LTC3638
High Efficiency, 140V
250mA Step-Down
Regulator
FEATURES
n
n
n
n
n
n
n
n
n
n
Wide Operating Input Voltage Range: 4V to 140V
Internal Low Resistance Power MOSFET
No Compensation Required
Adjustable 20mA to 250mA Maximum Output
Current
Low Dropout Operation: 100% Duty Cycle
Low Quiescent Current: 12µA
Wide Output Range: 0.8V to VIN
0.8V ±1% Feedback Voltage Reference
Precise RUN Pin Threshold
Internal or External Soft-Start
Programmable 1.8V, 3.3V, 5V or Adjustable Output
Few External Components Required
Programmable Input Overvoltage Lockout
Thermally Enhanced High Voltage MSOP Package
APPLICATIONS
Industrial Control Supplies
Medical Devices
n Distributed Power Systems
n Portable Instruments
n Battery-Operated Devices
n Avionics
n Automotive
n
n
The LTC®3638 is a high efficiency step-down DC/DC
regulator with internal power switch that draws only 12μA
typical DC supply current while maintaining a regulated
output voltage at no load.
The LTC3638 can supply up to 250mA load current and
features a programmable peak current limit that provides
a simple method for optimizing efficiency and for reducing output ripple and component size. The LTC3638’s
combination of Burst Mode® operation, integrated power
switch, low quiescent current, and programmable peak
current limit provides high efficiency over a broad range
of load currents.
With its wide input range of 4V to 140V and programmable
overvoltage lockout, the LTC3638 is a robust regulator
suited for regulating from a wide variety of power sources.
Additionally, the LTC3638 includes a precise run threshold
and soft-start feature to guarantee that the power system
start-up is well-controlled in any environment. A feedback
comparator output enables multiple LTC3638s to be connected in parallel for higher current applications.
The LTC3638 is available in a thermally enhanced high
voltage-capable 16-lead MSE package with four missing pins.
L, LT, LTC, LTM, Burst Mode, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their respective
owners.
TYPICAL APPLICATION
Efficiency and Power Loss vs Load Current
100
5V to 140V Input to 5V Output, 250mA Step-Down Regulator
CIN
1µF
250V
SW
VIN
LTC3638
RUN
80
VOUT
5V
250mA
VFB
OVLO
SS
VPRG1
VPRG2
GND
COUT
22µF
70
VIN = 12V
VIN = 48V
VIN = 140V
60
50
40
1000
100
30
20
POWER LOSS (mW)
VIN
5V TO 140V
L1
220µH
EFFICIENCY
90
EFFICIENCY (%)
n
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DESCRIPTION
10
POWER LOSS
10
3638 TA01a
0
0.1
1
10
100
LOAD CURRENT (mA)
1
1000
3638 TA01b
3638fa
For more information www.linear.com/LTC3638
1
LTC3638
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
VIN Supply Voltage.................................... –0.3V to 140V
RUN Voltage............................................. –0.3V to 140V
SS, FBO, OVLO, ISET Voltages....................... –0.3V to 6V
VFB, VPRG1, VPRG2 Voltages.......................... –0.3V to 6V
Operating Junction Temperature Range (Notes 2, 3, 4)
LTC3638E, LTC3638I.......................... –40°C to 125°C
LTC3638H........................................... –40°C to 150°C
LTC3638MP........................................ –55°C to 150°C
Storage Temperature Range................... –65°C to 150°C
Lead Temperature (Soldering, 10 sec).................... 300°C
TOP VIEW
SW 1
16 GND
VIN 3
FBO
VPRG2
VPRG1
GND
17
GND
5
6
7
8
14 RUN
12
11
10
9
OVLO
ISET
SS
VFB
MSE PACKAGE
VARIATION: MSE16 (12)
16-LEAD PLASTIC MSOP
TJMAX = 150°C, θJA = 40°C/W, θJC = 10°C/W
EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC3638EMSE#PBF
LTC3638EMSE#TRPBF
3638
16-Lead Plastic MSOP
–40°C to 125°C
LTC3638IMSE#PBF
LTC3638IMSE#TRPBF
3638
16-Lead Plastic MSOP
–40°C to 125°C
LTC3638HMSE#PBF
LTC3638HMSE#TRPBF
3638
16-Lead Plastic MSOP
–40°C to 150°C
LTC3638MPMSE#PBF
LTC3638MPMSE#TRPBF
3638
16-Lead Plastic MSOP
–55°C to 150°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping
container.Consult LTC Marketing for information on non-standard lead based finish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the specified operating
