LT3505EDD-PBF [Linear]
1.2A, Step-Down Switching Regulator in 3mm × 3mm DFN; 1.2A ，降压型开关稳压器采用3mm × 3mm DFN封装
| 型号： | LT3505EDD-PBF |
| 厂家： | 
Linear |
| 描述： | 1.2A, Step-Down Switching Regulator in 3mm × 3mm DFN |
| 文件： | 总24页 (文件大小：806K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LT3505
1.2A, Step-Down
Switching Regulator in
3mm × 3mm DFN
FEATURES
DESCRIPTION
The LT®3505 is a current mode PWM step-down DC/DC
converter with an internal 1.4A power switch. The wide
operating input range of 3.6V to 36V (40V maximum)
makes the LT3505 ideal for regulating power from a wide
variety of sources, including unregulated wall transform-
ers, 24V industrial supplies and automotive batteries. The
oscillatorcanbeprogrammedforhighfrequencyoperation
allowing the use of tiny, low cost external components or
it can be programmed for lower frequency operation to
maximize efficiency.
!
Wide Input Range: 3.6V to 36V Operating,
40V Maximum
!
Up to 1.2A Output Current
!
Resistor-Programmable Fixed-Frequency Operation
from 200kHz to 3MHz
!
Output Adjustable Down to 780mV
!
Short-Circuit Robust
!
Uses Tiny Capacitors and Inductors
!
Soft-Start
!
Low Shutdown Current: <2µA
!
Low VCESAT Switch: 350mV at 1A
Cycle-by-cycle current limit provides protection against
shorted outputs and soft-start eliminates input current
surge during start-up. The low current (<2µA) shutdown
mode provides output disconnect, enabling easy power
management in battery-powered systems.
!
Thermally Enhanced, Low Profile 3mm x 3mm
DFN-8 and MSOP-8 Packages
APPLICATIONS
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
!
Automotive Battery Regulation
!
Industrial Control Supplies
!
Wall Transformer Regulation
!
Distributed Supply Regulation
!
Battery-Powered Equipment
TYPICAL APPLICATION
750kHz, 3.3V Step-Down Converter
Efficiency
V
OUT
90
85
80
75
70
65
60
55
50
V
IN
3.3V
V
BOOST
SW
IN
4.2V TO 36V
1.1A, V > 5V
1.2A, V > 8V
IN
IN
0.1µF
10µH
36.5k
ON OFF
SHDN
LT3505
GND
22pF
FB
10µF
11.3k
R
V
C
T
V
V
f
= 12V
= 3.3V
= 750kHz
75.0k
69.8k
68pF
IN
OUT
SW
1µF
L = 10 H
0.8
LOAD CURRENT (A)
1.0
0
0.2
0.4
0.6
1.2
3505 TA01
3505fc
1
LT3505
ABSOLUTE MAXIMUM RATINGS
(Note 1)
Input Voltage (V )....................................................40V
Operating Temperature Range (Note 2)
IN
BOOST Pin Voltage ..................................................50V
BOOST Pin Above SW Pin.........................................25V
SHDN Pin..................................................................40V
FB Pin .........................................................................6V
LT3505E .............................................. –40°C to 85°C
LT3505I ............................................. –40°C to 125°C
Maximum Junction Temperature .......................... 125°C
Storage Temperature Range................... –65°C to 150°C
V Pin .........................................................................3V
C
R Pin .........................................................................3V
T
PIN CONFIGURATION
TOP VIEW
TOP VIEW
BOOST
SW
1
2
3
4
8
7
6
5
V
C
BOOST
SW
VIN
SHDN
1
2
3
4
8 VC
7 FB
FB
9
9
V
R
T
IN
6 R
T
5 GND
SHDN
GND
MS8E PACKAGE
8-LEAD PLASTIC MSOP
= 125°C, θ = 40°C/W, θ = 5°C/W
JA JC
EXPOSED PAD (PIN 9) IS GND, MUST BE SOLDERED TO PCB
DD PACKAGE
T
JMAX
8-LEAD (3mm × 3mm) PLASTIC DFN
T
= 125°C, θ = 43°C/W, θ = 5°C/W
JA JC
EXPOSED PAD (PIN 9) IS GND, MUST BE SOLDERED TO PCB
JMAX
ORDER INFORMATION
LEAD FREE FINISH
LT3505EDD#PBF
LT3505IDD#PBF
LT3505EMS8E#PBF
LT3505IMS8E#PBF
LEAD BASED FINISH
LT3505EDD
TAPE AND REEL
PART MARKING
LCHB
PACKAGE DESCRIPTION
8-Lead (3mm x 3mm) Plastic DFN
TEMPERATURE RANGE
–40°C to 85°C
LT3505EDD#TRPBF
LT3505IDD#TRPBF
LT3505EMS8E#TRPBF
LT3505IMS8E#TRPBF
TAPE AND REEL
LCHC
8-Lead (3mm x 3mm) Plastic DFN
8-Lead Plastic MSOP
–40°C to 125°C
–40°C to 85°C
LTCNX
LTCNY
8-Lead Plastic MSOP
–40°C to 125°C
TEMPERATURE RANGE
–40°C to 85°C
PART MARKING
LCHB
PACKAGE DESCRIPTION
8-Lead (3mm x 3mm) Plastic DFN
8-Lead (3mm x 3mm) Plastic DFN
8-Lead Plastic MSOP
LT3505EDD#TR
LT3505IDD
LT3505IDD#TR
LCHC
–40°C to 125°C
–40°C to 85°C
LT3505EMS8E
LT3505EMS8E#TR
LT3505IMS8E#TR
LTCNX
LT3505IMS8E
LTCNY
8-Lead Plastic MSOP
–40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
3505fc
2
LT3505
ELECTRICAL CHARACTERISTICS The " denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C, VIN = 12V, VBOOST = 17V, unless otherwise noted. (Note 2).
PARAMETER
CONDITIONS
MIN
3.6
TYP
MAX
36
UNITS
V
VIN Operating Range
Undervoltage Lockout
Feedback Voltage
3.1
3.35
780
55
3.6
795
150
2.7
2
V
"
"
765
mV
nA
FB Pin Bias Current
Quiescent Current
VFB = Measured VREF (Note 4)
Not Switching, RT = 75.0k
VSHDN = 0V
2.0
mA
µA
Quiescent Current in Shutdown
Reference Line Regulation
Switching Frequency
0.01
0.007
VIN = 5V to 36V
%/V
VFB = 0.7V, RT = 13.7k
2.70
675
180
3.01
750
200
3.30
825
220
MHz
kHz
kHz
V
FB = 0.7V, RT = 75.0k
FB = 0.7V, RT = 357k
V
"
Maximum Duty Cycle
Error Amp Transconductance
Error Amp Voltage Gain
VC Source Current
RT = 75.0k
90
94
200
400
10
%
µA/V
V/V
µA
VFB = 0.78V
VFB = 0.78V
VFB = 0V, VC = 1.5V
VFB = 1V, VC = 1.5V
IOUT = 0mA
VC Sink Current
14
µA
VC Switching Threshold Voltage
VC Clamp Voltage
0.9
1.7
V
VFB = 0V
V
RT Bias Voltage
VFB = 0.6V
0.5
50
V
mV
V
FB = 0V, RT = 75.0k
Switch Current Limit
(Note 3)
ISW = 1A
1.4
2.3
1.75
350
0.1
1.6
24
2.2
A
mV
µA
V
Switch VCESAT
Switch Leakage Current
Minimum Boost Voltage Above Switch
BOOST Pin Current
2
ISW = 1A
ISW = 1A
2.2
50
mA
V
SHDN Input Voltage High
SHDN Input Voltage Low
SHDN Bias Current
0.3
V
VSHDN = 2.3V (Note 5)
VSHDN = 0V
6
0.01
20
0.1
µA
µA
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 limit guaranteed by design and/or correlation to static test.
