LT3500 - Monolithic 2A Step-Down Regulator Plus Linear Regulator/Controller

LT3500
Monolithic 2A Step-Down
Regulator Plus Linear
Regulator/Controller
DESCRIPTION
FEATURES
n
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Wide Input Range: 3V to 36V Operating,
40V Maximum
Short-Circuit Protected Over Full Input Range
2A Output Current Capability
Adjustable/Synchronizable Fixed Frequency
Operation from 250kHz to 2.2MHz
Soft-Start/Tracking Capability
Output Adjustable Down to 0.8V
Adjustable Linear Regulator/Driver with 13mA
Output Capability
Power Good Comparator with Complimentary
Outputs
Low Shutdown Current: 12μA
Thermally Enhanced 3mm × 3mm DFN or 16-Pin
MSOP Package
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Resistor-programmable 250kHz to 2.2MHz frequency
range and synchronization capability enable optimization
between efficiency and external component size. Cycleby-cycle current limit, frequency foldback and thermal
shutdown provide protection against a shorted output.
The soft-start feature controls the ramp rate of the output
voltage, eliminating input current surge during start-up,
and also provides output tracking.
The LT3500 contains an internal NPN transistor with feedback control which can be configured as a linear regulator
or as a linear regulator controller.
APPLICATIONS
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The LT®3500 is a current mode PWM step-down DC/DC
converter with an internal 2.3A switch. The wide input range
of 3V to 40V makes the LT3500 suitable for regulating
power from a wide variety of sources, including automotive batteries, 24V industrial supplies and unregulated
wall adapters.
Automotive Battery Regulation
Industrial Control
Wall Transformer Regulation
Distributed Power Regulation
The LT3500’s low current shutdown mode (<12μA) enables
easy power management in battery-powered systems.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear
Technology Corporation. All other trademarks are the property of their respective owners.
TYPICAL APPLICATION
Dual Step-Down Converters
VIN
BST
VOUT1
5V
1A
SW
B240A
42.2k
SHDN
SS
0.47μF
53.6k
330pF
FB
PG
PG
RT/SYNC LDRV
VC
22μF
8.06k
80
VOUT1 = 5V AT 1A
AC COUPLED
2mV/DIV
75
70
65
VOUT2 = 3.3V AT 1A
AC COUPLED
2mV/DIV
60
8.06k
VIN = 12V
IOUT2 = 0A
FREQUENCY = 800kHz
55
1k
24.9k
LFB
40.2k
Output Voltage Ripple
85
0.47μF
6.8μH
LT3500
EFFICIENCY (%)
VIN
6V TO 36V
2.2μF
Switching Converter Efficiency
90
22μF
VOUT2
3.3V
1A
50
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
LOAD CURRENT (A)
500ns/DIV
3500 TA01c
3500 TA01b
3500 TA01a
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LT3500
ABSOLUTE MAXIMUM RATINGS
(Note 1)
VIN, PG, PG Operating .................................... 40V/–0.3V
SW .............................................................................VIN
BST ................................................................ 55V/–0.3V
BST Pin Above SW....................................................25V
LDRV, SHDN ..............................................................15V
FB, LFB, RT/SYNC .......................................................5V
SS, VC ......................................................................2.5V
Operating Junction Temperature Range
LT3500EDD (Note 2) .......................... –40°C to 125°C
LT3500IDD (Note 2) ........................... –40°C to 125°C
LT3500HDD (Note 2).......................... –40°C to 150°C
LT3500EMSE (Note 2)........................ –40°C to 125°C
LT3500IMSE (Note 3) ........................ –40°C to 125°C
LT3500HMSE (Note 2) ....................... –40°C to 150°C
Storage Temperature Range................... –65°C to 150°C
PIN CONFIGURATION
TOP VIEW
TOP VIEW
VIN
1
12 SW
SHDN
2
11 BST
SS
3
PG
4
9 LFB
VC
5
8 FB
RT/SYNC
6
7 PG
13
NC1
VIN
SHDN
SS
PG
VC
RT/SYNC
NC8
10 LDRV
1
2
3
4
5
6
7
8
17
16
15
14
13
12
11
10
9
NC16
SW
BST
LDRV
LFB
FB
PG
NC9
MSE PACKAGE
16-LEAD PLASTIC MSE
DD PACKAGE
12-LEAD (3mm × 3mm) PLASTIC DFN
θJA = 45°C/W, θJC(PAD) = 10°C/W
EXPOSED PAD (PIN 13) IS GND, MUST BE SOLDERED TO PCB
θJA = 45°C/W, θJC(PAD) = 10°C/W
EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3500EDD#PBF
LT3500EDD#TRPBF
LCRN
12-Lead (3mm × 3mm) Plastic DFN
–40°C to 125°C
LT3500IDD#PBF
LT3500IDD#TRPBF
LCRN
12-Lead (3mm × 3mm) Plastic DFN
–40°C to 125°C
LT3500HDD#PBF
LT3500HDD#TRPBF
LDCY
12-Lead (3mm × 3mm) Plastic DFN
–40°C to 150°C
LT3500EMSE#PBF
LT3500EMSE#TRPBF
3500
16-Lead Plastic MSE
–40°C to 125°C
LT3500IMSE#PBF
LT3500IMSE#TRPBF
3500
16-Lead Plastic MSE
–40°C to 125°C
LT3500HMSE#PBF
LT3500HMSE#TRPBF
3500
16-Lead Plastic MSE
–40°C to 150°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/
3500fc
2
LT3500
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TJ = 25°C. VVIN = 15V, VRT/SYNC = 2V, unless otherwise specified.
PARAMETER
CONDITIONS
l
SHDN Threshold
SHDN Source Current
VSHDN = 0.62V
SHDN Current Hysterisis
MIN
TYP
MAX
UNITS
710
760
780
mV
1.5
2.5
3.5
μA
1.25
2
3.25
μA
2.4
2.8
V
Minimum Input Voltage (Note 3)
VFB = 0V
l
Supply Shutdown Current
VSHDN = 0V
l
Supply Quiescent Current
VFB = 0.9V
FB Voltage
VVC = 1V
VVC = 0.8V to 1.6V, VIN = 3V to 40V
FB Bias Current
VFB = 0.8V, VVC = 1V
Error Amplifier gm
VVC = 1V, IVC = ±10μA
Error Amplifier Source Current
VFB = 0.6V, VVC = 1V
Error Amplifier Sink Current
VFB = 1V, VVC = 1V
14
Error Amplifier High Clamp
VFB = 0.6V
1.8
Error Amplifier Switching Threshold
VFB = 0.6V
0.6
l
l
12
30
μA
2.5
3.5
mA
0.8
0.8
0.816
0.824
V
V
50
150
nA
150
250
350
μmho
12
16
20
μA
18
22
μA
2.0
2.2
V
0.8
1.0
V
0.784
0.776
SS Source Current
VSS = 0.4V, VFB = 0.9V
2.25
2.75
3.75
μA
SS Sink Current
VFB = 0V, VSS = 2V
300
600
900
μA
SS POR Sink Current (Note 4)
VFB = 0V, VSS = 2V, Cycle SHDN
400
600
800
μA
50
100
150
mV
70
100
120
mV
SS POR Threshold
SS to FB Offset (VSS – VFB)
VVC = VFS, VSS = 0.4V
PG/PG Leakage
VFB = 0.9V, VPG/VPG = 40V
0.1
1
μA
PG/PG Threshold (Rising)
VPG = 0.4V
0.685
0.708
0.730
V
PG/PG Hysteresis (Falling)
VPG = 0.4V
20
30
40
mV
PG Sink Current
VPG = 0.4V, VFB = 0.7V
250
500
750
μA
PG Sink Current
VPG = 0.4V, VFB = 0.9V
500
800
1100
μA
RT/SYNC Reference Voltage
VFB = 0.9V, RRT/SYNC = 15k
0.75
0.850
0.975
Switching Frequency
RRT/SYNC = 90.9k
RRT/SYNC = 90.9k
RRT/SYNC = 15k
l
450
425
2
500
500
2.4
550
625
2.8
kHz
kHz
MHz
l
250
2500
kHz
SYNC Frequency Range
V
Minimum Switch On Time
VFB = 0.7V, RRT/SYNC = 90.9k
140
ns
Minimum Switch Off Time
VFB = 0.7V, RRT/SYNC = 90.9k
120
ns
Switch Leakage Current
VSW = 0V
Switch Saturation Voltage
ISW = 2A, VBST = 18V, VFB = 0.7V
Switch Peak Current DD Package
VBST = 18V, VFB = 0.7V
Switch Peak Current MSE Package
VBST = 18V, VFB = 0.7V
Boost Current
ISW = 2A, VBST = 20V, VFB = 0.7V
Minimum Boost Voltage (Note 5)
ISW = 2A, VFB = 0.7V
1
10
450
μA
mV
l
2.3
2.1
2.8
2.8
3.5
3.5
A
A
l
2.3
2.1
2.9
2.9
3.5
3.7
A
A
20
30
45
mA
2.2
3
V
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LT3500
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TJ = 25°C. VVIN = 15V, VRT/SYNC = 2V, unless otherwise specified.