junction temperature range, otherwise specifications are at TA = 25°C (Note 2). VIN = 12V, unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
140
V
Input Supply (VIN)
VIN
Input Voltage Operating Range
VOUT
Output Voltage Operating Range
UVLO
VIN Undervoltage Lockout
IQ
DC Supply Current (Note 5)
Active Mode
Sleep Mode
Shutdown Mode
4
0.8
VIN Rising
VIN Falling
Hysteresis
l
l
3.5
3.3
No Load
VRUN = 0V
V
4.0
3.8
V
V
mV
150
12
1.4
350
22
6
µA
µA
µA
1.21
1.10
110
1.25
1.14
V
V
mV
VRUN
RUN Pin Threshold
RUN Rising
RUN Falling
Hysteresis
IRUN
RUN Pin Leakage Current
RUN = 1.3V
–10
0
10
nA
VOVLO
OVLO Pin Threshold
OVLO Rising
OVLO Falling
Hysteresis
1.17
1.06
1.21
1.10
110
1.25
1.14
V
V
mV
2
1.17
1.06
VIN
3.75
3.5
250
3638fa
For more information www.linear.com/LTC3638
LTC3638
ELECTRICAL
CHARACTERISTICS
The l denotes the specifications which apply over the specified operating
junction temperature range, otherwise specifications are at TA = 25°C (Note 2). VIN = 12V, unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
V
V
Output Supply (VFB)
Feedback Comparator Threshold
(Adjustable Output)
VFB Rising, VPRG1 = VPRG2 = 0V
LTC3638E, LTC3638I
LTC3638H, LTC3638MP
l
l
0.792
0.788
0.800
0.800
0.808
0.812
VFBH
Feedback Comparator Hysteresis
(Adjustable Output)
VFB Falling, VPRG1 = VPRG2 = 0V
l
3
5
9
mV
IFB
Feedback Pin Current
VFB = 1V, VPRG1 = VPRG2 = 0V
VFB(FIXED)
Feedback Comparator Thresholds
(Fixed Output)
VFB(ADJ)
–10
0
10
nA
VFB Rising, VPRG1 = SS, VPRG2 = 0V
VFB Falling, VPRG1 = SS, VPRG2 = 0V
l
l
4.94
4.91
5.015
4.985
5.09
5.06
V
V
VFB Rising, VPRG1 = 0V, VPRG2 = SS
VFB Falling, VPRG1 = 0V, VPRG2 = SS
l
l
3.25
3.23
3.31
3.29
3.37
3.35
V
V
VFB Rising, VPRG1 = VPRG2 = SS
VFB Falling, VPRG1 = VPRG2 = SS
l
l
1.78
1.77
1.81
1.80
1.84
1.83
V
V
l
l
l
500
250
40
575
300
60
650
350
80
mA
mA
mA
Operation
IPEAK
Peak Current Comparator Threshold
ISET Floating
100k Resistor from ISET to GND
ISET Shorted to GND
RON
Power Switch On-Resistance
ISW = –100mA
1.8
ILSW
Switch Pin Leakage Current
VIN = 140V, SW = 0V
ISS
Soft-Start Pin Pull-Up Current
VSS < 2.5V
tINT(SS)
Internal Soft-Start Time
SS Pin Floating
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: The LTC3638 is tested under pulsed load conditions such that
TJ ≈ TA. The LTC3638E is guaranteed to meet performance specifications
from 0°C to 85°C. Specifications over the –40°C to 125°C operating
junction temperature range are assured by design, characterization and
correlation with statistical process controls. The LTC3638I is guaranteed
over the –40°C to 125°C operating junction temperature range, the
LTC3638H is guaranteed over the –40°C to 150°C operating junction
temperature range and the LTC3638MP is tested and guaranteed over the
–55°C to 150°C operating junction temperature range.
High junction temperatures degrade operating lifetimes; operating lifetime
is derated for junction temperatures greater than 125°C. Note that the
maximum ambient temperature consistent with these specifications is
determined by specific operating conditions in conjunction with board
layout, the rated package thermal impedance and other environmental
factors.
4
Ω
0.1
1
μA
5
6
μA
1
ms
Note 3: The junction temperature (TJ, in °C) is calculated from the ambient
temperature (TA, in °C) and power dissipation (PD, in Watts) according to
the formula:
TJ = TA + (PD • θJA)
where θJA is 40°C/W for the MSOP package.
Note that the maximum ambient temperature consistent with these
specifications is determined by specific operating conditions in
conjunction with board layout, the rated package thermal impedance and
other environmental factors.
Note 4: This IC includes overtemperature protection that is intended to
protect the device during momentary overload conditions. The maximum
rated junction temperature will be exceeded when this protection is active.
Continuous operation above the specified absolute maximum operating
junction temperature may impair device reliability or permanently damage
the device. The overtemperature protection level is not production tested.
Note 5: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency. See Applications Information.
