LT3502/LT3502A
1.1MHz/2.2MHz, 500mA
Step-Down Regulators in
Features
n
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
n
Soft-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
applications
n
Automotive Systems
n
Battery-Powered Equipment
n
Wall Transformer Regulation
n
Distributed Supply Regulation
2mm ×
2mm DFN and MS10
Description
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.
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 protec
tion in fault conditions. An internal boost diode reduces
component count.
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
3.3V Step-Down Converter
V
4.7V TO 40V
IN
1µF
OFF ON
V
IN
SHDN
BD
BOOST
SW
LT3502A
DA
FB
GND
10k
0.1µF
6.8µH
31.6k
V
OUT
3.3V
500mA
10µF
3502 TA01a
90
80
70
60
50
40
EFFICIENCY (%)
30
20
10
0
0
LT3502A 12VIN Efficiency
5V
OUT
0.1 0.3
0.2
LOAD CURRENT (A)
3.3V
OUT
0.4
0.5
3502 TA01b
3502fd
1
LT3502/LT3502A
TOP VIEW
absolute MaxiMuM ratings
Input Voltage (VIN) ....................................................40V
BOOST Voltage .........................................................50V
BOOST Pin Above SW Pin ...........................................7V
FB Voltage ...................................................................6V
SHDN Voltage ...........................................................40V
pin conFiguration
TOP VIEW
1
V
IN
2
BD
3
FB
SHDN
4
8-LEAD (2mm × 2mm) PLASTIC DFN
EXPOSED PAD (PIN 9) IS GND, MUST BE SOLDERED TO PCB
DC PACKAGE
= 102°C/W
θ
JA
8
SW
BOOST
7
9
DA
6
GND
5
(Note 1)
BD Voltage ..................................................................7V
Operating Junction Temperature Range (Note 2)
LT3502AE, LT 3502E .......................... –40°C to 125°C
LT3502AI, LT3502I ............................ –40°C to 125°C
Storage Temperature Range .................. –65°C to 150°C
10
1
SW
2
BOOST
3
NC
4
DA
5
GND
MS PACKAGE
10-LEAD PLASTIC MSOP
θ
= 110°C/W
JA
V
IN
NC
9
BD
8
FB
7
SHDN
6
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
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
.linear.com/leadfree/
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, V
PARAMETER CONDITIONS MIN TYP MAX UNITS
Undervoltage Lockout 2.6 2.8 3 V
Quiescent Current at Shutdown V
Quiescent Current Not Switching 1.5 2 mA
Feedback Voltage 2mm × 2mm DFN
Reference Voltage Line Regulation 0.005 %/V
FB Pin Bias Current (Note 5)
Switching Frequency I
Maximum Duty Cycle 100mA Load (LT3502A)
Switch V
Switch Current Limit (Note 3) 0.75 0.9 1.1 A
Switch Active Current SW = 10V (Note 4)
BOOST Pin Current I
Minimum BOOST Voltage Above Switch I
BOOST Schottky Forward Drop I
DA Pin Current to Stop OSC 500 650 mA
SHDN Bias Current V
SHDN Input Voltage High 2 V
SHDN Input Voltage Low 0.3 V
CESAT
= 0V 0.5 2 µA
SHDN
2mm × 2mm DFN
MS10
MS10
< 500mA (LT3502A)
DA
I
< 500mA (LT3502A)
DA
I
< 500mA (LT3502)
DA
I
< 500mA (LT3502)
DA
100mA Load (L
ISW = 500mA 450 mV
SW = 0V (Note 5)
= 500mA 10 13 mA
SW
= 500mA 1.9 2.2 V
SW
= 100mA 0.8 1 V
OUT
= 5V
SHDN
V
= 0V
SHDN
T3502)
SHDN
= 5V, V
BOOST
= 15V.
