LT3508HUF#TRPBF [Linear]
LT3508 - Dual Monolithic 1.4A Step-Down Switching Regulator; Package: QFN; Pins: 24; Temperature Range: -40°C to 125°C;
| 型号： | LT3508HUF#TRPBF |
| 厂家： | 
Linear |
| 描述： | LT3508 - Dual Monolithic 1.4A Step-Down Switching Regulator; Package: QFN; Pins: 24; Temperature Range: -40°C to 125°C 开关 输出元件 |
| 文件： | 总26页 (文件大小：328K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LT3508
Dual Monolithic 1.4A
Step-Down Switching
Regulator
FEATURES
DESCRIPTION
The LT®3508 is a dual current mode PWM step-down
DC/DC converter with internal power switches capable of
generatingtwo1.4Aoutputs. Thewideinputvoltagerange
of 3.7V to 36V makes the LT3508 suitable for regulating
power from a wide variety of sources, including automo-
tivebatteries,24Vindustrialsuppliesandunregulatedwall
adapters. Bothconvertersaresynchronizedtoasingleos-
cillatorprogrammableupto2.5MHzandrunwithopposite
phases, reducing input ripple current. Its high operating
frequency allows the use of small, low cost inductors and
ceramic capacitors, resulting in low, predictable output
ripple. Each regulator has independent tracking and soft-
start circuits and generates a power good signal when its
output is in regulation, easing power supply sequencing
and interfacing with microcontrollers and DSPs.
n
Wide Input Voltage Range: 3.7V to 36V
n
Two 1.4A Output Switching Regulators with Internal
Power Switches
n
Adjustable 250kHz to 2.5MHz Switching Frequency
n
Synchronizable over the Full Frequency Range
n
Anti-Phase Switching Reduces Ripple
n
Uses Small Inductors and Ceramic Capacitors
n
Accurate Programmable Undervoltage Lockout
n
Independent Tracking, Soft-Start and Power Good
Circuits Ease Supply Sequencing
Output Adjustable Down to 800mV
n
n
Small 4mm × 4mm 24-Pin QFN or 16-Pin Thermally
Enhanced TSSOP Surface Mount Packages
APPLICATIONS
Cycle-by-cycle current limit, frequency foldback and ther-
malshutdownprovideprotectionagainstshortedoutputs,
and soft-start eliminates input current surge during start-
up. The low current (<2μA) shutdown mode enables easy
power management in battery-powered systems.
n
Automotive
n
DSP Power Supplies
n
Wall Transformer Regulation
n
DSL and Cable Modems
n
PCI Express
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and
No R
is a trademark of Linear Technology Corporation. All other trademarks are the
SENSE
property of their respective owners.
TYPICAL APPLICATION
3.3V and 5V Dual Output Step-Down Converter with Output Sequencing
V
IN
ON OFF
5.6V TO 36V
Efficiency
4.7ꢀF
V
V
SHDN
IN1 IN2
95
90
85
80
75
70
65
OUT2
5V
V
= 12V
IN
BOOST1
BOOST2
V
V
= 5V
1.4A
OUT2
OUT1
0.22ꢀF
0.22ꢀF
6.8ꢀH
10ꢀH
OUT1
3.3V
1.4A
SW1
SW2
= 3.3V
LT3508
35.7k
56.2k
FB1
FB2
V
V
C2
C1
11.5k
22ꢀF
51k
TRACK/SS1
TRACK/SS2
PG1
PG2
43k
10ꢀF
10.7k
100k
GND RT/SYNC
52.3k
= 700kHz
150pF
1nF
100pF
0
0.5
1
1.5
LOAD CURRENT (A)
POWER
GOOD
f
SW
3508 TA01a
3508 TA01b
3508fd
1
LT3508
(Notes 1, 7)
ABSOLUTE MAXIMUM RATINGS
Storage Temperature Range
V Pin Voltage............................................(–0.3V), 40V
IN
QFN.................................................... –65°C to 150°C
TSSOP ............................................... –65°C to 150°C
Lead Temperature (Soldering, 10 sec)
BOOST Pin Voltage ...................................................60V
BOOST Above SW Voltage ........................................30V
SHDN, PG Voltage.....................................................40V
TSSOP .............................................................. 300°C
TRACK/SS, FB, RT/SYNC, V Voltage..........................6V
C
Operating Junction Temperature Range (Note 2)
LT3508E............................................. –40°C to 125°C
LT3508I.............................................. –40°C to 125°C
LT3508H ............................................ –40°C to 150°C
PIN CONFIGURATION
TOP VIEW
TOP VIEW
TRACK/SS1
BOOST1
SW1
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
FB1
24 23 22 21 20 19
V
C1
1
2
3
4
5
6
18 FB2
FB1
TRACK/SS1
GND
PG1
TRACK/SS2
17
16
V
RT/SYNC
SHDN
PG2
IN1
17
GND
GND
25
GND
V
15 GND
GND
IN2
GND
SW2
BOOST2
GND
14
13 GND
V
GND
C2
TRACK/SS2
FB2
7
8
9 10 11 12
FE PACKAGE
16-LEAD PLASTIC TSSOP
θ
= 40°C/W, θ = 10°C/W
JC
JA
UF PACKAGE
24-LEAD (4mm × 4mm) PLASTIC QFN
EXPOSED PAD (PIN 17) IS GND AND MUST BE SOLDERED TO PCB
θ
JA
= 40°C/W, θ = 10°C/W
JC
EXPOSED PAD (PIN 25) IS GND AND MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
LT3508EFE#PBF
LT3508IFE#PBF
LT3508HFE#PBF
LT3508EUF#PBF
LT3508IUF#PBF
LT3508HUF#PBF
TAPE AND REEL
PART MARKING*
3508FE
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3508EFE#TRPBF
LT3508IFE#TRPBF
LT3508HFE#TRPBF
LT3508EUF#TRPBF
LT3508IUF#TRPBF
LT3508HUF#TRPBF
16-Lead Plastic TSSOP
–40°C to 125°C
–40°C to 125°C
–40°C to 150°C
–40°C to 125°C
–40°C to 125°C
–40°C to 150°C
3508FE
16-Lead Plastic TSSOP
3508HFE
3508
16-Lead Plastic TSSOP
24-Lead (4mm × 4mm) Plastic QFN
24-Lead (4mm × 4mm) Plastic QFN
24-Lead (4mm × 4mm) Plastic QFN
3508
3508H
Consult LTC Marketing for parts specified with wider operating temperature ranges.*Temperature grades are 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/
3508fd
2
LT3508
ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 12V unless otherwise noted. (Note 2)
PARAMETER
CONDITIONS
MIN
TYP
3.4
MAX
3.7
3.0
5.2
500
2
UNITS
V
l
l
Minimum Operating Voltage, V
Minimum Operating Voltage, V
IN1
V
= 12V
2.5
V
IN2
IN1
V
V
Quiescent Current
Quiescent Current
Not Switching
Not Switching
4.3
mA
ꢀA
IN1
IN2
320
0.1
Shutdown Current (V + V
)
IN2
V
= 0.3V
SHDN
ꢀA
IN1
FB Voltage
0.790
0.784
0.800
0.814
0.816
V
V
l
l
FB Pin Bias Current (Note 3)
FB Voltage Line Regulation
Error Amp Transconductance
Error Amp Voltage Gain
V
= 0.800V, V = 0.5V
50
0.01
300
600
2.5
1
300
nA
%/V
ꢀS
FB
C
5V < V < 40V
IN
V/V
A/V
MHz
Deg
V to Switch Current Gain
C
l
l
Switching Frequency
Switching Phase
R = 33.2k
0.92
150
84
1.06
210
T
R = 33.2k
180
T
Maximum Duty Cycle (Note 4)
R = 33.2k
90
80
98
%
%
%
T
T
T
R = 7.50k
R = 169k
Foldback Frequency
R = 33.2k, V = 0V
120
2.6
kHz
A
T
FB
l
Switch Current Limit (Note 5)
Duty Cycle = 15%
2.0
3.2
Switch V
I
= 1.5A
300
0.01
1.7
mV
ꢀA
V
CESAT
SW
Switch Leakage Current
Minimum Boost Voltage
Boost Pin Current
1
2.5
50
I
= 1.5A, V
= 17V
35
mA
ꢀA
mV
V
SW
BOOST
TRACK/SS Pin Current
PG Threshold Offset
V
V
V
V
= 0V
0.8
56
1.2
2.2
110
0.4
1
TRACK/SS
Rising
75
FB
FB
PG
PG Voltage Output Low
PG Pin Leakage
= 0.6V, I = 250ꢀA
0.13
0.01
2.63
8
PG
= 2V
ꢀA
V
SHDN Threshold Voltage
SHDN Input Current (Note 6)
2.53
6
2.73
10
V
= 60mV Above Threshold Voltage
ꢀA
ꢀA
V
SHDN
SHDN Threshold Current Hysteresis
SYNC Threshold Voltage
5.5
1
7.5
9.5
1.5
2.5
1.25
SYNC Input Frequency
0.25
MHz
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 3: Current flows out of pin.