Slope compensation reduces current limit at higher duty cycle.
Note 4: Current flows out of pin.
Note 5: Current flows into pin.
Note 2: The LT3505E is guaranteed to meet performance specifications
from 0°C to 85°C. Specifications over the –40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls. The LT3505I specifications are
guaranteed over the –40°C to 125°C temperature range.
3505fc
3
LT3505
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency (VOUT = 5V, L = 10µH,
fSW = 750kHz)
Efficiency (VOUT = 3.3V, L = 10µH,
fSW = 750kHz)
Efficiency (VOUT = 3.3V,
L = 4.7µH, fSW = 2.2MHz)
95
90
85
80
75
70
65
60
55
50
95
90
85
80
75
70
65
60
55
50
95
90
85
80
75
70
65
60
55
50
T
= 25°C
T
= 25°C
A
T
= 25°C
A
A
V
V
V
= 8V
= 12V
= 24V
V
V
V
= 8V
= 12V
= 24V
IN
IN
IN
IN
IN
IN
V
V
= 8V
= 12V
IN
IN
0
0.2
0.4
0.6
1.2
0
0.2
0.4
0.6
1.2
0
0.2
0.4
0.6
1.2
0.8
1.0
0.8
1.0
0.8
1.0
LOAD CURRENT (A)
LOAD CURRENT (A)
LOAD CURRENT (A)
Efficiency (VOUT = 5V, L = 4.7µH,
fSW = 2.2MHz)
Max Load Current (VOUT = 3.3V,
L = 6.8µH, fSW = 750kHz)
Max Load Current (VOUT = 5V,
fSW = 750kHz)
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1.0
0.9
0.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1.0
0.9
95
90
85
80
75
70
65
60
55
50
T
= 25°C
T
= 25°C
T
= 25°C
A
A
A
TYPICAL, L = 22µH
TYPICAL
TYPICAL, L = 10µH
MINIMUM
MINIMUM, L = 10µH
*10% DROPOUT
*10% DROPOUT
V
V
= 8V
= 12V
IN
IN
0.8
11
13
5
7
9
15
17
19
6
8
10 12 14 16 18 20
30
22 24 26 28
0
0.2
0.4
0.6
1.2
0.8
1.0
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
LOAD CURRENT (A)
3505 G05
3505 G06
Max Load Current (VOUT = 3.3V,
L = 2.2µH, fSW = 2.2MHz)
Max Load Current (VOUT = 5V,
L = 3.3µH, fSW = 2.2MHz)
Switch Voltage Drop
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1.0
0.9
0.8
1.80
1.70
1.60
1.50
1.40
1.30
1.20
1.10
1.00
0.90
0.80
500
450
400
350
300
250
200
150
100
50
T
= 25°C
T = 25°C
A
A
TYPICAL
T
= 85°C
A
TYPICAL
T
= 25°C
A
MINIMUM
MINIMUM
T
= –45°C
A
*10% DROPOUT
*10% DROPOUT
0
5
7
8
9
10
11
12
7
11
INPUT VOLTAGE (V)
14 16
6
8
9
10
12
18
0
300
600
900
1200
1500
INPUT VOLTAGE (V)
SWITCH CURRENT (mA)
3505 G07
3505 G08
3505 G09
3505fc
4
LT3505
TYPICAL PERFORMANCE CHARACTERISTICS
Undervoltage Lockout
Switching Frequency
Frequency Foldback, RT = 75.0k
2.40
2.20
2.00
1.80
1.60
1.40
1.20
1.00
0.80
4.00
3.90
3.80
3.70
3.60
3.50
3.40
3.30
3.20
3.10
3.00
0.6
0.5
0.4
0.3
0.2
0.1
0
T
= 25°C
A
R
= 21k
T
R
= 30.1k
T
R
= 75.0k
T
0.60
0.5 0.6
0
0.1 0.2 0.3 0.4
0.7 0.8
–50 –25
0
25
50
75 100 125
–50 –25
0
25
50
75
125
100
FB VOLTAGE (V)
TEMPERATURE (°C)
TEMPERATURE (°C)
3505 G12
3505 G10
Typical Minimum Input Voltage,
(VOUT = 5V, fSW = 750kHz)
Soft-Start
SHDN Pin Current
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
7.2
7.0
6.8
6.6
50
45
40
35
30
25
20
15
10
5
T
= 25°C
T
= 25°C
T = 25°C
A
A
A
TO START
6.4
6.2
6.0
5.8
5.6
5.4
5.2
TO RUN
0
0
2
4
6
8
V
10 12 14 16 18 20
(V)
1
10
100
1000
0
0.25 0.50 0.75
1
1.25 1.50 1.75
2
SHDN PIN VOLTAGE (V)
LOAD CURRENT (mA)
SHDN
3505 G15
3505 G14
Typical Minimum Input Voltage,
(VOUT = 3.3V, fSW = 750kHz)
Typical Minimum Input Voltage,
(VOUT = 3.3V, fSW = 2.2MHz)
Typical Minimum Input Voltage,
(VOUT = 5V, fSW = 2.2MHz)
7.0
6.8
6.6
6.4
6.2
6.0
5.8
5.6
5.4
5.2
5.0
5.5
5.3
5.1
4.9
T = 25°C
A
T
= 25°C
A
5.5
5.0
4.5
4.0
3.5
TO START
TO RUN
TO START
TO START
4.7
4.5
TO RUN
4.3
4.1
3.9
3.7
3.5
TO RUN
T
= 25°C
A
1
10
100
1000
1
10
100
1000
1
10
100
1000
LOAD CURRENT (mA)
LOAD CURRENT (mA)
LOAD CURRENT (mA)
3505fc
5
LT3505
TYPICAL PERFORMANCE CHARACTERISTICS
Switch Current Limit
Switch Current Limit, RT = 75.0k
Typical Minimum On Time
2.0
1.9
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1.0
1.8
1.7
160
140
120
100
80
T
= 25°C
A
1.6
1.5
60
1.4
1.3
1.2
40
20
0
50
75
–50 –25
0
25
100 125
0
10 20 30 40 50 60 70 80 90 100
DUTY CYCLE (%)
–50 –25
0
25
50
75 100 125
TEMPERATURE (°C)
TEMPERATURE (°C)
3505 G20
RT Pin Bias Voltage
Switching Frequency
Switching Frequency
505
500
495
490
485
480
3.0
2.5
2.0
1.5
1.0
0.5
0
V
= 0.78V
T
A
= 25°C
FB
T
= 25°C
A
1
0.1
50
TEMPERATURE(°C)
–50 –25
0
25
75 100 125
10
100
PIN RESISTANCE (kΩ)
0
5
35
10
15
20
25
30
R
R
T
PIN BIAS CURRENT (µA)
T
3505 G24
Operating Waveforms,
Discontinuous Mode
Operating Waveforms
V
V
SW
5V/DIV
SW
5V/DIV
I
L
I
L
0.5A/DIV
0.5A/DIV
0
0
V
V
OUT
OUT
20mV/DIV
20mV/DIV
3505 G18
3505 F26
V
V
= 12V
1µs/DIV
V
V
= 12V
IN
1µs/DIV
IN
= 3.3V
OUT
= 0.5A
OUT
= 3.3V
OUT
OUT
I
I
= 50mA
L = 10µH
C
R
L = 10µH
= 10µF
C
= 10µF
OUT
OUT
= 75.0k
R = 75.0k
T
T
3505fc
6
LT3505
PIN FUNCTIONS
BOOST (Pin 1): The BOOST pin is used to provide a drive
voltage,higherthantheinputvoltage,totheinternalbipolar
NPN power switch.