3638fa
For more information www.linear.com/LTC3638
3
LTC3638
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency vs Load Current,
VOUT = 5V
100
100
FIGURE 14 CIRCUIT
90
90
100
FIGURE 14 CIRCUIT
80
70
70
40
30
20
0
0.1
1
10
100
LOAD CURRENT (mA)
60
50
40
30
20
VIN = 12V
VIN = 48V
VIN = 140V
10
EFFICIENCY (%)
80
50
0
0.1
1000
1
10
100
LOAD CURRENT (mA)
3638 G01
60
50
40
30
ILOAD = 250mA
ILOAD = 10mA
ILOAD = 1mA
0
0
75
50
100
INPUT VOLTAGE (V)
25
125
800
799
798
–55
125
155
PEAK CURRENT (mA)
100
50
1.14
1.12
75
100 125 150 175 200
RISET (kΩ)
3638 G07
FALLING
1.10
1.08
125
155
3638 G06
600
500
400
RISET = 100kΩ
300
200
0
–55
65
35
95
5
TEMPERATURE (°C)
700
ISET = GND
100
25
1.16
Peak Current Trip Threshold
vs Input Voltage
ISET OPEN
600
500
0
1.20
1.18
1.06
–55 –25
700
600
PEAK CURRENT TRIP THRESHOLD (mA)
95
5
35
65
TEMPERATURE (°C)
RISING
1.22
Peak Current Trip Threshold
vs Temperature and ISET
200
1000
1.24
3638 G05
Peak Current Trip Threshold
vs RISET
0
–25
3638 G04
300
10
100
LOAD CURRENT (mA)
RUN and OVLO Thresholds vs
Temperature
801
150
400
1
3638 G03
RUN OR OVLO THRESHOLD VOLTAGE (V)
EFFICIENCY (%)
70
VIN = 12V
VIN = 48V
VIN = 140V
0
0.1
1000
802
THRESHOLD VOLTAGE (mV)
80
10
30
Feedback Comparator Trip
Threshold vs Temperature
FIGURE 14 CIRCUIT
20
40
3638 G02
Efficiency vs Input Voltage,
VOUT = 5V
90
50
10
PEAK CURRENT (mA)
100
60
20
VIN = 12V
VIN = 48V
VIN = 140V
10
FIGURE 14 CIRCUIT
90
70
60
4
Efficiency vs Load Current,
VOUT = 1.8V
80
EFFICIENCY (%)
EFFICIENCY (%)
Efficiency vs Load Current,
VOUT = 3.3V
–25
5
35
65
95
TEMPERATURE (°C)
ISET OPEN
500
400
RISET = 100kΩ
300
200
ISET = GND
100
125
155
3638 G08
0
0
30
60
90
VIN VOLTAGE (V)
120
150
3638 G09
3638fa
For more information www.linear.com/LTC3638
LTC3638
TYPICAL PERFORMANCE CHARACTERISTICS
Quiescent Supply Current
vs Temperature
Quiescent Supply Current
vs Input Voltage
15
35
5
SHUTDOWN
30
60
90
VIN VOLTAGE (V)
25
20
SLEEP
15
10
SW = 0.8V
CURRENT INTO SW
5
0
SW = 0V
CURRENT OUT OF SW
–5
SHUTDOWN
0
–55
150
10
VIN = 140V
SLEEP MODE
–10
5
120
SWITCH PIN CURRENT (µA)
VIN SUPPLY CURRENT (µA)
VIN SUPPLY CURRENT (µA)
10
0
15
VIN = 140V
30
SLEEP
0
Switch Pin Current
vs Temperature
–25
65
35
5
95
TEMPERATURE (°C)
125
155
–15
–55 –25
5
65
95
35
TEMPERATURE (°C)
125
3638 G10
155
3638 G12
3638 G11
Switch On-Resistance
vs Input Voltage
Switch On-Resistance
vs Temperature
4
SWITCH ON-RESISTANCE (Ω)
SWITCH ON-RESISTANCE (Ω)
3.0
2.5
2.0
1.5
1.0
0
30
60
90
VIN VOLTAGE (V)
120
150
3
LOAD
CURRENT
100mA/DIV
2
VIN = 48V
200µs/DIV
VOUT = 3.3V
1mA TO 250mA LOAD STEP
FIGURE 15 CIRCUIT
1
0
–55
3638 G13
Operating Waveforms, VIN = 48V
–25
5
65
95
35
TEMPERATURE (°C)
125
SWITCH
VOLTAGE
20V/DIV
SWITCH
VOLTAGE
50V/DIV
INDUCTOR
CURRENT
500mA/DIV
INDUCTOR
CURRENT
500mA/DIV
3638 G15
155
3638 G14
Operating Waveforms, VIN = 140V
OUTPUT
VOLTAGE
50mV/DIV
3638 G16
OUTPUT
VOLTAGE
50mV/DIV
ISW = 250mA
OUTPUT
VOLTAGE
50mV/DIV
VIN = 48V
10µs/DIV
VOUT = 3.3V
IOUT = 250mA
FIGURE 15 CIRCUIT
Load Step Transient Response
Short-Circuit and Recovery
OUTPUT
VOLTAGE
1V/DIV
INDUCTOR
CURRENT
500mA/DIV
VIN = 140V
10µs/DIV
VOUT = 3.3V
IOUT = 250mA
FIGURE 15 CIRCUIT
3638 G17
500µs/DIV
FIGURE 15 CIRCUIT
3638 G18
3638fa
For more information www.linear.com/LTC3638
5
LTC3638
PIN FUNCTIONS
SW (Pin 1): Switch Node Connection to Inductor and
Catch Diode Cathode. This pin connects to the drain of
the internal power MOSFET switch.
VIN (Pin 3): Main Supply Pin. A ceramic bypass capacitor
should be tied between this pin and GND.
FBO (Pin 5): Feedback Comparator Output. Connect to the
VFB pins of additional LTC3638s to combine the output
current. The typical pull-up current is 20µA. The typical pulldown impedance is 70Ω. See Applications Information.
VPRG2, VPRG1 (Pins 6, 7): Output Voltage Selection. Short
both pins to ground for a resistive divider programmable
output voltage. Short VPRG1 to SS and short VPRG2 to
ground for a 5V output voltage. Short VPRG1 to ground
and short VPRG2 to SS for a 3.3V output voltage. Short
both pins to SS for a 1.8V output voltage.
GND (Pin 8, 16, Exposed Pad Pin 17): Ground. The exposed pad must be soldered to the PCB ground plane for
rated thermal performance.