Note 4: V =12V. Circuitry increases the maximum duty cycle of the
BOOST
LT3508 when V
> V + 2.5V. See Minimum Operating Voltage in the
BOOST
IN
Applications Information section for details.
Note 2: The LT3508E is guaranteed to meet performance specifications
from 0°C to 125°C junction temperature. Specifications over the –40°C
to 125°C operating junction temperature range are assured by design,
characterization and correlation with statistical process controls. The
LT3508I is guaranteed over the full –40°C to 125°C operating junction
temperature range. The LT3508H is guaranteed over the full –40°C to
150°C operating junction temperature range. High junction temperatures
degrade operating lifetimes. Operating lifetime is derated at junction
temperatures greater than 125°C.
Note 5: Current limit is guaranteed by design and/or correlation to static
test. Slope compensation reduces current limit at higher duty cycles.
Note 6: Current flows into pin.
Note 7: This IC includes overtemperature protection that is intended
to protect the device during momentary overload conditions. Junction
temperature will exceed the maximum operating junction temperature
range when overtemperature protection is active. Continuous operation
above the specified maximum operating junction temperature may impair
device reliability.
3508fd
3
LT3508
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency, VOUT = 3.3V
Efficiency, VOUT = 1.8V
Efficiency, VOUT = 5V
90
85
80
75
70
65
60
95
90
85
80
75
70
65
85
80
75
70
65
60
55
T
= 25°C
T
= 25°C
T = 25°C
A
A
A
f = 700kHz
f = 700kHz
f = 1MHz
V
= 12V
= 24V
IN
V
= 3.3V
V
= 12V
= 24V
IN
IN
V
IN
V
IN
V
= 5V
IN
V
= 32V
IN
V
= 12V
IN
V
= 32V
IN
0
0.5
1
1.5
0
0.5
1
1.5
0
0.5
1
1.5
LOAD CURRENT (A)
LOAD CURRENT (A)
LOAD CURRENT (A)
3508 G01
3508 G02
3508 G03
Switch Current Limit
vs Temperature
Switch Current Limit
vs Duty Cycle
Feedback Voltage
3.0
2.5
3.0
2.5
2.0
1.5
0.810
0.805
0.800
0.795
T
= 25°C
A
TYPICAL
2.0
1.5
MINIMUM
1.0
0.5
0
1.0
0.5
0
0.790
0
20
40
60
80
100
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3508 G05
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
DUTY CYCLE (%)
3508 G06
3508 G04
Switching Frequency
vs Temperature
Switching Frequency vs RT
Switching Frequency Foldback
175
150
125
100
75
1.2
1.0
0.8
0.6
3.0
2.5
2.0
1.5
1.0
0.5
0
T
= 25°C
R
= 33.2k
T
= 25°C
A
T
A
R
= 7.50k
= 33.2k
T
R
T
0.4
0.2
0
50
25
R
T
= 169k
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3508 G08
0
0.5
1.0
1.5
2.0
2.5
0
100 200 300 400 500 600 700 800
FEEDBACK VOLTAGE (mV)
FREQUENCY (MHz)
3508 G07
3508 G09
3508fd
4
LT3508
TYPICAL PERFORMANCE CHARACTERISTICS
Quiescent Current
VC Voltages
Error Amp Output Current
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
35
30
2.5
2.0
1.5
1.0
0.5
0
T
= 25°C
A
V
IN1
CLAMP VOLTAGE
SINKING
25
20
15
10
5
SOURCING
TO SWITCH
V
IN2
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3508 G12
0
30 35
5
10 15
20
INPUT VOLTAGE (V)
25
40
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3508 G10
3508 G11
Switch Voltage Drop
Boost Pin Current
35
30
25
20
15
10
5
350
300
250
200
150
100
50
T
= 25°C
T = 25°C
A
A
0
0
1
1.5
1
1.5
0
0.5
0
0.5
SWITCH CURRENT (A)
SWITCH CURRENT (A)
3508 G14
3508 G13
SHDN Pin Current
Undervoltage Lockout
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
120
100
80
60
40
20
0
V
T
= –45°C
IN1
A
T
= 125°C
A
V
IN2
T
= 25°C
A
25 30
10 15 20
SHDN PIN VOLTAGE (V)
0
5
35 40
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3508 G16
3508 G15
3508fd
5
LT3508
PIN FUNCTIONS
BOOST1, BOOST2: The BOOST pins are used to provide
drivevoltages,higherthantheinputvoltage,totheinternal
NPN power switches. Tie through a diode to a 2.8V or
from operating until the input voltage has reached the
programmed level. Do not drive SHDN more than 6V
above V
.
IN1
higher supply, such as V
or V .
OUT
IN
SW1, SW2: The SW pins are the outputs of the internal
powerswitches.Connectthesepinstotheinductors,catch
diodes and boost capacitors.
Exposed Pad: The exposed pad metal of the package pro-
vides both electrical contact to ground and good thermal
contacttotheprintedcircuitboard. Theexposedpadmust
be soldered to the circuit board for proper operation.
TRACK/SS1, TRACK/SS2: The TRACK/SS pins are used
to soft-start the two channels, to allow one channel to
track the other output, or to allow both channels to track
another output. For tracking, tie a resistor divider to this
pin from the tracked output. For soft-start, tie a capacitor
to this pin. An internal 1.2ꢀA soft-start current charges
the capacitor to create a voltage ramp at the pin. In the
TSSOP package, if these pins are unused, tie a 47pF or
greater capacitor from each pin to GND.
FB1, FB2: The LT3508 regulates each feedback pin to
0.800V. Connect the feedback resistor divider taps to
these pins.
GND: Tie the GND pins directly to the Exposed Pad and
ground plane.
PG1, PG2: The power good pins are the open-collector
outputs of an internal comparator. PG remains low until
the FB pin is within 10% of the final regulation voltage.
As well as indicating output regulation, the PG pins can
be used to sequence the two switching regulators. These
pins can be left unconnected. The PG outputs are valid
V , V : The V pins are the outputs of the internal error
C1 C2
C
amps. The voltages on these pins control the peak switch
currents. These pins are normally used to compensate the
control loops, but can also be used to override the loops.
Pull these pins to ground with an open drain to shut down
each switching regulator separately.
when V is greater than 3.7V and SHDN is high. The PG
IN1
comparators are disabled in shutdown.
V
: The V pin supplies current to the LT3508 internal
IN1
IN1
RT/SYNC: The RT/SYNC pin is used to set the internal
oscillator frequency. Tie a 33.2k resistor from RT/SYNC
to GND for a 1MHz switching frequency. To synchronize
the part to an external frequency, drive the RT/SYNC pin
with a logic-level signal with positive and negative pulse
widths of at least 80ns.
circuitry and to the internal power switch connected to
SW1 and must be locally bypassed. V must be greater
IN1
than 3.7V for channel 1 or channel 2 to operate. If V
IN1
is greater than 3.7V, channel 2 can operate with V as
IN2
low as 3V.
V
: The V pin supplies current to the internal power
IN2
IN2
SHDN: The shutdown pin is used to put the LT3508 in
shutdown mode. Pull the pin below 0.3V to shut down the
LT3508. The 2.63V threshold can function as an accurate
undervoltage lockout (UVLO), preventing the regulator
switch connected to SW2 and must be locally bypassed.
Connect this pin directly to V unless power for chan-
IN1
nel 2 is coming from a different source. V must be
IN2
greater than 3V and V must be greater than 3.7V for
IN1
channel 2 to operate.