l
l
l
l
l
0.785
0.79
0.780
0.786
1.9
1.8
0.9
0.8
70
80
0.8
0.8
0.8
0.8
15 50 nA
2.25
2.25
1.1
1.1
80
90
95
55 80
0.813
0.81
0.816
0.813
2.7
2.8
1.3
1.4
8
130
30
1
MHz
MHz
MHz
MHz
µA
µA
µA
µA
V
V
V
V
%
%
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
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
V
(mV)
LOAD CURRENT (A)
LOAD CURRENT (A)
EFFICIENCY (%)
LOAD CURRENT (A)
3502 G05
LOAD CURRENT (A)
3502 G06
EFFICIENCY (%)
0.5
0.5
90
3502 G02
typical perForMance characteristics
LT3502A 3.3V
90
80
70
60
50
40
30
20
10
0
0
LT3502 5V
100
90
80
70
60
50
40
30
20
10
0
0
0.1 0.3
0.1
Efficiency LT3502A 5V
OUT
12V
IN
24V
IN
EFFICIENCY (%)
0.3
0.4
0.4
0.5
3502 G04
0.2
LOAD CURRENT (A)
Efficiency
OUT
12V
IN
24V
IN
0.2
LOAD CURRENT (A)
Efficiency LT3502 3.3V
OUT
80
70
60
50
40
30
20
10
0
0
12V
0.1 0.3
0.2
LOAD CURRENT (A)
24V
IN
IN
LT3502A Maximum Load Current
V
= 3.3V, L = 6.8µH
OUT
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
TYPICAL
10
MINIMUM
20
VIN (V)
30
(TA = 25°C unless otherwise noted)
100
90
80
70
60
50
40
EFFICIENCY (%)
30
20
10
0
0.1
0.4
0
LOAD CURRENT (A)
LT3502A Maximum Load Current
V
= 5V, L = 10µH
OUT
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
40
0
10
Efficiency
OUT
5V
IN
12V
0.2
TYPICAL
MINIMUM
20
VIN (V)
24V
IN
IN
0.3
0.4
0.5
3502 G03
30
40
4
LT3502 Maximum Load Current
V
= 3.3V, L = 15µH
OUT
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
TYPICAL
MINIMUM
10
20
VIN (V)
30
3502 G07
LT3502 Maximum Load Current
V
= 5V, L = 22µH Switch Voltage Drop
OUT
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
40
0
0
TYPICAL
MINIMUM
10
20
VIN (V)
30
40
3502 G08
700
600
500
400
CE
300
200
100
0
0
–40°C
25°C
125°C
0.2 0.4 0.6 1.0
SWITCH CURRENT (A)
0.8
3502 G09
3502fd
LT3502/LT3502A
3502 G12
1600
3502 G13
3502 G14
150
45
0.7
V
(V)
45
45
typical perForMance characteristics
UVLO
3.5
3.0
2.5
2.0
(V)
IN
V
1.5
1.0
0.5
0
–50
SHDN Pin Current
300
250
200
150
100
SHDN PIN CURRENT (µA)
50
0
5 15
0
0 50 150
TEMPERATURE (°C)
10
20
SHDN PIN VOLTAGE (V)
100
3502 G10
35
25 45
40
30
Switching Frequency Soft-Start (SHDN)
2.5
2.0
1.5
1.0
FREQUENCY (MHz)
0.5
0
–50
LT3502A
LT3502
0
50
TEMPERATURE (°C)
100
Switch Current Limit
1.0
0.9
0.8
0.7
0.6
0.5
0.4
CURRENT LIMIT (A)
0.3
0.2
0.1
0
–50
SW PEAK CURRENT LIMIT
DA VALLEY CURRENT LIMIT
0
50
TEMPERATURE (°C)
100
(TA = 25°C unless otherwise noted)
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
SWITCH CURRENT LIMIT (A)
0.1
0
–0.1
3502 G11
150
0
200 600
400
SHDN PIN VOLTAGE (mV)
800
Switch Current Limit
1.2
1.0
0.8
0.6
0.4
CURRENT LIMIT (A)
0.2
0
0
50 100
DUTY CYCLE (%)
1000
LT3502
LT3502A
1200
1400
3502 G15
LT3502A Maximum V
Frequency (V
40
35
30
25
(V)
IN
20
V
15
10
5
0
0
0.1
OUT
TA = 25°C
TA = 85°C
0.2
0.3
LOAD CURRENT (A)
for Full
IN
= 3.3V)
0.4
0.5 0.6
3502 G16
LT3502A Maximum VIN for Full
Frequency (V
40
35
30
25
IN
20
15
10
5
0
0
TA = 25°C
TA = 85°C
0.2
0.1
= 5V)
OUT
0.3 0.7
LOAD CURRENT (A)
0.4
0.5 0.6
3502 G17
LT3502 Maximum VIN for Full
Frequency (V
40
35
30
25
(V)
IN
20
V
15
10
5
0
0
TA = 25°C
TA = 85°C
0.1
0.2
= 3.3V)
OUT
0.3 0.7
LOAD CURRENT (A)
0.4
0.5 0.6
3502 G18
3502fd
5
LT3502/LT3502A
OUT
OUT
typical perForMance characteristics
LT3502A Typical Minimum Input
(V)
IN
V
7
6
5
4
3
2
1
0
0.001
Voltage (V
= 3.3V)
OUT
0.01
LOAD CURRENT (A)
LT3502 Typical Minimum Input
(V)
IN
V
8
7
6
5
4
3
2
1
0
0.001
Voltage (V
= 5V)
OUT
0.01 0.1 1
