LT3502/LT3502A - 1.1MHz/2.2MHz, 500mA Step-Down Regulators in 2mm × 2mm DFN and MS10

LT3502/LT3502A
1.1MHz/2.2MHz, 500mA
Step-Down Regulators in
2mm × 2mm DFN and MS10
Description
Features
3V to 40V Input Voltage Range
n 500mA Output Current
n Switching Frequency: 2.2MHz (LT3502A),
1.1MHz (LT3502)
n 800mV Feedback Voltage
n Short-Circuit Robust
nSoft-Start
n Low Shutdown Current: <2µA
n Internally Compensated
n Internal Boost Diode
n Thermally Enhanced 2mm × 2mm 8-Lead DFN
and 10-Lead MS10 Package
The LT®3502/LT3502A are current mode PWM step-down
DC/DC converters with an internal 500mA power switch,
in tiny 8-lead 2mm × 2mm DFN and 10-lead MS10
packages. The wide input voltage range of 3V to 40V makes
the LT3502/LT3502A suitable for regulating power from a
wide variety of sources, including 24V industrial supplies
and automotive batteries. Its high operating frequency
allows the use of tiny, low cost inductors and capacitors,
resulting in a very small solution. Constant frequency
above the AM band avoids interfering with radio reception,
making the LT3502A particularly suitable for automotive
applications.
n
Cycle-by-cycle current limit and frequency foldback
provide protection against shorted outputs. Soft-start
and frequency foldback eliminates input current surge
during start-up. DA current sense provides further protection in fault conditions. An internal boost diode reduces
component count.
Applications
n
n
n
n
Automotive Systems
Battery-Powered Equipment
Wall Transformer Regulation
Distributed Supply Regulation
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
LT3502A 12VIN Efficiency
3.3V Step-Down Converter
90
80
BD
BOOST
VIN
1µF
SW
0.1µF 6.8µH
LT3502A
VOUT
3.3V
500mA
DA
OFF ON
SHDN
GND
31.6k
FB
10k
5VOUT
70
10µF
EFFICIENCY (%)
VIN
4.7V TO 40V
3.3VOUT
60
50
40
30
20
10
3502 TA01a
0
0
0.1
0.3
0.2
LOAD CURRENT (A)
0.4
0.5
3502 TA01b
3502fd
1
LT3502/LT3502A
Absolute Maximum Ratings
(Note 1)
Input Voltage (VIN).....................................................40V
BOOST Voltage..........................................................50V
BOOST Pin Above SW Pin............................................7V
FB Voltage....................................................................6V
SHDN Voltage............................................................40V
BD Voltage...................................................................7V
Operating Junction Temperature Range (Note 2)
LT3502AE, LT3502E........................... –40°C to 125°C
LT3502AI, LT3502I............................. –40°C to 125°C
Storage Temperature Range................... –65°C to 150°C
Pin Configuration
TOP VIEW
BD 2
FB 3
TOP VIEW
8 SW
VIN 1
9
SHDN 4
SW
BOOST
NC
DA
GND
7 BOOST
6 DA
5 GND
10
9
8
7
6
1
2
3
4
5
VIN
NC
BD
FB
SHDN
MS PACKAGE
10-LEAD PLASTIC MSOP
DC PACKAGE
8-LEAD (2mm × 2mm) PLASTIC DFN
θJA = 110°C/W
θJA = 102°C/W
EXPOSED PAD (PIN 9) IS GND, MUST BE SOLDERED TO PCB
Order Information
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3502EDC#PBF
LT3502EDC#TRPBF
LCLV
8-Lead 2mm × 2mm Plastic DFN
–40°C to 125°C
LT3502IDC#PBF
LT3502IDC#TRPBF
LCLV
8-Lead 2mm × 2mm Plastic DFN
–40°C to 125°C
LT3502AEDC#PBF
LT3502AEDC#TRPBF
LCLT
8-Lead 2mm × 2mm Plastic DFN
–40°C to 125°C
LT3502AIDC#PBF
LT3502AIDC#TRPBF
LCLT
8-Lead 2mm × 2mm Plastic DFN
–40°C to 125°C
LT3502EMS#PBF
LT3502EMS#TRPBF
LTDTR
10-Lead Plastic MSOP
–40°C to 125°C
LT3502IMS#PBF
LT3502IMS#TRPBF
LTDTR
10-Lead Plastic MSOP
–40°C to 125°C
LT3502AEMS#PBF
LT3502AEMS#TRPBF
LTDTS
10-Lead Plastic MSOP
–40°C to 125°C
LT3502AIMS#PBF
LT3502AIMS#TRPBF
LTDTS
10-Lead Plastic MSOP
–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/
2
3502fd
LT3502/LT3502A
Electrical
Characteristics
The
l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 10V, VSHDN = 5V, VBOOST = 15V.
PARAMETER
CONDITIONS
MIN
Undervoltage Lockout
2.6
TYP
MAX
UNITS
2.8
3
V
Quiescent Current at Shutdown
VSHDN = 0V
0.5
2
µA
Quiescent Current
Not Switching
1.5
2
mA
Feedback Voltage
2mm × 2mm DFN
2mm × 2mm DFN
MS10
MS10
l
0.8
0.8
0.8
0.8
0.813
0.81
0.816
0.813
FB Pin Bias Current
(Note 5)
l
Switching Frequency
IDA < 500mA (LT3502A)
IDA < 500mA (LT3502A)
IDA < 500mA (LT3502)
IDA < 500mA (LT3502)
l
0.785
0.79
0.780
0.786
Reference Voltage Line Regulation
0.005
l
l
%/V
15
50
nA
1.9
1.8
0.9
0.8
2.25
2.25
1.1
1.1
2.7
2.8
1.3
1.4
MHz
MHz
MHz
MHz
70
80
80
90
%
%
450
mV
Maximum Duty Cycle
100mA Load (LT3502A)
100mA Load (LT3502)
Switch VCESAT
ISW = 500mA
Switch Current Limit
(Note 3)
Switch Active Current
SW = 10V (Note 4)
SW = 0V (Note 5)
BOOST Pin Current
ISW = 500mA
Minimum BOOST Voltage Above Switch
ISW = 500mA
BOOST Schottky Forward Drop
IOUT = 100mA
0.8
1
DA Pin Current to Stop OSC
SHDN Bias Current
0.75
500
VSHDN = 5V
VSHDN = 0V
SHDN Input Voltage High
SHDN Input Voltage Low
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 LT3502EDC and LT3502AEDC are guaranteed to meet
performance specifications from 0°C to 125°C junction temperature
range. Specifications over the –40°C to 125°C operating junction
temperature range are assured by design, characterization and correlation
V
V
V
V
0.9
1.1
A
95
8
130
30
µA
µA
10
13
mA
1.9
2.2
V
650
55
V
mA
80
1
2
µA
µA
V
0.3
V
with statistical process controls. The LT3502IDC and LT3502AIDC are
guaranteed over the – 40°C to 125°C operating junction temperature
range.