VFB (Pin 9): Output Voltage Feedback. When configured
for an adjustable output voltage, connect to an external
resistive divider to divide the output voltage down for
comparison to the 0.8V reference. For the fixed output
configuration, directly connect this pin to the output.
SS (Pin 10): Soft-Start Control Input. A capacitor to
ground at this pin sets the output voltage ramp time. A
50µA current initially charges the soft-start capacitor until
switching begins, at which time the current is reduced to
its nominal value of 5µA. The output voltage ramp time
from zero to its regulated value is 1ms for every 6.25nF
of capacitance from SS to GND. If left floating, the ramp
time defaults to an internal 1ms soft-start.
6
ISET (Pin 11): Peak Current Set Input. A resistor from this
pin to ground sets the peak current comparator threshold.
Leave floating for the maximum peak current (575mA
typical) or short to ground for minimum peak current
(60mA typical). The maximum output current is one-half
the peak current. The 5µA current that is sourced out of
this pin when switching is reduced to 1µA in sleep. Optionally, a capacitor can be placed from this pin to GND
to trade off efficiency for light load output voltage ripple.
See Applications Information.
OVLO (Pin 12): Overvoltage Lockout Input. Connect to
the input supply through a resistor divider to set the overvoltage lockout level. A voltage on this pin above 1.21V
disables the internal MOSFET switch. Normal operation
resumes when the voltage on this pin decreases below
1.10V. Exceeding the OVLO lockout threshold triggers a
soft-start reset, resulting in a graceful recovery from an
input supply transient. Tie this pin to ground if the overvoltage is not used.
RUN (Pin 14): Run Control Input. A voltage on this pin
above 1.21V enables normal operation. Forcing this pin
below 0.7V shuts down the LTC3638, reducing quiescent
current to approximately 1.4µA. Optionally, connect to the
input supply through a resistor divider to set the undervoltage lockout.
3638fa
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LTC3638
BLOCK DIAGRAM
1.3V
11
ACTIVE: 5µA
SLEEP: 1µA
ISET
VIN
+
3
VIN
CIN
PEAK CURRENT
COMPARATOR
+
–
14
RUN
+
1.21V
–
SW
LOGIC
12
OVLO
–
VOUT
D1 COUT
GND
1.21V
L1
1
+
16
+
5V
20µA
5
FEEDBACK
COMPARATOR
FBO
+
+
–
70Ω
8
17
VOLTAGE
REFERENCE
5V
START-UP: 50µA
NORMAL: 5µA
0.800V
R1
R2
GND
GND
–
SWITCH NODE
COMPARATOR
VPRG2 VPRG1
GND
GND
SS
SS
GND
SS
GND
SS
VOUT
ADJUSTABLE
5V FIXED
3.3V FIXED
1.8V FIXED
R1
VFB
VPRG1
VPRG2
R2
1.0M ∞
4.2M 800k
2.5M 800k
1.0M 800k
SS
10
9
7
6
IMPLEMENT DIVIDER
EXTERNALLY FOR
ADJUSTABLE VERSION
3638 BD
3638fa
For more information www.linear.com/LTC3638
7
LTC3638
OPERATION
(Refer to Block Diagram)
The LTC3638 is a step-down DC/DC regulator with internal
power switch that uses Burst Mode control, combining
low quiescent current with high switching frequency,
which results in high efficiency across a wide range of
load currents. Burst Mode operation functions by using short “burst” cycles to switch the inductor current
through the internal power MOSFET, followed by a sleep
cycle where the power switch is off and the load current is
supplied by the output capacitor. During the sleep cycle,
the LTC3638 draws only 12µA of supply current. At light
loads, the burst cycles are a small percentage of the total
cycle time which minimizes the average supply current,
greatly improving efficiency. Figure 1 shows an example
of Burst Mode operation. The switching frequency is dependent on the inductor value, peak current, input voltage
and output voltage.
SLEEP
CYCLE
BURST
CYCLE
SWITCHING
FREQUENCY
INDUCTOR
CURRENT
BURST
FREQUENCY
OUTPUT
VOLTAGE
∆VOUT
3638 F01
Figure 1. Burst Mode Operation
Main Control Loop
The LTC3638 uses the VPRG1 and VPRG2 control pins to
connect internal feedback resistors to the VFB pin. This
enables fixed outputs of 1.8V, 3.3V or 5V without increasing component count, input supply current or exposure to
noise on the sensitive input to the feedback comparator.
8
External feedback resistors (adjustable mode) can be used
by connecting both VPRG1 and VPRG2 to ground.
In adjustable mode the feedback comparator monitors
the voltage on the VFB pin and compares it to an internal
800mV reference. If this voltage is greater than the reference, the comparator activates a sleep mode in which
the power switch and current comparators are disabled,
reducing the VIN pin supply current to only 12µA. As the
load current discharges the output capacitor, the voltage
on the VFB pin decreases. When this voltage falls 5mV
below the 800mV reference, the feedback comparator
trips and enables burst cycles.
At the beginning of the burst cycle, the internal high side
power switch (P-channel MOSFET) is turned on and the
inductor current begins to ramp up. The inductor current
increases until either the current exceeds the peak current comparator threshold or the voltage on the VFB pin
exceeds 800mV, at which time the switch is turned off
and the inductor current is carried by the external catch
diode. The inductor current ramps down until the switch
node rises, indicating that the current in the catch diode
is zero. If the voltage on the VFB pin is still less than the
800mV reference, the power switch is turned on again and
another cycle commences. The average current during a
burst cycle will normally be greater than the average load
current. For this architecture, the maximum average output
current is equal to half of the peak current.