PARAMETER
CONDITIONS
LFB Voltage
VLDRV = VLFB
LFB Line/Load Regulation
VVIN = 3V to 40V, VLDRV = VLFB
SS to LFB Offset (VSS – VLFB)
VSS = 0.8V, VLDRV = VLFB
MIN
TYP
MAX
UNITS
l
0.784
0.8
0.816
V
l
0.776
0.8
0.824
V
90
115
140
mV
LFB Bias Current
VLFB = 0.8V
115
300
nA
LDRV Dropout (VVIN – VLDRV)
VLDRV = 3V, ILDRV = 5mA
l
0.8
1.2
1.6
V
LDRV Maximum Current
VLDRV = 0V
l
9
13
18
mA
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.
Note2: The LT3500EDD/LT3500EMSE is guaranteed to meet performance
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 LT3500IDD/LT3500IMSE is guaranteed over the full –40°C to 125°C
operating junction temperature range. The LT3500HDD/LT3500HMSE is
guaranteed over the full –40°C to 150°C operating junction temperature
range. High junction temperatures degrade operating lifetimes. Operating
lifetime is derated at junction temperatures greater than 125°C.
Note 3: Minimum input voltage is defined as the voltage where internal
bias lines are regulated so that the reference voltage and oscillator remain
constant. Actual minimum input voltage to maintain a regulated output
will depend upon output voltage and load current. See Applications
Information.
Note 4: An internal power-on reset (POR) latch is set on the positive
transition of the SHDN pin through its threshold. The output of the latch
activates a current source on the SS pin which typically sinks 600μA,
discharging the SS capacitor. The latch is reset when the SS pin is driven
below the soft-start POR threshold or the SHDN pin is taken below its
threshold.
Note 5: This is the minimum voltage across the boost capacitor needed to
guarantee full saturation of the internal power switch.
Note 6: This IC includes overtemperature protection that is intended
to protect the device during momentary overload conditions. Junction
temperature will exceed the maximum operating junction temperature
when overtemperature protection is active. Continuous operation above
the specified maximum operating junction temperature may impair device
reliability.
TYPICAL PERFORMANCE CHARACTERISTICS
Feedback Voltage vs Temperature
1.10
0.820
VOLTAGE (V)
1.04
0.805
FB
0.800
LFB
0.795
1.02
1.00
0.98
0.96
0.790
0.94
0.785
2.5
MINIMUM INPUT VOLTAGE
RRT/SYNC = 90.9k
VOLTAGE (V)
1.06
0.810
VOLTAGE (V)
3.0
1.08
0.815
0.780
–50 –25
Shutdown Threshold and Minimum
Input Voltage vs Temperature
RT/SYNC Voltage vs Temperature
2.0
1.5
1.0
SHUTDOWN THRESHOLD
RRT/SYNC = 15k
0.5
0.92
0
25 50 75 100 125 150
TEMPERATURE (°C)
3500 G01
0.90
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3500 G02
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3500 G03
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LT3500
TYPICAL PERFORMANCE CHARACTERISTICS
Shutdown Quiescent Current
vs Temperature
Error Amplifier gm vs Temperature
15.0
400
5
12.5
350
4
VSHDN = 0.9V
3
VSHDN = 0.7V
2
TRANSCONDUCTANCE (μmhos)
6
CURRENT (μA)
CURRENT (μA)
Shutdown Input Currents
vs Temperature
10.0
7.5
5.0
2.5
1
0
–50 –25
0
0
–50 –25
25 50 75 100 125 150
TEMPERATURE (°C)
0
200
150
25 50 75 100 125 150
TEMPERATURE (°C)
0
3500 G05
Soft-Start Source Current
vs Temperature
3500 G06
Soft-Start Feedback Offset
vs Temperature
3.5
VC Switching Threshold
vs Temperature
150
0.95
3.3
0.90
3.1
0.85
125
2.7
2.5
2.3
2.1
LFB
VOLTAGE (V)
VOLTAGE (mV)
2.9
CURRENT (μA)
250
100
–50 –25
25 50 75 100 125 150
TEMPERATURE (°C)
3500 G04
100
FB
0.80
0.75
0.70
0.65
75
0.60
1.9
0.55
1.7
1.5
–50 –25
0
50
–50 –25
25 50 75 100 125 150
TEMPERATURE (°C)
0
25 50 75 100 125 150
TEMPERATURE (°C)
3500 G07
0.50
–50 –25
Power Good Sink Currents
vs Temperature
Frequency vs Temperature
1000
600
0.74
900
580
0.73
PG
800
RISING EDGE
FALLING EDGE
0.69
FREQUENCY (kHz)
CURRENT (μA)
0.70
600
PG
500
400
540
520
500
480
460
0.68
300
0.67
200
440
0.66
100
420
0.65
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3500 G10
0
–50 –25
0
RRT/SYNC = 90.9k
560
700
0.71
25 50 75 100 125 150
TEMPERATURE (°C)
3500 G09
0.75
0.72
0
3500 G08
Power Good Thresholds
vs Temperature
VOLTAGE (V)
300
25 50 75 100 125 150
TEMPERATURE (°C)
3500 G11
400
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3500 G12
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LT3500
TYPICAL PERFORMANCE CHARACTERISTICS
Peak Switch Current
vs Temperature
LDRV Short-Circuit Current
vs Temperature
20
100
3.4
19
90
18
80
17
70
CURRENT (A)
3.2
CURRENT (mA)
MSE PACKAGE
3.1
3.0
DD PACKAGE
2.9
DUTY CYCLE (%)
3.5
3.3
16
15
14
12
20
2.6
11
10
10
–50 –25
0
Switch Saturation Voltage
vs Switch Current
2000
225
1750
1500
175
1250
MINIMUM ON TIME
1000
125
750
500
MINIMUM OFF TIME
75
250
50
–50 –25
0
0
25 50 75 100 125 150
TEMPERATURE (°C)
600
TA = 25°C
SWITCH SATURATION VOLTAGE (mV)
2250
250
FREQUENCY (kHz)
2500
275
200
0
Minimum Input Voltage
8
6
2.3
2.1
1.9
MINIMUM BOOST FOR
SWITCH SATURATION
VOUT1 = 5V
5
VOUT1 = 3.3V
4
3
2
10
1.7
1
5
0
TA = 25°C
7
INPUT VOLTAGE (V)
BOOST VOLTAGE (V)
25°C
15
150°C
3500 G18
2.5
40
20
25°C
100
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
SWITCH CURRENT (A)
45
25
–50°C
200
20 40 60 80 100 120 140 160 180 200
RRT/SYNC (kΩ)
2.7
30
300
Minimum Boost Voltages
vs Temperature
50
–50°C
400
3500 G17
Boost Current vs Switch Current
35
500
0
3500 G16
150°C
2250
750
1750
1250
SYNCHRONIZATION FREQUENCY (kHz)
3500 G15
Frequency vs RRT/SYNC
300
100
MINIMUM DUTY CYCLE
3500 G14
Minimum Switching Times
150
30
0
250
25 50 75 100 125 150
TEMPERATURE (°C)
3500 G13
BOOST CURRENT (mA)
40
2.7
25 50 75 100 125 150
TEMPERATURE (°C)
TA = 25°C
50
13
0
MAXIMUM DUTY CYCLE
60
2.8
2.5
–50 –25
TIME (ns)
External Sync Duty Cycle Range
vs External Sync Frequency
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
SWITCH CURRENT (A)
3500 G19
1.5
–50 –25
fSW = 1MHz
L = 3.3μH
0
0
25 50 75 100 125 150
TEMPERATURE (°C)
3500 G20
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
LOAD CURRENT (A)
3500 G21
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LT3500
TYPICAL PERFORMANCE CHARACTERISTICS
LDRV Dropout Voltage
vs Temperature
1.45
Switcher Dropout Operation
6
ILDRV = 5mA
5
OUTPUT VOLTAGE (V)
1.40
VOLTAGE (V)
1.35
1.30
1.25
1.20
1.15
1.10
2500
IVOUT1 = 1A
TA = 25°C
L = 1μH
2000
VVOUT1 = 5V
4
VVOUT1 = 3.3V
3
2
L = 1.5μH
1500
L = 3.3μH
1250
L = 4.7μH
1000
L = 6.8μH
500
0
0
25 50 75 100 125 150
TEMPERATURE (°C)
2.5
3.0
3.5
4.0
4.5
INPUT VOLTAGE (V)
3500 G22
5.0
5.5
3500 G23
L = 2.2μH
1750
750
1
1.05
1.00
–50 –25
TA = 25°C
2250
FREQUENCY (kHz)
1.50
Inductor Value for 2A Maximum
Load Current (VOUT1 = 3.3V,
IRIPPLE = 250mA)
L = 10μH
250
5
10
15
20
25
30
INPUT VOLTAGE (V)
35
40
3500 G24
PIN FUNCTIONS
VIN: The VIN pin powers the internal control circuitry and
is monitored by an undervoltage comparator. The VIN pin
is also connected to the collectors of the internal power
NPN switch and linear output NPN. The VIN pin has high
dI/dt edges and must be decoupled to ground close to
the pin of the device.