RT (Pin 6): The RT pin is used to program the switching
frequency of the LT3505 by connecting a resistor from
thispintoground.TheApplicationsInformationsectionof
the data sheet includes a table to determine the resistance
value based on the desired switching frequency. Minimize
capacitance at this pin.
SW (Pin 2): The SW pin is the output of the internal power
switch. Connect this pin to the inductor, catch diode and
boost capacitor.
FB(Pin7):TheLT3505regulatesitsfeedbackpinto780mV.
Connect the feedback resistor divider tap to this pin. Set
the output voltage by selecting R1 according to:
VIN (Pin 3): The VIN pin supplies current to the LT3505’s
internal regulator and to the internal power switch. This
pin must be locally bypassed.
VOUT
0.78V
SHDN (Pin 4): The SHDN pin is used to put the LT3505 in
shutdown mode. Tie to ground to shut down the LT3505.
Tie to 2.3V or more for normal operation. If the shutdown
feature is not used, tie this pin to the VIN pin. SHDN also
provides a soft-start function; see the Applications Infor-
mation section.
R1= R2
– 1
A good value for R2 is 10.0k.
VC (Pin 8): The VC pin is used to compensate the LT3505
control loop by tying an external RC network from this
pin to ground.
GND (Pin 5): Tie the GND pin to a local ground plane
below the LT3505 and the circuit components. Return the
feedback divider to this pin.
Exposed Pad (Pin 9): The Exposed Pad must be soldered
to the PCB and electrically connected to ground. Use a
large ground plane and thermal vias to optimize thermal
performance.
3505fc
7
LT3505
BLOCK DIAGRAM
V
IN
V
3
IN
C2
INT REG
AND
UVLO
D2
BOOST
Σ
1
2
ON OFF
SLOPE
COMP
R
S
Q
Q
R3
SHDN
C3
L1
4
DRIVER
Q1
C4
SW
OSC
V
OUT
C1
D1
FREQUENCY
FOLDBACK
V
C
g
m
780mV
GND
V
C
FB
R
T
5
8
7
6
R1
3505 BD
R2
OPERATION (Refer to Block Diagram)
The LT3505 is a constant frequency, current mode step-
downregulator.Aresistor-programmedoscillatorenables
an RS flip-flop, turning on the internal 1.4A power switch
Q1. An amplifier and comparator monitor the current
flowing between the VIN and SW pins, turning the switch
off when this current reaches a level determined by the
voltage at the VC pin. An error amplifier measures the
output voltage through an external resistor divider tied to
the FB pin and servos the VC node. If the error amplifier’s
output increases, more current is delivered to the output;
if it decreases, less current is delivered. An active clamp
(not shown) on the VC node provides current limit. The
VC node is also clamped to the voltage on the SHDN pin;
soft-start is implemented by generating a voltage ramp at
the SHDN pin using an external resistor and capacitor.
Aninternalregulatorprovidespowertothecontrolcircuitry.
Thisregulatorincludesanundervoltagelockouttoprevent
switching when VIN is less than ~3.4V. The SHDN pin is
used to place the LT3505 in shutdown, disconnecting the
output and reducing the input current to less than 2µA.
The switch driver operates from either the input or from
the BOOST pin. An external capacitor and 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 opera-
tion.
When the FB pin is low, the voltage at the RT pin decreases
toreducetheoscillatorfrequency.Thisfrequencyfoldback
helps to control the output current during start-up and
overload.
3505fc
8
LT3505
APPLICATIONS INFORMATION
FB Resistor Network
wherefSWistheswitchingfrequencyinhertzandtON(MIN)is
theworst-caseminimumon-timeinseconds.Theminimum
on-time of the LT3505 is a strong function of temperature.
The typical performance characteristics section of the
datasheet contains a graph of minimum on-time versus
temperature to help determine the worst-case minimum
on-time for the intended application.
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the 1% resis-
tors according to:
VOUT
0.78V
R1= R2
– 1
If the input voltage is high enough that the duty-cycle
requirement is lower than DCMIN, the part enters pulse-
skipping mode. Specifically, the onset of pulse-skipping
occurs at:
R2 should be 20k or less to avoid bias current errors.
Reference designators refer to the Block Diagram.
Input Voltage Range
VIN(PS) = (VOUT + VD) / DCMIN – VD + VSW
The input voltage range for LT3505 applications depends
on the output voltage, on the absolute maximum ratings
of the VIN and BOOST pins, and on the programmed
switching frequency.
Above VIN(PS) the part turns on for brief periods of time
to control the inductor current and regulate the output
voltage, possibly producing a spectrum of frequencies
below the programmed switching frequency. To remain
in constant-frequency operation the input voltage should
remain below VIN(PS). See the “Minimum On Time” sec-
tion of the data sheet for more information on operating
The minimum input voltage is determined by either the
LT3505’s minimum operating voltage of 3.6V, 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:
above VIN(PS)
.
Notethatthisisarestrictionontheoperatinginputvoltage
to remain in constant-frequency operation; the circuit will
tolerate brief transient inputs up to the absolute maximum
ratings of the VIN and BOOST pins when the output is in
regulation. The input voltage should be limited to VIN(PS)
during overload conditions (short-circuit or start-up).
VOUT + VD
DC =
V – VSW + VD
IN
where VD is the forward voltage drop of the catch diode
(~0.4V) and VSW is the voltage drop of the internal switch
(~0.4V at maximum load). This leads to a minimum input
voltage of:
Minimum On Time
VOUT + VD
DCMAX
For switching frequencies less than 750kHz, the part
will still regulate the output at input voltages that exceed
VIN(PS) (up to 40V), however, the output voltage ripple
increases as the input voltage is increased. Figure 1 il-
lustrates switching waveforms in continuous mode for a
3V output application near VIN(PS) = 33V.
V
=
– VD + VSW
IN(MIN)
with DCMAX = 1 – fSW/8.33, where fSW is in MHz.
The maximum input voltage is determined by the abso-
lute maximum ratings of the VIN and BOOST pins. For
constant-frequencyoperation,themaximuminputvoltage
is determined by the minimum duty cycle requirement.
As the input voltage increases, the required duty cycle
to regulate the output voltage decreases. The minimum
duty-cycle is:
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 determine the minimum on
time of the part. The worst-case typical minimum on-time
is 130ns. Figure 2 illustrates the switching waveforms
when the input voltage is increased to VIN = 35V.
DCMIN = fSW
t
ON(MIN)
•
3505fc
9
LT3505
APPLICATIONS INFORMATION
Now the required on time has decreased below the
minimum on time of 130ns. Instead of the switch pulse
width becoming narrower to accommodate the lower duty
cycle requirement, the switch pulse width remains fixed
at 130ns. In Figure 2 the inductor current ramps up to a
value exceeding the load current and the output ripple
increases to ~200mV. The part then remains off until the
output voltage dips below 100% of the programmed value
before it begins switching again.
V
SW
20V/DIV
I
L
0.5A/DIV
V
OUT
200mV/DIV
AC COUPLED
3505 F01
2 s/DIV
= 0.75A
Forswitchingfrequenciesabove750kHz,theinputvoltage
mustnotexceedVIN(PS).Seethe“InputVoltageFrequency
Foldback” section of the datasheet for a circuit solution
that provides safe operation above VIN(PS) at switching
frequencies exceeding 750kHz. For switching frequencies
below 750kHz, operation above VIN(PS) is safe and will
not damage the part as long as the output voltage stays
in regulation and the inductor does not saturate. Figure
3 shows the switching waveforms of a 750kHz applica-
tion when the input voltage is increased to its absolute
maximum rating of 40V.