LOAD CURRENT (A)
0.1 1
3502 G19
200mA/DIV
20mV/DIV
3502 G22
LT3502A Typical Minimum Input
(V)
IN
V
8
7
6
5
4
3
2
1
0
0.001
Voltage (V
= 5V)
OUT
0.01 0.1 1
LOAD CURRENT (A)
Continuous Mode Waveform Discontinuous Mode Waveform
V
SW
5V/DIV
I
L
V
OUT
VIN = 12V
= 3.3V
V
OUT
L = 6.8µH
= 10µF
C
OUT
= 250mA
I
200ns/DIV
(TA = 25°C unless otherwise noted)
LT3502 Typical Minimum Input
3502 G20
3502 G23
(V)
IN
V
V
5V/DIV
200mA/DIV
V
OUT
20mV/DIV
Voltage (V
7
6
5
4
3
2
1
0
0.001
SW
I
L
VIN = 12V
V
OUT
L = 6.8µH
C
OUT
I
= 3.3V
= 10µF
= 30mA
= 3.3V)
OUT
0.01
LOAD CURRENT (A)
200ns/DIV
0.1 1
3502 G21
3502 G24
3502fd
6
LT3502/LT3502A
pin Functions
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 di
vider tap to this pin. Set the output voltage according to
= 0.8 (1 + R1/R2). A good value for R2 is 10k.
V
OUT
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
pin. The SHDN pin also provides soft-start and
V
to the
frequency foldback. To use the soft-start feature, connect
R3 and C4 to the SHDN pin. SHDN Pin voltage should
not be higher than V
IN
IN
(DFN/MS)
-
.
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
LT3502/LT3502A
block DiagraM
BD
OUT
V
3502 BD
C1
L1
C3
7
2
BOOST
Q1
DRIVER
Q
Q
SW
D1
5
DA
6
GND
FB
3
R2 R1
0.8V
8
R
S
m
g
C
V
∑
COMP
SLOPE
AND
UVLO
INT REG
IN
V
1
C2
IN
V
SHDN
4
R3
ON OFF
OSC
FREQUENCY
FOLDBACK
C4
8
3502fd
operation
LT3502/LT3502A
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 V
this current reaches a level determined by the voltage at
. An error amplifier measures the output voltage through
V
C
an external resistor divider tied to the FB pin and servos
node. If the error amplifier’s output increases, more
the V
C
current is delivered to the output; if it decreases, less
current is delivered. An active clamp (not shown) on the V
node provides current limit. The V
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 cir
cuitry. This regulator includes an undervoltage lockout to
prevent switching when V
pin is used to place the LT3502/LT3502A in shutdown,
disconnecting the output and reducing the input current
to less than 2µA.
and SW pins, turning the switch off when
IN
node is also clamped
C
is less than ~3V. The SHDN
IN
C
-
The switch driver operates from either V
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 L
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.
T3502/LT3502A. The
or from the
IN
3502fd
9
LT3502/LT3502A
V
V
D
V
V
MAX
V
V
MIN
applications inForMation
FB Resistor Network
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the 1% resis
-
tors according to:
R1= R2
⎛
V
⎜
⎝
0.8V
OUT
– 1
⎞
⎟
⎠
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 applica
tions depends on the output voltage and on the absolute
maximum ratings of the V
and BOOST pins.
IN
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:
+
D
DC =
VIN– VSW+ V
OUT
where VD is the forward voltage drop of the catch diode
(~0.4V) and V
is the voltage drop of the internal switch
SW
(~0.45V at maximum load). This leads to a minimum input
voltage of:
+
D
– VD+ V
SW
V
IN(MIN)
OUT
=
DC
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 V
limited to the V
and BOOST pins. The input voltage should be
IN
operating range (40V) during overload
IN
conditions.
Minimum On-Time
The LT3502/LT3502A will still regulate the output at input
voltages that exceed V
IN(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).