SHDN: The SHDN pin is used to shut down the LT3500
and reduce quiescent current to a typical value of
12μA. The accurate 0.76V threshold and input current
hysteresis can be used as an undervoltage lockout, preventing the regulator from operating until the input voltage has
reached a predetermined level. Force the SHDN pin above
its threshold or let it float for normal operation.
SS: The SS pin is used to control the slew rate of the
output of both the switching and linear regulators. A
single capacitor from the SS pin to ground determines
the regulators’ ramp rate. For soft-start details see the
Applications Information section.
PG: The power good pin is an open-collector output that
sinks current when the FB or LFB falls below 90% of its
nominal regulating voltage. For VIN above 2V, its output
state remains true, although during SHDN, VIN undervoltage
lockout, or thermal shutdown, its current sink capability
is reduced
VC: The VC pin is the output of the error amplifier and the
input to the peak switch current comparator. It is normally
used for frequency compensation, but can also be used
as a current clamp or control loop override. If the error
amplifier drives VC above the maximum switch current
level, a voltage clamp activates. This indicates that the
output is overloaded and current to be pulled from the
SS pin reducing the regulation point.
RT/SYNC: This RT/SYNC pin provides two modes of setting
the constant switch frequency.
Connecting a resistor from the RT/SYNC pin to ground
will set the RT/SYNC pin to a typical value of 1V. The
resultant switching frequency will be set by the resistor
value. The minimum value of 15kΩ and maximum value
of 200kΩ set the switching frequency to 2.5MHz and
250kHz respectively.
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LT3500
PIN FUNCTIONS
Driving the RT/SYNC pin with an external clock signal
will synchronize the switch to the applied frequency.
Synchronization occurs on the rising edge of the clock
signal after the clock signal is detected. Each rising clock
edge initiates an oscillator ramp reset. A gain control loop
servos the oscillator charging current to maintain a constant oscillator amplitude. Hence, the slope compensation
remains unchanged. If the clock signal is removed, the
oscillator reverts to resistor mode and reapplies the 1V
bias to the RT/SYNC pin after the synchronization detection
circuitry times out. The clock source impedance should
be set such that the current out of the RT/SYNC pin in
resistor mode generates a frequency roughly equivalent
to the synchronization frequency. Floating or holding the
RT/SYNC pin above 1.1V will not damage the device, but
will halt oscillation.
PG: The power good bar pin is an open-collector output
that sinks current when the FB or LFB rises above 90%
of its nominal regulating voltage.
FB: The FB pin is the negative input to the switcher error
amplifier. The output switches to regulate this pin to 0.8V
with respect to the exposed ground pad. Bias current
flows out of the FB pin.
LFB: The LFB pin is the negative input to the linear error
amplifier. The LDRV pin servo’s to regulate this pin to 0.8V
with respect to the exposed ground pad. Bias current flows
out of the LFB pin.
LDRV: The LDRV pin is the emitter of an internal NPN that
can be configured as an output of a linear regulator or as
the drive for an external NPN high current regulator. Current flows out of the LDRV pin when the LFB pin voltage is
below 0.8V. The LDRV pin has a typical maximum current
capability of 13mA.
BST: The BST pin provides a higher than VIN base drive to
the power NPN to ensure a low switch drop. A comparator to VIN imposes a minimum off time on the SW pin if
the BST pin voltage drops too low. Forcing a SW off time
allows the boost capacitor to recharge.
SW: The SW pin is the emitter of the on-chip power NPN.
At switch off, the inductor will drive this pin below ground
with a high dV/dt. An external catch diode to ground, close
to the SW pin and respective VIN decoupling capacitor’s
ground, must be used to prevent this pin from excessive
negative voltages.
Exposed Pad: GND. The exposed pad is the only ground
connection for the device. The exposed pad should be
soldered to a large copper area to reduce thermal resistance. The GND pin also serves as small-signal ground.
For ideal operation all small-signal ground paths should
connect to the GND pin at a single point, avoiding any
high current ground returns.
NC Pins (MSE Package Only): No Connection. The NC pins
are electrically isolated from the LT3500. The NC pins may
be connected to PCB traces to aid PCB layout.
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8
R6
C3
C2
C4
R5
100mV
SS
VC
–
+
2.75μA
0.76V
RT/SYNC
SHDN
GND
VIN
–
+
2μA
2.5μA
R
S
PRE
Q
OSCILLATOR
AND AGC
R
S
Q
+
+
–
LFB
0.7V
+
–
–
0.8V 100mV
DRIVER
CIRCUITRY
–
+
+
SS
115mV
+
0.8V
Figure 1. LT3500 Block Diagram
THERMAL
OVERLOAD
POWER ON RESET
SLOPE
COMPENSATION
INTERNAL
REGULATOR
AND REFERENCES
+
+
–
C1
SS
3500 BD
PG
PG
FB
SW
LFB
LDRV
R4
L1
C6
R3
D1
R2
R1
C5
C7
D2
VOUT1
VOUT2
LT3500
BLOCK DIAGRAM
3500fc
9
LT3500
OPERATION
The LT3500 is a constant frequency, current mode buck
converter with an internal 2.3A switch plus a linear regulator with 13mA output capability. Control of both outputs
is achieved with a common SHDN pin, internal regulator,
oscillator, undervoltage detect, soft-start, thermal shutdown and power-on reset.
During power up, the POR signal sets the soft-start latch,
which discharges the SS pin to ensure proper start-up
operation. When the SS pin voltage drops below 100mV,
the VC pin is driven low disabling switching and the softstart latch is reset. Once the latch is reset the soft-start
capacitor starts to charge with a typical value of 2.75μA.
If the SHDN pin is taken below its 0.8V threshold, the
LT3500 will be placed in a low quiescent current mode.
In this mode the LT3500 typically draws 12μA from the
VIN pin.
As the voltage rises above 100mV on the SS pin, the VC
pin will be driven high by the error amplifier. When the
voltage on the VC pin exceeds 0.8V, the clock set-pulse sets
the driver flip-flop which turns on the internal power NPN
switch. This causes current from VIN, through the NPN
switch, inductor and internal sense resistor, to increase.
When the voltage drop across the internal sense resistor
exceeds a predetermined level set by the voltage on the
VC pin, the flip-flop is reset and the internal NPN switch
is turned off. Once the switch is turned off the inductor
will drive the voltage at the SW pin low until the external
Schottky diode starts to conduct, decreasing the current
in the inductor. The cycle is repeated with the start of each
clock cycle. However, if the internal sense resistor voltage
exceeds the predetermined level at the start of a clock cycle,
the flip-flop will not be set resulting in a further decrease in
inductor current. Since the output current is controlled by
the VC voltage, output regulation is achieved by the error
amplifier continually adjusting the VC pin voltage.
When the SHDN pin is floated or driven above 0.76V, the
internal bias circuits turn on generating an internal regulated voltage, 0.8(VFB) and 1V(RT/SYNC) references, and
a POR signal which sets the soft-start latch.
As the RT/SYNC pin reaches its 1V regulation point, the
internal oscillator will start generating a clock signal at a
frequency determined by the resistor from the RT/SYNC
pin to ground. Alternatively, if a synchronization signal is
detected by the LT3500 at the RT/SYNC pin, a clock signal
will be generated at the incoming frequency on the rising
edge of the synchronization pulse. In addition, the internal
slope compensation will be automatically adjusted to prevent subharmonic oscillation during synchronization.
The LT3500 is a constant frequency, current mode stepdown converter. Current mode regulators are controlled
by an internal clock and two feedback loops that control
the duty cycle of the power switch. In addition to the
normal error amplifier, there is a current sense amplifier
that monitors switch current on a cycle-by-cycle basis.
This technique means that the error amplifier commands
current to be delivered to the output rather than voltage.
A voltage fed system will have low phase shift up to the
resonant frequency of the inductor and output capacitor,
then an abrupt 180° shift will occur. The current fed system
will have 90° phase shift at a much lower frequency, but
will not have the additional 90° shift until well beyond
the LC resonant frequency. This makes it much easier to
frequency compensate the feedback loop and also gives
much quicker transient response.
The error amplifier is a transconductance amplifier that
compares the FB voltage to either the SS pin voltage minus
100mV or an internally regulated 800mV, whichever is
lowest. Compensation of the loop is easily achieved with
a simple capacitor or series resistor/capacitor from the
VC pin to ground.