The hysteretic nature of this control architecture results
in a switching frequency that is a function of the input
voltage, output voltage, and inductor value. This behavior
provides inherent short-circuit protection. If the output is
shorted to ground, the inductor current will decay very
slowly during a single switching cycle. Since the high side
switch turns on only when the inductor current is near
zero, the LTC3638 inherently switches at a lower frequency
during start-up or short-circuit conditions.
3638fa
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LTC3638
OPERATION
(Refer to Block Diagram)
Start-Up and Shutdown
If the voltage on the RUN pin is less than 0.7V, the LTC3638
enters a shutdown mode in which all internal circuitry is
disabled, reducing the DC supply current to 1.4µA. When the
voltage on the RUN pin exceeds 1.21V, normal operation of
the main control loop is enabled. The RUN pin comparator
has 110mV of internal hysteresis, and therefore must fall
below 1.1V to disable the main control loop.
An internal 1ms soft-start function limits the ramp rate of
the output voltage on start-up to prevent excessive input
supply droop. If a longer ramp time and consequently less
supply droop is desired, a capacitor can be placed from
the SS pin to ground. The 5µA current that is sourced
out of this pin will create a smooth voltage ramp on the
capacitor. If this ramp rate is slower than the internal 1ms
soft-start, then the output voltage will be limited by the
ramp rate on the SS pin. The internal and external softstart functions are reset on start-up, after an undervoltage
or overvoltage event on the input supply, and after an
overtemperature shutdown.
Peak Inductor Current Programming
The peak current comparator nominally limits the peak
inductor current to 575mA. This peak inductor current
can be adjusted by placing a resistor from the ISET pin to
ground. The 5µA current sourced out of this pin through
the resistor generates a voltage that adjusts the peak current comparator threshold.
During sleep mode, the current sourced out of the ISET pin
is reduced to 1µA. The ISET current is increased back to 5µA
on the first switching cycle after exiting sleep mode. The
ISET current reduction in sleep mode, along with adding
a filtering network, RISET and CISET, from the ISET pin to
ground, provides a method of reducing light load output
voltage ripple at the expense of lower efficiency and slightly
degraded load step transient response.
For applications requiring higher output current, the
LTC3638 provides a feedback comparator output pin (FBO)
for combining the output current of multiple LTC3638s.
By connecting the FBO pin of a master LTC3638 to the VFB
pin of one or more slave LTC3638s, the output currents
can be combined to source 250mA times the number of
LTC3638s.
Dropout Operation
When the input supply decreases toward the output supply, the duty cycle increases to maintain regulation. The
P-channel MOSFET switch in the LTC3638 allows the duty
cycle to increase all the way to 100%. At 100% duty cycle,
the P-channel MOSFET stays on continuously, providing
output current equal to the peak current, which is twice
the maximum load current when not in dropout.
Input Voltage and Overtemperature Protection
When using the LTC3638, care must be taken not to
exceed any of the ratings specified in the Absolute Maximum Ratings section. As an added safeguard, however,
the LTC3638 incorporates an overtemperature shutdown
feature. If the junction temperature reaches approximately
180°C, the LTC3638 will enter thermal shutdown mode.
The power switch will be turned off and the SW node will
become high impedance. After the part has cooled below
160°C, it will restart. The overtemperature level is not
production tested.
The LTC3638 additionally implements protection features
which inhibit switching when the input voltage is not within
a programmable operating range. By use of a resistive
divider from the input supply to ground, the RUN and
OVLO pins serve as a precise input supply voltage monitor.
Switching is disabled when either the RUN pin falls below
1.1V or the OVLO pin rises above 1.21V, which can be
configured to limit switching to a specific range of input
supply voltage. Furthermore, if the input voltage falls below
3.5V typical (3.8V maximum), an internal undervoltage
detector disables switching.
When switching is disabled, the LTC3638 can safely sustain
input voltages up to the absolute maximum rating of 140V.
Input supply undervoltage or overvoltage events trigger a
soft-start reset, which results in a graceful recovery from
an input supply transient.
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9
LTC3638
APPLICATIONS INFORMATION
The basic LTC3638 application circuit is shown on the
front page of this data sheet. External component selection is determined by the maximum load current requirement and begins with the selection of the peak current
programming resistor, RISET. The inductor value L can
then be determined, followed by capacitors CIN and COUT.
Peak Current Resistor Selection
The peak current comparator has a maximum current
limit of at least 500mA, which guarantees a maximum
average current of 250mA. For applications that demand
less current, the peak current threshold can be reduced
to as little as 40mA. This lower peak current allows the
efficiency and component selection to be optimized for
lower current applications.
The peak current threshold is linearly proportional to the
voltage on the ISET pin, with 100mV and 1V corresponding
to 40mA and 500mA peak current respectively. This pin
may be driven by an external voltage source to modulate
the peak current, which may be beneficial in some applications. Usually, the peak current is programmed with an
appropriately chosen resistor (RISET) between the ISET pin
and ground. The voltage generated on the ISET pin by RISET
and the internal 5µA current source sets the peak current.