3508fd
6
LT3508
BLOCK DIAGRAM
SHDN
V
IN1
RT/SYNC
CLK1
CLK2
INT REG
AND REF
MASTER
OSC
1.2ꢀA
V
IN
TRACK/SS
V
IN
C
IN
0.75V
+
Σ
SLOPE
R
BOOST
SW
D2
L1
S
Q
C1
SLAVE
OSC
C3
CLK
OUT
D1
C1
+
–
ERROR
AMP
0.625V
FB
R1
–
V
C
C
+
+
TRACK/SS
0.80V
R2
R
C
–
C
F
C
+
+
75mV
I
LIMIT
CLAMP
PG
GND
+
–
3508 F01
Figure 1. Block Diagram of the LT3508 with Associated External Components (One of Two Switching Regulators Shown)
3508fd
7
LT3508
OPERATION
The LT3508 is a dual constant frequency, current mode
regulator with internal power switches. Operation can be
best understood by referring to the Block Diagram. If the
SHDN pin is tied to ground, the LT3508 is shut down and
draws minimal current from the input source tied to the
between the two modes. Unique circuitry generates the
appropriate slope compensation ramps and generates the
180° out-of-phase clocks for the two channels.
The switching regulator performs frequency foldback
during overload conditions. An amplifier senses when
V
pins. If the SHDN pin exceeds 1V, the internal bias
IN
V
is less than 0.625V and begins decreasing the oscil-
FB
circuits turn on, including the internal regulator, reference
and oscillator. The switching regulators will only begin to
operate when the SHDN pin exceeds 2.63V.
lator frequency down from full frequency to 12% of the
nominal frequency when V = 0V. The FB pin is less than
FB
0.8Vduringstart-up,short-circuitandoverloadconditions.
Frequency foldback helps limit switch current under these
conditions.
Theswitcherisacurrentmoderegulator.Insteadofdirectly
modulatingthedutycycleofthepowerswitch,thefeedback
loop controls the peak current in the switch during each
cycle. Compared to voltage mode control, current mode
control improves loop dynamics and provides cycle-by-
cycle current limit. A pulse from the oscillator sets the
RS flip-flop and turns on the internal NPN power switch.
Current in the switch and the external inductor begins to
increase. When this current exceeds a level determined
The switch driver operates either from V or from the
IN
BOOST pin. An external capacitor and Schottky diode are
used to generate a voltage at the BOOST pin that is higher
than the input supply. This allows the driver to saturate the
internal bipolar NPN power switch for efficient operation.
The TRACK/SS pin serves as an alternative input to the
erroramplifier.Theamplifierwillusethelowestvoltageof
either the reference of 0.8V or the voltage on the TRACK/
SS pin as the positive input of error amplifier. Since the
TRACK/SS pin is driven by a constant current source, a
single capacitor on the pin will generate a linear ramp on
the output voltage. Tying the TRACK/SS pin to a resistor
dividerfromtheoutputofoneoftheswitchingregulators
allows one output to track another.
by the voltage at V , current comparator C1 resets the
C
flip-flop, turning off the switch. The current in the inductor
flows through the external Schottky diode and begins to
decrease.Thecyclebeginsagainatthenextpulsefromthe
oscillator. In this way, the voltage on the V pin controls
C
thecurrentthroughtheinductortotheoutput. Theinternal
error amplifier regulates the output current by continually
adjusting the V pin voltage. The threshold for switching
C
on the V pin is 0.8V, and an active clamp of 1.75V limits
C
The PG output is an open-collector transistor that is off
when the output is in regulation, allowing an external re-
sistor to pull the PG pin high. Power good is valid when
the output current.
The switching frequency is set either by the resistance to
GND at the RT/SYNC pin or the frequency of the logic-level
signaldrivingtheRT/SYNCpin.Adetectioncircuitmonitors
for the presence of a SYNC signal on the pin and switches
the LT3508 is enabled (SHDN is high) and V is greater
IN1
than 3.7V.
3508fd
8
LT3508
APPLICATIONS INFORMATION
Setting the Output Voltage
where V is the forward voltage drop of the catch diode
F
(~0.4V) and V is the voltage drop of the internal switch
SW
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the 1% resis-
tors according to:
(~0.4V at maximum load).
Example: I = 1.5A and I
SW
= 50mA, V
= 3.3V,
OUT
SW
BOOST
β
= 1.5A/50mA = 30, DC
= 1/(1+1/30) = 96%:
MAX
⎛
⎞
VOUT
0.8V
R1= R2
– 1
3.3V + 0.4V
⎜
⎝
⎟
V
=
– 0.4V + 0.4V = 3.8V
⎠
IN(MIN)
96%
R2 should be 20k or less to avoid bias current errors.
Reference designators refer to the Block Diagram.
Maximum Operating Voltage
The maximum operating voltage is determined by the
Minimum Operating Voltage
Absolute Maximum Ratings of the V and BOOST pins,
IN
The minimum operating voltage is determined either by
theLT3508’sundervoltagelockoutorbyitsmaximumduty
and by the minimum duty cycle:
DC
= t
• f
MIN
ON(MIN)
cycle. If V and V are tied together, the undervoltage
IN1
IN2
where t
is equal to 130ns (for T > 125°C t
J ON(MIN)
lockout is at 3.7V or below. If the two inputs are used
ON(MIN)
is equal to 150ns) and f is the switching frequency.
Running at a lower switching frequency allows a lower
minimum duty cycle. The maximum input voltage before
pulse skipping occurs depends on the output voltage and
the minimum duty cycle:
separately, then V has an undervoltage lockout of 3.7V
IN1
or below and V has an undervoltage lockout of 3V or
IN2
below. Because the internal supply runs off V , chan-
IN1
nel 2 will not operate unless V > 3.7V. The duty cycle
IN1
is the fraction of time that the internal switch is on and is
determined by the input and output voltages:
VOUT + V
DCMIN
F
V
=
– V + V
F
SW
IN(PS)
VOUT + V
F
DC =
V – V + V
IN
F
SW
Example:f=790kHz, V
= 0.103:
=3.3V, DC =130ns•790kHz
OUT
MIN
Unlike many fixed frequency regulators, the LT3508 can
extend its duty cycle by turning on for multiple cycles.
The LT3508 will not switch off at the end of each clock
cycle if there is sufficient voltage across the boost capaci-
tor (C3 in Figure 1). Eventually, the voltage on the boost
capacitor falls and requires refreshing. Circuitry detects
this condition and forces the switch to turn off, allowing
the inductor current to charge up the boost capacitor. This
places a limitation on the maximum duty cycle as follows:
3.3V + 0.4V
V
=
– 0.4V + 0.4V = 36V
IN(PS)
0.103
TheLT3508willregulatetheoutputcurrentatinputvoltages
greater than V . For example, an application with an
IN(PS)
output voltage of 1.8V and switching frequency of 1.5MHz
hasaV of11.3V,asshowninFigure2.Figure3shows
IN(PS)
operation at 18V. Output ripple and peak inductor current
have significantly increased. Exceeding V is safe if
1
DCMAX
=
IN(PS)
1
theoutputisinregulation,iftheexternalcomponentshave
adequate ratings to handle the peak conditions and if the
peak inductor current does not exceed 3.2A. A saturating
inductor may further reduce performance. Do not exceed
1+
βSW
where β is equal to the SW pin current divided by the
SW
BOOST pin current as shown in the Typical Performance
Characteristics section. This leads to a minimum input
voltage of:
V
duringstart-uporoverloadconditions(foroutputs
IN(PS)
greater than 5V, use V
= 5V to calculate V
). For
OUT
IN(PS)
operation above 20V in pulse skipping mode, program
the switching frequency to 1.1MHz or less.