V
SW
20V/DIV
I
L
500mA/DIV
V
OUT
100mV/DIV
1µs/DIV
V
= 33V, V
IN
L = 6.8µH, C
Figure 1. Continuous Mode Operation Near
Minimum On-Time of 60ns
OUT
OUT
= 3.3V
= 10µF, I
OUT
= 250mA
3502 F01
with DC
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 DC
V
IN(MAX )
DC
MIN
= 0.80 for the LT3502A and 0.90 for the
MAX
:
MIN
+
OUT
=
DC
D
– VD+ V
SW
= 0.15 for the LT3502A and 0.08 for the LT3502.
When the required on-time decreases below the mini
mum 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 exceed
ing the load current and the output ripple increases. The
rt then remains off until the output voltage dips below
pa
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 V
IN(MAX)
is safe and will not
damage the part.
3502fd
10
applications inForMation
VIN– V
L
LT3502/LT3502A
V
SW
20V/DIV
I
L
500mA/DIV
V
OUT
100mV/DIV
3502 F02
V
= 40V, V
IN
L = 6.8µH, C
OUT
OUT
= 3.3V
= 10µF, I
1µs/DIV
OUT
= 250mA
Figure 2. Pulse-Skipping Occurs when
Required On-Time is Below 60ns
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
V
IN(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
pulse-skipping
in
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:
900mA+
OUT
• 60ns
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 per
formance
during this operating regime.
V
SW
20V/DIV
I
L
500mA/DIV
V
OUT
100mV/DIV
3502 F03
V
= 40V, V
IN
L = 6.8µH, C
OUT
OUT
= 3.3V
= 10µF, I
1µs/DIV
OUT
= 500mA
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
Inductor Selection and Maximum Output Current
A good first choice for the inductor value is:
L = 1.6(V
L = 4.6(V
where V
D
+ VD) for the LT3502A
OUT
+ VD) for the LT3502
OUT
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 URL PART SERIES INDUCTANCE RATE (µH) SIZE (mm)
Sumida www.sumida.com CDRH4D28
CDRH5D28
CDRH8D28
Toko www.toko.com A916CY
D585LC
Würth Elektronik www.we-online.com WE-TPC(M)
WE-PD2(M)
WE-PD(S)
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
4.5
4.5 ×
5.5 × 5.5
8.3 × 8.3
6.2
6.3 ×
8.1 × 8
4.8
4.8 ×
5.2 × 5.8
7.3 × 7.3
3502fd
11
LT3502/LT3502A
33
25
OUT
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.
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.
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:
C
=
OUT
=
C
OUT
where C
OUT
in mind that a ceramic capacitor biased with V
for the LT3502A
V
OUT
66
for the LT3502
V
OUT
is in µF. Use an X5R or X7R type and keep
will
OUT
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).
Step-down regulators draw current from the input supply
in pulses with very fast rise and fall times. The input ca
pacitor 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 volt
age rating of the LT3502/LT3502A. A ceramic input capacitor combined with trace or cable inductance forms a high
quality (underdamped) tank cir
cuit. If the L
T3502/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.
For small size, the output capacitor can be chosen
according to:
C
=
OUT
where C
V
is in µF. However, using an output capacitor
OUT
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
12
LT3502/LT3502A
applications inForMation
Table 2
VENDOR PHONE URL PART SERIES COMMENTS
Panasonic (714) 373-7366 www.panasonic.com Ceramic
Kemet (864) 963-6300 www.kemet.com Ceramic,
Sanyo (408)794-9714 www.sanyovideo.com
Murata (404) 436-1300 www.murata.com Ceramic
X www.avxcorp.com Ceramic,
AV
T
uden (864) 963-6300 www.taiyo-yuden.com Ceramic
aiyo Y
Polymer,
Tantalum
antalum
T
Ceramic
Polymer,
Tantalum
Tantalum
EEF Series
T494,T495
POSCAP
TPS Series
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 sec
ond 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 V
. The
D