The value of resistor for a particular peak current can be
computed by using Figure 2 or the following equation:
RISET = IPEAK • 400k
CURRENT (mA)
500
TYPICAL PEAK
INDUCTOR
CURRENT
200
MAXIMUM
LOAD
CURRENT
0
25
50
The inductor, input voltage, output voltage, and peak current
determine the switching frequency during a burst cycle of
the LTC3638. For a given input voltage, output voltage,
and peak current, the inductor value sets the switching
frequency during a burst cycle when the output is in regulation. Generally, switching at a frequency between 50kHz
and 200kHz yields high efficiency, and 100kHz is a good
first choice for many applications. The inductor value can
be determined by the following equation:
An additional constraint on the inductor value is the
LTC3638’s 150ns minimum on-time of the switch.
Therefore, in order to keep the current in the inductor
well-controlled, the inductor value must be chosen so that
300
0
Inductor Selection
The variation in switching frequency during a burst cycle
with input voltage and inductance is shown in Figure 3. For
lower values of IPEAK, multiply the frequency in Figure 3
by 575mA/IPEAK.
600
100
The peak current is internally limited to be within the range
of 40mA to 500mA. Shorting the ISET pin to ground programs the current limit to 40mA, and leaving it floating sets
the current limit to the maximum value of 500mA. When
selecting this resistor value, be aware that the maximum
average output current for this architecture is limited to
half of the peak current. Therefore, be sure to select a value
that sets the peak current with enough margin to provide
adequate load current under all conditions. Selecting the
peak current to be 2.2 times greater than the maximum
load current is a good starting point for most applications.
V
V
L = OUT • 1– OUT
VIN
f •IPEAK
where 40mA < IPEAK < 500mA.
400
The internal 5μA current source is reduced to 1μA in sleep
mode to maximize efficiency and to facilitate a tradeoff
between efficiency and light load output voltage ripple, as
described in the Optimizing Output Voltage Ripple section.
75 100 125 150 175 200
RISET (kΩ)
3638 F02
Figure 2. RISET Selection
10
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APPLICATIONS INFORMATION
10000
ISET OPEN
140
120
L = 47µH
INDUCTOR VALUE (µH)
SWITCHING FREQUENCY (kHz)
160
100
80
L = 100µH
60
L = 220µH
40
1000
100
20
0
0
30
60
90
120
VIN INPUT VOLTAGE (V)
150
10
10
3638 F03
100
PEAK INDUCTOR CURRENT (mA)
1000
3638 F04
Figure 3. Switching Frequency for VOUT = 3.3V
Figure 4. Recommended Inductor Values for Maximum Efficiency
it is larger than a minimum value which can be computed
as follows:
For applications where board area is not a limiting factor,
inductors with larger cores can be used, which extends
the recommended range of Figure 4 to larger values.
L>
VIN(MAX) • tON(MIN)
IPEAK
•1.2
Inductor Core Selection
where VIN(MAX) is the maximum input supply voltage when
switching is enabled, tON(MIN) is 150ns, IPEAK is the peak
current, and the factor of 1.2 accounts for typical inductor
tolerance and variation over temperature.
For applications that have large input supply transients,
the OVLO pin can be used to disable switching above the
maximum operating voltage VIN(MAX) so that the minimum
inductor value is not artificially limited by a transient
condition. Inductor values that violate the above equation
will cause the peak current to overshoot and permanent
damage to the part may occur.
Although the previous equation provides the minimum
inductor value, higher efficiency is generally achieved with
a larger inductor value, which produces a lower switching
frequency. For a given inductor type, however, as inductance is increased DC resistance (DCR) also increases.
Higher DCR translates into higher copper losses and lower
current rating, both of which place an upper limit on the
inductance. The recommended range of inductor values
for small surface mount inductors as a function of peak
current is shown in Figure 4. The values in this range are a
good compromise between the trade-offs discussed above.
Once the value for L is known, the type of inductor must
be selected. High efficiency regulators generally cannot
afford the core loss found in low cost powdered iron cores,
forcing the use of the more expensive ferrite cores. Actual
core loss is independent of core size for a fixed inductor
value but is very dependent of the inductance selected.
As the inductance increases, core losses decrease. Unfortunately, increased inductance requires more turns of
wire and therefore copper losses will increase.
Ferrite designs have very low core losses and are preferred at high switching frequencies, so design goals
can concentrate on copper loss and preventing saturation. Ferrite core material saturates “hard,” which means
that inductance collapses abruptly when the peak design
current is exceeded. This results in an abrupt increase in
inductor ripple current and consequently output voltage
ripple. Do not allow the core to saturate!
Different core materials and shapes will change the size/
current and price/current relationship of an inductor. Toroid
or shielded pot cores in ferrite or permalloy materials are
small and do not radiate energy but generally cost more
than powdered iron core inductors with similar characteristics. The choice of which style inductor to use mainly
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LTC3638
APPLICATIONS INFORMATION
depends on the price versus size requirements and any
radiated field/EMI requirements. New designs for surface
mount inductors are available from Coiltronics, Coilcraft,
TDK, Toko, and Sumida.