C
V
V
= 10 F
= 3V
I
LOAD
L = 10 H
R = 75.0k
T
OUT
OUT
IN
= 30V
Figure 1
V
SW
20V/DIV
I
L
0.5A/DIV
As the input voltage increases, the inductor current ramp
rate increases, the number of skipped pulses increases
and the output voltage ripple increases. The part is robust
enoughtosurviveprolongedoperationunderthesecondi-
tions as long as the programmed switching frequency is
less than 750kHz and the peak inductor current does not
exceed 2.2A. Inductor current saturation may further limit
performance in this operating regime.
V
OUT
200mV/DIV
AC COUPLED
3505 F02
2 s/DIV
= 0.75A
C
V
V
= 10 F
= 3V
I
LOAD
OUT
OUT
IN
L = 10 H
= 35V
R
= 75.0k
T
Figure 2
Frequency Selection
The maximum frequency that the LT3505 can be pro-
grammed to is 3MHz. The minimum frequency that the
LT3505 can be programmed to is 200kHz. The switching
frequencyisprogrammedbytyinga1%resistorfromtheRT
pintoground.Table1canbeusedtoselectthevalueofRT.
Minimumon-timeandedgelossmustbetakenintoconsid-
erationwhenselectingtheintendedfrequencyofoperation.
Higher switching frequency increases power dissipation
andlowersefficiency.
V
SW
20V/DIV
I
L
0.5A/DIV
V
OUT
200mV/DIV
AC COUPLED
3505 F03
2µs/DIV
C
V
V
= 10µF
I
= 0.75A
OUT
OUT
IN
LOAD
= 3V
L = 10µH
= 40V
R
= 75.0k
T
Figure 3
3505fc
10
LT3505
APPLICATIONS INFORMATION
When the input voltage is below 16V, the zener diode
path conducts no current and the current flowing out
of the RT pin (and through R4) is nominally 0.5V/20k =
25µA, which programs a 2.2MHz switching frequency.
As the input voltage is increased above 16V, the zener
diode begins to conduct and gradually reduces the cur-
rent flowing out of the RT pin. This mechanism reduces
the switching frequency as the input voltage is increased
above 16V (up to 36V) to ensure that the part constantly
operates in continuous mode without skipping pulses,
thereby preventing the excessive die temperature rise
encountered in pulse-skipping mode.
Finite transistor bandwidth limits the speed at which the
power switch can be turned on and off, effectively setting
theminimumon-timeoftheLT3505.Foragivenoutputvolt-
age,theminimumon-timedeterminesthemaximuminput
voltage to remain in continuous mode operation, VIN(PS)
.
See the “Input Voltage Range” section of the datasheet for
more information on determining VIN(PS). For switching
frequencies below 750kHz, operation above VIN(PS) (up
to 40V) is safe provided that the system will tolerate the
pulse-skipping behavior outlined in the “Minimum On
Time” section of the datasheet. At switching frequencies
exceeding 750kHz, edge loss limits operation to input
voltages below VIN(PS)
.
Although the circuit can be operated indefinitely above
VZENER, this frequency foldback method is intended to
protect circuits during temporary periods of high input
voltage. For example, in many automotive systems, the
normal operating input range might be 9V to 16V, and
the LT3505 can be programmed to operate above the
AM band (>1.8MHz). At the same time, the circuit must
be able to withstand higher input voltages due to load
dump or double-battery jump starts. During these brief
periods, it is usually acceptable to switch at a frequency
within the AM band.
Finite transition time results in a small amount of power
dissipation each time the power switch turns on and off
(edge loss). Edge loss increases with frequency, switch
current, and input voltage.
Input Voltage Frequency Foldback
In constant frequency operation (below VIN(PS)) edge
loss only reduces the application efficiency. However, at
high switching frequencies exceeding 750kHz and input
voltagesexceedingVIN(PS),thepartoperatesinpulse-skip-
ping mode and the switch current can increase above the
current limit of the part, 1.75A. This further increases the
power dissipated during switch transitions and increases
die temperature. To remedy the situation a single resis-
tor (R5) and a zener diode (D3) can be added to a typical
LT3505 circuit as shown in Figure 4.
Iftheoutputisshortedwhiletheinputvoltageisgreaterthan
VZENER, the switching frequency will be reduced to 30kHz
and the part will not be able to recover from the short until
theinputvoltageisreducedbelowVZENER(seethefollowing
discussion).
2.50
D2
1N4148
2.25
V
V
OUT
IN
V
BOOST
SW
IN
5V
6.7V TO 36V
2.00
C3
0.1µF
1.75
ON OFF
SHDN
L1
C5
22pF
R1
1.50
LT3505
GND
6.8µH
61.9k
D3
1.25
16V
FB
BZT52C16T
1.00
C1
10µF
D1
R2
11.5k
R5
806k
R
T
V
C
MBRM140
0.75
0.50
0.25
0
Switching
Frequency
R4
R3
Maximum
20.0k
100k
Load Current
C2
1µF
C4
22pF
0
30 35
5
10 15
20
Input Voltage [V]
25
40
LTC3505 • F04b
3505 F04
Figure 4. 2.2MHz, 5V Application with Input Voltage Frequency Foldback Circuit
3505fc
11
LT3505
APPLICATIONS INFORMATION
Component Selection for Input Voltage Frequency
Foldback Circuit
and VSW is the voltage drop of the internal power switch
(~0.4V at maximum load), VIN(MAX) is the maximum input
voltage for the application (must be less than 36V), and
tON(MIN) is the worst-case minimum on-time for the in-
tended application. The worst-case minimum on-time can
bedeterminedfromthegraphsinthe“TypicalPerformance
Characteristics” section of the datasheet. Next look up the
resistance that corresponds to fSW(MIN) in Table 1. This
resistance is RT(MAX), the effective resistance from the RT
pin to ground at VIN(MAX) that programs the oscillator to
To determine the values of R4, R5, and D3 for a specific
application follow the procedure outlined in this section.
First select the value of R4 from Table 1.
Table 1. RT Pin Resistance
SWITCHING FREQUENCY (MHz)
RT PIN RESISTANCE (kΩ)
357
237
165
124
100
0.20
0.30
0.40
0.50
0.60
0.69
0.80
0.91
1.00
1.11
1.21
1.31
1.39
1.50
1.60
1.70
1.80
1.90
2.02
2.10
2.22
2.31
2.39
2.48
2.62
2.71
2.81
2.90
3.01
a switching frequency equal to fSW(MIN)
.
Finally determine R5 from the following equation:
R5 = 2 • (VIN(MAX) – VZENER)/(1/R4 – 1/RT(MAX)
84.5
71.5
61.9
54.9
48.7
44.2
40.2
37.4
34.0
31.6
29.4
27.4
25.5
23.7
22.6
21.0
20.0
19.1
18.2
16.9
16.2
15.4
14.7
13.7
)
where V
IN(MAX)
applied to the V pin. V
is the zener diode breakdown voltage,
ZENER
and V
is the maximum input voltage that will be
must not exceed 36V, the
IN
IN(MAX)
maximum operating input voltage of the LT3505. The
equation to determine R5 assumes that R5 will com-
pensate a percentage of the current flowing through R4
equal to R4/R
. Be careful not to select a value of
T(MAX)
R5muchlessthanthatdeterminedbytheequationabove
because it may become possible for R5 to compensate
100% of the current flowing through R4 reducing the
frequency to 30kHz. In this state the part is not able to
start into large output current loads.
