VADJ = 0.38, IOUT = 1.5A
l
IADJ
ADJ Input Current
VADJ = 0.4V
l
–100
100
IOUT
Continuous Output Current
VSHDN = VIN
l
1.5
ILIM
Output Current Current Limit
en
Output Voltage Noise
A
3
f = 10Hz to 100kHz, IL = 800mA
Boost Disabled
Boost Enabled
110
210
A
µVRMS
µVRMS
3026ff
3
LTC3026
electrical characteristics
(BOOST ENABLED or DISABLED)
The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at
TJ = 25°C. VIN = 1.5V, VOUT = 1.2V, CIN = CBST = 1µF, COUT = 10µF (all capacitors ceramic) unless otherwise noted.
SYMBOL PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
VIHSHDN
SHDN Input High Voltage
1.14V ≤ VIN ≤ 3.5V
3.5V ≤ VIN ≤ 5.5V
l
l
1.0
1.2
VILSHDN
SHDN Input Low Voltage
1.14V ≤ VIN ≤ 5.5V
l
0.4
V
IIHSHDN
SHDN Input High Current
SHDN = VIN
–1
1
µA
IILSHDN
SHDN Input Low Current
SHDN = 0V
–1
1
µA
VOLPG
PG Output Low Voltage
IPG = 2mA
0.1
0.4
V
IOHPG
PG Output High Leakage Current VPG = 5.5V
0.01
1
µA
PG
Output Threshold (Note 4)
–9
–7
–6
–4
%
%
l
PG High to Low
PG Low to High
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. This IC has overtemperature protection that is
intended to protect the device during momentary overload conditions.
Junction temperatures will exceed 125°C when overtemperature is active.
Continuous operation above the specified maximum operating junction
temperature may impair device reliability.
Note 2: Minimum Operating Voltage required for regulation is:
VIN ≥ VOUT(MIN) + VDROPOUT
Note 3: When using BST to drive loads other than LTC3026s, the load
must be high impedance during start-up (i.e. prior to PG going high).
Note 4: PG threshold expressed as a percentage difference from the
“VADJ Regulation Voltage” as given in the table.
Note 5: 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.
–12
–10
V
V
Note 6: Dropout voltage is minimum input to output voltage differential
needed to maintain regulation at a specified output current. In dropout, the
output voltage will be equal to VIN – VDROPOUT.
Note 7: To maintain correct regulation
VOUT ≤ VBST – 2.4V
Note 8: The LTC3026 is tested under pulsed load conditions such
that TJ ≈ TA. The LTC3026E is guaranteed to meet specifications from
0°C to 125°C junction temperature. Specifications over the –40°C to
125°C operating junction temperature range are assured by design,
characterization and correlation with statistical process controls. The
LTC3026I is guaranteed over the –40°C to 125°C operating junction
temperature range. Note that the maximum ambient temperature
consistent with these specifications is determined by specific operating
conditions in conjunction with board layout, the rated package thermal
impedance and other environmental factors. The junction temperature
(TJ, in °C) is calculated from the ambient temperature (TA, in °C) and
power dissipation (PD, in watts) according to the formula:
TJ = TA + (PD • θJA), where θJA (in °C/W) is the package thermal
impedance.