Note 3: Current limit guaranteed by design and/or correlation to static test.
Slope compensation reduces current limit at higher duty cycle.
Note 4: Current flows into pin.
Note 5: Current flows out of pin.
3502fd
3
LT3502/LT3502A
Typical Performance Characteristics
LT3502A 3.3VOUT Efficiency
70
80
12VIN
70
24VIN
EFFICIENCY (%)
60
LT3502 3.3VOUT Efficiency
100
90
12VIN
80
EFFICIENCY (%)
LT3502A 5VOUT Efficiency
50
40
30
80
60
50
40
30
40
30
20
10
10
0.1
0.3
0.2
LOAD CURRENT (A)
0
0.5
0.4
0
0.1
0.3
0.2
LOAD CURRENT (A)
0
0.5
0.4
40
30
MINIMUM
0.7
0.6
0.5
0.4
0.3
0.7
0.5
0.4
0.3
20
0.2
10
0.1
0.1
0
0
0.1
0.3
0.4
0.2
LOAD CURRENT (A)
0.5
0
10
20
VIN (V)
30
3502 G04
LT3502 Maximum Load Current
VOUT = 3.3V, L = 15µH
LT3502 Maximum Load Current
VOUT = 5V, L = 22µH
0.8
TYPICAL
0.7
MINIMUM
0.6
0.5
0.4
0.3
0.6
0.1
20
30
40
VIN (V)
MINIMUM
500
0
40
25°C
400
–40°C
125°C
300
200
100
0
10
20
30
40
0
0
0.2
0.4
0.6
0.8
1.0
SWITCH CURRENT (A)
VIN (V)
3502 G07
4
600
0.3
0.1
10
TYPICAL
0.5
0.2
0
30
Switch Voltage Drop
0.4
0.2
0
20
VIN (V)
700
VCE (mV)
0.8
10
3502 G06
0.9
0.7
0
3502 G05
LOAD CURRENT (A)
0.9
0
40
MINIMUM
0.6
0.2
0
TYPICAL
0.8
LOAD CURRENT (A)
LOAD CURRENT (A)
50
0.9
TYPICAL
0.8
60
0.5
1.0
0.9
24VIN
70
0.3
0.4
0.2
LOAD CURRENT (A)
LT3502A Maximum Load Current
VOUT = 5V, L = 10µH
1.0
12VIN
80
0.1
3502 G03
LT3502A Maximum Load Current
VOUT = 3.3V, L = 6.8µH
100
90
0
3502 G02
LT3502 5VOUT Efficiency
EFFICIENCY (%)
50
10
0
24VIN
12VIN
60
20
3502 G01
LOAD CURRENT (A)
70
20
0
5VIN
90
24VIN
EFFICIENCY (%)
90
(TA = 25°C unless otherwise noted)
3502 G08
3502 G09
3502fd
LT3502/LT3502A
Typical Performance Characteristics
UVLO
(TA = 25°C unless otherwise noted)
Switching Frequency
3.5
Soft-Start (SHDN)
0.9
2.5
LT3502A
3.0
2.0
1.5
1.0
1.5
LT3502
1.0
0.5
0.5
0
50
0
–50
150
100
TEMPERATURE (°C)
0
50
100
TEMPERATURE (°C)
3502 G10
0.5
0.4
0.3
0.2
0.1
–0.1
150
200 400 600 800 1000 1200 1400 1600
SHDN PIN VOLTAGE (mV)
3502 G12
Switch Current Limit
SHDN Pin Current
Switch Current Limit
1.0
1.2
0.9
CURRENT LIMIT (A)
0.8
200
150
100
1.0
SW PEAK CURRENT LIMIT
0.7
CURRENT LIMIT (A)
250
DA VALLEY CURRENT LIMIT
0.6
0.5
0.4
0.3
0.2
50
0
5
10
15 20 25 30 35
SHDN PIN VOLTAGE (V)
40
0
–50
45
0
0
150
50
100
TEMPERATURE (°C)
40
40
35
35
VIN (V)
30
TA = 25°C
25
TA = 85°C
20
15
15
10
10
5
5
5
0.2 0.3 0.4 0.5
LOAD CURRENT (A)
0.6
0.7
3502 G16
0
0
0.1
0.2 0.3 0.4 0.5
LOAD CURRENT (A)
0.6
0.7
3502 G17
TA = 85°C
20
15
0.1
TA = 25°C
25
10
0
100
45
30
20
50
DUTY CYCLE (%)
LT3502 Maximum VIN for Full
Frequency (VOUT = 3.3V)
VIN (V)
TA = 85°C
25
0
3502 G15
45
TA = 25°C
30
0.4
LT3502A Maximum VIN for Full
Frequency (VOUT = 5V)
45
40
LT3502A
0.6
3502 G14
LT3502A Maximum VIN for Full
Frequency (VOUT = 3.3V)
35
LT3502
0.8
0.2
0.1
3502 G13
0
0
3502 G11
300
SHDN PIN CURRENT (µA)
0.7
0.6
0
0
–50
VIN (V)
SWITCH CURRENT LIMIT (A)
FREQUENCY (MHz)
VIN (V)
2.5
0
0.8
2.0
0
0
0.1
0.2 0.3 0.4 0.5
LOAD CURRENT (A)
0.6
0.7
3502 G18
3502fd
5
LT3502/LT3502A
Typical Performance Characteristics
LT3502A Typical Minimum Input
Voltage (VOUT = 3.3V)
LT3502A Typical Minimum Input
Voltage (VOUT = 5V)
8
7
6
7
6
6
5
4
3
VIN (V)
5
VIN (V)
VIN (V)
LT3502 Typical Minimum Input
Voltage (VOUT = 3.3V)
7
5
4
3
2
0.1
0.01
LOAD CURRENT (A)
1
0
0.001
0.01
0.1
LOAD CURRENT (A)
LT3502 Typical Minimum Input
Voltage (VOUT = 5V)
5
3
3502 G21
IL
200mA/DIV
VOUT
20mV/DIV
VOUT
20mV/DIV
2
1
1
1
VSW
5V/DIV
IL
200mA/DIV
4
0.1
0.01
LOAD CURRENT (A)
Discontinuous Mode Waveform
VSW
5V/DIV
6
0.01
0.1
LOAD CURRENT (A)
0
0.001
Continuous Mode Waveform
7
0
0.001
1
3502 G20
3502 G19
8
3
1
1
0
0.001
4
2
2
1
VIN (V)
(TA = 25°C unless otherwise noted)
VIN = 12V
VOUT = 3.3V
L = 6.8µH
COUT = 10µF
IOUT = 250mA
200ns/DIV
3502 G23
VIN = 12V
VOUT = 3.3V
L = 6.8µH
COUT = 10µF
IOUT = 30mA
200ns/DIV
3502 G24
3502 G22
6
3502fd
LT3502/LT3502A
Pin Functions
(DFN/MS)
VIN (Pin 1/Pin 10): The VIN pin supplies current to the
LT3502/LT3502A’s internal regulator and to the internal
power switch. This pin must be locally bypassed.