Whenever the voltage at the FB pin is below 600mV, the
LT3505 folds back the switching frequency by reducing
the bias voltage at the RT pin. If the input voltage is higher
than the zener voltage, the reduced voltage at the RT pin
results in a larger voltage drop across R5, and a reduced
voltage drop across R4. The current carried by R5 may
be large enough to completely compensate the current
flowing through R4, reducing the frequency to 30kHz. In
thissituationtheinputvoltagewillhavetobereduceduntil
the input voltage is less than the zener voltage.
Second, determine the value of VIN(PS) from the equation
in the “Input Voltage Range” section of the data sheet.
Select the zener diode, D3, to have a breakdown voltage
(VZENER)belowVIN(PS).Nextdeterminethedesiredfoldback
frequency from the following equation:
Note that when VIN is above VZENER and the frequency is
reduced, the inductor ripple current will be higher and the
maximum load that the LT3505 can regulate will be lower.
See the Inductor Selection and Maximum Output Current
section of this data sheet for more information.
fSW(MIN) =(VOUT +VD)/[tON(MIN) •(VIN(MAX) +VD –VSW)]
where VD is the forward drop of the catch diode (~0.4V),
3505fc
12
LT3505
APPLICATIONS INFORMATION
Inductor Selection and Maximum Output Current
A good first choice for the inductor value is:
L = 1.2 (VOUT + VD)/fSW
Catch Diode
Depending on load current, a 1A to 2A Schottky diode is
recommended for the catch diode, D1. The diode must
have a reverse voltage rating equal to or greater than the
maximuminputvoltage.TheONSemiconductorMBRM140
is a good choice; it is rated for 1A continuous forward
current and a maximum reverse voltage of 40V.
where VD is the voltage drop of the catch diode (~0.4V),
L is in µH and fSW is in MHz. 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 your maximum load current and
its saturation current should be about 30% higher. For
robustoperationinfaultconditions, thesaturationcurrent
should be above 2.2A. To keep efficiency high, the series
resistance (DCR) should be less than 0.1 . Table 2 lists
several vendors and types that are suitable.
Input Capacitor
The input of the LT3505 circuit must be bypassed with a
X7R or X5R type ceramic capacitor. Y5V types have poor
performance over temperature and applied voltage and
should not be used. For switching frequencies higher than
750kHz,bypasstheinputwitha1µForhighervalueceramic
capacitor.Forswitchingfrequenciesbelow750kHz,bypass
the input with a 2.2µF or higher value ceramic capacitor.
If the input power source has high impedance, or there is
significantinductanceduetolongwiresorcables,additional
bulk capacitance may be necessary. This can be provided
with a low performance electrolytic capacitor.
Of course, such a simple design guide will not always
result in the optimum inductor for your application. A
larger value provides a higher maximum load current and
reduces output voltage ripple at the expense of slower
transient response. If your load is lower than 1.2A, then
you can decrease the value of the inductor and operate
with higher ripple current. This allows you to use a physi-
cally smaller inductor, or one with a lower DCR resulting
in higher efficiency. There are several graphs in the Typical
PerformanceCharacteristicssectionofthisdatasheetthat
show the maximum load current as a function of input
voltage and inductor value for several popular output volt-
ages. Low inductance may result in discontinuous mode
operation, which is okay, but further reduces maximum
load current. For details on maximum output current and
discontinuous mode operation, see Linear Technology
Application Note 44.
Step-down regulators draw current from the input sup-
ply in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage
ripple at the LT3505 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
To accomplish this task, the input bypass capacitor must
be placed close to the LT3505 and the catch diode; see
the PCB Layout section. A second precaution regarding
the ceramic input capacitor concerns the maximum input
voltage rating of the LT3505. A ceramic input capacitor
combined with trace or cable inductance forms a high
quality (underdamped) tank circuit. If the LT3505 circuit
is plugged into a live supply, the input voltage can ring to
Table 2. Inductor Vendors
VENDOR
URL
PART SERIES
INDUCTANCE RANGE (µH)
Size (mm)
Sumida
www.sumida.com
CDRH4D28
CDRH5D28
CDRH5D28
1.2 to 4.7
2.5 to 10
2.5 to 33
4.5 × 4.5
5.5 × 5.5
8.3 × 8.3
Toko
www.toko.com
A916CY
D585LC
2 to 12
1.1 to 39
6.3 × 6.2
8.1 × 8.0
Würth Elektronik
www.we-online.com
WE-TPC(M)
WE-PD2(M)
WE-PD(S)
1 to 10
2.2 to 22
1 to 27
4.8 × 4.8
5.2 × 5.8
7.3 × 7.3
3505fc
13
LT3505
APPLICATIONS INFORMATION
twice its nominal value, possibly exceeding the LT3505’s
voltage rating. This situation can be easily avoided; see
the Hot Plugging Safely section.
High performance electrolytic capacitors can be used for
theoutputcapacitor. LowESRisimportant, sochooseone
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 3 lists
several capacitor vendors.
Output Capacitor
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated by
the LT3505 to produce the DC output. In this role it deter-
mines 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
LT3505’s control loop.
Figure 5 shows the transient response of the LT3505 with
several output capacitor choices. The output is 3.3V. The
load current is stepped from 500mA to 1.2A and back
to 500mA and the oscilloscope traces show the output
voltage. The upper photo shows the recommended value.
The second photo shows the improved response (less
voltage drop) resulting from a larger output capacitor
and a larger phase lead capacitor. The last photo shows
the response to a high performance electrolytic capaci-
tor. Transient performance is improved due to the large
output capacitance.
Ceramic capacitors have very low equivalent series re-
sistance (ESR) and provide the best ripple performance.
A good value is:
COUT = 49/(VOUT • fSW
)
where COUT is in µF and fSW is in MHz. Use X5R or X7R
types and keep in mind that a ceramic capacitor biased
with VOUT will have less than its nominal capacitance. This
choice will provide low output ripple and good transient
response. Transient performance can be improved with a
high value capacitor, if the compensation network is also
adjusted to maintain the loop bandwidth.
BOOST Pin Considerations
Capacitor C3 and diode D2 are used to generate a boost
voltage that is higher than the input voltage. In most cases
a 0.1µF capacitor and fast switching diode (such as the
1N4148 or 1N914) will work well. Figure 6 shows two
ways to arrange the boost circuit. The BOOST pin must
be at least 2.3V above the SW pin for best efficiency. For
outputs of 3.3V and above, the standard circuit (Figure 6a)
is best. For outputs between 3V and 3.3V, use a 0.22µF
capacitor. For outputs between 2.5V and 3V, use a 0.47µF
A lower value of output capacitor can be used, but tran-
sient performance will suffer unless the compensation
network is adjusted to reduce the loop gain. Also, a lower
value output capacitor may result in increased sensitivity
to noise which can be alleviated by adding a 22pF phase
lead capacitor from FB to VOUT
.