Catch Diode Selection
The catch diode (D1 from Block Diagram) conducts current
only during the switch off time. Average forward current
in normal operation can be calculated from:
ID(AVG) =IOUT
VIN – VOUT
VIN
where IOUT is the output load current. The maximum average diode current occurs with a shorted output at the
high line. For this worst-case condition, the diode current
will approach half of the programmed peak current. The
diode reverse voltage rating should be greater than the
maximum operating input voltage. When the OVLO pin is
used to limit the maximum operating input voltage, the
diode reverse voltage should be greater than the OVLO
pin setting, but may be lower the maximum input voltage
during overvoltage lockout.
For high efficiency at full load, it is important to select a
catch diode with a low reverse recovery time and low forward voltage drop. As a result, Schottky diodes are often
used as catch diodes. However, Schottky diodes generally
exhibit much higher leakage than silicon diodes. In sleep,
the catch diode leakage current will appear as load current,
and may significantly reduce light load efficiency. Diodes
with low leakage often have larger forward voltage drops
at a given current, so a trade-off can exist between light
load and full load efficiency.
The selection of Schottky diodes with high reverse voltage
ratings is limited relative to that of silicon diodes. Therefore, for low reverse leakage and part availability, some
applications may prefer a silicon diode. If a silicon diode
is necessary, be sure to select a diode with a specified low
reverse recovery time to maximize efficiency.
12
CIN and COUT Selection
The input capacitor, CIN, is needed to filter the trapezoidal
current at the source of the high side MOSFET. CIN should
be sized to provide the energy required to magnetize the
inductor without causing a large decrease in input voltage
(∆VIN). The relationship between CIN and ∆VIN is given by:
CIN >
L •IPEAK 2
2 • VIN • ∆VIN
It is recommended to use a larger value for CIN than
calculated by the previous equation since capacitance
decreases with applied voltage. In general, a 1µF X7R ceramic capacitor is a good choice for CIN in most LTC3638
applications.
To prevent large ripple voltage, a low ESR input capacitor
sized for the maximum RMS current should be used. RMS
current is given by:
IRMS =IOUT(MAX) •
VOUT
VIN
•
–1
VIN
VOUT
This formula has a maximum at VIN = 2VOUT, where IRMS =
IOUT/2. This simple worst-case condition is commonly used
for design because even significant deviations do not offer
much relief. Note that ripple current ratings from capacitor
manufacturers are often based only on 2000 hours of life
which makes it advisable to further derate the capacitor,
or choose a capacitor rated at a higher temperature than
required. Several capacitors may also be paralleled to meet
size or height requirements in the design.
The output capacitor, COUT, filters the inductor’s ripple
current and stores energy to satisfy the load current when
the LTC3638 is in sleep. The output ripple has a lower limit
of VOUT/160 due to the 5mV typical hysteresis of the feedback comparator. The time delay of the comparator adds
an additional ripple voltage that is a function of the load
current. During this delay time, the LTC3638 continues to
switch and supply current to the output. The output ripple
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can be approximated by:
–6
V
IPEAK
4 •10
∆VOUT ≈
–ILOAD •
+ OUT
2
COUT
160
The output ripple is a maximum at no load and approaches
lower limit of VOUT/160 at full load. Choose the output
capacitor COUT to limit the output voltage ripple ∆VOUT
using the following equation:
COUT ≥
IPEAK • 2 •10 –6
V
∆VOUT – OUT
160
The value of the output capacitor must also be large enough
to accept the energy stored in the inductor without a large
change in output voltage during a single switching cycle.
Setting this voltage step equal to 1% of the output voltage,
the output capacitor must be:
2
100%
L I
COUT > • PEAK •
2 VOUT
1%
Typically, a capacitor that satisfies the voltage ripple requirement is adequate to filter the inductor ripple. To avoid
overheating, the output capacitor must also be sized to
handle the ripple current generated by the inductor. The
worst-case ripple current in the output capacitor is given
by IRMS = IPEAK/2. Multiple capacitors placed in parallel
may be needed to meet the ESR and RMS current handling
requirements.
Dry tantalum, special polymer, aluminum electrolytic,
and ceramic capacitors are all available in surface mount
packages. Special polymer capacitors offer very low ESR
but have lower capacitance density than other types.
Tantalum capacitors have the highest capacitance density
but it is important only to use types that have been surge
tested for use in switching power supplies. Aluminum
electrolytic capacitors have significantly higher ESR but
can be used in cost-sensitive applications provided that
consideration is given to ripple current ratings and longterm reliability. Ceramic capacitors have excellent low ESR
characteristics but can have high voltage coefficient and
audible piezoelectric effects. The high quality factor (Q)
of ceramic capacitors in series with trace inductance can
also lead to significant input voltage ringing.
Input Voltage Steps
If the input voltage falls below the regulated output voltage,
the body diode of the internal MOSFET will conduct current
from the output supply to the input supply. If the input
voltage falls rapidly, the voltage across the inductor will be
significant and may saturate the inductor. A large current
will then flow through the MOSFET body diode, resulting
in excessive power dissipation that may damage the part.
If rapid voltage steps are expected on the input supply, put
a small silicon or Schottky diode in series with the VIN pin
to prevent reverse current and inductor saturation, shown
below as D1 in Figure 5. The diode should be sized for a
reverse voltage of greater than the regulated output voltage, and to withstand repetitive currents higher than the
maximum peak current of the LTC3638.