VOUT + V
DCMAX
F
V
=
– V + V
F
SW
IN(MIN)
3508fd
9
LT3508
APPLICATIONS INFORMATION
Table 1. Programming the Switching Frequency
V
OUT
100mV/DIV
(AC)
SWITCHING FREQUENCY (MHz)
R (kΩ)
T
2.5
2.2
2
7.50
9.76
11.5
14
I
L
500mA/DIV
1.8
1.6
1.4
1.2
1
3508 F02
2ꢀs/DIV
16.9
20.5
26.1
33.2
38.3
44.2
52.3
61.9
76.8
88.7
100
Figure 2. Operation Below VIN(PS). VIN = 10V, VOUT = 1.8V and
fSW = 1.5MHz
0.9
0.8
0.7
0.6
0.5
0.45
0.4
0.35
0.3
0.25
V
OUT
100mV/DIV
(AC)
I
L
500mA/DIV
3508 F03
2ꢀs/DIV
115
Figure 3. Operation Above VIN(PS). VIN = 18V, VOUT = 1.8V
and fSW = 1.5MHz. Output Ripple and Peak Inductor Current
Increase
140
169
Inductor Selection and Maximum Output Current
A good first choice for the inductor value is:
1.2µH
Setting the Switching Frequency
The switching frequency is programmed either by driving
the RT/SYNC pin with a logic level SYNC signal or by tying
a resistor from the RT/SYNC pin to ground. A graph for
L = V
+ V •
(
)
F
OUT
f
where V is the voltage drop of the catch diode (~0.4V)
F
selecting the value of R for a given operating frequency
T
and f is in MHz. The inductor’s RMS current rating must be
greater than the maximum load current and its saturation
currentshouldbeatleast30%higher.Forhighestefficiency,
the series resistance (DCR) should be less than 0.1Ω.
Table 2 lists several vendors and types that are suitable.
is shown in the Typical Application section. Suggested
programming resistors for various switching frequencies
are shown in Table 1.
Choosingahighswitchingfrequencywillallowthesmallest
overall solution size. However, at high input voltages the
efficiency can drop significantly with increasing switching
frequency. The choice of switching frequency will also
impact the input voltage range, inductor and capacitor
selection, and compensation. See the related sections
for details.
Table 2. Inductor Vendors
VENDOR
Coilcraft
Murata
TDK
URL
PART SERIES
MSS7341
LQH55D
TYPE
Shielded
Open
www.coilcraft
www.murata.com
www.component.tdk.com
SLF7045
SLF10145
Shielded
Shielded
Toko
www.toko.com
DC62CB
D63CB
D75C
Shielded
Shielded
Shielded
Open
D75F
Sumida
www.sumida.com
CR54
CDRH74
CDRH6D38
CR75
Open
Shielded
Shielded
Open
3508fd
10
LT3508
APPLICATIONS INFORMATION
The optimum inductor for a given application may differ
fromtheoneindicatedbythissimpledesignguide.Alarger
value inductor provides a higher maximum load current,
and reduces the output voltage ripple. If your load is lower
than the maximum load current, then you can relax the
value of the inductor and operate with higher ripple cur-
rent. This allows you to use a physically smaller inductor,
or one with a lower DCR resulting in higher efficiency.
Be aware that if the inductance differs from the simple
rule above, then the maximum load current will depend
on input voltage. In addition, low inductance may result
in discontinuous mode operation, which further reduces
maximumloadcurrent. Fordetailsofdiscontinuousmode
operation, see Application Note 44. Finally, for duty cycles
to 1.55A at DC = 90%. The maximum output current is a
function of the chosen inductor value:
ΔIL
2
ΔIL
2
IOUT(MAX) = ILIM
–
= 2A • 1– 0.25 •DC –
(
)
Choosing an inductor value so that the ripple current is
smallwillallowamaximumoutputcurrentneartheswitch
current limit.
One approach to choosing the inductor is to start with the
simplerulegivenabove,lookattheavailableinductors,and
choose one to meet cost or space goals. Then use these
equations to check that the LT3508 will be able to deliver
therequiredoutputcurrent.Noteagainthattheseequations
assumethattheinductorcurrentiscontinuous.Discontinu-
greaterthan50%(V /V >0.5), aminimuminductance
OUT IN
is required to avoid sub-harmonic oscillations:
ous operation occurs when I
is less than ∆I /2.
OUT
L
0.8µH
LMIN = V
+ V •
Input Capacitor Selection
(
)
F
OUT
f
Bypass the V pins of the LT3508 circuit with a ceramic
IN
where f is in MHz. The current in the inductor is a triangle
wave with an average value equal to the load current. The
peak switch current is equal to the output current plus
half the peak-to-peak inductor ripple current. The LT3508
limits its switch current in order to protect itself and the
system from overload faults. Therefore, the maximum
output current that the LT3508 will deliver depends on
the switch current limit, the inductor value, and the input
and output voltages.
capacitor of X7R or X5R type. For switching frequen-
cies above 500kHz, use a 4.7ꢀF capacitor or greater. For
switchingfrequenciesbelow500kHz, usea10ꢀForhigher
capacitor. If the V pins are tied together only a single
IN
capacitor is necessary. If the V pins are separated, each
IN
pin will need its own bypass. The following paragraphs
describetheinputcapacitorconsiderationsinmoredetail.
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 LT3508 input and to force this switching current into a
tight local loop, minimizing EMI. The input capacitor must
have low impedance at the switching frequency to do this
effectively, and it must have an adequate ripple current
rating.Withtwoswitchersoperatingatthesamefrequency
but with different phases and duty cycles, calculating the
input capacitor RMS current is not simple. However, a
conservativevalueistheRMSinputcurrentforthechannel
When the switch is off, the potential across the inductor
is the output voltage plus the catch diode drop. This gives
the peak-to-peak ripple current in the inductor:
1–DC V
+ V
F
(
)(
)
OUT
ΔIL =
L • f
where f is the switching frequency of the LT3508 and L
is the value of the inductor. The peak inductor and switch
current is:
that is delivering most power (V
times I ):
OUT
OUT
ΔIL
2
ISW(PK) = IL(PK) = IOUT
+
VOUT V – V
(
)
IOUT
OUT
IN
ICIN(RMS) = IOUT
•
<
V
2
IN
To maintain output regulation, this peak current must be
less than the LT3508’s switch current limit I . I is
LIM LIM
and is largest when V = 2V
(50% duty cycle). As
IN
OUT
at least 2A for at low duty cycles and decreases linearly
the second, lower power channel draws input current,
3508fd
11
LT3508
APPLICATIONS INFORMATION
the input capacitor’s RMS current actually decreases as
the out-of-phase current cancels the current drawn by
the higher power channel. Considering that the maximum
load current from a single channel is ~1.4A, RMS ripple
current will always be less than 0.7A.
LT3508’s control loop. Ceramic capacitors have very low
equivalent series resistance (ESR) and provide the best
ripple performance. A good value is:
50V 1MHz
COUT
=
•
VOUT
f
The high frequency of the LT3508 reduces the energy stor-
age requirements of the input capacitor. The combination
of small size and low impedance (low equivalent series
resistance or ESR) of ceramic capacitors makes them the
preferred choice. The low ESR results in very low voltage
ripple. Ceramic capacitors can handle larger magnitudes
of ripple current than other capacitor types of the same
value. Use X5R and X7R types.
where C
is in ꢀF. Use X5R or X7R types. This choice
OUT
willprovidelowoutputrippleandgoodtransientresponse.
Transient performance can be improved with a high value
capacitor if the compensation network is also adjusted to
maintaintheloopbandwidth.Alowervalueofoutputcapaci-
torcanbeused, buttransientperformancewillsuffer. With
an external compensation network, the loop gain can be
loweredtocompensateforthelowercapacitorvalue.Look
carefully at the capacitor’s data sheet to find out what the
actual capacitance is under operating conditions (applied
voltage and temperature). A physically larger capacitor, or
one with a higher voltage rating, may be required. 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 0.05Ω
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 3 lists
several capacitor vendors.
An alternative to a high value ceramic capacitor is a lower
valueceramicalongwithalargerelectrolyticcapacitor.The
electrolytic capacitor likely needs to be greater than 10ꢀF
in order to meet the ESR and ripple current requirements.
The input capacitor is likely to see high surge currents
when the input source is applied. Tantalum capacitors
can fail due to an oversurge of current. Only use tantalum
capacitors with the appropriate surge current rating. The
manufacturer may also recommend operation below the
rated voltage of the capacitor.
A final caution is in order regarding the use of ceramic
capacitors at the input. A ceramic input capacitor can
combine with stray inductance to form a resonant tank
circuit.Ifpowerisappliedquickly(forexamplebyplugging
the circuit into a live power source), this tank can ring,
doubling the input voltage and damaging the LT3508. The
solution is to either clamp the input voltage or dampen
the tank circuit by adding a lossy capacitor in parallel with
the ceramic capacitor. For details see Application Note 88.