BD (Pin 2/Pin 8): The BD pin is used to provide current
to the internal boost Schottky diode.
FB (Pin 3/Pin 7): The LT3502/LT3502A regulate their
feedback pin to 0.8V. Connect the feedback resistor divider tap to this pin. Set the output voltage according to
VOUT = 0.8 (1 + R1/R2). A good value for R2 is 10k.
SHDN (Pin 4/Pin 6): The SHDN pin is used to put the
LT3502 in shutdown mode. Tie to ground to shut down
the LT3502/LT3502A. Tie to 2V or more for normal
operation. If the shutdown feature is not used, tie this pin
to the VIN pin. The SHDN pin also provides soft-start and
frequency foldback. To use the soft-start feature, connect
R3 and C4 to the SHDN pin. SHDN Pin voltage should
not be higher than VIN.
GND (Pin 5/Pin 5): Ground Pin.
DA (Pin 6/Pin 4): Connect the catch diode (D1) anode to
this pin. This pin is used to provide frequency foldback
in extreme situations.
BOOST (Pin 7/Pin 2): The BOOST pin is used to provide a
drive voltage, higher than the input voltage, to the internal
bipolar NPN power switch. Connect a boost capacitor from
this pin to SW Pin.
SW (Pin 8/Pin 1): The SW pin is the output of the internal
power switch. Connect this pin to the inductor, catch diode
and boost capacitor.
3502fd
7
8
ON OFF
VIN
C4
R3
C2
4
1
SHDN
VIN
SLOPE
COMP
FREQUENCY
FOLDBACK
OSC
INT REG
AND
UVLO
∑
VC
gm
S
R
R2
0.8V
Q
Q
3
R1
FB
DRIVER
Q1
GND
DA
SW
BOOST
BD
5
6
8
7
2
C1
L1
D1
C3
3502 BD
VOUT
LT3502/LT3502A
Block Diagram
3502fd
LT3502/LT3502A
Operation
The LT3502/LT3502A are constant frequency, current
mode step-down regulators. An oscillator enables an RS
flip-flop, turning on the internal 500mA power switch Q1.
An amplifier and comparator monitor the current flowing
between the VIN and SW pins, turning the switch off when
this current reaches a level determined by the voltage at
VC. An error amplifier measures the output voltage through
an external resistor divider tied to the FB pin and servos
the VC node. If the error amplifier’s output increases, more
current is delivered to the output; if it decreases, less
current is delivered. An active clamp (not shown) on the VC
node provides current limit. The VC node is also clamped
to the voltage on the SHDN pin; soft-start is implemented
by generating a voltage ramp at the SHDN pin using an
external resistor and capacitor. The SHDN pin voltage
during soft-start also reduces the oscillator frequency to
avoid hitting current limit during start-up.
An internal regulator provides power to the control circuitry. This regulator includes an undervoltage lockout to
prevent switching when VIN is less than ~3V. The SHDN
pin is used to place the LT3502/LT3502A in shutdown,
disconnecting the output and reducing the input current
to less than 2µA.
The switch driver operates from either VIN or from the
BOOST pin. An external capacitor and the internal diode
are used to generate a voltage at the BOOST pin that is
higher than the input supply. This allows the driver to
fully saturate the internal bipolar NPN power switch for
efficient operation.
A comparator monitors the current flowing through
the catch diode via the DA pin and reduces the LT3502/
LT3502A’s operating frequency when the DA pin current
exceeds the 650mA valley current limit. This frequency
foldback helps to control the output current in fault
conditions such as shorted output with high input voltage. The DA comparator works in conjunction with the
switch peak current limit comparator to determine the
maximum deliverable current of the LT3502/LT3502A. The
peak current limit comparator is used in normal current
mode operations and is used to turn off the switch. The DA
valley current comparator monitors the catch diode current
and will delay switching until the catch diode current is
below the 650mA limit. Maximum deliverable current to
the output is therefore limited by both switch peak current
limit and DA valley current limit.
3502fd
9
LT3502/LT3502A
Applications Information
FB Resistor Network
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 ⎜ OUT – 1⎟
⎝ 0.8V ⎠
R2 should be 20k or less to avoid bias current errors.
Reference designators refer to the Block Diagram.
Input Voltage Range
The input voltage range for the LT3502/LT3502A applications depends on the output voltage and on the absolute
maximum ratings of the VIN and BOOST pins.
The minimum input voltage is determined by either the
LT3502/LT3502A’s minimum operating voltage of 3V, or
by its maximum duty cycle. The duty cycle is the fraction
of time that the internal switch is on and is determined
by the input and output voltages:
VOUT + VD
DC =
VIN – VSW + VD
where VD is the forward voltage drop of the catch diode
(~0.4V) and VSW is the voltage drop of the internal switch
(~0.45V at maximum load). This leads to a minimum input
voltage of:
V +V
VIN(MIN) = OUT D – VD + VSW
DCMAX
with DCMAX = 0.80 for the LT3502A and 0.90 for the
LT3502.
The maximum input voltage is determined by the
absolute maximum ratings of the VIN and BOOST pins. For
fixed frequency operation, the maximum input voltage is
determined by the minimum duty cycle DCMIN:
VIN(MAX) =
VOUT + VD
– VD + VSW
DCMIN
DCMIN = 0.15 for the LT3502A and 0.08 for the LT3502.
10
Note that this is a restriction on the operating input voltage for fixed frequency operation; the circuit will tolerate
transient inputs up to the absolute maximum ratings of
the VIN and BOOST pins. The input voltage should be
limited to the VIN operating range (40V) during overload
conditions.
Minimum On-Time
The LT3502/LT3502A will still regulate the output at input
voltages that exceed VIN(MAX) (up to 40V), however, the
output voltage ripple increases as the input voltage is
increased.