following equations can be used to calculate and minimize
boost capacitance in μF:
0.012/(V
0.030/(V
is the forward drop of the boost diode, and V
V
D
BD
BD
+ V
+ V
– VD – 2.2) for the LT3502A
CATCH
– VD– 2.2) for the LT3502
CATCH
CATCH
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
(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 V
IN
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 V
. At higher load currents,
OUT
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
V
applications inForMation
V
OUT
32.4k
10µFFB
10k
0.2A/DIV
V
OUT
0.1V/DIV
AC COUPLED
I
L
32.4k
FB
10k
32.4k
FB
10k
50pF
+
V
OUT
10µF
×2
V
OUT
100µF
SANYO
4TPB100M
0.2A/DIV
V
OUT
0.1V/DIV
AC COUPLED
0.2A/DIV
V
OUT
0.1V/DIV
AC COUPLED
10µs/DIV
I
L
10µs/DIV
I
L
10µs/DIV
3502 F04a
3502 F04b
3502 F04c
Figure 4. Transient Load Response of the LT3502A with Different Output Capacitors
as the Load Current is Stepped from 150mA to 400mA. VIN = 12V, V
DD
BD
BOOST
V
IN
V
MAX V
BOOST
LT3502
V
IN
GND
– VSW ≅ V
≅ VIN + V
BOOST
SW
DA
OUT
OUT
(5a)
V
3502 F05a
OUT
V
IN
= 3.3V, L = 6.8µH
OUT
LT3502
V
IN
V
– VSW ≅ V
BOOST
MAX V
BOOST
BD
BOOST
GND
DD
≅ VIN + V
SW
DA
DD
(5b)
V
3502 F05b
OUT
Figure 5
3502fd
14
applications inForMation
7
V
(V)
6
5
4
IN
3
2
1
0
0.001
START
RUN
0.01
LOAD CURRENT (A)
0.1 1
3502 G19
(V)
IN
V
8
7
6
5
4
3
2
1
0
0.001
LT3502/LT3502A
START
RUN
0.01 0.1 1
LOAD CURRENT (A)
3502 G20
(6a) LT3502A Typical Minimum Input Voltage, V
7
6
(V)
IN
V
5
4
3
2
1
0
0.001
START
RUN
0.01
LOAD CURRENT (A)
0.1 1
3502 G21
(6c) LT3502 Typical Minimum Input Voltage, V
= 3.3V (6b) LT3502A Typical Minimum Input Voltage, V
OUT
= 3.3V (6d) LT3502 Typical Minimum Input Voltage, V
OUT
Soft-Start
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.
OUT
8
(V)
IN
V
7
6
5
4
3
2
1
0
0.001
START
RUN
0.01 0.1 1
LOAD CURRENT (A)
3502 G22
OUT
Figure 6
Short and Reverse Protection
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 V
pin is allowed to
IN
float and the SHDN pin is held high (either by a logic signal
= 5V
= 5V
3502fd
15
LT3502/LT3502A
OUT
applications inForMation
RUN
SHDN
GND
3502 F07a
RUN
50k
SHDN
0.1µF
GND
3502 F07b
V
SW
10V/DIV
500mA/DIV
V
OUT
2V/DIV
V
SW
10V/DIV
500mA/DIV
V
OUT
2V/DIV
I
L
V
V
L = 6.8µH
C
I
L
V
V
L = 6.8µH
C
IN
OUT
OUT
IN
OUT
OUT
= 12V
= 3.3V
= 10µF
= 12V
= 3.3V
= 10µF
5µs/DIV
50µs/DIV
3502 F07
Figure 7. To Soft-Start the LT3502A , Add a Resistor and Capacitor to the SHDN Pin
V
SW
10V/DIV
I
L
500mA/DIV
= 40V
IN
= 0V
V
OUT
L = 6.8µH
= 10µF
C
2µs/DIVV
3502 F08a
Figure 8a. The LT3502A Reduces its Frequency to Below 500kHz
to Protect Against Shorted Output with 40V Input
D4
V
IN
V
IN
SHDN
BD
LT3502A
GND
BOOST
SW
DA
V
OUT
+
FB
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
16
3502fd
applications inForMation
CLOSING SWITCH
LT3502/LT3502A
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
pin is grounded while the output is held high, then
V
IN
parasitic diodes inside the LT3502/LT3502A can pull large
currents from the output through the SW pin and the V
IN
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
SIMULATES HOT PLUG
+
LOW
IMPEDANCE
ENERGIZED
24V SUPPLY
I
IN
STRAY
INDUCTANCE
DUE TO 6 FEET
(2 METERS) OF
TWISTED PAIR
V
IN
LT3502
2.2µF
(9a)
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 V
pin of the LT3502/LT3502A
IN
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 L
T3502/L
T3502A 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
V
20V/DIV
5A/DIV
IN
I
IN
DANGER!
RINGING VIN MAY EXCEED
ABSOLUTE MAXIMUM
RATING OF THE LT3502
20µs/DIV
(9b)
(9c)
V
20V/DIV
5A/DIV
V
20V/DIV
5A/DIV
IN
I
IN
20µs/DIV
IN
I
IN
20µs/DIV
LT3502
+
10µF
35V
AI.EI.