Table 3. Capacitor Vendors
VENDOR
PART SERIES
COMMENTS
Panasonic
Ceramic
Polymer
Tantalum
EEF Series
Kemet
Sanyo
Ceramic
Tantalum
T494, T495
POSCAP
Ceramic
Polymer
Tantalum
Output Capacitor Selection
Murata
AVX
Ceramic
The output capacitor has two essential functions. Along
withtheinductor,itfiltersthesquarewavegeneratedbythe
LT3508toproducetheDCoutput. Inthisroleitdetermines
the output ripple, and low impedance at the switching
frequency is important. The second function is to store
energy in order to satisfy transient loads and stabilize the
Ceramic
Tantalum
TPS Series
Taiyo Yuden
TDK
Ceramic
Ceramic
3508fd
12
LT3508
APPLICATIONS INFORMATION
Diode Selection
and higher, the standard circuit (Figure 4a) is best. For
outputs between 2.8V and 3.3V, use a small Schottky
diode (such as the BAT-54). For lower output voltages,
the boost diode can be tied to the input (Figure 4b). The
circuit in Figure 4a is more efficient because the boost pin
current comes from a lower voltage source. Finally, the
anode of the boost diode can be tied to another source
The catch diode (D1 from Figure 1) conducts current only
during switch off time. Average forward current in normal
operation can be calculated from:
IOUT V – V
(
)
IN
OUT
ID(AVG)
=
V
IN
(V ) that is at least 3V (Figure 4c). For example, if you
AUX
are generating a 3.3V output, and the 3.3V output is on
whenever the particular channel is on, the anode of the
BOOST diode can be connected to the 3.3V output. In any
case, be sure that the maximum voltage at the BOOST pin
is both less than 60V and the voltage difference between
the BOOST and SW pins is less than 30V.
The only reason to consider a diode with a larger current
rating than necessary for nominal operation is for the
worst-case condition of shorted output. The diode cur-
rent will then increase to the typical peak switch current.
Peakreversevoltageisequaltotheregulatorinputvoltage.
Use a diode with a reverse voltage rating greater than the
inputvoltage.Table4listsseveralSchottkydiodesandtheir
manufacturers.Ifoperatingathighambienttemperatures,
consider using a Schottky with low reverse leakage.
D2
C3
C3
C3
BOOST
LT3508
V
V
OUT
V
SW
IN
IN
Table 4. Schottky Diodes
V
I
V at 1A V at 2A
R
AVE
F
F
GND
PART NUMBER
On Semiconductor
MBR0520L
MBR0540
MBRM120E
MBRM140
Diodes Inc.
B0530W
(V)
(A)
(mV)
(mV)
V
– V ≅ V
SW OUT
BOOST
MAX V
≅ V + V
IN OUT
BOOST
20
40
20
40
0.5
0.5
1
(4a)
620
530
550
D2
1
BOOST
LT3508
V
V
OUT
V
SW
IN
IN
30
20
30
40
40
40
0.5
1
B120
500
500
GND
B130
1
B140HB
1
V
– V ≅ V
BOOST
SW
BOOST
IN
IN
MAX V
≅ 2V
DFLS140
1.1
2
510
(4b)
B240
500
D2
V
> 3V
AUX
BOOST Pin Considerations
BOOST
LT3508
The capacitor and diode tied to the BOOST pin generate
a voltage that is higher than the input voltage. In most
cases, a 0.22ꢀF capacitor and fast switching diode (such
as the CMDSH-3 or MMSD914LT1) will work well. For ap-
plications 1MHz or faster, a 0.1ꢀF capacitor is sufficient.
Use a 0.47ꢀF capacitor or greater for applications running
below 500kHz. Figure 4 shows three ways to arrange the
boost circuit. The BOOST pin must be more than 2.5V
above the SW pin for full efficiency. For outputs of 3.3V
V
V
OUT
V
SW
IN
IN
GND
V
– V ≅ V
SW AUX
BOOST
3508 F04
MAX V
≅ V
+ V
IN
V
BOOST
AUX
MINIMUM VALUE FOR AUX = 3V
(4c)
Figure 4. Generating the Boost Voltage
3508fd
13
LT3508
APPLICATIONS INFORMATION
many cases the discharged output capacitor will present
a load to the switcher that will allow it to start. The plots
The minimum operating voltage of an LT3508 applica-
tion is limited by the undervoltage lockout (≈3.7V) and
by the maximum duty cycle. The boost circuit also limits
the minimum input voltage for proper start-up. If the
input voltage ramps slowly, or the LT3508 turns on when
the output is already in regulation, the boost capacitor
may not be fully charged. Because the boost capacitor
charges with the energy stored in the inductor, the circuit
will rely on some minimum load current to get the boost
circuit running properly. This minimum load will depend
on input and output voltages, and on the arrangement of
the boost circuit. The minimum load current generally
goes to zero once the circuit has started. Figure 5 shows
a plot of minimum load to start and to run as a function
of input voltage. Even without an output load current, in
show the worst case, where V is ramping very slowly.
IN
Frequency Compensation
The LT3508 uses current mode control to regulate the
output.Thissimplifiesloopcompensation.Inparticular,the
LT3508 does not require the ESR of the output capacitor
for stability, so you are free to use ceramic capacitors to
achieve low output ripple and small circuit size.
Frequency compensation is provided by the components
tied to the V pin, as shown in Figure 1. Generally a capaci-
C
tor (C ) and a resistor (R ) in series to ground are used. In
C
C
addition, there may be a lower value capacitor in parallel.
This capacitor (C ) is not part of the loop compensation
F
Minimum Input Voltage, VOUT = 3.3V
but is used to filter noise at the switching frequency, and
is required only if a phase-lead capacitor is used or if the
output capacitor has high ESR.
6.5
T
= 25°C
OUT
A
V
= 3.3V
6.0
5.5
Loop compensation determines the stability and transient
performance.Designingthecompensationnetworkisabit
complicatedandthebestvaluesdependontheapplication
and in particular the type of output capacitor. A practical
approach is to start with one of the circuits in this data
sheet that is similar to your application and tune the com-
pensation network to optimize the performance. Stability
should then be checked across all operating conditions,
includingloadcurrent, inputvoltageandtemperature. The
LT1375datasheetcontainsamorethoroughdiscussionof
loop compensation and describes how to test the stability
using a transient load.
5.0
4.5
4.0
3.5
3.0
TO START
TO RUN
10
100
10000
1
1000
LOAD CURRENT (mA)
3508 F05a
Minimum Input Voltage, VOUT = 5V
9
8
7
6
5
4
T
V
= 25°C
= 5V
A
OUT
Figure6showsanequivalentcircuitfortheLT3508control
loop. The error amplifier is a transconductance amplifier
withfiniteoutputimpedance.Thepowersection,consisting
of the modulator, power switch and inductor, is modeled
as a transconductance amplifier generating an output
TO START
TO RUN
current proportional to the voltage at the V pin. Note that
C
the output capacitor integrates this current, and that the
capacitor on the V pin (C ) integrates the error amplifier
C
C
output current, resulting in two poles in the loop. In most
1
10
100
1000
10000
cases a zero is required and comes from either the output
LOAD CURRENT (mA)
capacitor ESR or from a resistor R in series with C .
C
C
3508 G05b
This simple model works well as long as the value of the
Figure 5. The Minimum Input Voltage Depends on Output
Voltage, Load Current and Boost Circuit
inductor is not too high and the loop crossover frequency
3508fd
14
LT3508
APPLICATIONS INFORMATION
is much lower than the switching frequency. A phase-lead
If an adjustable UVLO threshold is required, the SHDN
pin can be used. The threshold voltage of the SHDN pin
comparatoris2.63V.Currenthysteresisisaddedabovethe
SHDNthreshold.Thiscanbeusedtosetvoltagehysteresis
of the UVLO using the following:
capacitor (C ) across the feedback divider may improve
PL
the transient response.
LT3508
CURRENT MODE
POWER STAGE
V
SW
OUTPUT
VH – VL
R3 =
ERROR
AMPLIFIER
g
= 2.5S
m
C
PL
R1
7.5µA
FB
–
g
=
m
300ꢀS
ESR
2.63V
R4 =
+
2M
0.8V
C1
+
VL – 2.63V
– 8µA
GND
V
C
C1
R3
POLYMER
OR
TANTALUM
CERAMIC
R2
R
C
C
F
Example:switchingshouldnotstartuntiltheinputisabove
4.75V and is to stop if the input falls below 4V.