As the input voltage is increased, the part is required to
switch for shorter periods of time. Delays associated with
turning off the power switch dictate the minimum on-time
of the part. The minimum on-time for the LT3502/LT3502A
is 60ns (Figure 1).
VSW
20V/DIV
IL
500mA/DIV
VOUT
100mV/DIV
1µs/DIV
VIN = 33V, VOUT = 3.3V
L = 6.8µH, COUT = 10µF, IOUT = 250mA
3502 F01
Figure 1. Continuous Mode Operation Near
Minimum On-Time of 60ns
When the required on-time decreases below the minimum on-time of 60ns, instead of the switch pulse width
becoming narrower to accommodate the lower duty cycle
requirement, the switch pulse width remains fixed at
60ns. The inductor current ramps up to a value exceeding the load current and the output ripple increases. The
part then remains off until the output voltage dips below
the programmed value before it begins switching again
(Figure 2).
Provided that the load can tolerate the increased output
voltage ripple and that the components have been properly
selected, operation above VIN(MAX) is safe and will not
damage the part.
3502fd
LT3502/LT3502A
Applications Information
VSW
20V/DIV
VSW
20V/DIV
IL
500mA/DIV
IL
500mA/DIV
VOUT
100mV/DIV
VOUT
100mV/DIV
1µs/DIV
VIN = 40V, VOUT = 3.3V
L = 6.8µH, COUT = 10µF, IOUT = 250mA
1µs/DIV
3502 F02
VIN = 40V, VOUT = 3.3V
L = 6.8µH, COUT = 10µF, IOUT = 500mA
Figure 2. Pulse-Skipping Occurs when
Required On-Time is Below 60ns
Figure 3. Pulse-Skipping with Large Load Current Will be
Limited by the DA Valley Current Limit. Notice the Flat Inductor
Valley Current and Reduced Switching Frequency
As the input voltage increases, the inductor current ramps
up quicker, the number of skipped pulses increases and
the output voltage ripple increases. For operation above
VIN(MAX) the only component requirement is that the
components be adequately rated for operation at the
intended voltage levels.
Inductor current may reach current limit when operating
in pulse-skipping mode with small valued inductors. In
this case, the LT3502/LT3502A will periodically reduce its
frequency to keep the inductor valley current to 650mA
(Figure 3). Peak inductor current is therefore peak current
plus minimum switch delay:
VIN – VOUT
• 60ns
L
The part is robust enough to survive prolonged operation
under these conditions as long as the peak inductor current does not exceed 1.2A. Inductor current saturation
and junction temperature may further limit performance
during this operating regime.
900mA +
3502 F03
Inductor Selection and Maximum Output Current
A good first choice for the inductor value is:
L = 1.6(VOUT + VD) for the LT3502A
L = 4.6(VOUT + VD) for the LT3502
where VD is the voltage drop of the catch diode (~0.4V) and
L is in µH. With this value there will be no subharmonic
oscillation for applications with 50% or greater duty cycle.
The inductor’s RMS current rating must be greater than the
maximum load current and its saturation current should
be about 30% higher. For robust operation during fault
conditions, the saturation current should be above 1.2A.
To keep efficiency high, the series resistance (DCR) should
be less than 0.1Ω. Table 1 lists several vendors and types
that are suitable.
There are several graphs in the Typical Performance
Characteristics section of this data sheet that show the
maximum load current as a function of input voltage and
inductor value for several popular output voltages. Low
inductance may result in discontinuous mode opera-
Table 1
VENDOR
Sumida
URL
www.sumida.com
Toko
www.toko.com
Würth Elektronik
www.we-online.com
PART SERIES
CDRH4D28
CDRH5D28
CDRH8D28
A916CY
D585LC
WE-TPC(M)
WE-PD2(M)
WE-PD(S)
INDUCTANCE RATE (µH)
1.2 to 4.7
2.5 to 10
2.5 to 33
2 to 12
1.1 to 39
1 to 10
2.2 to 22
1 to 27
SIZE (mm)
4.5 × 4.5
5.5 × 5.5
8.3 × 8.3
6.3 × 6.2
8.1 × 8
4.8 × 4.8
5.2 × 5.8
7.3 × 7.3
3502fd
11
LT3502/LT3502A
Applications Information
tion, which is okay, but further reduces maximum load
current. For details of the maximum output current and
discontinuous mode operation, see Linear Technology
Application Note 44.
Catch Diode
A low capacitance 500mA Schottky diode is recommended
for the catch diode, D1. The diode must have a reverse
voltage rating equal to or greater than the maximum input
voltage. The Diodes Inc. SBR1U40LP, ON Semi MBRM140,
and Diodes Inc. DFLS140 are good choices for the catch
diode.
Output Capacitor
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated by
the LT3502/LT3502A to produce the DC output. In this role
it determines the output ripple so low impedance at the
switching frequency is important. The second function is to
store energy in order to satisfy transient loads and stabilize
the LT3502/LT3502A’s control loop. Ceramic capacitors
have very low equivalent series resistance (ESR) and
provide the best ripple performance. A good value is:
Input Capacitor
Bypass the input of the LT3502/LT3502A circuit with a 1µF
or higher value ceramic capacitor of X7R or X5R type. Y5V
types have poor performance over temperature and applied
voltage and should not be used. A 1µF ceramic is adequate
to bypass the LT3502/LT3502A and will easily handle the
ripple current. However, if the input power source has
high impedance, or there is significant inductance due to
long wires or cables, additional bulk capacitance may be
necessary. This can be provided with a low performance
electrolytic capacitor.
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 LT3502/LT3502A and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
A 1µF capacitor is capable of this task, but only if it is placed
close to the LT3502/LT3502A and the catch diode (see the
PCB Layout section). A second precaution regarding the
ceramic input capacitor concerns the maximum input voltage rating of the LT3502/LT3502A. A ceramic input capacitor combined with trace or cable inductance forms a high
quality (underdamped) tank circuit. If the LT3502/LT3502A
circuit is plugged into a live supply, the input voltage can
ring to twice its nominal value, possibly exceeding the
LT3502/LT3502A’s voltage rating. This situation is easily
avoided; see the Hot Plugging Safely section.
12
COUT =
33
for the LT3502A
VOUT
COUT =
66
for the LT3502
VOUT
where COUT is in µF. Use an X5R or X7R type and keep
in mind that a ceramic capacitor biased with VOUT will
have less than its nominal capacitance. This choice will
provide low output ripple and good transient response.
Transient performance can be improved with a high value
capacitor, but a phase lead capacitor across the feedback
resistor, R1, may be required to get the full benefit (see
the Compensation section).