+
2.2µF
1Ω
LT3502
+
2.2µF0.1µF
Figure 9. A Well Chosen Input Network Prevents Input Voltage Overshoot and
Ensures Reliable Operation When the LT3502 is Connected to a Live Supply
3502 F09
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.
and that the capacitor on the VC node (CC) integrates the
error amplifier output current, resulting in two poles in the
loop. R
capacitor, the loop crossover occurs above the R
provides a zero. With the recommended output
C
zero.
CCC
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 (C
) across the feedback
PL
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.
If the output capacitor is different than the recommended
capacitor, stability should be checked across all operat
ing 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.
PCB Layout
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 V
node.
C
Note that the output capacitor integrates this current,
AMPLIFIER
1M
CURRENT MODE
–
POWER STAGE
=
g
m
1A/V
+
gm =
100µA/V
ERROR
SW
R1
–
FB
+
800mV
R2
C
ESR
C1
OUT
PL
+
C1
3502 F10
LT3502
R
150k
C
70pF
GND
0.5V
V
C
C
C
For proper operation and minimum EMI, care must
be taken during printed cir
cuit 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 V
IN
and
SW pins, the catch diode (D1) and the input capacitor (C2).
V
OUT
BD
= VIA
C1
C2
V
IN
FB
R1
SHDN
R2
BST
DA
L1
C3
D1
GND
3502 F11
18
Figure 10. Model for Loop Response
Figure 11
3502fd
applications inForMation
LT3502/LT3502A
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
R1
180k
L1
33µH
0.1µF
10V
22pF
V
15V
500mA
C1
10µF
3502 F12
OUT
V
20V TO 40V
1N4148
OR OTHER
SIMILAR
DIODES
IN
OFF ON
C2
1µF
V
IN
SHDN
BD
BOOST
SW
LT3502A
DA
FB
GND
Figure 12. 15V Step-Down Converter
R2
10k
C3
0.1µF
3502fd
19
LT3502/LT3502A
V
V
V
typical applications
0.8V Step-Down Converter
3V TO 7V
V
3V TO 40V
3V TO 7V
V
3V TO 40V
BD
IN
BD
IN
OFF ON
OFF ON
0.1µF
C2
1µF
0.1µF
C2
1µF
BD
V
IN
LT3502A
SHDN
GND
C1: JMK212BJ476MG
C3: HMK212BJ104MG
L1: LQH43CN3R3M03
BD
V
IN
LT3502A
SHDN
GND
C1: JMK212BJ226MG
L1: LQH43CN4R7M03
BOOST
BOOST
SW
DA
FB
SW
DA
BD
3V TO 7V
V
IN
L1
C3
3.3µH
0.1µF
D1
FB
V
OUT
0.8V
500mA
C1
47µF
3502 TA02a
3V TO 40V
OFF ON
0.1µF
C2
1µF
BD
V
BOOST
IN
LT3502
SHDN
GND
C1: JMK316BJ107ML
L1: LQH43CN100K03
SW
DA
L1
C3
10µH
0.1µF
D1
FB
V
OUT
0.8V
500mA
C1
100µF
3502 TA02b
1.8V Step-Down Converter
V
BD
R2
10k
C3
0.1µF
D1
R1
12.5k
L1
4.7µH
V
OUT
1.8V
500mA
C1
22µF
3502 TA03a
3V TO 7V
V
3V TO 40V
IN
OFF ON
0.1µF
C2
1µF
BD
V
IN
LT3502
SHDN
GND
C1: JMK212BJ476MG
L1: LQH55DN150M03
BOOST
SW
DA
L1
C3
15µH
0.1µF
D1
R1
FB
12.5k
R2
10k
V
OUT
1.8V
500mA
C1
47µF
3502 TA03b
3502fd
20
V
typical applications
LT3502/LT3502A
2.5V Step-Down Converter
3V TO 7V
V
3.5V TO 40V
BD
IN
OFF ON
0.1µF
C2
1µF
BD
V
BOOST
IN
LT3502A