C
C
3508 F06
VH = 4.75V, VL = 4.0V
Figure 6. Model for Loop Response
4.75V – 4V
R3 =
R4 =
= 100k
Shutdown and Undervoltage Lockout
7.5µA
Figure 7 shows how to add undervoltage lockout (UVLO)
to the LT3508. Typically, UVLO is used in situations where
the input supply is current limited, or has a relatively high
source resistance. A switching regulator draws constant
power from the source, so source current increases as
sourcevoltagedrops. Thislookslikeanegativeresistance
load to the source and can cause the source to current
limit or latch low under low source voltage conditions.
UVLO prevents the regulator from operating at source
voltages where the problems might occur.
2.63V
4V – 2.63V
100k
= 461k
– 8µA
Keep the connection from the resistor to the SHDN pin
short and make sure the interplane or surface capacitance
toswitchingnodesisminimized.Ifhighresistorvaluesare
used, theSHDNpinshouldbebypassedwitha1nFcapaci-
tor to prevent coupling problems from the switch node.
Soft-Start
An internal comparator will force the part into shutdown
belowtheminimumV of3.7V.Thisfeaturecanbeusedto
The output of the LT3508 regulates to the lowest voltage
present at either the TRACK/SS pin or an internal 0.8V
reference. A capacitor from the TRACK/SS pin to ground
is charged by an internal 1.2μA current source resulting
in a linear output ramp from 0V to the regulated output
whose duration is given by:
IN1
preventexcessivedischargeofbattery-operatedsystems.
LT3508
V
IN
V
C
–
+
2.6V
R3
R4
SHDN
C1
C
SS • 0.8V
1.2µA
TRACK/SS
tRAMP
=
8ꢀA
7.5ꢀA
At power up, internal open-collector outputs discharge
both TRACK/SS pins. The pins clamp at 1.3V.
3508 F07
Figure 7. Undervoltage Lockout
3508fd
15
LT3508
APPLICATIONS INFORMATION
Output Tracking and Sequencing
Independent soft-start for each channel is shown in Fig-
ure 8a. Theoutputramptimeforeachchannelissetbythe
soft-start capacitor as described in the soft-start section.
Complexoutputtrackingandsequencingbetweenchannels
can be implemented using the LT3508’s TRACK/SS and
PG pins. Figure 8 shows several configurations for output
tracking and sequencing of 5V and 3.3V applications.
Independent Start-Up
Ratiometric Start-Up
Coincident Start-Up
V
V
V
V
V
OUT1
OUT1
OUT1
OUT2
V
OUT2
OUT2
1V/DIV
1V/DIV
1V/DIV
20ms/DIV
20ms/DIV
20ms/DIV
5V
5V
5V
TRACK/SS1 V
LT3508
TRACK/SS1 V
LT3508
TRACK/SS1 V
LT3508
OUT1
OUT1
OUT1
0.1ꢀF
0.22ꢀF
0.1ꢀF
3.3V
3.3V
3.3V
TRACK/SS2 V
TRACK/SS2 V
TRACK/SS2 V
OUT2
OUT2
OUT2
0.047ꢀF
R1
28.7k
R2
10.0k
(8a)
(8b)
(8c)
Output Sequencing
Controlled Power Up and Down
V
V
V
V
OUT1
OUT1
OUT2
OUT2
1V/DIV
1V/DIV
EXTERNAL SOURCE
20ms/DIV
20ms/DIV
5V
5V
TRACK/SS1 V
LT3508
TRACK/SS1 V
LT3508
OUT1
PG1
OUT2
OUT1
0.1ꢀF
EXTERNAL
SOURCE
+
–
3.3V
3.3V
TRACK/SS2 V
TRACK/SS2 V
OUT2
0.047ꢀF
R1
28.7k
R2
10.0k
(8d)
(8e)
Figure 8
3508fd
16
LT3508
APPLICATIONS INFORMATION
If precise output ramp up and down is required, drive the
TRACK/SS pins as shown in Figure 8e.
RatiometrictrackingisachievedinFigure8bbyconnecting
both the TRACK/SS pins together. In this configuration the
TRACK/SS pin source current is doubled (2.4μA) which
must be taken into account when calculating the output
rise time.
Multiple Inputs
For applications requiring large inductors due to high V
IN
to V
ratios, a 2-stage step down approach may reduce
OUT
Do not tie TRACK/SS1 and TRACK/SS2 together if using
inductor size by allowing an increase in frequency. A dual
multiple inputs. If V is below 3V, TRACK/SS2 pulls low
IN2
step-down application (Figure 9) steps down the input
and would hold TRACK/SS1 low as well if the two pins
are tied together, which would prevent channel 1 from
operating.
voltage (V ) to the highest output voltage, then uses that
IN1
voltage to power the second output (V ). V
must
IN2
OUT1
be able to provide enough current for its output plus the
input current at V when V is at its maximum load.
By connecting a feedback network from V
to the
OUT2
IN2
OUT2
OUT1
TRACK/SS2 pin with the same ratio that set the V
For applications with multiple input voltages, the LT3508
can accommodate input voltages as low as 3V on V
voltage, absolute tracking shown in Figure 8c is imple-
mented. A small V voltage offset will be present due
.
IN2
OUT2
This can be useful in applications regulating outputs from
a PCI Express bus, where the 12V input is power limited
and the 3.3V input has power available to drive other
to the TRACK/SS2 1.2μA source current. This offset can
be corrected for by slightly reducing the value of R2. Use
a resistor divider such that when V
is in regulation,
OUT1
outputs. In this case, tie the 12V input to V and the
IN1
TRACK/SS2 is pulled up to 1V or greater. If TRACK/SS is
below 1V, the output may regulate FB to a voltage lower
than the 800mV reference voltage.
3.3V input to V . See the Typical Application section for
IN2
an example circuit.
Shorted and Reverse Input Protection
Figure 8d illustrates output sequencing. When V
is
OUT1
within 10% of its regulated voltage, PG1 releases the
TRACK/SS2 soft-start pin allowing V to soft-start.
If the inductor is chosen so that it won’t saturate exces-
sively,anLT3508step-downregulatorwilltolerateashorted
output. There is another situation to consider in systems
where the output will be held high when the input to the
OUT2
In this case PG1 will be pulled up to 1.3V by the TRACK/
SS pin.
V
IN
OUT1
5.7V TO 36V
C1
4.7ꢀF
V
V
IN2
IN1
D2
ON OFF
C2
SHDN
D1
BOOST1
BOOST2
SW2
L2
C3
L1 6.8ꢀH
3.3ꢀH
OUT1
5V
0.9A
0.1ꢀF
0.1ꢀF
OUT2
1.8V
1A
SW1
D3
D4
LT3508
R1
56.2k
R2
18.7k
FB1
FB2
V
V
C2
C1
R3
R5
R4
15.0k
R6
47k
C5
47ꢀF
TRACK/SS1
TRACK/SS2
PG1
PG2
10.7k
39k
R7
100k
C6
100pF
C4
10ꢀF
GND RT/SYNC
C7
330pF
R8
33.2k
C8
1nF
C9
3.3nF
POWER
GOOD
f
= 1MHz
SW
3508 F09
Figure 9. 1MHz, Wide Input Range 5V and 1.8V Outputs
3508fd
17
LT3508
APPLICATIONS INFORMATION
PCB Layout
LT3508 is absent. This may occur in battery charging
applicationsorinbatteryback-upsystemswhereabattery
or some other supply is diode OR-ed with the LT3508’s
ForproperoperationandminimumEMI,caremustbetaken
during printed circuit board layout. Figure 11 shows the
recommendedPCBlayoutwithtraceandvialocations.Note
that large, switched currents flow in the LT3508’s V and
SWpins,thecatchdiode(D1)andtheinputcapacitor(C ).
output. If the V pin is allowed to float and the SHDN pin
IN
is held high (either by a logic signal or because it is tied
IN
to V ), then the LT3508’s internal circuitry will pull its
IN
IN
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
The loop formed by these components should be as small
as possible. These components, along with the inductor
and output capacitor, should be placed on the same side
of the circuit board, and their connections should be made
on that layer. Place a local, unbroken ground plane below
these components. The SW and BOOST nodes should be
zero. However, if the V pin is grounded while the output
IN
is held high, then parasitic diodes inside the LT3508 can
pull large currents from the output through the SW pin
and the V pin. Figure 10 shows a circuit that will run
IN
as small as possible. Finally, keep the FB and V nodes
C
only when the input voltage is present and that protects
small so that the ground traces will shield them from the
SW and BOOST nodes. The exposed pad on the bottom of
the package must be soldered to ground so that the pad
acts as a heat sink. To keep thermal resistance low, extend
the ground plane as much as possible, and add thermal
viasunderandneartheLT3508toadditionalgroundplanes
within the circuit board and on the bottom side.
against a shorted or reversed input.