For small size, the output capacitor can be chosen
according to:
25
COUT =
VOUT
where COUT is in µF. However, using an output capacitor
this small results in an increased loop crossover frequency
and increased sensitivity to noise.
High performance electrolytic capacitors can be used for
the output capacitor. Low ESR is important, so choose
one that is intended for use in switching regulators. The
ESR should be specified by the supplier and should be
0.1Ω or less. Such a capacitor will be larger than a ceramic
capacitor and will have a larger capacitance, because the
capacitor must be large to achieve low ESR. Table 2 lists
several capacitor vendors.
3502fd
LT3502/LT3502A
Applications Information
Table 2
VENDOR
Panasonic
PHONE
(714) 373-7366
URL
www.panasonic.com
Kemet
(864) 963-6300
www.kemet.com
Sanyo
(408)794-9714
www.sanyovideo.com
Murata
AVX
(404) 436-1300
www.murata.com
www.avxcorp.com
Taiyo Yuden
(864) 963-6300
www.taiyo-yuden.com
Figure 4 shows the transient response of the LT3502A with
several output capacitor choices. The output is 3.3V. The
load current is stepped from 150mA to 400mA and back to
150mA, and the oscilloscope traces show the output voltage.
The upper photo shows the recommended value. The second photo shows the improved response (less voltage drop)
resulting from a larger output capacitor and a phase lead
capacitor. The last photo shows the response to a high
performance electrolytic capacitor. Transient performance
is improved due to the large output capacitance.
BOOST Pin Considerations
Capacitor C3 and the internal boost diode are used to
generate a boost voltage that is higher than the input
voltage. In most cases a 0.1μF capacitor will work well.
Figure 5 shows two ways to arrange the boost circuit. The
BOOST pin must be at least 2.2V above the SW pin for
best efficiency. For outputs of 3V and above, the standard
circuit (Figure 5a) is best. For outputs less than 3V and
above 2.5V, place a discrete Schottky diode (such as the
BAT54) in parallel with the internal diode to reduce VD. The
following equations can be used to calculate and minimize
boost capacitance in μF:
0.012/(VBD + VCATCH – VD – 2.2) for the LT3502A
0.030/(VBD + VCATCH – VD– 2.2) for the LT3502
VD is the forward drop of the boost diode, and VCATCH is
the forward drop of the catch diode (D1).
For lower output voltages the BD pin can be tied to an
external voltage source with adequate local bypassing
PART SERIES
Ceramic
Polymer,
Tantalum
Ceramic,
Tantalum
Ceramic
Polymer,
Tantalum
Ceramic
Ceramic,
Tantalum
Ceramic
COMMENTS
EEF Series
T494,T495
POSCAP
TPS Series
(Figure 5b). The above equations still apply for calculating
the optimal boost capacitor for the chosen BD voltage.
The absence of BD voltage during start-up will increase
minimum voltage to start and reduce efficiency. You must
also be sure that the maximum voltage rating of BOOST
pin is not exceeded.
The minimum operating voltage of an LT3502/LT3502A
application is limited by the undervoltage lockout (3V) 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
LT3502/LT3502A is turned on with its SHDN 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 the input and output voltages, and on the arrangement
of the boost circuit. The minimum load generally goes to
zero once the circuit has started. Figure 6 shows plots of
minimum load to start and to run as a function of input
voltage. 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. At light loads, the inductor current
becomes discontinuous and the effective duty cycle can
be very high. This reduces the minimum input voltage to
approximately 400mV above VOUT. At higher load currents,
the inductor current is continuous and the duty cycle is
limited by the maximum duty cycle of the LT3502/LT3502A,
requiring a higher input voltage to maintain regulation.
3502fd
13
LT3502/LT3502A
Applications Information
VOUT
32.4k
FB
IL
0.2A/DIV
10µF
VOUT
0.1V/DIV
AC COUPLED
10k
10µs/DIV
3502 F04a
10µs/DIV
3502 F04b
10µs/DIV
3502 F04c
VOUT
32.4k
50pF
IL
0.2A/DIV
10µF
×2
FB
VOUT
0.1V/DIV
AC COUPLED
10k
VOUT
32.4k
FB
10k
+
IL
0.2A/DIV
100µF
VOUT
0.1V/DIV
AC COUPLED
SANYO
4TPB100M
Figure 4. Transient Load Response of the LT3502A with Different Output Capacitors
as the Load Current is Stepped from 150mA to 400mA. VIN = 12V, VOUT = 3.3V, L = 6.8µH
VDD
BD
VIN
BD
BOOST
VIN LT3502 SW
GND
VOUT
VIN
DA
VBOOST – VSW ≅ VOUT
MAX VBOOST ≅ VIN + VOUT
BOOST
VIN LT3502 SW
GND
DA
VBOOST – VSW ≅ VDD
MAX VBOOST ≅ VIN + VDD
3502 F05a
(5a)
VOUT
3502 F05b
(5b)
Figure 5
14
3502fd
LT3502/LT3502A
Applications Information
7
8
6
7
6
START
4
3
RUN
4
3
2
2
1
0
0.001
START
RUN
5
VIN (V)
VIN (V)
5
1
0.1
0.01
LOAD CURRENT (A)
0
0.001
1
0.01
0.1
LOAD CURRENT (A)
3502 G20
3502 G19
(6a) LT3502A Typical Minimum Input Voltage, VOUT = 3.3V
(6b) LT3502A Typical Minimum Input Voltage, VOUT = 5V
7
8
6
7
6
RUN
START
3
START
4
3
2
2
1
0
0.001
RUN
5
VIN (V)
VIN (V)
5
4
1
1
0.1
0.01
LOAD CURRENT (A)
0
0.001
1
0.01
0.1
LOAD CURRENT (A)
1
3502 G22
3502 G21
(6c) LT3502 Typical Minimum Input Voltage, VOUT = 3.3V
(6d) LT3502 Typical Minimum Input Voltage, VOUT = 5V
Figure 6
Soft-Start
Short and Reverse Protection
The SHDN pin can be used to soft start the LT3502/LT3502A,
reducing the maximum input current during start-up. The
SHDN pin is driven through an external RC filter to create
a voltage ramp at this pin. Figure 7 shows the start-up
waveforms with and without the soft-start circuit. By
choosing a large RC time constant, the peak start-up
current can be reduced to the current that is required to
regulate the output, with no overshoot. Choose the value
of the resistor so that it can supply 80µA when the SHDN
pin reaches 2V.