SHDN
GND
C1: JMK212BJ226MG
L1: LQH43DN6R8M03
SW
DA
V
BD
3V TO 7V
V
IN
L1
C3
6.8µH
0.1µF
D1
R1
FB
21.3k
R2
10k
V
OUT
2.5V
500mA
C1
22µF
3502 TA04a
3.5V TO 40V
OFF ON
0.1µF
C2
1µF
BD
V
IN
LT3502
SHDN
GND
C1: JMK212BJ226MG
L1: LQH55DN150M03
BOOST
SW
DA
L1
C3
15µH
0.1µF
D1
R1
FB
21.3k
R2
10k
V
OUT
2.5V
500mA
C1
22µF
3502 TA04b
3.3V Step-Down Converter
V
4.7V TO 40V
IN
OFF ON
C2
1µF
BD
V
BOOST
IN
SW
LT3502A
SHDN
GND
C1: LMK316BJ106ML-BR
L1: LQH43CN6R8M03
DA
V
IN
L1
C3
6.8µH
0.1µF
D1
R1
FB
31.6k
R2
10k
V
OUT
3.3V
500mA
C1
10µF
3502 TA05a
4.5V TO 40V
C2
1µF
OFF ON
BD
V
BOOST
IN
LT3502
SHDN
GND
C1: JMK212BJ226MG
L1: LQH55DN150M03
SW
DA
L1
C3
15µH
0.1µF
D1
R1
FB
31.6k
R2
10k
V
OUT
3.3V
500mA
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)
1.37 ±0.05
(2 SIDES)
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
PIN 1 BAR
TOP MARK
(SEE NOTE 6)
0.200 REF
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
PACKAGE
OUTLINE
0.25 ± 0.05
0.45 BSC
R = 0.05
2.00 ±0.10
(4 SIDES)
0.75 ±0.05
0.00 – 0.05
R = 0.115
TYP
TYP
0.64 ± 0.10
(2 SIDES)
4
1.37 ±0.10
BOTTOM VIEW—EXPOSED PAD
(2 SIDES)
85
1
0.45 BSC
0.40 ± 0.10
PIN 1 NOTCH
R = 0.20 OR
0.25 × 45°
CHAMFER
(DC8) DFN 0106 REVØ
0.23 ± 0.05
22
3502fd
package Description
0.889 ± 0.127
LT3502/LT3502A
MS Package
10-Lead Plastic MSOP
(Reference LTC DWG # 05-08-1661 Rev E)
(.035 ± .005)
0.305 ± 0.038
(.0120 ± .0015)
TYP
GAUGE PLANE
5.23
(.206)
MIN
RECOMMENDED SOLDER PAD LAYOUT
0.254
(.010)
0.18
(.007)
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
3.20 – 3.45
(.126 – .136)
DETAIL “A”
DETAIL “A”
0.50
(.0197)
BSC
0° – 6° TYP
0.53 ± 0.152
(.021 ± .006)
SEATING
PLANE
3.00 ± 0.102
(.118 ± .004)
(NOTE 3)
4.90 ± 0.152
(.193 ± .006)
0.17 –0.27
(.007 – .011)
TYP
1.10
(.043)
MAX
1 2
0.50
(.0197)
BSC
8910
7
6
4 5
3
0.497 ± 0.076
(.0196 ± .003)
REF
3.00 ± 0.102
(.118 ± .004)
(NOTE 4)
0.86
(.034)
REF
0.1016 ± 0.0508
(.004 ± .002)
MSOP (MS) 0307 REV E
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.
3502fd
23
LT3502/LT3502A
typical application
5V Step-Down Converter
V
6.7V TO 40V
IN
C2
1µF
OFF ON
BD
V
BOOST
IN
LT3502A
SHDN
GND
C1: LMK316BJ106ML-BR
L1: LQH43CN100K03
SW
DA
V
IN
L1
C3
10µH
0.1µF
D1
R1
FB
52.3k
R2
10k
V
OUT
5V
500mA
C1
10µF
3502 TA06a
6.4V TO 40V
C2
1µF
OFF ON
relateD parts
PART NUMBER DESCRIPTION COMMENTS
LT1766 60V, 1.2A (I
), 200kHz, High Efficiency Step-Down
OUT
DC/DC Converter
LT1933 500mA (I
), 500kHz, Step-Down Switching Regulator in
OUT
SOT-23
LT1936 36V
, 1.4A (I
), 500kHz, High Efficiency Step-Down
OUT
DC/DC Converter
LT1940 Dual 25V
, 1.4A (I
), 1.1MHz, High Efficiency Step-Down
OUT
DC/DC Converter
LT1976/
L
T1977
3407/
LT C
LTC3407-2
LT3434/
L
T3435
60V, 1.2A (I
DC/DC Converters with Burst Mode
Dual 600mA/800mA, 1.5MHz/2.25MHz