PARASITIC DIODE
D4
V
SW
IN
V
V
IN
OUT
LT3508
3508 F10
Figure 10. Diode D4 Prevents a Shorted Input from Discharging
a Backup Battery Tied to the Output
(11a) Example Layout for FE16 Package
(11b) Example Layout for QFN Package
Figure 11. A Good PCB Layout Ensures Proper Low EMI Operation
3508fd
18
LT3508
APPLICATIONS INFORMATION
High Temperature Considerations
was 13°C; for 24V to 3.3V
the rise was 18°C; for
IN
OUT
12V to 5V
the rise was 14°C and for 24V to 5V
IN
OUT
IN OUT
The die temperature of the LT3508 must be lower than the
maximum rating of 125°C (150°C for the H-grade). This is
generally not a concern unless the ambient temperature is
above85°C.Forhighertemperatures,careshouldbetaken
in the layout of the circuit to ensure good heat sinking of
the LT3508. The maximum load current should be derated
as the ambient temperature approaches 125°C (150°C
for the H-grade). The die temperature is calculated by
multiplying the LT3508 power dissipation by the thermal
resistance from junction to ambient. Power dissipation
within the LT3508 can be estimated by calculating the total
power loss from an efficiency measurement and subtract-
ing the catch diode loss. Thermal resistance depends on
the layout of the circuit board, but values from 30°C/W to
60°C/W are typical. Die temperature rise was measured
on a 4-layer, 6.5cm × 7.5cm circuit board in still air at a
the rise was 19°C.
Outputs Greater Than 6V
For outputs greater than 6V, add a resistor of 1k to 2.5k
across the inductor to damp the discontinuous ringing of
the SW node, preventing unintended SW current. The 12V
output circuit in the Typical Applications section shows
the location of this resistor.
Other Linear Technology Publications
Application Notes 19, 35 and 44 contain more detailed
descriptions and design information for step-down regu-
lators and other switching regulators. The LT1376 data
sheet has a more extensive discussion of output ripple,
loop compensation and stability testing. Design Note 318
showshowtogenerateadualpolarityoutputsupplyusing
a step-down regulator.
load current of 1.4A (f = 700kHz). For a 12V input to
SW
3.3V output the die temperature elevation above ambient
3508fd
19
LT3508
TYPICAL APPLICATIONS
1MHz, 3.3V and 1.8V Outputs with Sequencing
V
IN
ON OFF
3.9V TO 16V
C1
4.7ꢀF
D1
V
V
SHDN
D2
IN1 IN2
OUT2
3.3V
1.4A
OUT2
BOOST1
BOOST2
C2
C3
0.1ꢀF
L1 3.3ꢀH
L2 4.7ꢀH
OUT1
1.8V
1.4A
0.1ꢀF
SW1
SW2
D3
D4
LT3508
R1
18.7k
R2
35.7k
FB1
FB2
V
V
C2
C1
R3
R5
R4
11.5k
R6
39k
C5
10ꢀF
TRACK/SS1
TRACK/SS2
PG1
PG2
15.0k
47k
R7
100k
C6
330pF
C4
47ꢀF
GND RT/SYNC
C7
150pF
R8
33.2k
C8
1nF
POWER
GOOD
C1 TO C5: X5R OR X7R
D1, D2: MMSD4148
f
= 1MHz
SW
3508 TA02
D3: DIODES INC. B140
D4: DIODES INC. B240A
3.3V and 5V Dual Output Step-Down Converter with Output Sequencing
V
IN
ON OFF
5.7V TO 36V
C1
4.7ꢀF
V
V
IN2
SHDN
D2
D1
IN1
OUT2
5V
1.4A
BOOST1
BOOST2
SW2
C2
0.22ꢀF
C3
0.22ꢀF
L2 10ꢀH
L1 6.8ꢀH
OUT1
3.3V
1.4A
SW1
D3
D4
LT3508
R1
35.7k
R2
56.2k
FB1
FB2
V
V
C2
C1
R3
R5
R4
10.7k
R6
43k
C5
10ꢀF
TRACK/SS1
TRACK/SS2
PG1
PG2
11.5k
51k
R7
100k
C6
150pF
C4
22ꢀF
GND RT/SYNC
C7
100pF
R8
C8
52.3k
= 700kHz
1nF
POWER
GOOD
f
C1 TO C5: X5R OR X7R
D1, D2: MMSD4148
SW
3508 TA03
D3: DIODES INC. B140
D4: DIODES INC. B240A
3508fd
20
LT3508
TYPICAL APPLICATIONS
1MHz, Wide Input Range 5V and 1.8V Outputs
V
IN
OUT1
5.7V TO 36V
C1
4.7ꢀF
V
V
IN2
IN1
D2
ON OFF
C2
SHDN
D1
BOOST1
BOOST2
SW2
L2
C3
L1 6.8ꢀH
3.3ꢀH
OUT1
5V
0.9A
0.1ꢀF
0.1ꢀF
OUT2
1.8V
1A
SW1
D3
D4
LT3508
R1
56.2k
R2
18.7k
FB1
FB2
V
V
C2
C1
R3
R5
R4
15.0k
R6
47k
C5
47ꢀF
TRACK/SS1
TRACK/SS2
PG1
PG2
10.7k
39k
R7
100k
C6
100pF
C4
10ꢀF
GND RT/SYNC
C7
330pF
R8
33.2k
C8
1nF
C9
3.3nF
POWER
GOOD
f
= 1MHz
C1 TO C5: X5R OR X7R
D1, D2: MMSD4148
SW
3508 TA04
D3: DIODES INC. B240A
D4: DIODES INC. B120
3508fd
21
LT3508
TYPICAL APPLICATIONS
1MHz, 5V and 12V Outputs
V
IN
ON OFF
14V TO 36V
C1
4.7ꢀF
D1
V
V
IN2
SHDN
D2
IN1
OUT2
5V
1.4A*
OUT2
BOOST1
BOOST2
SW2
C3
C2
L2 6.8ꢀH
L1 15ꢀH
R2 1k
0.1ꢀF
OUT1
12V
1.4A*
0.1ꢀF
SW1
D3
D4
LT3508
R3
56.2k
R1 154k
FB1
FB2
V
V
C2
C1
R4
R5
C5
10ꢀF
R7
10.7k
R6
39k
TRACK/SS1
TRACK/SS2
PG1
PG2
11.0k
43k
R8
100k
C6
100pF
C4
4.7ꢀF
GND RT/SYNC
C7
100pF
R9
33.2k
C8
1nF
POWER
GOOD
f
= 1MHz
SW
C1 TO C5: X5R OR X7R
D1, D2: MMSD4148
3508 TA06
D3: DIODES INC. B240A
D4: DIODES INC. B140
R2: USE 0.25W RESISTOR. FOR CONTINUOUS OPERATION
ABOVE 30V, USE TWO 2k, 0.25W RESISTORS IN PARALLEL
*DERATE OUTPUT CURRENT AT HIGHER AMBIENT TEMPERATURES
AND INPUT VOLTAGES TO MAINTAIN JUNCTION TEMPERATURE
BELOW THE ABSOLUTE MAXIMUM
3508fd
22
LT3508
PACKAGE DESCRIPTION
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
FE Package
16-Lead Plastic TSSOP (4.4mm)
(Reference LTC DWG # 05-08-1663 Rev I)
Exposed Pad Variation BA
4.90 – 5.10*
(.193 – .201)
2.74
(.108)
2.74
(.108)
16 1514 13 12 1110
9
6.60 t0.10
4.50 t0.10
2.74
(.108)
6.40
(.252)
BSC
SEE NOTE 4
2.74
(.108)
0.45 t0.05
1.05 t0.10
0.65 BSC
5
7
8
1
2
3
4
6
RECOMMENDED SOLDER PAD LAYOUT
1.10
(.0433)
MAX
4.30 – 4.50*
(.169 – .177)
0.25
REF
0s – 8s
0.65
(.0256)
BSC
0.09 – 0.20
(.0035 – .0079)
0.50 – 0.75
(.020 – .030)
0.05 – 0.15
(.002 – .006)
0.195 – 0.30
FE16 (BA) TSSOP REV I 0211
(.0077 – .0118)
TYP
NOTE:
1. CONTROLLING DIMENSION: MILLIMETERS 4. RECOMMENDED MINIMUM PCB METAL SIZE
FOR EXPOSED PAD ATTACHMENT
*DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.150mm (.006") PER SIDE