If the inductor is chosen so that it won’t saturate excessively,
the LT3502/LT3502A will tolerate a shorted output. When
operating in short-circuit condition, the LT3502/LT3502A
will reduce their frequency until the valley current is
650mA (Figure 8a). There is another situation to consider
in systems where the output will be held high when the
input to the LT3502/LT3502A is absent. This may occur in
battery charging applications or in battery backup systems
where a battery or some other supply is diode OR-ed with
the LT3502/LT3502A’s output. If the VIN pin is allowed to
float and the SHDN pin is held high (either by a logic signal
3502fd
15
LT3502/LT3502A
Applications Information
VSW
10V/DIV
RUN
SHDN
IL
500mA/DIV
GND
3502 F07a
VOUT
2V/DIV
VIN = 12V
VOUT = 3.3V
L = 6.8µH
COUT = 10µF
5µs/DIV
VIN = 12V
VOUT = 3.3V
L = 6.8µH
COUT = 10µF
50µs/DIV
RUN
VSW
10V/DIV
50k
SHDN
0.1µF
IL
500mA/DIV
GND
3502 F07b
VOUT
2V/DIV
3502 F07
Figure 7. To Soft-Start the LT3502A, Add a Resistor and Capacitor to the SHDN Pin
D4
VIN
VSW
10V/DIV
BD
BOOST
VIN
VOUT
SW
LT3502A
DA
IL
500mA/DIV
SHDN
VIN = 40V
VOUT = 0V
L = 6.8µH
COUT = 10µF
2µs/DIV
FB
3502 F08a
Figure 8a. The LT3502A Reduces its Frequency to Below 500kHz
to Protect Against Shorted Output with 40V Input
16
GND
+
3502 F08b
Figure 8b. Diode D4 Prevents a Shorted Input from Discharging
a Backup Battery Tied to the Output; it Also Protects the Circuit
from a Reversed Input. The LT3502/LT3502A Runs Only When
the Input is Present
3502fd
LT3502/LT3502A
Applications Information
or because it is tied to VIN), then the LT3502/LT3502A’s
internal circuitry will pull its quiescent current through
its SW pin. This is fine if your system can tolerate a few
mA in this state. If you ground the SHDN pin, the SW
pin current will drop to essentially zero. However, if the
VIN pin is grounded while the output is held high, then
parasitic diodes inside the LT3502/LT3502A can pull large
currents from the output through the SW pin and the VIN
pin. Figure 8b shows a circuit that will run only when the
input voltage is present and that protects against a shorted
or reversed input.
Hot Plugging Safely
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LT3502/LT3502A circuits. However,
these capacitors can cause problems if the LT3502/LT3502A
CLOSING SWITCH
SIMULATES HOT PLUG
IIN
VIN
+
LOW
IMPEDANCE
ENERGIZED
24V SUPPLY
+
LT3502
are plugged into a live supply (see Linear Technology
Application Note 88 for a complete discussion). The low
loss ceramic capacitor combined with stray inductance in
series with the power source forms an underdamped tank
circuit, and the voltage at the VIN pin of the LT3502/LT3502A
can ring to twice the nominal input voltage, possibly exceeding the LT3502/LT3502A’s rating and damaging the
part. If the input supply is poorly controlled or the user
will be plugging the LT3502/LT3502A into an energized
supply, the input network should be designed to prevent
this overshoot. Figure 9 shows the waveforms that result
when an LT3502/LT3502A circuit is connected to a 24V
supply through six feet of 24-gauge twisted pair. The first
plot is the response with a 2.2µF ceramic capacitor at the
input. The input voltage rings as high as 35V and the input
current peaks at 20A. One method of damping the tank
circuit is to add another capacitor with a series resistor to
VIN
20V/DIV
2.2µF
20µs/DIV
(9a)
LT3502
+
RINGING VIN MAY EXCEED
ABSOLUTE MAXIMUM
RATING OF THE LT3502
IIN
5A/DIV
STRAY
INDUCTANCE
DUE TO 6 FEET
(2 METERS) OF
TWISTED PAIR
10µF
35V
AI.EI.
DANGER!
VIN
20V/DIV
2.2µF
IIN
5A/DIV
(9b)
1Ω
+
0.1µF
LT3502
20µs/DIV
VIN
20V/DIV
2.2µF
IIN
5A/DIV
(9c)
20µs/DIV
3502 F09
Figure 9. A Well Chosen Input Network Prevents Input Voltage Overshoot and
Ensures Reliable Operation When the LT3502 is Connected to a Live Supply
3502fd
17
LT3502/LT3502A
Applications Information
the circuit. In Figure 9b an aluminum electrolytic capacitor
has been added. This capacitor’s high equivalent series
resistance damps the circuit and eliminates the voltage
overshoot. The extra capacitor improves low frequency
ripple filtering and can slightly improve the efficiency of the
circuit, though it is likely to be the largest component in the
circuit. An alternative solution is shown in Figure 9c. A 1Ω
resistor is added in series with the input to eliminate the
voltage overshoot (it also reduces the peak input current).
A 0.1µF capacitor improves high frequency filtering. This
solution is smaller and less expensive than the electrolytic
capacitor. For high input voltages its impact on efficiency
is minor, reducing efficiency less than one half percent for
a 5V output at full load operating from 24V.
Frequency Compensation
The LT3502/LT3502A use current mode control to regulate
the output. This simplifies loop compensation. In particular,
the LT3502/LT3502A does not require the ESR of the output
capacitor for stability allowing the use of ceramic capacitors
to achieve low output ripple and small circuit size.
Figure 10 shows an equivalent circuit for the LT3502/
LT3502A 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 current proportional to the voltage at the VC node.
Note that the output capacitor integrates this current,
CURRENT MODE
POWER STAGE
SW
gm =
+1A/V
LT3502
For proper operation and minimum EMI, care must
be taken during printed circuit board layout. Figure 11
shows the recommended component placement with
trace, ground plane and via locations. Note that large,
switched currents flow in the LT3502/LT3502A’s VIN and
SW pins, the catch diode (D1) and the input capacitor (C2).
–
+
ERROR
AMPLIFIER
C1
OUT
R1
L1
CPL
C2
FB
VIN
ESR
800mV
C1
1M
+
BD
R2
Figure 10. Model for Loop Response
C3
C1
R1
BST
FB
SHDN
R2
3502 F10
18
PCB Layout
VOUT
gm =
100µA/V
VC
GND
If the output capacitor is different than the recommended
capacitor, stability should 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.
–
0.5V
RC
150k
CC
70pF
and that the capacitor on the VC node (CC) integrates the
error amplifier output current, resulting in two poles in the
loop. RC provides a zero. With the recommended output
capacitor, the loop crossover occurs above the RCCC zero.