Since the SS pin is driven by a constant current source, a
single capacitor on the soft-start pin will generate controlled
linear ramp on the output voltage.
If the current demanded by the output exceeds the maximum current dictated by the VC pin clamp, the SS pin
will be discharged, lowering the regulation point until the
output voltage can be supported by the maximum current.
When overload is removed, the output will soft-start from
the overload regulation point.
3500fc
10
LT3500
OPERATION
VIN undervoltage detection or thermal shutdown will
set the soft-start latch, resulting in a complete soft-start
sequence.
in the same manner as the switching regulator. The linear
output can also be configured to drive an external NPN to
provide a linear regulator with higher current capability.
The switch driver operates from either the VIN or BST voltage. An external diode and capacitor are used to generate
a drive voltage higher than VIN to saturate the output NPN
and maintain high efficiency.
A power good comparator with 30mV of hysteresis trips
when both FB and LFB are above 90% of the 0.8V reference. The PG output is an open collector NPN that is off
when the output is in regulation allowing a resistor to pull
the PG pin to a desired voltage. The PG output is an opencollector NPN that is on when the output is in regulation
providing either drive for an output disconnect transistor
or inverted power good logic.
In addition to the switching regulator, the LT3500 contains
a NPN linear regulator with a 0.8V reference, and 13mA
current capability. The 0.8 reference will track the SS pin
APPLICATIONS INFORMATION
Choosing the Output Voltage
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the 1% resistors according to:
V
R1= R2 OUT1 – 1
0.8V R2 should be 10.0k or less to avoid bias current errors.
Reference designators refer to the Block Diagram in
Figure 1.
maximum recommended frequency can be approximated
by the equation:
Frequency (Hz) =
VOUT1 + VD
1
•
VIN − VSW + VD tON(MIN)
where VD is the forward voltage drop of the catch diode
(D1 Figure 1), VSW is the voltage drop of the internal
switch, and tON(MIN) is the minimum on time of the
switch, all at maximum load current.
2500
2250
The LT3500 switching frequency is set by resistor R5 in
Figure 1. The RT/SYNC pin is internally regulated at 1V.
Setting resistor R5 sets the current in the RT/SYNC pin
which determines the oscillator frequency as illustrated
in Figure 2.
The switching frequency is typically set as high as possible to reduce overall solution size. The LT3500 employs
techniques to enhance dropout at high frequencies but
efficiency and maximum input voltage decrease due to
switching losses and minimum switch on times. The
2000
1750
FREQUENCY (kHz)
Choosing the Switching Frequency
1500
1250
1000
750
500
250
0
0
20 40 60 80 100 120 140 160 180 200
RRT/SYNC (kΩ)
3500 F02
Figure 2. Frequency vs RT/SYNC Resistance
3500fc
11
LT3500
APPLICATIONS INFORMATION
where VSW is the voltage drop of the internal switch,
and
The following example along with the data in Table 1
illustrates the tradeoffs of switch frequency selection.
DCMAX = 1 – tOFF(MIN) • Frequency.
Example.
Figure 3 shows a typical graph of minimum input voltage
vs load current for 3.3V and 5V applications.
VIN = 25V, VOUT1 = 3.3V, IOUT1 = 2.0A,
Temperature = 0°C to 85°C
The maximum input voltage is determined by the absolute
maximum ratings of the VIN and BST pins and by the
frequency and minimum duty cycle.
tON(MIN) = 185ns (85°C from Typical Characteristics
graph), VD = 0.6V, VSW = 0.4V (85°C)
Max Frequency =
3.3 + 0.6
1
•
~ 835kHz
25 − 0.4 + 0.6 185ns
The minimum duty cycle is defined as:
DCMIN = tON(MIN) • Frequency
RT/SYNC ~ 49.9k
Maximum input voltage as:
Frequency ≅ 820kHz
Input Voltage Range
Once the switching frequency has been determined, the
input voltage range of the regulator can be determined.
The minimum input voltage is determined by either the
LT3500’s minimum operating voltage of ~2.8V or by its
maximum duty cycle. The duty cycle is the fraction of time
that the internal switch is on during a clock cycle. The
maximum duty cycle can be determined from the clock
frequency and the minimum off time from the typical
characteristics graph.
8
fSW = 1MHz
L = 3.3μH
INPUT VOLTAGE (V)
7
6
5
4
VOUT1 = 5V START-UP
VOUT1 = 5V RUNNING
VOUT1 = 3.3V START-UP
VOUT1 = 3.3V RUNNING
3
This leads to a minimum input voltage of:
VIN(MIN) =
VOUT1 + VD
− VD + VSW
DCMIN
VIN(MAX) =
2
VOUT1 + VD
− VD + VSW
DCMAX
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
0
LOAD CURRENT (A)
3500 F03
Figure 3. Minimum Input Voltage vs Load Current
Table 1. Efficiency and Size Comparisons for Different RRT/SYNC Values, VOUT1 = 3.3V
FREQUENCY
RT/SYNC
EFFICIENCY
VIN(MAX)
2.5MHz
15k
73.6
12
2.0MHz
20k
81.5
14
1.5MHz
24.9k
84.5
18
1.0MHz
40.2k
87.3
500kHz
90.9k
88.9
L
C
C + L AREA
(mm2)
1μ
10μ
24
1.5μ
10μ
24
2.2μ
10μ
24
28
3.3μ
22μ
34
36
4.7μ
47μ
40
3500fc
12
LT3500
APPLICATIONS INFORMATION
Note that the LT3500 will regulate if the input voltage is
taken above the calculated maximum voltage as long as
maximum ratings of the VIN and BST pins are not violated.
However operation in this region of input voltage will exhibit
pulse skipping behavior.
Example:
VOUT1 = 3.3V, IOUT1 = 1A, Frequency = 1MHz,
Temperature = 25°C,
VSW = 0.3V, VD = 0.4V, tON(MIN) = 150ns,
tOFF(MIN) = 110ns
DCMAX = 1− (110ns)1MHz = 89%
VIN(MIN) =
3.3 + 0.4
− 0.4 + 0.3 = 4.06V
0.89
DCMIN = tON(MIN) • Frequency = 15%
VIN(MAX) =
3.3 + 0.4
− 0.4 + 0.3 = 24.57V
0.15
Inductor Selection and Maximum Output Current
A good first choice for the inductor value is:
)• V
(V − V
L = IN OUT1 OUT1
VIN • f
where f is frequency in MHz and L is in μH.
With this value the maximum load current will be ~2A,
independent of input voltage. The inductor’s RMS current
rating must be greater than your maximum load current
and its saturation current should be about 30% higher. To
keep efficiency high, the series resistance (DCR) should
be less than 0.05Ω.
For applications with a duty cycle of about 50%, the inductor value should be chosen to obtain an inductor ripple
current less than 40% of peak switch current.
Of course, such a simple design guide will not always result
in the optimum inductor for your application. A larger value
provides a slightly higher maximum load current, and will
reduce the output voltage ripple. If your load is lower than
1.5A, then you can decrease the value of the inductor and
operate with higher ripple current. This allows you to use
a physically smaller inductor, or one with a lower DCR
resulting in higher efficiency.
The current in the inductor is a triangle wave with an
average value equal to the load current. The peak switch
current is equal to the output current plus half the peak-to
peak inductor ripple current. The LT3500 limits its switch
current in order to protect itself and the system from
overload faults. Therefore, the maximum output current
that the LT3500 will deliver depends on the current limit,
the inductor value, switch frequency, and the input and
output voltages. The inductor is chosen based on output
current requirements, output voltage ripple requirements,
size restrictions and efficiency goals.
When the switch is off, the inductor sees the output voltage plus the catch diode drop. This gives the peak-to-peak
ripple current in the inductor:
ΔIL =
(1− DC)( VOUT1 + VD )
L•f
where f is the switching frequency of the LT3500 and L
is the value of the inductor. The peak inductor and switch
current is:
ISW(PK) =ILPK =IOUT1 +
ΔIL
2
3500fc
13
LT3500
APPLICATIONS INFORMATION
IOUT1(MAX) =ILIM −
ΔIL
ΔI
=2.3 – L
2
2
If the inductor value is chosen so that the ripple current
is small, then the available output current will be near the
switch current limit.
One approach to choosing the inductor is to start with the
simple rule given above, look at the available inductors
and choose one to meet cost or space goals. Then use
these equations to check that the LT3500 will be able to
deliver the required output current. Note again that these
equations assume that the inductor current is continuous.
Discontinuous operation occurs when IOUT1 is less than
IL/2 as calculated above.