Table 3. Capacitor Vendors
VENDOR
PHONE
URL
PART SERIES
COMMENTS
Panasonic
(714) 373-7366
www.panasonic.com
Ceramic,
Polymer,
Tantalum
EEF Series
Kemet
Sanyo
(864) 963-6300
(408) 749-9714
www.kemet.com
Ceramic,
Tantalum
T494, T495
POSCAP
www.sanyovideo.com
Ceramic,
Polymer,
Tantalum
Murata
AVX
(404) 436-1300
(864) 963-6300
www.murata.com
www.avxcorp.com
Ceramic
Ceramic,
Tantalum
TPS Series
Taiyo Yuden
www.taiyo-yuden.com
Ceramic
3505fc
14
LT3505
APPLICATIONS INFORMATION
I
LOAD
1A/DIV
V
OUT
22pF
32.4k
10.0k
FB
10µF
V
V
V
V
OUT
20mV/DIV
C
AC COUPLED
100k
22pF
3505 F05a
3505 F05b
3505 F05c
10µs/DIV
10µs/DIV
10µs/DIV
I
LOAD
1A/DIV
V
OUT
32.4k
44pF
10µF
×2
FB
C
V
OUT
10.0k
20mV/DIV
100k
AC COUPLED
22pF
I
LOAD
V
1A/DIV
OUT
32.4k
10.0k
66pF
+
FB
120µF
C
V
OUT
KEMET
A700D127M006ATE015
301k
20mV/DIV
AC COUPLED
22pF
Figure 5. Transient Load Response of the LT3505 with Different Output Capacitors as the
Load Current is Stepped from 500mA to 1.2A. VIN = 12V, VOUT = 3.3V, L = 2µH, RT = 20.0k
D2
D2
C3
C3
BOOST
LT3505
BOOST
LT3505
V
V
V
V
V
SW
V
SW
IN
OUT
IN
OUT
IN
IN
GND
GND
V
– V ≅ V
V
– V ≅ V
3505 F06a
BOOST
SW
OUT
BOOST
SW
IN
IN
3505 F06b
MAX V
≅ V + V
MAX V
≅ 2V
BOOST
BOOST
IN
OUT
(6a)
(6b)
Figure 6. Two Circuits for Generating the Boost Voltage
3505fc
15
LT3505
APPLICATIONS INFORMATION
capacitorandasmallSchottkydiode(suchastheBAT-54).
For lower output voltages tie a Schottky diode to the input
(Figure6b).ThecircuitinFigure6aismoreefficientbecause
theBOOSTpincurrentcomesfromalowervoltagesource.
You must also be sure that the maximum voltage rating
of the BOOST pin is not exceeded.
of the BOOST pin.
At light loads, the inductor current becomes discontinu-
ous and the effective duty cycle can be very high. This
reduces the minimum input voltage to approximately
400mV above VOUT. At higher load currents, the inductor
current is continuous and the duty cycle is limited by the
maximum duty cycle of the LT3505, requiring a higher
input voltage to maintain regulation.
The minimum operating voltage of an LT3505 applica-
tion is limited by the undervoltage lockout (3.6V) 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 LT3505 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 7 shows a plot of
minimum load to start and to run as a function of input
voltage. In many cases the discharged output capacitor
will present a load to the switcher which will allow it to
start. The plots show the worst-case situation where VIN
is ramping verly slowly. For lower start-up voltage, the
boost diode can be tied to VIN; however this restricts the
input range to one-half of the absolute maximum rating
Soft-Start
TheSHDNpincanbeusedtosoft-starttheLT3505,reducing
themaximuminputcurrentduringstart-up. TheSHDNpin
is driven through an external RC filter to create a voltage
ramp at this pin. Figure 8 shows the start-up waveforms
with and without the soft-start circuit. By choosing a large
RCtimeconstant, thepeakstartupcurrentcanbereduced
to the current that is required to regulate the output, with
no overshoot. Choose the value of the resistor so that it
can supply 20µA when the SHDN pin reaches 2.3V.
Shorted and Reversed Input Protection
If the inductor is chosen so that it won’t saturate exces-
sively, an LT3505 buck regulator will tolerate a shorted
output. There is another situation to consider in systems
where the output will be held high when the input to the
LT3505 is absent. This may occur in battery charging ap-
7.2
5.5
T
= 25°C
T
= 25°C
A
A
7.0
6.8
6.6
5.3
5.1
4.9
TO START
TO START
6.4
6.2
4.7
4.5
6.0
5.8
5.6
5.4
5.2
4.3
4.1
3.9
3.7
3.5
TO RUN
TO RUN
1
10
100
1000
1
10
100
1000
LOAD CURRENT (mA)
LOAD CURRENT (mA)
3505 G15
(7a) Typical Minimum Input Voltage, VOUT = 5V, fSW = 750kHz
(7b) Typical Minimum Input Voltage, VOUT = 3.3V, fSW = 750kHz
Figure 7
3505fc
16
LT3505
APPLICATIONS INFORMATION
plications or in battery backup systems where a battery
or some other supply is diode OR-ed with the LT3505’s
output. If the VIN pin is allowed to float and the SHDN pin
is held high (either by a logic signal or because it is tied
to VIN), then the LT3505’s internal circuitry will pull its
quiescent current through its SW pin. This is fine if your
system can tolerate a few mA in this state. If you ground
the SHDN pin, the SW pin current will drop to essentially
zero. However, if the VIN pin is grounded while the output
is held high, then parasitic diodes inside the LT3505 can
pull large currents from the output through the SW pin
and the VIN pin. Figure 9 shows a circuit that will run only
whentheinputvoltageispresentandthatprotectsagainst
a shorted or reversed input.
RUN
SHDN
GND
V
SW
5V/DIV
I
L
1A/DIV
V
OUT
2V/DIV
3505 F08a
10 s/DIV
V
V
= 12V
IN
OUT
= 3.3V
L = 2.5 H
= 10 F
C
D4
OUT
= 20.0k
R
T
V
OUT
V
IN
V
IN
BOOST SW
LT3505
RUN
15k
SHDN
FB
R
GND
V
C
T
BACKUP
SHDN
GND
0.068 F
3505 F09
Figure 9. 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 LT3505 Runs Only When the Input
is Present
V
SW
5V/DIV
Hot Plugging Safely
I
L
1A/DIV
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LT3505 circuits. However, these ca-
pacitors can cause problems if the LT3505 is plugged into
a live supply (see Linear Technology Application Note 88
foracompletediscussion).Thelowlossceramiccapacitor
combined with stray inductance in series with the power
sourceformsanunderdampedtankcircuitandthevoltage
at the VIN pin of the LT3505 can ring to twice the nominal
input voltage, possibly exceeding the LT3505’s rating and
V
OUT
2V/DIV
3505 F08b
10 s/DIV
V
V
= 12V
IN
OUT
= 3.3V
L = 2.5 H
= 10 F
C
OUT
= 20.0k
R
T
Figure 8. To Soft-Start the LT3505, Add a Resistor and Capacitor
to the SHDN pin. VIN = 12V, VOUT = 3.3V, COUT = 10µF, RLOAD
=
5Ω, RT = 20.0k, L = 2.5µH
3505fc
17
LT3505
APPLICATIONS INFORMATION
damaging the part. If the input supply is poorly controlled
or the user will be plugging the LT3505 into an energized
supply, the input network should be designed to prevent
this overshoot.
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
capacitorimproveshighfrequencyfiltering.Thissolutionis
smaller and less expensive than the electrolytic capacitor.
For high input voltages its impact on efficiency is minor,
reducing efficiency only one percent for a 5V output at full
load operating from 24V.
Figure10showsthewaveformsthatresultwhenanLT3505
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 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
Frequency Compensation
The LT3505 uses current mode control to regulate the
output. This simplifies loop compensation. In particular,
the LT3505 does not require the ESR of the output capaci-
tor for stability allowing the use of ceramic capacitors to
achieve low output ripple and small circuit size.
Frequency compensation is provided by the components
tied to the VC pin, as shown in Figure 10. Generally a
CLOSING SWITCH
SIMULATES HOT PLUG
I
IN
V
IN
DANGER!