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. With a larger
ceramic capacitor (very low ESR), crossover may be lower
and a phase lead capacitor (CPL) across the feedback
divider may improve the phase margin and transient
response. Large electrolytic capacitors may have an ESR
large enough to create an additional zero, and the phase
lead may not be necessary.
D1
DA
GND
= VIA
3502 F11
Figure 11
3502fd
LT3502/LT3502A
Applications Information
The loop formed by these components should be as
small as possible and tied to system ground in only one
place. 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 capacitor C1. The SW and BOOST nodes should
be as small as possible. Finally, keep the FB node small so
that the ground pin and ground traces will shield it from
the SW and BOOST nodes. Include vias near the exposed
GND pad of the LT3502/LT3502A to help remove heat from
the LT3502/LT3502A to the ground plane.
High Temperature Considerations
The die temperature of the LT3502/LT3502A must be lower
than the maximum rating of 125°C. This is generally not
a concern unless the ambient temperature is above 85°C.
For higher temperatures, care should be taken in the layout
of the circuit to ensure good heat sinking of the LT3502/
LT3502A. The maximum load current should be derated
as the ambient temperature approaches 125°C. The die
temperature is calculated by multiplying the LT3502/
LT3502A power dissipation by the thermal resistance from
junction to ambient. Power dissipation within the LT3502/
LT3502A can be estimated by calculating the total power
loss from an efficiency measurement and subtracting
the catch diode loss. Thermal resistance depends on the
layout of the circuit board, but 102°C/W and 110ºC/W are
typical for the (2mm × 2mm) DFN and MS10 packages
respectively.
Outputs Greater Than 7V
Note that for outputs above 7V, the input voltage range
will be limited by the maximum rating of the BOOST pin.
The sum of input and output voltages cannot exceed the
BOOST pin’s 50V rating. The 15V circuit (Figure 12) shows
how to overcome this limitation using an additional Zener
diode.
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 100 shows how to generate a bipolar output supply
using a buck regulator.
C4
0.1µF
1N4148
OR OTHER
SIMILAR
DIODES
10V
BD
VIN
20V TO 40V
VIN
BOOST
C2
1µF
SW
C3
0.1µF
L1
33µH
LT3502A
22pF
DA
OFF ON
SHDN
GND
FB
VOUT
15V
500mA
R1
180k
R2
10k
C1
10µF
3502 F12
Figure 12. 15V Step-Down Converter
3502fd
19
LT3502/LT3502A
Typical Applications
0.8V Step-Down Converter
VBD
3V TO 7V
VBD
3V TO 7V
0.1µF
BD
VIN
3V TO 40V
C2
1µF
VIN
BOOST
L1
C3
0.1µF 3.3µH
SW
LT3502A
D1
0.1µF
BD
VIN
3V TO 40V
C2
1µF
VOUT
0.8V
500mA
VIN
BOOST
SW
LT3502
DA
OFF ON
SHDN
GND
C3
0.1µF
L1
10µH
D1
VOUT
0.8V
500mA
DA
FB
OFF ON
C1
47µF
C1: JMK212BJ476MG
C3: HMK212BJ104MG
L1: LQH43CN3R3M03
SHDN
GND
FB
C1
100µF
C1: JMK316BJ107ML
L1: LQH43CN100K03
3502 TA02a
3502 TA02b
1.8V Step-Down Converter
VBD
3V TO 7V
VBD
3V TO 7V
0.1µF
BD
VIN
3V TO 40V
C2
1µF
VIN
BOOST
SW
LT3502A
D1
DA
OFF ON
SHDN
GND
L1
C3
0.1µF 4.7µH
FB
C1: JMK212BJ226MG
L1: LQH43CN4R7M03
20
BD
VIN
3V TO 40V
C2
1µF
VOUT
1.8V
500mA
VIN
BOOST
SW
LT3502
C1
22µF
3502 TA03a
OFF ON
SHDN
GND
C3
0.1µF
D1
DA
R1
12.5k
R2
10k
0.1µF
FB
L1
15µH
VOUT
1.8V
500mA
R1
12.5k
R2
10k
C1: JMK212BJ476MG
L1: LQH55DN150M03
C1
47µF
3502 TA03b
3502fd
LT3502/LT3502A
Typical Applications
2.5V Step-Down Converter
VBD
3V TO 7V
VBD
3V TO 7V
0.1µF
BD
VIN
3.5V TO 40V
C2
1µF
VIN
BOOST
SW
LT3502A
D1
DA
OFF ON
SHDN
GND
L1
C3
0.1µF 6.8µH
FB
C1: JMK212BJ226MG
L1: LQH43DN6R8M03
BD
VIN
3.5V TO 40V
C2
1µF
VOUT
2.5V
500mA
VIN
BOOST
SW
LT3502
SHDN
OFF ON
C1
22µF
FB
GND
C3
0.1µF
L1
15µH
D1
DA
R1
21.3k
R2
10k
0.1µF
R1
21.3k
R2
10k
C1
22µF
C1: JMK212BJ226MG
L1: LQH55DN150M03
3502 TA04a
VOUT
2.5V
500mA
3502 TA04b
3.3V Step-Down Converter
BD
VIN
4.7V TO 40V
VIN
BOOST
C2
1µF
SW
LT3502A
D1
DA
OFF ON
SHDN
GND
L1
C3
0.1µF 6.8µH
FB
C1: LMK316BJ106ML-BR
L1: LQH43CN6R8M03
VIN
BOOST
C2
1µF
VOUT
3.3V
500mA
SW
LT3502
C1
10µF
3502 TA05a
OFF ON
SHDN
GND
C3
0.1µF
D1
DA
R1
31.6k
R2
10k
BD
VIN
4.5V TO 40V
FB
L1
15µH
VOUT
3.3V
500mA
R1
31.6k
R2
10k
C1: JMK212BJ226MG
L1: LQH55DN150M03
C1
22µF
3502 TA05b
3502fd
21
LT3502/LT3502A
Package Description
DC8 Package
8-Lead Plastic DFN (2mm × 2mm)
(Reference LTC DWG # 05-08-1719 Rev A)
0.70 ±0.05
2.55 ±0.05
1.15 ±0.05 0.64 ±0.05
(2 SIDES)
PACKAGE
OUTLINE
0.25 ± 0.05
0.45 BSC
1.37 ±0.05
(2 SIDES)
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
R = 0.05
TYP
2.00 ±0.10
(4 SIDES)
PIN 1 BAR
TOP MARK
(SEE NOTE 6)
R = 0.115
TYP
5
8
0.40 ± 0.10
0.64 ± 0.10
(2 SIDES)
PIN 1 NOTCH
R = 0.20 OR
0.25 × 45°
CHAMFER
(DC8) DFN 0106 REVØ
4
0.200 REF
1
0.23 ± 0.05
0.45 BSC
0.75 ±0.05
1.37 ±0.10
(2 SIDES)
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 SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE
TOP AND BOTTOM OF PACKAGE
22
3502fd
LT3502/LT3502A
Package Description
MS Package
10-Lead Plastic MSOP
(Reference LTC DWG # 05-08-1661 Rev E)
0.889 ± 0.127
(.035 ± .005)
5.23
(.206)
MIN
3.20 – 3.45
(.126 – .136)
3.00 ± 0.102
(.118 ± .004)
(NOTE 3)
0.50
0.305 ± 0.038
(.0197)
(.0120 ± .0015)
BSC
TYP
RECOMMENDED SOLDER PAD LAYOUT
0.254
(.010)
10 9 8 7 6
3.00 ± 0.102
(.118 ± .004)
(NOTE 4)
4.90 ± 0.152
(.193 ± .006)
DETAIL “A”
0.497 ± 0.076
(.0196 ± .003)
REF
0° – 6° TYP
GAUGE PLANE
1 2 3 4 5
0.53 ± 0.152
(.021 ± .006)
DETAIL “A”
0.18
(.007)
SEATING
PLANE
1.10
(.043)
MAX
0.86
(.034)
REF
0.17 – 0.27
(.007 – .011)
TYP
0.50
(.0197)
NOTE:
BSC
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
0.1016 ± 0.0508
(.004 ± .002)
MSOP (MS) 0307 REV E
3502fd
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.