Figure 4 illustrates the inductance value needed for a 3.3V
output with a maximum load capability of 2A. Referring
to Figure 4, an inductor value between 3.3μH and 4.7μH
will be sufficient for a 15V input voltage and a switch
frequency of 750kHz. There are several graphs in the
Typical Performance Characteristics section of this data
sheet that show inductor selection as a function of input
voltage and switch frequency for several popular output
voltages and output ripple currents. Also, low inductance
may result in discontinuous mode operation, which is
okay, but further reduces maximum load current. For
details of maximum output current and discontinuous
mode operation, see Linear Technology Application Note
44. Finally, for duty cycles greater than 50% (VOUT1/VIN
> 0.5), there is a minimum inductance required to avoid
subharmonic oscillations. See Application Note 19 for
more information.
2500
2250
L = 1μH
L = 1.5μH
2000
FREQUENCY (kHz)
To maintain output regulation, this peak current must be
less than the LT3500’s switch current limit, ILIM. ILIM is
guaranteed to be greater than 2.3A over the entire duty
cycle range. The maximum output current is a function
of the chosen inductor value:
1750
L = 2.2μH
1500
1250
L = 3.3μH
1000
750
L = 4.7μH
500
L = 6.8μH
250
5
10
15
20
25
30
INPUT VOLTAGE (V)
35
40
3500 F04
Figure 4. Inductor Values for 2A Maximum Load Current
(VOUT1 = 3.3V, IRIPPLE = 1A)
Input Capacitor Selection
Bypass the input of the LT3500 circuit with a 4.7μF or
higher ceramic capacitor of X7R or X5R type. A lower
value or a less expensive Y5V type can be used if there
is additional bypassing provided by bulk electrolytic or
tantalum capacitors. The following paragraphs describe
the input capacitor considerations in more detail.
Step-down regulators draw current from the input supply in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage
ripple at the LT3500 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
The input capacitor must have low impedance at the
switching frequency to do this effectively, and it must
have an adequate ripple current rating.
A conservative value is the RMS input current is given
by:
ICIN(RMS) =
IOUT1 ⎡⎣ VOUT1 • ( VIN − VOUT1 ) ⎤⎦
VIN
0.5
<
IOUT1
2
and is largest when VIN = 2VOUT1 (50% duty cycle).
3500fc
14
LT3500
APPLICATIONS INFORMATION
The frequency, VIN to VOUT1 ratio, and maximum load
current requirement of the LT3500 along with the input
supply source impedance, determine the energy storage
requirements of the input capacitor. Determine the worstcase condition for input ripple current and then size the
input capacitor such that it reduces input voltage ripple to
an acceptable level. Typical values for input capacitors run
from 10μF at low frequencies to 2.2μF at higher frequencies.
The combination of small size and low impedance (low
equivalent series resistance or ESR) of ceramic capacitors
make them the preferred choice. The low ESR results in
very low voltage ripple and the capacitors can handle plenty
of ripple current. They are also comparatively robust and
can be used in this application at their rated voltage. X5R
and X7R types are stable over temperature and applied
voltage, and give dependable service. Other types (Y5V and
Z5U) have very large temperature and voltage coefficients
of capacitance, so they may have only a small fraction of
their nominal capacitance in your application. While they
will still handle the RMS ripple current, the input voltage
ripple may become fairly large, and the ripple current may
end up flowing from your input supply or from other bypass capacitors in your system, as opposed to being fully
sourced from the local input capacitor. An alternative to a
high value ceramic capacitor is a lower value along with
a larger electrolytic capacitor, for example a 1μF ceramic
capacitor in parallel with a low ESR tantalum capacitor.
For the electrolytic capacitor, a value larger than 10μF will
be required to meet the ESR and ripple current requirements. Because the input capacitor is likely to see high
surge currents when the input source is applied, tantalum
capacitors should be surge rated. The manufacturer may
also recommend operation below the rated voltage of the
capacitor. Be sure to place the 1μF ceramic as close as
possible to the VIN and GND pins on the IC for optimal
noise immunity.
A final caution regarding the use of ceramic capacitors for
input bypassing. A ceramic input capacitor can combine
with stray inductance to form a resonant tank circuit. If
power is applied quickly (for example, by plugging the
circuit into a live power source) this tank can ring, doubling
the input voltage and damaging the LT3500. The solution is
to either clamp the input voltage or dampen the tank circuit
by adding a lossy capacitor in parallel with the ceramic
capacitor. For details see Application Note 88.
Output Capacitor Selection
Typically step-down regulators are easily compensated with
an output crossover frequency that is 1/10 of the switching frequency. This means that the time that the output
capacitor must supply the output load during a transient
step is ~2 or 3 switching periods. With an allowable 5%
drop in output voltage during the step, a good starting
value for the output capacitor can be expressed by:
C VOUT1 =
Max Load Step
Frequency • 0.05 • VOUT1
Example:
VOUT1 = 3.3V, Frequency = 1MHz, Max Load Step =
2A
C VOUT1 =
2
= 12μF
1MHz • 0.05 • 3.3
The calculated value is only a suggested starting value.
Increase the value if transient response needs improvement
or reduce the capacitance if size is a priority. The output
capacitor filters the inductor current to generate an output
with low voltage ripple. It also stores energy in order to
satisfy transient loads and to stabilize the LT3500’s control
loop. The switching frequency of the LT3500 determines
3500fc
15
LT3500
APPLICATIONS INFORMATION
the value of output capacitance required. Also, the current
mode control loop doesn’t require the presence of output
capacitor series resistance (ESR). For these reasons, you
are free to use ceramic capacitors to achieve very low
output ripple and small circuit size. Estimate output ripple
with the following equations:
VRIPPLE =
ΔIL
8 • Frequency • COUT1
For ceramic capacitors and,
VRIPPLE = ∆IL • ESR
For electrolytic (tantalum and aluminum)
where ∆IL is the peak-to-peak ripple current in the
inductor.
The RMS content of this ripple is very low, and the RMS
current rating of the output capacitor is usually not of
concern.
Another constraint on the output capacitor is that it must
have greater energy storage than the inductor; if the stored
energy in the inductor is transferred to the output, you
would like the resulting voltage step to be small compared
to the regulation voltage. For a 5% overshoot, this requirement becomes:
I
COUT1 > 10 • L LIM VOUT1 2
Finally, there must be enough capacitance for good transient
performance. The last equation gives a good starting point.
Alternatively, you can start with one of the designs in this
data sheet and experiment to get the desired performance.
This topic is covered more thoroughly in the section on
loop compensation.
The high performance (low ESR), small size and robustness of ceramic capacitors make them the preferred type
for LT3500 applications. However, all ceramic capacitors
are not the same. As mentioned above, many of the high
value capacitors use poor dielectrics with high temperature and voltage coefficients. In particular, Y5V and Z5U
types lose a large fraction of their capacitance with applied voltage and temperature extremes. Because the loop
stability and transient response depend on the value of
COUT1, you may not be able to tolerate this loss. Use X7R
and X5R types. You can also use electrolytic capacitors.
The ESRs of most aluminum electrolytics are too large to
deliver low output ripple. Tantalum and newer, lower ESR
organic electrolytic capacitors intended for power supply
use, are suitable and the manufacturers will specify the
ESR. The choice of capacitor value will be based on the
ESR required for low ripple. Because the volume of the
capacitor determines its ESR, both the size and the value
will be larger than a ceramic capacitor that would give you
similar ripple performance. One benefit is that the larger
capacitance may give better transient response for large
changes in load current.
Catch Diode
The diode D1 conducts current only during switch off
time. Use a Schottky diode to limit forward voltage drop to
increase efficiency. The Schottky diode must have a peak
reverse voltage that is equal to regulator input voltage and
sized for average forward current in normal operation.
Average forward current can be calculated from:
ID(AVG) =
IOUT1
• ( VIN − VOUT1)
VIN
3500fc
16
LT3500
APPLICATIONS INFORMATION
The only reason to consider a larger diode is the worstcase condition of a high input voltage and shorted output.
With a shorted condition, diode current will increase to a
typical value of 3A, determined by the peak switch current
limit of the LT3500. This is safe for short periods of time,
but it would be prudent to check with the diode manufacturer if continuous operation under these conditions
can be tolerated.
BST Pin Considerations
The capacitor and diode tied to the BST pin generate
a voltage that is higher than the input voltage. In most
cases a 0.47μF capacitor and fast switching diode (such
as the CMDSH-3 or FMMD914) will work well. Almost
any type of film or ceramic capacitor is suitable, but the
ESR should be <1Ω to ensure it can be fully recharged
during the off time of the switch. The capacitor value can
be approximated by:
C BST =
VIN
(
IOUT1(MAX) • DC
)
50 • VOUT1 − VBST(MIN) • f
LDRV
VIN
LT3500
where IOUT1(MAX) is the maximum load current, and
VBST(MIN) is the minimum boost voltage to fully saturate
the switch.
Figure 5 shows four ways to arrange the boost circuit.
The BST pin must be more than 2.2V above the SW pin
for full efficiency.
Generally, for outputs of 3.3V and higher the standard
circuit (Figure 5a) is the best. For outputs between 2.8V
and 3.3V, replace the D2 with a small Schottky diode such
as the PMEG4005.