Typical Performance Characteristics
IN Supply Current with Boost
Converter Enabled
BST Supply Current with Boost
Converter Disabled
1.50
IN Supply Current with Boost
Converter Disabled
200
200
150
150
0.75
100
0.50
0.25
0
50
–40°C
25°C
85°C
1.0
1.5
2.0
2.5
VIN (V)
3.0
3.5
3026 G01
IIN (µA)
1.00
IBST (µA)
INPUT CURRENT (mA)
1.25
VBST = 5V
–40°C
25°C
85°C
125°C
0
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
VIN (V)
3026 G02
100
50
VBST = 5V
–40°C
25°C
85°C
125°C
0
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
VIN (V)
3026 G03
3026ff
4
LTC3026
Typical Performance Characteristics
ADJ Voltage vs Temperature
IN Shutdown Current
4.5
403
4.0
1mA
400
1.5A
399
398
397
–25
0
25
50
100
75
TEMPERATURE (°C)
2.5
2.0
125
160
50
RIPPLE REJECTION (dB)
120
100
80
60
–40°C
25°C
85°C
125°C
40
20
1.8
2.0
2.4
2.2
VIN (V)
4.950
–50
20
VBST = 5V
VOUT =1.2V
IOUT = 800mA
COUT = 10µF
1.4
1.6
1.8 2.0
VIN (V)
40
30
–40°C
25°C
125°C
4
5
3026 G10
1000
3026 G08
10000 100000 1000000 1E+07
FREQUENCY (Hz)
3026 G09
BST to OUT Headroom Voltage
2.22
2.20
2.18
VBST – VOUT (V)
3.0
CURRENT LIMIT
1.0
1.0
2.16
2.14
2.12
2.10
2.08
2.06
THERMAL LIMIT
1.5
6
VBST = 5V
VIN = 1.5V
VOUT =1.2V
IOUT = 800mA
COUT = 10µF
20
0
100
2.6
3.5
2.0
VIN (V)
2.4
4.0
2.5
600
3
2.2
50
10
VOUT = 0V
TA = 25°C
4.5
IOUT (A)
FALL
125
3026 G06
Output Current Limit
5.0
900
100
60
30
3026 G07
RISE
RISE
FALL
FALL
RISE
0
25
50
75
TEMPERATURE (°C)
Ripple Rejection
100kHz
0
1.2
2.6
–25
3026 G05
1MHz
40
Shutdown Threshold
2
125
70
10
1200
1
100
10kHz
140
DROPOUT (mV)
1.2V
0
25
50
75
TEMPERATURE (°C)
Ripple Rejection
VFB = 0.38V
180 I
OUT =1.5A
VSHDN THRESHOLD (mV)
–25
3026 G04
60
1.6
2.5V
0
–50
Dropout Voltage vs Input Voltage
1.4
4.975
0.5
200
0
1.2
3.5V
1.5
5.000
RIPPLE REJECTION (dB)
396
–50
3.0
1.0
VBST = 5V
VIN = 1.5V
VOUT =1.2V
VIN = 1.5V
5.025
3.5
BST VOLTAGE (V)
401
INPUT CURRENT (µA)
ADJUST VOLTAGE (mV)
402
300
BST Voltage vs Temperature
5.050
5.0
404
2.04
1.5
2.0
2.5
VIN (V)
3.0
3.5
3026 G11
2.02
–50
–25
50
25
0
75
TEMPERATURE (°C)
100
125
3026 G12
3026ff
5
LTC3026
Typical Performance Characteristics
Delay from Enable to PG with
Boost Disabled
Delay from Enable to PG with
Boost Enabled
5.0
400
VOUT = 0.8V
ROUT = 8Ω
–40°C
25°C
85°C
4.5
375
4.0
3.5
DELAY (ms)
DELAY (µs)
350
325
300
250
2mA
OUT
AC 20mV/DIV
3.0
2.5
2.0
1.0
0.5
0
1.0
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
VIN (V)
1.5
3026 G13
IN Supply Transient Response
2.5
2.0
VIN (V)
3.0
3.5
VOUT = 1.5V
COUT = 10µF
VIN = 1.7V
VBST = 5V
3026 G14
50µs/DIV
3026 G15
BST Ripple and Feedthrough
to OUT
BST/OUT Start-Up
SHDN
VIN
IOUT
1.5
VOUT = 0.8V
ROUT = 8Ω
–40°C
25°C
85°C
275
Output Load Transient Response
1.5A
HI
2V
LO
1.5V
5V
VBST
AC 20mV/DIV
BST
1V
1.5V
VOUT
AC
10mV/DIV
VOUT
AC 5mV/DIV
OUT
VOUT = 1.2V
IOUT = 800mA
COUT = 10µF
VBST = 5V
TA = 25°C
10µs/DIV
3026 G16
0V
TA = 25°C
ROUT = 1Ω
VIN = 1.7V
200µs/DIV
3026 G17
VOUT = 1.2V
VIN = 1.5V
IOUT = 1A
COUT = 10µF
LSW = 10µH
TA = 25°C
20µs/DIV
3026 G18
3026ff
6
LTC3026
Pin Functions
IN (Pins 1, 2): Input Supply Voltage. Output load current
is supplied directly from IN. The IN pin should be locally
bypassed to ground if the LTC3026 is more than a few
inches away from another source of bulk capacitance.
In general, the output impedance of a battery rises with
frequency, so it is usually advisable to include an input
bypass capacitor when supplying IN from a battery. A
capacitor in the range of 0.1µF to 4.7µF is usually sufficient.
GND (Pin 3, Exposed Pad Pin 11): Ground and Heat Sink.
Connect the exposed pad to the PCB ground plane or large
pad for optimum thermal performance.