23
LT3502/LT3502A
Typical Application
5V Step-Down Converter
BD
VIN
6.7V TO 40V
VIN
BOOST
C2
1µF
SW
LT3502A
C3
0.1µF
D1
DA
OFF ON
SHDN
GND
L1
10µH
FB
BD
VIN
6.4V TO 40V
VIN
BOOST
C2
1µF
VOUT
5V
500mA
SW
LT3502
R2
10k
C1
10µF
C1: LMK316BJ106ML-BR
L1: LQH43CN100K03
OFF ON
SHDN
GND
L1
22µH
D1
DA
R1
52.3k
C3
0.1µF
FB
R1
52.3k
R2
10k
C1: LMK316BJ106ML-BR
L1: LQH43CN100K03
3502 TA06a
VOUT
5V
500mA
C1
22µF
3502 TA06b
Related Parts
PART NUMBER
DESCRIPTION
COMMENTS
LT1766
60V, 1.2A (IOUT), 200kHz, High Efficiency Step-Down
DC/DC Converter
VIN: 5.5V to 60V, VOUT(MIN) = 1.2V, IQ = 2.5mA, ISD = 25µA,
TSSOP16/TSSOP16E Packages
LT1933
500mA (IOUT), 500kHz, Step-Down Switching Regulator in
SOT-23
VIN: 3.6V to 36V, VOUT(MIN) = 1.2V, IQ = 1.6mA, ISD < 1µA,
ThinSOT™ Package
LT1936
36V, 1.4A (IOUT), 500kHz, High Efficiency Step-Down
DC/DC Converter
VIN: 3.6V to 36V, VOUT(MIN) = 1.2V, IQ = 1.9mA, ISD < 1µA,
MS8E Package
LT1940
Dual 25V, 1.4A (IOUT), 1.1MHz, High Efficiency Step-Down
DC/DC Converter
VIN: 3.6V to 25V, VOUT(MIN) = 1.20V, IQ = 3.8mA, ISD < 30µA,
TSSOP16E Package
LT1976/
LT1977
60V, 1.2A (IOUT), 200kHz/500kHz High Efficiency Step-Down
DC/DC Converters with Burst Mode® Operation
VIN: 3.3V to 60V, VOUT(MIN) = 1.20V, IQ = 100µA, ISD < 1µA,
TSSOP16E Package
LTC 3407/
LTC3407-2
Dual 600mA/800mA, 1.5MHz/2.25MHz, Synchronous
Step-DownDC/DC Converters
VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 40µA, ISD <1µA,
3mm × 3mm DFN, MS10E Package
LT3434/
LT3435
60V, 1.2A (IOUT), 200kHz/500kHz High Efficiency Step-Down
DC/DC Converters with Burst Mode Operation
VIN: 3.3V to 60V, VOUT(MIN) = 1.20V, IQ = 100µA, ISD < 1µA,
TSSOP16E Package
LT3437
60V, 400mA (IOUT), Micropower Step-Down DC/DC Converter
with Burst Mode Operation
VIN: 3.3V to 60V, VOUT(MIN) = 1.25V, IQ = 100µA, ISD < 1µA,
DFN Package
LT3493
36V, 1.4A (IOUT), 750kHz, High Efficiency Step-Down
DC/DC Converter
VIN: 3.6V to 36V, VOUT(MIN) = 0.8V, IQ = 1.9mA, ISD < 1µA,
DFN Package
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,
TSSOP20E 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,
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,
3mm × 3mm DFN, MS8E Packages
LT3506/
LT3506A
Dual 25V, 1.6A (IOUT), 575kHz/1.1MHz, High Efficiency StepDown DC/DC Converters
VIN: 3.6V to 25V, VOUT(MIN) = 0.8V, IQ = 3.8mA, ISD < 30µA,
4mm × 5mm DFN Package
LT3508
Dual 36V, 1.4A (IOUT), 2.5MHz, High Efficiency Step-Down
DC/DC Converter
VIN: 3.6V to 36V, VOUT(MIN) = 0.8V, IQ = 4.3mA, ISD < 1µA,
4mm × 4mm QFN, TSSOP16E Packages
LT3510
Dual 25V, 2A (IOUT), 1.5MHz, High Efficiency Step-Down
DC/DC Converter
VIN: 3.3V to 25V, VOUT(MIN) = 0.8V, IQ = 3.7mA, ISD < 10µA,
TSSOP20E Package
LTC3548
Dual 400mA + 800mA, 2.25MHz Synchronous Step-Down
DC/DC Converter
VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 40µA, ISD < 1µA,
3mm × 3mm DFN, MS10E Packages
Burst Mode is a registered trademark of Linear Technology Corporation. ThinSOT is a trademark of Linear Technology Corporation.
24 Linear Technology Corporation
3502fd
LT 0809 REV D • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com
 LINEAR TECHNOLOGY CORPORATION 2007