For lower output voltages the boost diode can be tied to
the input (Figure 5b). The circuit in Figure 5a is more efficient because the BST pin current comes from a lower
voltage source.
Figure 5c shows the boost voltage source from the linear
output that is set to greater than 2.5V (any available DC
sources that are greater than 2.5V is sufficient). The highest
efficiency is attained by choosing the lowest boost voltage above 2.5V. You must also be sure that the maximum
voltage at the BST pin is less than the maximum specified
in the Absolute Maximum Ratings section.
VIN
LDRV
VIN
LT3500
BST
C3
BST
D2
C3
VOUT1
SW
D2
VOUT1
SW
D1
D1
VBST – VSW = VOUT1
VBST(MAX) = VIN + VOUT1
VBST – VSW = VIN
VBST(MAX) = 2 • VIN
(5a)
VIN
(5b)
VOUT2
LDRV
VIN
VIN
LDRV
VIN
D2
D2
LT3500
LT3500
BST
VX > VIN + 3V
BST
C3
VOUT1
SW
VBST – VSW = VOUT2
VBST(MAX) = VIN + VOUT2
VOUT2 ≥ 2.5V
VOUT1
SW
D1
D1
VBST – VSW = VX
VBST(MAX) = VX
(5c)
3500 F05
(5d)
Figure 5. BST Pin Considerations
3500fc
17
LT3500
APPLICATIONS INFORMATION
The boost circuit can also run directly from a DC voltage
that is higher than the input voltage by more than 2.5V, as
in Figure 5d. The diode is used to prevent damage to the
LT3500 in case VX is held low while VIN is present. The
circuit eliminates a capacitor, but efficiency may be lower
and dissipation in the LT3500 may be higher. Also, if VX is
absent, the LT3500 will still attempt to regulate the output,
but will do so with very low efficiency and high dissipation
because the switch will not be able to saturate, dropping
1.5V to 2V in conduction.
Frequency Compensation
The LT3500 uses current mode control to regulate the
output. This simplifies loop compensation. In particular, the
LT3500 does not require the ESR of the output capacitor
for stability so you are free to use ceramic capacitors to
achieve low output ripple and small circuit size. Frequency
compensation is provided by the components tied to the
VC pin. Generally a capacitor and a resistor in series to
ground determine loop gain. In addition, there is a lower
value capacitor in parallel. This capacitor is not part of
the loop compensation but is used to filter noise at the
switching frequency.
The minimum input voltage of an LT3500 application is
limited by the minimum operating voltage (<2.8V) and by
the maximum duty cycle as outlined above. For proper
start-up, the minimum input voltage is also limited by
the boost circuit. If the input voltage is ramped slowly, or
the LT3500 is turned on with its SS pin when the output
is already in regulation, then the boost capacitor may not
be fully charged. Because the boost capacitor is charged
with the energy stored in the inductor, the circuit will rely
on some minimum load current to get the boost circuit
running properly. This minimum load will depend on
input and output voltages and on the arrangement of the
boost circuit.
Loop compensation determines the stability and transient
performance. Designing the compensation network is a bit
complicated and the best values depend on the application
and in particular the type of output capacitor. A practical
approach is to start with one of the circuits in this data
sheet that is similar to your application and tune the compensation network to optimize the performance. Stability
should then be checked across all operating conditions,
including load current, input voltage and temperature.
The LT1375 data sheet contains a more thorough discussion of loop compensation and describes how to test the
stability using a transient load.
The Typical Performance Characteristics section shows
plots of the minimum load current to start and to run as a
function of input voltage for 3.3V and 5V outputs. In many
cases the discharged output capacitor will present a load
to the switcher which will allow it to start. The plots show
the worst-case situation where VIN is ramping very slowly.
Use a Schottky diode for the lowest start-up voltage.
Figure 6 shows an equivalent circuit for the LT3500 control
loop. The error amp is a transconductance amplifier with
finite output impedance. The power section, consisting
of the modulator, power switch, and inductor, is modeled
as a transconductance amplifier generating an output
LT3500
CURRENT MODE
POWER STAGE
gm = 3mho
SW
VOUT1
R1
+
–
VC
CC
RC
CF
CPL
ESR
FB
0.8V
4M
ERROR AMP
gm = 250μmhos
C1
R2
TANTALUM
OR
POLYMER
C1
CERAMIC
3500 F06
Figure 6. Model for Loop Response
3500fc
18
LT3500
APPLICATIONS INFORMATION
current proportional to the voltage at the VC pin. Note that
the output capacitor integrates this current, and that the
capacitor on the VC pin (CC) integrates the error amplifier
output current, resulting in two poles in the loop. In
most cases a zero is required and comes from either the
output capacitor ESR or from a resistor in series with CC.
This simple model works well as long as the value of the
inductor is not too high and the loop crossover frequency
is much lower than the switching frequency. A phase lead
capacitor (CPL) across the feedback divider may improve
the transient response.
Synchronization
The RT/SYNC pin can be used to synchronize the LT3500
to an external clock source. Driving the RT/SYNC resistor
with a clock source triggers the synchronization detection
circuitry. Once synchronization is detected, the rising edge
of SW will be synchronized to the rising edge of the RT/SYNC
pin signal. An AGC loop will adjust slope compensation
to avoid subharmonic oscillation.
The synchronizing clock signal input to the LT3500 must
have a frequency between 250kHz and 2.5MHz, a duty
cycle between 20% and 80%, a low state below 0.5V and
a high state above 1.6V. Synchronization signals outside
of these parameters will cause erratic switching behavior.
The RT/SYNC resistor should be set such that the free
running frequency ((VRT/SYNC – VSYNCLO)/RRT/SYNC) is
approximately equal to the synchronization frequency. If
the synchronization signal is halted, the synchronization
detection circuitry will timeout in typically 10μs at which
time the LT3500 reverts to the free-running frequency based
on the current through RT/SYNC. If the RT/SYNC pin is held
above 1.1V at any time, switching will be disabled.
If the synchronization signal is not present during regulator start-up (for example, the synchronization circuitry
is powered from the regulator output) the RT/SYNC pin
must see an equivalent resistance to ground between 15k
and 200k until the synchronization circuitry is active for
proper start-up operation.
If the synchronization signal powers up in an undetermined
state (VOL, VOH, Hi-Z), connect the synchronization clock
to the LT3500 as shown in Figure 7. The circuit as shown
will isolate the synchronization signal when the output
voltage is below 90% of the regulated output. The LT3500
will start-up with a switching frequency determined by the
resistor from the RT/SYNC pin to ground.
LDRV
LT3500
PG
RT/SYNC
VCC
SYNCHRONIZATION
CIRCUITRY
CLK
3500 F07
Figure 7. Synchronous Signal Powered from Regulator’s Output
If the synchronization signal powers up in a low impedance
state (VOL), connect a resistor between the RT/SYNC pin
and the synchronizing clock. The equivalent resistance
seen from the RT/SYNC pin to ground will set the start-up
frequency.
If the synchronization signal powers up in a high impedance
state (Hi-Z), connect a resistor from the RT/SYNC pin to
ground. The equivalent resistance seen from the RT/SYNC
pin to ground will set the start-up frequency.
3500fc
19
LT3500
APPLICATIONS INFORMATION
If the synchronization signal changes between high and
low impedance states during power up (VOL, Hi-Z), connect
the synchronization circuitry to the LT3500 as shown in
the Typical Applications section. This will allow the LT3500
to start up with a switching frequency determined by the
equivalent resistance from the RT/SYNC pin to ground.
Shutdown and Undervoltage Lockout
Figure 8 shows how to add an undervoltage lockout (UVLO)
to the LT3500. Typically, UVLO is used in situations where
the input supply is current limited, or has a relatively high
source resistance. A switching regulator draws constant
power from the source, so source current increases as
source voltage drops. This looks like a negative resistance
load to the source and can cause the source to current limit
or latch low under low source voltage conditions. UVLO
prevents the regulator from operating at source voltages
where these problems might occur.
VIN
2.5μA
R1
2μA
SHDN
C1
R2
0.76V
+
–
3500 F08
Figure 8. Undervoltage Lockout
An internal comparator will force the part into shutdown
below the minimum VIN of 2.8V. This feature can be
used to prevent excessive discharge of battery-operated
systems.
If an adjustable UVLO threshold is required, the SHDN
pin can be used. The threshold voltage of the SHDN pin
comparator is 0.76V. A 2.5μA internal current source defaults the open-pin condition to be operating (see Typical
Performance Characteristics). Current hysteresis is added
above the SHDN threshold. This can be used to set voltage
hysteresis of the UVLO using the following:
R1=
R2 =
VH − VL
2μA
0.76
VH − 0.76
+ 2.5μA
R1
VH = Turn-on threshold
VL = Turn-off threshold
Example: switching should not start until the input is above
4.75V and is to stop if the input falls below 3.75V.