SW (Pin 4): Boost Switching Pin. This is the boost converter
switching pin. A 4.7µH to 40µH inductor able to handle a
peak current of 150mA is connected from this pin to VIN.
The boost converter can be disabled by floating this pin.
This allows the use of an external boosted supply from
a second LTC3026 or other source. See Operating with
Boost Converter Disabled section for more information.
BST (Pin 5): Boost Output Voltage Pin. With boost converter enabled bypass the BST pin with a ≥4.7µF low ESR
ceramic capacitor to GND (CBST). BST does not load VIN
when in shutdown, but is diode connected to IN through
the external inductor, thus, will not go to ground with VIN
present. Users should not present any loads to the BST
pin (with boost enabled) until PG signals that regulation
has been achieved. When providing an external BST voltage (i.e. boost converter disabled) a 1µF low ESR ceramic
capacitor can be used.
SHDN (Pin 6): Shutdown Input Pin, Active Low. This pin
is used to put the LTC3026 into shutdown. The SHDN pin
current is typically less than 10nA. The SHDN pin cannot
be left floating and must be tied to a valid logic level (such
as IN) if not used.
PG (Pin 7): Power Good Pin. When PG is high impedance
OUT is in regulation, and low impedance when OUT is in
shutdown or out of regulation.
ADJ (Pin 8): Output Adjust Pin. This is the input to the error
amplifier. It has a typical bias current of 0.1nA flowing into
the pin. The ADJ pin reference voltage is 0.4V referenced
to ground. The output voltage range is 0.4V to 2.6V and is
typically set by connecting ADJ to a resistor divider from
OUT to GND. See Figure 2.
OUT (Pins 9, 10): Regulated Output Voltage. The OUT pins
supply power to the load. A minimum output capacitance
of 5µF is required to ensure stability. Larger output capacitors may be required for applications with large transient
loads to limit peak voltage transients. See the Applications Information section for more information on output
capacitance.
3026ff
7
LTC3026
Block Diagram
4
SHDN
6
BOOST
CONVERTER
5 BST
SWITCHING
LOGIC
–
SW
EN
+
SHDN
–
UVLO
7
–
–
PG
IN
1,2
+
0.4V
REFERENCE
0.372V
VOFF
OUT
9,10
+ –
+
+
8 ADJ
OVERSHOOT DETECT
GND
3,11
3026 BD
3026ff
8
LTC3026
Operation
The LTC3026 is a VLDO (very low dropout) linear regulator
which operates from input voltages as low as 1.14V. The
LDO uses an internal NMOS transistor as the pass device
in a source-follower configuration. The BST pin provides
the higher supply necessary for the LDO circuitry while the
output current comes directly from the IN input for high
efficiency regulation. The BST pin can either be supplied
off-chip by an external 5V source or it can be generated
through the internal boost converter of the LTC3026.
Boost Converter Operation
For applications where an external 5V supply is not available, the LTC3026 contains an internal boost converter to
produce the necessary 5V supply for the LDO. The boost
converter utilizes Burst Mode® operation to achieve high
efficiency for the relatively low current levels needed for
the LDO circuitry. The boost converter requires only a
small chip inductor between the IN and SW pins and a
small 4.7µF capacitor at BST.
The operation of the boost converter is described as follows. During the first half of the switching cycle, an internal
NMOS switch between SW and GND turns on, ramping
the inductor current. A peak comparator senses when the
inductor current reaches 100mA, at which point the NMOS
is turned off and an internal PMOS between SW and BST
turns on, transferring the inductor current to the BST pin.
The PMOS switch continues to deliver power to BST until
the inductor current approaches zero, at which point the
PMOS turns off and the NMOS turns back on, repeating
the switching cycle.
A burst comparator with hysteresis monitors the voltage
on the BST pin. When BST is above the upper threshold
of the comparator, no switching occurs. When BST falls
below the comparator’s lower threshold, switching commences and the BST pin gets charged. The upper and lower
thresholds of the burst comparator are set to maintain a 5V
supply at BST with approximately 40mV to 50mV of ripple.
Care must be taken not to short the BST pin to GND, since
the body diode of the internal PMOS transistor connects
the BST and SW pins. Shorting BST to GND with an inductor connected between IN and SW can ramp the inductor
current to destructive levels, potentially destroying the
inductor and/or the part.
Operating with Boost Converter Disabled
The LTC3026 has an option to disable the internal boost
converter. With the boost converter disabled, the LTC3026
becomes a bootstrapped device and the BST pin must be
driven by an external 5V supply, or driven by the BST pin
of a second LTC3026 with the boost converter enabled. The
recommended method for disabling the boost converter
is to simply float the SW pin. With the SW pin floating no
energy can be transferred to BST which effectively disables
the boost converter.