LT3505
2.2µF
V
IN
20V/DIV
RINGING V MAY EXCEED
IN
ABSOLUTE MAXIMUM
RATING OF THE LT3505
+
I
IN
5A/DIV
LOW
STRAY
IMPEDANCE
ENERGIZED
24V SUPPLY
INDUCTANCE
DUE TO 6 FEET
(2 METERS) OF
TWISTED PAIR
20µs/DIV
(9a)
V
LT3505
2.2µF
IN
20V/DIV
+
+
+
10µF
35V
AI.EI.
I
IN
5A/DIV
(9b)
20µs/DIV
1Ω
V
LT3505
2.2µF
IN
20V/DIV
0.1µF
I
IN
5A/DIV
3505 F10
20µs/DIV
(9c)
Figure 10. A Well Chosen Input Network Prevents Input Voltage Overshoot and
Ensures Reliable Operation When the LT3505 is Connected to a Live Supply
3505fc
18
LT3505
APPLICATIONS INFORMATION
capacitor (CC) and a resistor (RC) in series to ground are
used. In addition, a lower value filter capacitor (CF) may be
addedinparallel.Thefiltercapacitorisnotapartoftheloop
compensation 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.
CURRENT MODE
POWER STAGE
LT3505
–
0.8V
SW
g
=
m
OUT
1.1A/V
+
C
PL
R1
–
+
FB
g
=
V
m
C
200µA/V
ESR
780mV
C1
ERROR
+
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.
AMPLIFIER
2M
C1
R2
R
C
C
C
C
F
3505 F11
Figure 11. Model for Loop Response
PCB Layout
ForproperoperationandminimumEMI,caremustbetaken
during printed circuit board layout. Figure 12 shows the
recommended component placement with trace, ground
plane and via locations. Note that large, switched currents
flow in the LT3505’s VIN and SW pins, the catch diode (D1)
and the input capacitor (C2). The loop formed by these
components should be as small as possible and tied to
Figure11showsanequivalentcircuitfortheLT3505control
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 cur-
rent proportional to the voltage at the VC node. Note that
the output capacitor integrates this current and that the
capacitor on the VC node (CC) integrates the error ampli-
fier output current, resulting in two poles in the loop. RC
provides a zero. With the recommended output capacitor,
theloopcrossoveroccursabovetheRCCCzero.Thissimple
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
phaseleadcapacitor(CPL)acrossthefeedbackdividermay
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.
SYSTEM
V
GROUND OUT
: VIAS TO LOCAL GROUND PLANE
: OUTLINE OF LOCAL GROUND PLANE
C1
V
OUT
BOOST
SW
1
8
7
6
5
V
C
FB
2
3
4
R
T
D1
C2
POWER
GROUND
SIGNAL
GROUND
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.
3505 F12
V
IN
SHUTDOWN
Figure 12. A Good PCB Layout Ensures Proper, Low EMI Operation
3505fc
19
LT3505
APPLICATIONS INFORMATION
ing 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 43°C/W is
typical for the (3mm × 3mm) DFN (DD) package.
systemgroundinonlyoneplace.Thesecomponents,along
with the inductor and output capacitor, should be placed
onthesamesideofthecircuitboardandtheirconnections
shouldbemadeonthatlayer.Placealocal,unbrokenground
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 LT3505 to help remove
heat from the LT3505 to the ground plane.
Outputs Greater Than 6V
For outputs greater than 6V, add a 1k to 2.5k resistor
across the inductor to damp the discontinuous ringing
of the SW node, preventing unintended SW current. The
12V Step-Down Converter circuit in the Typical Applica-
tions section shows the location of this resistor. Also note
that for outputs above 10V, the input voltage range will
be limited by the maximum rating of the BOOST pin. The
12V circuit shows how to overcome this limitation using
an additional zener diode.
High Temperature Considerations
The die temperature of the LT3505 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 LT3505. The
maximum load current should be derated as the ambient
temperature approaches 125°C. The die temperature is
calculated by multiplying the LT3505 power dissipation
bythethermalresistancefromjunctiontoambient. Power
dissipationwithintheLT3505canbeestimatedbycalculat-
Other Linear Technology Publications
Application notes AN19, AN35 and AN44 contain more
detailed descriptions and design information for Buck
regulators and other switching regulators. The LT1376
data sheet has a more extensive discussion of output
ripple, loop compensation and stability testing. Design
Note DN100 shows how to generate a bipolar output
supply using a Buck regulator.
3505fc
20
LT3505
TYPICAL APPLICATIONS
2.2MHz, 3.3V Step-Down Converter
1N4148
2.50
2.25
2.00
1.75
1.50
1.25
1.0
V
V
OUT
IN
V
BOOST
SW
IN
3.3V
6V TO 36V
Switching
Frequency
0.1µF
3.3µH
ON OFF
SHDN
Maximum
Load Current
LT3505
GND
36.5k
11.3k
10V
CMPZ5240B
22pF
FB
MBRM140
10µF
R
V
C
T
698k
0.75
0.50
0.25
0.00
20.0k
100k
22pF
1µF
5
15
20
25
30
35
40
10
3505 TA02
Input Voltage [V]
LTC3505 • TA02b
1.2MHz, 1.8V Step-Down Converter
BAT54
1.60
1.40
1.20
1.00
V
IN
V
BOOST
SW
IN
3.6V TO 25V
0.1µF
4.7µH
V
1.8V
1.2A
OUT
ON OFF
SHDN
LT3505
GND
26.1k
20.0k
68pF
12V
CMPZ5242B
FB
0.80
0.60
MBRM140
22µF
R
V
C
T
1.5M
Switching
Frequency
0.40
0.20
0.00
44.2k
60.4k
120pF
Maximum
Load Current
2.2µF
5
10
20
0
25
15
INPUT VOLTAGE (V)
3505 TA03
LT3505 • TA03b
3505fc
21
LT3505
TYPICAL APPLICATIONS
750kHz, 3.3V Step-Down Converter
1N4148
BOOST
V
OUT
V
IN
3.3V
V
IN
4.2V TO 36V
1.1A, V > 5V
1.2A, V > 8V
IN
IN
0.1µF
10µH
SW
ON OFF
SHDN
LT3505
GND
36.5k
11.3k
68pF
FB
10µF
R
V
C
T
MBRM140
75.0k
69.8k
70pF
1µF
3505 TA04
1MHz, 12V Step-Down Converter
CMDZ5235B
6V
1N4148
0.1µF
1k*
0.25W
BOOST
V
OUT
15µH
12V
V
IN
SW
FB
V
IN
1A, V > 16.5V
IN
13.5V TO 36V
1.1A, V > 20.5V
IN
LT3505
GND
71.5k
22pF
ON OFF
SHDN
MBRM140
4.99k
10µF
R
V
C
T
54.9k
100k
22pF
3.3µF
*FOR CONTINUOUS OPERATION ABOVE 30V,
USE TWO 2k, 0.25W RESISTORS IN PARALLEL
3505 TA05
3505fc
22
LT3505
PACKAGE DESCRIPTION
DD Package
8-Lead Plastic DFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1698)
R = 0.115
0.38 0.10
8
TYP
5
0.675 0.05
3.5 0.05
2.15 0.05 (2 SIDES)
1.65 0.05
3.00 0.10
(4 SIDES)
1.65 0.10
(2 SIDES)
PIN 1
TOP MARK
(NOTE 6)
PACKAGE
OUTLINE
(DD) DFN 1203
4
1
0.75 0.05
0.25 0.05
0.200 REF
0.25 0.05
0.50 BSC
0.50
BSC
2.38 0.05
(2 SIDES)
2.38 0.10
(2 SIDES)
0.00 – 0.05
BOTTOM VIEW—EXPOSED PAD
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
NOTE:
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 TOP AND BOTTOM OF PACKAGE
1. DRAWING TO BE MADE A JEDEC PACKAGE
OUTLINE M0-229 VARIATION OF (WEED-1)
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
MS8E Package
8-Lead Plastic MSOP
(Reference LTC DWG # 05-08-1662)
0.889 0.127
(.035 .005)
BOTTOM VIEW OF
EXPOSED PAD OPTION
2.794 0.102
(.110 .004)
3.00 0.102
0.52
(.0205)
REF
(.118 .004)
(NOTE 3)
2.06 0.102
(.081 .004)
1
8
7 6
5
1.83 0.102
(.072 .004)
5.23
(.206)
MIN
3.20 – 3.45
(.126 – .136)
2.083 0.102
(.082 .004)
3.00 0.102
(.118 .004)
(NOTE 4)
4.90 0.152
(.193 .006)
0.65
(.0256)
BSC
0.42 0.038
(.0165 .0015)
TYP
8
1
2
3
4
RECOMMENDED SOLDER PAD LAYOUT
1.10
(.043)
MAX
0.86
(.034)
REF
DETAIL “A”
DETAIL “A”
0° – 6° TYP
0.254
(.010)
0.18
(.007)
SEATING
PLANE
GAUGE PLANE
0.22 – 0.38
(.009 – .015)
TYP
0.127 0.076
(.005 .003)
0.65
(.0256)
BSC
0.53 0.152
(.021 .006)
MSOP (MS8E) 0603
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
3505fc
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.