VH = 4.75V
VL = 3.75
4.75 − 3.75
~ 499k
2μA
0.76
R2 =
~ 71.5k
4.75 − 0.76
+ 2.5μA
499k
R1=
Keep the connections from the resistors to the SHDN
pin short and make sure that the interplane or surface
capacitance to switching nodes is minimized. If high resistor values are used, the SHDN pin should be bypassed
with a 1nF capacitor to prevent coupling problems from
the switch node.
Soft-Start
The outputs of the LT3500 regulate to either the SS pin
voltage minus 100mV or an internally regulated 800mV,
whichever is lowest. A capacitor from the SS pin to ground
is charged by an internal 2.75μA current source resulting
in a linear output ramp from 0V to the regulated output
whose duration is given by:
tRAMP =
CSS • 0.9V
2.75μA
3500fc
20
LT3500
APPLICATIONS INFORMATION
At power-up, a reset signal sets the soft-start latch and
discharges the SS pin to approximately 0V to ensure
proper start-up. When the SS pin is fully discharged the
latch is reset and the internal 2.75μA current source starts
to charge the SS pin.
When the SS pin voltage is below 100mV, the VC pin is
pulled low which disables switching. As the SS pin voltage
rises above 100mV, the VC pin is released and the outputs
are regulated to the SS voltage. When the SS pin voltage
minus 100mV exceeds the internal 0.8V reference, the
outputs are regulated to the reference. The SS pin voltage
will continue to rise until it is clamped at 2V.
In the event of a VIN undervoltage lockout, the SHDN pin
driven below 0.8V, or the internal die temperature exceeding
its maximum rating during normal operation, the soft-start
latch is set, triggering a start-up sequence.
In addition, if the load exceeds the maximum output switch
current (switching regulator only), the output will start to
drop causing the VC pin clamp to be activated. As long as
the VC pin is clamped, the SS pin will be discharged. As
a result, the output will be regulated to the highest voltage that the maximum output current can support. For
example, if a 6V output is loaded by 1Ω the SS pin will
drop to 0.5V, regulating the output at 3V (typical current
limit time load, 3A • 1Ω). Once the overload condition is
removed, the output will soft-start from the temporary
voltage level to the normal regulation point.
Since the SS pin is clamped at 2V and has to discharge to
0.9V before taking control of regulation, momentary overload conditions will be tolerated without a soft-start recovery. The typical time before the SS pin takes control is:
C • 1.1V
t SS(CONTROL) = SS
600μA
Power Good Indicators
The PG and PG pins are collector outputs of an internal
comparator. The comparator compares the voltages of
the FB and LFB pins to 90% of the reference voltage with
30mV of hysterisis.
The PG pin has a sink capability of 400μA when the FB and
LFB pins are below the threshold and can withstand 40V
when the outputs are in regulation. The PG pin is typically
connected to the output with a resistor and is used as an
error flag. The resistor value should be chosen to allow the
PG voltage to drop below 0.4V in an error condition.
Example:
VOUT1 = 5V, PGSINK(MIN) = 200μA
RPG = (5 – 0.4)/200μA = 23kΩ
The PG pin has a sink capability of 800μA when the FB
and LFB pins are above the threshold and can withstand
40V when the outputs are not in regulation. The PG pin is
typically used as a drive signal for an output disconnect
device. The PG pull-up resistor should be sized in the
same manner as the PG pull-up resistor.
Linear Regulator
The LT3500 contains an error amplifier and a NPN output
device which can be configured as a linear regulator or as
a linear regulator controller.
With the LFB and LDRV pins configured as shown in
Figure 1, the LDRV pin outputs a regulated voltage with a
typical current limit of 13mA.
The LDRV voltage is programmed with a resistor divider
between the output and the LFB pin. Choose the 1% resistors according to:
V
R3 = R4 LDRV – 1
0.8V
R4 should be 10.0k or less to avoid bias current errors.
Reference designators refer to the Block Diagram in
Figure 1.
The reference voltage for the linear regulator (LFB pin)
will track the SS pin in the same manner as the FB pin of
the switching regulator.
3500fc
21
LT3500
APPLICATIONS INFORMATION
NMOS times the output current of the regulator. This also
increases the overall efficiency of the system. However,
the minimum VIN increases to 2V plus the VGS at full load
of the transistor. Additionally, due to a lack of beta current
limiting, a shorted output can cause the switcher output
of the LT3500 to collapse.
VOUT2
AC COUPLED
20mV/DIV
LOAD STEP
2.5mA TO 7.5mA
5mA/DIV
To compensate the linear regulator, simply add a ceramic
capacitor from the LDRV pin to ground. Typical values
range from 0.01μF to 1μF. Figure 9 illustrates the transient
response with a 0.47μF output capacitor.
Since the collector of the LDRV npn is connected internally
to VIN, you must consider the impact of LDRV current on
efficiency and die temperature when configuring the linear
regulator/controller. For example, with VIN = 25V, LDRV =
3.3V and ILDRV = 10mA, power dissipation on the die will
be 217mW. For a typical 3.3V/1A switcher application,
this represents an additional 7% efficiency loss and approximately 10 degrees rise in die temperature.
Linear Controller
If the linear output of the LT3500 is not used, the LDRV
pin should be shorted to the LFB Pin.
3500 F09
20μs/DIV
Figure 9. Linear Regulator Transient Response
By adding an external follower (NPN or NMOS), the LFB
and LDRV pins can be configured as a controller (Figure 10) for a low dropout regulator with increased output
capability.
The output current capability of Figure 10’s circuit is a
product of the LDRV current limit and beta of the external
NPN which is normally less than the current capability of
the LT3500. The dropout voltage for the circuit is set by the
saturation voltage of the external NPN, which is typically
300mV. The minimum VIN for the circuit to function properly is 2V plus the base emitter drop of the external NPN.
Replacing the NPN in Figure 10 with a NMOS transistor
can reduce the dropout voltage down to the RDS(ON) of the
PCB Layout
For proper operation and minimum EMI, care must be
taken during printed circuit board (PCB) layout. Figure 11
shows the high di/dt paths in the buck regulator circuit.
Note that large switched currents flow in the power switch,
the catch diode and the input capacitor. The loop formed
by these components should be as small as possible.
These components, along with the inductor and output
capacitor, should be placed on the same side of the circuit
board and their connections should be made on that layer.
Place a local, unbroken ground plane below these components, and tie this ground plane to system ground at
one location, ideally at the ground terminal of the output
D2
BAT54
4.5V TO 36V
C1
2.2μF
VIN
BST
LT3500
C5
L1
0.47μF 3.3μH
SW
C2
0.47μF
C3
220pF
R6
40.2k
D1
B240A
SHDN
SS
R1
27.4k
R2
8.06k
C7
22μF
VOUT1
3.5V
FB
RT/SYNC LDRV
PG
VC
PG
LFB
R5
49.9k
R3
24.9k
VOUT2
3.3V
1A
C6
22μF
R4
8.06k
3500 F10
Figure 10. Linear Controller
3500fc
22
LT3500
APPLICATIONS INFORMATION
VIN LT3500 SW
VIN LT3500 SW
VIN LT3500 SW
GND
GND
GND
3500 F11
(11a)
(11b)
(11c)
Figure 11. Subtracting the Current when the Switch is On (11a) from the Current when the Switch is Off (11b) Reveals the Path of the
High Frequency Switching Current (11c). Keep this Loop Small. The Voltage on the SW and BST Traces will Also be Switched; Keep
These Traces as Short as Possible. Finally, Make Sure the Circuit is Shielded with a Local Ground Plane
should be tied to other copper layers below with thermal
vias; these layers will spread the heat dissipated by the
LT3500. Place additional vias near the catch diodes. Adding
more copper to the top and bottom layers and tying this
copper to the internal planes with vias can further reduce
thermal resistance. With these steps, the thermal resistance from die (or junction) to ambient can be reduced to
θJA = 45°C/W for the DD Package, and θJA = 45°C/W for
the MSE Package.
Power dissipation within the LT3500 can be estimated
by calculating the total power loss from an efficiency
measurement and subtracting the catch diode loss. The
die temperature is calculated by multiplying the LT3500
power dissipation by the thermal resistance from junction
to ambient.
Figure 12. LT3500 Demonstration Circuit Board DC1069A
capacitor C2. Additionally, the SW and BST traces should
be kept as short as possible. The topside metal from the
DC1069A demonstration board in Figure 12 illustrates
proper component placement and trace routing.
Thermal Considerations
The PCB must also provide heat sinking to keep the
LT3500 cool. The exposed metal on the bottom of the
package must be soldered to a ground plane. This ground
The power dissipation in the other power components
such as catch diodes, boost diodes and inductors, cause
additional copper heating and can further increase what
the IC sees as ambient temperature. See the LT1767 data
sheet’s Thermal Considerations section.