MILLIMETERS
(INCHES)
2. DIMENSIONS ARE IN
3. DRAWING NOT TO SCALE
3508fd
23
LT3508
PACKAGE DESCRIPTION
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
UF Package
24-Lead Plastic QFN (4mm × 4mm)
(Reference LTC DWG # 05-08-1697)
0.70 0.05
4.50 0.05
3.10 0.05
2.45 0.05
(4 SIDES)
PACKAGE OUTLINE
0.25 0.05
0.50 BSC
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
BOTTOM VIEW—EXPOSED PAD
R = 0.115
PIN 1 NOTCH
R = 0.20 TYP OR
0.35 × 45° CHAMFER
0.75 0.05
4.00 0.10
(4 SIDES)
TYP
23 24
PIN 1
TOP MARK
(NOTE 6)
0.40 0.10
1
2
2.45 0.10
(4-SIDES)
(UF24) QFN 0105
0.200 REF
0.25 0.05
0.00 – 0.05
0.50 BSC
NOTE:
1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WGGD-X)—TO BE APPROVED
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, IF PRESENT
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
3508fd
24
LT3508
REVISION HISTORY (Revision history begins at Rev D)
REV
DATE
DESCRIPTION
PAGE NUMBER
D
8/11
Revised Typical Application drawing TA01a
1
Changed R /SYNC pin name to RT/SYNC
1, 2, 6, 7, 8,
10, 17, 20, 21,
22, 26
T
Updated Pin Configuration drawings
2
6
Revised Exposed Pad, V , V and V pin descriptions in Pin Functions section
C1 C2
IN1
Made minor text edits in Operation and Applications Information sections
8, 10
Updated equations in Input Capacitor and Shutdown and Undervoltage Lockout sections and revised Figure 4 of
Applications Information
11, 13, 15
17
Moved last paragraph of Multiple Inputs section to the Soft-Start section in Applications Information
3508fd
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 representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
25
LT3508
TYPICAL APPLICATION
5V, 1.8V Output from PCI Express
V
V
IN
IN2
12V
R9
3.3V
C1
4.7ꢀF
C2
4.7ꢀF
40.2k
V
IN1
V
IN2
D2
SHDN
D1
R10
14.7k
BOOST1
BOOST2
SW2
L2
3.3ꢀH
C3
C4
0.1ꢀF
L1 6.8ꢀH
OUT1
0.1ꢀF
OUT2
1.8V
1.4A
5V
SW1
0.9A
D3
D4
LT3508
R1
52.3k
R2
18.7k
FB1
FB2
V
V
C2
C1
R3
R5
R4
15.0k
R6
47k
C5
47ꢀF
TRACK/SS1
TRACK/SS2
PG1
PG2
10k
43k
C8
0.047ꢀF
R7
100k
C9
100pF
C6
10ꢀF
GND RT/SYNC
C7
330pF
R8
33.2k
C10
0.047ꢀF
POWER
GOOD
f
= 1MHz
C1 TO C6: X5R OR X7R
D1, D2: MMSD4148
SW
3508 TA05
D3: DIODES INC. B140
D4: DIODES INC. B120
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LT1765
25V, 2.75A (I ), 1.25MHz, High Efficiency Step-Down V : 3V to 25V, V
= 1.2V, I = 1mA, S8, TSSOP16E Packages
OUT(MIN) Q
OUT
IN
DC/DC Converter
LT1766
LT1767
60V, 1.2A (I ), 200kHz, High Efficiency Step-Down
V : 5.5V to 60V, V
= 1.2V, I = 2.5mA, TSSOP16/TSSOP16E
OUT
IN
OUT(MIN) Q
DC/DC Converter
Packages
25V, 1.2A (I ), 1.25MHz, High Efficiency Step-Down
V : 3V to 25V, V
= 1.2V, I = 1mA, MS8, MS8E Packages
OUT
IN
OUT(MIN) Q
DC/DC Converter
LT1940/LT1940L Dual Monolithic 1.4A, 1.1MHz Step-Down Switching
Regulators
V : 3.6V to 25V, V
= 1.25V, I = 3.8mA, TSSOP16E Packages
OUT(MIN) Q
IN
LTC3407
Dual 600mA, 1.5MHz, Synchronous Step-Down
Regulator
V : 2.5V to 5.5V, V
= 0.6V, I = 40ꢀA, MSE Package
Q
IN
OUT(MIN)
OUT(MIN)
OUT(MIN)
LT3493
1.2A, 750kHz Step-Down Switching Regulator in
2mm × 3mm DFN
V : 3.6V to 36V, V
IN
= 0.78V, I = 1.9mA, 2mm × 3mm DFN Package
Q
LT3501/LT3510
Dual 3A/2A, 1.5MHz High Efficiency Step-Down
Switching Regulators
V : 3.6V to 25V, V
= 0.8V, I = 3.7mA, I < 10ꢀA,
Q SD
IN
TSSOP20E Package
LT3506/LT3506A Dual Monolithic 1.6A, 1.1MHz Step-Down Switching
Regulators
V : 3.6V to 25V, V
= 0.8V, I = 3.8mA, 16-Lead DFN and 16-Lead
Q
IN
OUT(MIN)
TSSOPE Packages
LTC3701
LTC3736
LTC3737
Two Phase, Dual, 500kHz, Constant Frequency, Current V : 2.5V to 10V, V
= 0.8V, I = 460ꢀA, SSOP-16 Package
IN
OUT(MIN)
Q
Mode, High Efficiency Step-Down DC/DC Controller
Dual Two Phase, No R ™, Synchronous Controller
SENSE
with Output Tracking
V : 2.75V to 9.8V, V
= 0.6V, I = 300ꢀA, 4mm × 4mm QFN or
IN
OUT(MIN)
Q
SSOP-24 Packages
Dual Two Phase, No R
Output Tracking
DC/DC Controller with
V : 2.75V to 9.8V, V
= 0.6V, I = 220ꢀA, 4mm × 4mm QFN or
SENSE
IN
OUT(MIN)
Q
SSOP-24 Packages
3508fd
LT 0811 REV D • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
26
●
●
© LINEAR TECHNOLOGY CORPORATION 2007
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
相关型号：
LT3508IFE
IC 3.2 A DUAL SWITCHING CONTROLLER, 1060 kHz SWITCHING FREQ-MAX, PDSO16, 4.40 MM, PLASTIC, TSSOP-16, Switching Regulator or Controller
Linear
LT3508IFE#PBF
LT3508 - Dual Monolithic 1.4A Step-Down Switching Regulator; Package: TSSOP; Pins: 16; Temperature Range: -40°C to 85°C
Linear
LT3508IFE#TR
IC 3.2 A DUAL SWITCHING CONTROLLER, 1060 kHz SWITCHING FREQ-MAX, PDSO16, 4.40 MM, PLASTIC, TSSOP-16, Switching Regulator or Controller
Linear
LT3508IFE#TRPBF
LT3508 - Dual Monolithic 1.4A Step-Down Switching Regulator; Package: TSSOP; Pins: 16; Temperature Range: -40°C to 85°C
Linear
LT3508IUF
IC 3.2 A DUAL SWITCHING CONTROLLER, 1060 kHz SWITCHING FREQ-MAX, PQCC24, 4 X 4 MM, PLASTIC, MO-220WGGD-X, QFN-24, Switching Regulator or Controller
Linear
LT3508IUF#PBF
LT3508 - Dual Monolithic 1.4A Step-Down Switching Regulator; Package: QFN; Pins: 24; Temperature Range: -40°C to 85°C
Linear
LT3508IUF#TR
IC 3.2 A DUAL SWITCHING CONTROLLER, 1060 kHz SWITCHING FREQ-MAX, PQCC24, 4 X 4 MM, PLASTIC, MO-220WGGD-X, QFN-24, Switching Regulator or Controller
Linear
LT3508IUF#TRPBF
LT3508 - Dual Monolithic 1.4A Step-Down Switching Regulator; Package: QFN; Pins: 24; Temperature Range: -40°C to 85°C
Linear
©2020 ICPDF网 联系我们和版权申明