Step-DownDC/DC Converters
60V, 1.2A (I
DC/DC Converters with Burst Mode Operation
LT3437 60V
, 400mA (I
), 200kHz/500kHz High Efficiency Step-Down
OUT
®
Operation
, Synchronous
), 200kHz/500kHz High Efficiency Step-Down
OUT
), Micropower Step-Down DC/DC Converter
OUT
with Burst Mode Operation
LT3493 36V
, 1.4A (I
), 750kHz, High Efficiency Step-Down
OUT
DC/DC Converter
LT3501 Dual 25V
, 3A (I
), 1.5MHz, High Efficiency Step-Down
OUT
DC/DC Converter
LT3503 20V
, 1A (I
), 2.2MHz, High Efficiency Step-Down
OUT
DC/DC Converter
LT3505 36V
, 1.2A (I
), 3MHz, High Efficiency Step-Down
OUT
DC/DC Converter
LT3506/
L
T3506A
Dual 25V, 1.6A (I
Down DC/DC Converters
LT3508 Dual 36V
, 1.4A (I
), 575kHz/1.1MHz, High Efficiency Step-
OUT
), 2.5MHz, High Efficiency Step-Down
OUT
DC/DC Converter
LT3510 Dual 25V
, 2A (I
), 1.5MHz, High Efficiency Step-Down
OUT
DC/DC Converter
LTC3548
Dual 400mA + 800mA, 2.25MHz Synchronous Step-Down
DC/DC Converter
Burst Mode is a registered trademark of Linear Technology Corporation. ThinSOT is a trademark of Linear Technology Corporation.
Linear Technology Corporation
24
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com
VIN: 5.5V to 60V, V
TSSOP16/TSSOP16E Packages
VIN: 3.6V to 36V, V
ThinSOT™ Package
VIN: 3.6V to 36V, V
MS8E Package
VIN: 3.6V to 25V, V
TSSOP16E Package
: 3.3V to 60V, V
V
IN
TSSOP16E Package
: 2.5V to 5.5V, V
V
IN
3mm × 3mm DFN, MS10E Package
VIN: 3.3V to 60V, V
TSSOP16E Package
VIN: 3.3V to 60V, V
DFN Package
VIN: 3.6V to 36V, V
DFN Package
VIN: 3.3V to 25V, V
TSSOP20E Package
VIN: 3.6V to 20V, V
2mm × 3mm DFN Package
VIN: 3.6V to 36V, V
3mm × 3mm DFN, MS8E Packages
VIN: 3.6V to 25V, V
4mm × 5mm DFN Package
VIN: 3.6V to 36V, V
4mm × 4mm QFN, TSSOP16E Packages
VIN: 3.3V to 25V, V
TSSOP20E Package
: 2.5V to 5.5V, V
V
IN
3mm × 3mm DFN, MS10E Packages
BD
V
BOOST
IN
SW
LT3502
DA
SHDN
OUT(MIN)
OUT(MIN)
OUT(MIN)
OUT(MIN)
OUT(MIN)
OUT(MIN)
OUT(MIN)
OUT(MIN)
OUT(MIN)
OUT(MIN)
OUT(MIN)
OUT(MIN)
OUT(MIN)
OUT(MIN)
OUT(MIN)
OUT(MIN)
FB
GND
C1: LMK316BJ106ML-BR
L1: LQH43CN100K03
= 1.2V, IQ = 2.5mA, ISD = 25µA,
= 1.2V, IQ = 1.6mA, ISD < 1µA,
= 1.2V, IQ = 1.9mA, ISD < 1µA,
= 1.20V, IQ = 3.8mA, ISD < 30µA,
= 1.20V, IQ = 100µA, ISD < 1µA,
= 0.6V, IQ = 40µA, ISD <1µA,
= 1.20V, IQ = 100µA, ISD < 1µA,
= 1.25V, IQ = 100µA, ISD < 1µA,
= 0.8V, IQ = 1.9mA, ISD < 1µA,
= 0.8V, IQ = 3.7mA, ISD < 10µA,
= 0.78V, IQ = 1.9mA, ISD < 1µA,
= 0.78V, IQ = 2mA, ISD < 2µA,
= 0.8V, IQ = 3.8mA, ISD < 30µA,
= 0.8V, IQ = 4.3mA, ISD < 1µA,
= 0.8V, IQ = 3.7mA, ISD < 10µA,
= 0.6V, IQ = 40µA, ISD < 1µA,
LT 0809 REV D • PRINTED IN USA
LINEAR TECHNOLOGY CORPORATION 2007
R2
10k
C3
0.1µF
D1
R1
52.3k
L1
22µH
V
OUT
5V
500mA
C1
22µF
3502 TA06b
3502fd
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