A single LTC3026 boost converter can be used to drive
multiple bootstrapped LTC3026s with the internal boost
converters disabled. Thus a single inductor can be used
to power two (or possibly more) functioning LTC3026s.
In cases where all LTC3026s have the same input supply
(IN) the internal boost converters of the bootstrapped
LTC3026s can be disabled by floating the SW pin. If the
LTC3026s are not all connected to the same input supply
then the internal boost converters of the bootstrapped
LTC3026s are disabled by floating the SW pin.
LDO Operation
An undervoltage lockout comparator (UVLO) senses the
BST pin voltage to ensure that the bias supply for the LDO
is greater than 4.2V before enabling the LDO. If BST is
below 4.2V, the UVLO shuts down the LDO, and OUT is
pulled to GND through the external divider.
3026ff
9
LTC3026
operation
The LDO provides a high accuracy output capable of
supplying 1.5A of output current with a typical dropout
voltage of only 100mV. A single ceramic capacitor as
small as 10µF is all that is required for output bypassing.
A low reference voltage allows the LTC3026 output to be
programmed to much lower voltages than available in
common LDOs (range of 0.4V to 2.6V).
The devices also include current limit and thermal overload
protection, and will survive an output short-circuit indefinitely. The fast transient response of the follower output
stage overcomes the traditional trade-off between dropout
voltage, quiescent current and load transient response
inherent in most LDO regulator architectures, see Figure 1.
1.5A
IOUT
SHDN
HI
LO
1.5V
OUT
0V
1.5V
PG
0V
TA = 25°C
ROUT = 1Ω
VIN = 1.7V
VB = 5V
100µs/DIV
3026 F02
Figure 2. Soft-Start with Boost Disable
Adjustable Output Voltage
0mA
OUT
AC 20mV/DIV
VOUT = 1.5V
COUT = 10µF
VIN = 1.7V
VB = 5V
100µs/DIV
3026 F01
Figure 1. Output Load Step Response
The LTC3026 also includes a soft-start feature to prevent
excessive current flow at VIN during start-up. When the
LDO is enabled, the soft-start circuitry gradually increases
the LDO reference voltage from 0V to 0.4V over a period
of approximately 200µs, see Figure 2.
The output voltage is set by the ratio of two external resistors as shown in Figure 3. The device servos the output
to maintain the ADJ 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 80µA, thus, the value of R1 should
be no greater than 5k. The current in R2 is the current in
R1 plus the ADJ pin bias current. Since the ADJ pin bias
current is typically 125°C) should
be avoided as it can degrade the performance or shorten
the life of the part.
Reverse Input Current Protection
The LTC3026 features reverse input current protection to
limit current draw from any supplementary power source
at the output. Figure 6 shows the reverse output current
limit for constant input and output voltages cases. Note:
Positive input current represents current flowing into the
VIN pin of LTC3026.
With VOUT held at or below the output regulation voltage
and VIN varied, IN current flow will follow Figure 6’s curves.
IIN reverse current ramps up to about 16µA as the VIN
approaches VOUT. Reverse input current will spike up as
VIN approaches within about 30mV of VOUT as the reverse
current protection circuitry is disabled and normal operation resumes. As VIN transitions above VOUT the reverse
current transitions into short-circuit current as long as
VOUT is held below the regulation voltage.
30
Connection from BST and OUT pins to their respective ceramic bypass capacitor should be kept as short
as possible. The ground side of the bypass capacitors
should be connected directly to the ground plane for best
results or through short traces back to the GND pin of the
part. Long traces will increase the effective series ESR
and inductance of the capacitor which can degrade
performance.
With the boost converter enabled, the SW pin will be
switching between ground and 5V whenever the BST pin
needs to be recharged. The transition edge rates of the SW
pin can be quite fast (~10ns). Thus care must be taken to
make sure the SW node does not couple capacitively to
other nodes (especially the ADJ pin). Additionally, stray
capacitance to this node reduces the efficiency and amount
of current available from the boost converter. For these
reasons it is recommended that the SW pin be connected
to the switching inductor with as short a trace as possible.
If the user has any sensitive nodes near the SW node, a
ground shield may be placed between the two nodes to
reduce coupling.
Because the ADJ pin is relatively high impedance (depending on the resistor divider used), stray capacitance at this
pin should be minimized (
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