Other Linear Technology Publications
Application notes AN19, AN35 and AN44 contain more
detailed descriptions and design information for buck
regulators and other switching regulators. The LT1376
data sheet has a more extensive discussion of output
ripple, loop compensation and stability testing. Design
note DN100 shows how to generate a dual (+ and –) output
supply using a buck regulator.
3500fc
23
LT3500
TYPICAL APPLICATIONS
High Efficiency Linear Regulator
Efficiency vs Load Current
90
D2
BAT54
VIN
C1
2.2μF
BST
L1
C5
0.47μF 3.3μH
LT3500
80
EFFICIENCY (%)
4.5V TO 36V
SW
D1
B240A
SHDN
SS
C2
0.47μF
R1
25.5k
FB
RT/SYNC LDRV
C3
220pF
R6
40.2k
C7
22μF
R2
8.06k
VC
R5
49.9k
GND
PG
PG
LFB
R3
24.9k
M1
ZXMN2A03E6
R7
10k
70
60
VOUT2
3.3V
50
C8
22μF
R4
8.06k
0
0.4 0.6 0.8 1.0 1.2
LOAD CURRENT (A)
0.2
1.4
3500 TA02a
3500 TA02b
5V/1.5A, 3.3V/0.5A Step-Down with Output Disconnect
D2
BAT54
6V TO 32V
VIN
C1
2.2μF
BST
LT3500
L1
C5
0.47μF 4.7μH
SW
D1
B240A
SHDN
SS
C2
0.47μF
R1
42.2k
R2
8.06k
C7
22μF
FB
Q1
ZXTCM322
RT/SYNC LDRV
C3
220pF
R7
40.2k
R8
100k
VOUT1
5V
1.5A
VC
R6
49.9k
GND
PG
PG
LFB
R5
8.06k
C6
22μF
R4
24.9k
VOUT2
3.3V
0.5A
I89
ZXMP3A17E6
3500 TA03
5V/2A Step-Down with Power Good LED
D2
BAT54
6V TO 32V
VIN
C1
2.2μF
BST
LT3500
L1
C5
0.47μF 4.7μH
SW
C2
0.47μF
C3
220pF
R7
40.2k
D1
B240A
SHDN
SS
FB
PG
RT/SYNC LDRV
VC
R6
49.9k
GND
LFB
PG
R1
42.2k
R2
8.06k
C7
22μF
VOUT1
5V
2A
R8
8.06k
R4
R3
42.2k 8.06k
C8
1μF
R5
100k
M1
ZXM61N02F
1
3500 TA04
3500fc
24
LT3500
PACKAGE DESCRIPTION
DD Package
12-Lead Plastic DFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1725 Rev A)
0.70 ±0.05
3.50 ±0.05
2.10 ±0.05
2.38 ±0.05
1.65 ±0.05
PACKAGE
OUTLINE
0.25 ± 0.05
0.45 BSC
2.25 REF
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
3.00 ±0.10
(4 SIDES)
R = 0.115
TYP
7
0.40 ± 0.10
12
2.38 ±0.10
1.65 ± 0.10
PIN 1 NOTCH
R = 0.20 OR
0.25 × 45°
CHAMFER
PIN 1
TOP MARK
(SEE NOTE 6)
6
0.200 REF
1
0.23 ± 0.05
0.45 BSC
0.75 ±0.05
2.25 REF
(DD12) DFN 0106 REV A
0.00 – 0.05
BOTTOM VIEW—EXPOSED PAD
NOTE:
1. DRAWING IS NOT A JEDEC PACKAGE OUTLINE
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 AND TIE BARS SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE
TOP AND BOTTOM OF PACKAGE
3500fc
25
LT3500
PACKAGE DESCRIPTION
MSE Package
16-Lead Plastic MSOP, Exposed Die Pad
(Reference LTC DWG # 05-08-1667 Rev A)
BOTTOM VIEW OF
EXPOSED PAD OPTION
2.845 p 0.102
(.112 p .004)
5.23
(.206)
MIN
2.845 p 0.102
(.112 p .004)
0.889 p 0.127
(.035 p .005)
8
1
1.651 p 0.102
(.065 p .004)
1.651 p 0.102 3.20 – 3.45
(.065 p .004) (.126 – .136)
0.305 p 0.038
(.0120 p .0015)
TYP
16
0.50
(.0197)
BSC
4.039 p 0.102
(.159 p .004)
(NOTE 3)
RECOMMENDED SOLDER PAD LAYOUT
0.254
(.010)
0.35
REF
0.12 REF
DETAIL “B”
CORNER TAIL IS PART OF
DETAIL “B” THE LEADFRAME FEATURE.
FOR REFERENCE ONLY
9
NO MEASUREMENT PURPOSE
0.280 p 0.076
(.011 p .003)
REF
16151413121110 9
DETAIL “A”
0o – 6o TYP
3.00 p 0.102
(.118 p .004)
(NOTE 4)
4.90 p 0.152
(.193 p .006)
GAUGE PLANE
0.53 p 0.152
(.021 p .006)
1234567 8
DETAIL “A”
1.10
(.043)
MAX
0.18
(.007)
SEATING
PLANE
0.17 – 0.27
(.007 – .011)
TYP
0.50
(.0197)
BSC
NOTE:
1. DIMENSIONS IN MILLIMETER/(INCH)
2. DRAWING NOT TO SCALE
3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.
MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.
INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
0.86
(.034)
REF
0.1016 p 0.0508
(.004 p .002)
MSOP (MSE16) 0608 REV A
3500fc
26
LT3500
REVISION HISTORY
(Revision history begins at Rev C)
REV
DATE
DESCRIPTION
PAGE NUMBER
C
01/11
Corrected Pin Configuration for MSE package
2
3500fc
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
LT3500
TYPICAL APPLICATION
1.8V/2A Step-Down Regulator
4.5V TO 36V
C1
2.2μF
VIN
VOUT2
3.3V
10mA
LDRV
R3
R4
24.9k 8.06k
LT3500
C6
1μF
D2
LFB
C2
R5 0.47μF
40.2k
C3
220pF
SHDN
SS
BST
C5
0.47μF
RT/SYNC SW
FB
VC
PG
PG
D1
L1
2.2μH
R6
49.9k
R1
10k
R2
8.06k
VOUT1
1.8V
2A
C7
22μF
3500 TA05
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LT3437
60V, 400mA (IOUT), Micropower Step-Down DC/DC Converter with
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VIN: 3.3V to 60V, VOUT(MIN) = 1.25V, IQ = 100μA, ISD < 1μA,
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LT3493
36V, 1.4A (IOUT), 750kHz High Efficiency Step-Down DC/DC
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LT3501
Dual 25V, 3A (IOUT), 1.5MHz High Efficiency Step-Down DC/DC
Converter
VIN: 3.3V to 25V, VOUT(MIN) = 0.8V, IQ = 3.7mA, ISD < 10μA,
20-Lead TSSOPE Package
LT3502/LT3502A
40V, 500mA (IOUT), 1.1MHz/2.2MHz High Efficiency Step-Down
DC/DC Converter
VIN: 3V to 40V, VOUT(MIN) = 0.8V, IQ = 1.5mA, ISD < 2μA,
8-Lead 2mm × 2mm DFN Package
LT3503
20V, 1A (IOUT), 2.2MHz High Efficiency Step-Down DC/DC Converter VIN: 3.6V to 20V, VOUT(MIN) = 0.78V, IQ = 1.9mA, ISD < 1μA,
6-Lead 2mm × 3mm DFN Package
LT3505
36V, 1.2A (IOUT), 3MHz High Efficiency Step-Down DC/DC Converter VIN: 3.6V to 36V, VOUT(MIN) = 0.78V, IQ = 2mA, ISD < 2μA,
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LT3506/LT3506A
Dual 25V, 1.6A (IOUT), 575kHz/1.1MHz High Efficiency Step-Down
DC/DC Converters
VIN: 3.6V to 25V, VOUT(MIN) = 0.8V, IQ = 3.8mA, ISD < 30μA,
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LT3508
Dual 36V, 1.4A (IOUT), 2.5MHz High Efficiency Step-Down DC/DC
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Dual 25V, 2A (IOUT), 1.5MHz High Efficiency Step-Down DC/DC
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VIN: 3.3V to 25V, VOUT(MIN) = 0.8V, IQ = 3.7mA, ISD < 10μA,
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Dual 400mA/800mA, 2.25MHz Synchronous Step-Down DC/DC
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VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 40μA, ISD < 1μA,
3mm × 3mm DFN and 10-Lead MSE Packages
LT3680
36V, 3.5A (IOUT), 2.4MHz High Efficiency Step-Down DC/DC
Converter
VIN: 3.6V to 36V, VOUT(MIN) = 0.79V, IQ = 75μA, ISD < 1μA,
3mm × 3mm DFN and MS10E Packages
ThinSOT is a trademark of Linear Technology Corporation
3500fc
28 Linear Technology Corporation
LT 0111 REV C • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
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FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2007