23
LT3505
TYPICAL APPLICATIONS
300kHz, 3.3V Step-Down Converter
1N4148
BOOST
V
OUT
V
IN
3.3V
V
IN
4V TO 36V
1A, V > 5V
1.2A, V > 8.5V
IN
IN
0.47µF
22µH
SW
ON OFF
SHDN
LT3505
36.5k
11.3k
100pF
FB
68µF
KEMET
A700D686M010ATE015
R
GND
V
C
T
MBRM140
226k
100k
150pF
2.2µF
3505 TA06
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LT1766
60V, 1.2A (IOUT), 200kHz, High Efficiency Step-Down
DC/DC Converter
VIN: 5.5V to 60V, VOUT(MIN) = 1.2V, IQ = 2.5mA, ISD < 25µA,
TSSOP16/TSSOP16E Packages
LT1767
25V, 1.2A (IOUT), 1.25MHz, High Efficiency Step-Down
DC/DC Converter
VIN: 3V to 25V, VOUT(MIN) = 1.20V, IQ = 1mA, ISD < 6µA,
MS8E Package
LT1933
500mA (IOUT), 500kHz, Step-Down Switching Regulator in VIN: 3.6V to 36V, VOUT(MIN) = 1.25V, IQ = 1.6mA, ISD < 1µA,
SOT-23
TSSOP16/TSSOP16E Packages
LT1936
36V, 1.4A (IOUT), 500kHz, High Efficiency Step-Down
DC/DC Converter
VIN: 3.6V to 36V, VOUT(MIN) = 1.20V, IQ = 1.9mA, ISD < 1µA,
MS8E Package
LT1940
Dual 25V, 1.4A (IOUT), 1.1MHz, High Efficiency Step-Down VIN: 3.6V to 25V, VOUT(MIN) = 1.25V, IQ = 3.8mA, ISD < 30µA,
DC/DC Converter
TSSOP16E Package
LT1976/LT1977
LT3434/LT3435
LT3437
60V, 1.2A (IOUT), 200kHz/500kHz, High Efficiency Step-
Down DC/DC Converters with Burst Mode® Operation
VIN: 3.3V to 60V, VOUT(MIN) = 1.25V, IQ = 100µA, ISD < 1µA,
TSSOP16E Package
60V, 2.4A (IOUT), 200kHz/500kHz, High Efficiency Step-
Down DC/DC Converters with Burst Mode Operation
VIN: 3.3V to 60V, VOUT(MIN) = 1.25V, IQ = 100µA, ISD < 1µA,
TSSOP16E Package
60V, 400mA (IOUT), Micropower Step-Down DC/DC
Converter with Burst Mode Operation
VIN: 3.3V to 60V, VOUT(MIN) = 1.25V, IQ = 100µA, ISD = <1µA,
DFN Package
LT3493
36V, 1.2A (IOUT), 750kHz, High Efficiency Step-Down
DC/DC Converter
VIN: 3.6V to 36V, VOUT(MIN) = 0.78V, IQ = 1.9mA, ISD < 2µA,
DFN Package
Burst Mode is a registered trademark of Linear Technology Corporation.
3505fc
LT 0807 REV C • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
24
"
#"#
LINEAR TECHNOLOGY CORPORATION 2006
(408)432-1900 FAX: (408) 434-0507 www.linear.com
相关型号：
SI9130DB
5- and 3.3-V Step-Down Synchronous ConvertersWarning: Undefined variable $rtag in /www/wwwroot/website_ic37/www.icpdf.com/pdf/pdf/index.php on line 217
-
VISHAY
SI9135LG-T1
SMBus Multi-Output Power-Supply ControllerWarning: Undefined variable $rtag in /www/wwwroot/website_ic37/www.icpdf.com/pdf/pdf/index.php on line 217
-
VISHAY
SI9135LG-T1-E3
SMBus Multi-Output Power-Supply ControllerWarning: Undefined variable $rtag in /www/wwwroot/website_ic37/www.icpdf.com/pdf/pdf/index.php on line 217
-
VISHAY
SI9135_11
SMBus Multi-Output Power-Supply ControllerWarning: Undefined variable $rtag in /www/wwwroot/website_ic37/www.icpdf.com/pdf/pdf/index.php on line 217
-
VISHAY
SI9136_11
Multi-Output Power-Supply ControllerWarning: Undefined variable $rtag in /www/wwwroot/website_ic37/www.icpdf.com/pdf/pdf/index.php on line 217
-
VISHAY
SI9130CG-T1-E3
Pin-Programmable Dual Controller - Portable PCsWarning: Undefined variable $rtag in /www/wwwroot/website_ic37/www.icpdf.com/pdf/pdf/index.php on line 217
-
VISHAY
SI9130LG-T1-E3
Pin-Programmable Dual Controller - Portable PCsWarning: Undefined variable $rtag in /www/wwwroot/website_ic37/www.icpdf.com/pdf/pdf/index.php on line 217
-
VISHAY
SI9130_11
Pin-Programmable Dual Controller - Portable PCsWarning: Undefined variable $rtag in /www/wwwroot/website_ic37/www.icpdf.com/pdf/pdf/index.php on line 217
-
VISHAY
SI9137
Multi-Output, Sequence Selectable Power-Supply Controller for Mobile ApplicationsWarning: Undefined variable $rtag in /www/wwwroot/website_ic37/www.icpdf.com/pdf/pdf/index.php on line 217
-
VISHAY
SI9137DB
Multi-Output, Sequence Selectable Power-Supply Controller for Mobile ApplicationsWarning: Undefined variable $rtag in /www/wwwroot/website_ic37/www.icpdf.com/pdf/pdf/index.php on line 217
-
VISHAY
SI9137LG
Multi-Output, Sequence Selectable Power-Supply Controller for Mobile ApplicationsWarning: Undefined variable $rtag in /www/wwwroot/website_ic37/www.icpdf.com/pdf/pdf/index.php on line 217
-
VISHAY
SI9122E
500-kHz Half-Bridge DC/DC Controller with Integrated Secondary Synchronous Rectification DriversWarning: Undefined variable $rtag in /www/wwwroot/website_ic37/www.icpdf.com/pdf/pdf/index.php on line 217
-
VISHAY
©2020 ICPDF网 联系我们和版权申明