LTC3638
INPUT
SUPPLY
D1
VIN
SW
L
VOUT
COUT
CIN
3638 F05
Figure 5. Preventing Current Flow to the Input
Ceramic Capacitors and Audible Noise
Higher value, lower cost ceramic capacitors are now becoming available in smaller case sizes. Their high ripple
current, high voltage rating, and low ESR make them ideal
for switching regulator applications. However, care must
be taken when these capacitors are used at the input and
output. When a ceramic capacitor is used at the input and
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13
LTC3638
APPLICATIONS INFORMATION
the power is supplied by a wall adapter through long wires,
a load step at the output can induce ringing at the input,
VIN. At best, this ringing can couple to the output and be
mistaken as loop instability. At worst, a sudden inrush
of current through the long wires can potentially cause
a voltage spike at VIN large enough to damage the part.
For applications with inductive source impedance, such
as a long wire, a series RC network may be required in
parallel with CIN to dampen the ringing of the input supply.
Figure 6 shows this circuit and the typical values required
to dampen the ringing. Refer to Application Note 88 for additional information on suppressing input supply transients.
Ceramic capacitors are also piezoelectric. The LTC3638’s
burst frequency depends on the load current, and in some
applications the LTC3638 can excite the ceramic capacitor at audio frequencies, generating audible noise. This
noise is typically very quiet to a casual ear; however, if the
noise is unacceptable, use a high performance tantalum
or electrolytic capacitor at the output.
LIN
For the adjustable output mode (VPRG1 = VPRG2 = GND),
the output voltage is set by an external resistive divider
according to the following equation:
R1
VOUT = 0.8V • 1+
R2
The resistive divider allows the VFB pin to sense a fraction
of the output voltage as shown in Figure 7. The output
voltage can range from 0.8V to VIN. Be careful to keep
the divider resistors very close to the VFB pin to minimize
noise pick-up on the sensitive VFB trace.
VOUT
VFB
LTC3638
VPRG1
VPRG2
LTC3638
VIN
L
R = IN
CIN
CIN
3638 F06
4 • CIN
Figure 6. Series RC to Reduce VIN Ringing
Output Voltage Programming
The LTC3638 has three fixed output voltage modes and
an adjustable mode that can be selected with the VPRG1
and VPRG2 pins. The fixed output modes use an internal
feedback divider which enables higher efficiency, higher
noise immunity, and lower output voltage ripple for 5V,
3.3V, and 1.8V applications. To select the fixed 5V output
14
voltage, connect VPRG1 to SS and VPRG2 to GND. For 3.3V,
connect VPRG1 to GND and VPRG2 to SS. For 1.8V, connect
both VPRG1 and VPRG2 to SS. For any of the fixed output
voltage options, directly connect the VFB pin to VOUT.
0.8V
R1
R2
3638 F07
Figure 7. Setting the Output Voltage with External Resistors
To minimize the no-load supply current, resistor values in
the megohm range may be used; however, large resistor
values should be used with caution. The feedback divider
is the only load current when in shutdown. If PCB leakage
current to the output node or switch node exceeds the load
current, the output voltage will be pulled up. In normal
operation, this is generally a minor concern since the load
current is much greater than the leakage.
To avoid excessively large values of R1 in high output voltage applications (VOUT ≥ 10V), a combination of external
and internal resistors can be used to set the output voltage. This has an additional benefit of increasing the noise
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APPLICATIONS INFORMATION
immunity on the VFB pin. Figure 8 shows the LTC3638
with the VFB pin configured for a 5V fixed output with an
external divider to generate a higher output voltage. The
internal 5M resistance appears in parallel with R2, and
the value of R2 must be adjusted accordingly. R2 should
be chosen to be less than 200k to keep the output voltage variation less than 1% due to the tolerance of the
LTC3638’s internal resistor.
The RUN and OVLO pins can alternatively be configured
as precise undervoltage (UVLO) and overvoltage (OVLO)
lockouts on the VIN supply with a resistive divider from VIN
to ground. A simple resistive divider can be used as shown
in Figure 10 to meet specific VIN voltage requirements.
VIN
R3
RUN
VOUT
LTC3638
VFB
R4
OVLO
R1
5V
4.2M
R5
Figure 10. Adjustable UV and OV Lockout
800k
SS
VPRG1
VPRG2
3638 F08
Figure 8. Setting the Output Voltage with
External and Internal Resistors
RUN Pin and Overvoltage/Undervoltage Lockout
The LTC3638 has a low power shutdown mode controlled
by the RUN pin. Pulling the RUN pin below 0.7V puts the
LTC3638 into a low quiescent current shutdown mode
(IQ ~ 1.4µA). When the RUN pin is greater than 1.21V,
switching is enabled. Figure 9 shows examples of configurations for driving the RUN pin from logic.
VIN
LTC3638
4.7M
RUN
1k
3638 F10
R2
0.8V
SUPPLY
LTC3638
LTC3638
R5 = R TOTAL •
1.21V
Rising VIN OVLO Threshold
R4 = R TOTAL •
1.21V
–R5
Rising VIN UVLO Threshold
R3 = R TOTAL –R5 –R4
RUN
1k
3638 F09
Figure 9. RUN Pin Interface to Logic
The current that flows through the R3-R4-R5 divider will
directly add to the shutdown, sleep, and active current of
the LTC3638, and care should be taken to minimize the
impact of this current on the overall efficiency of the application circuit. Resistor values in the megohm range may
be required to keep the impact on quiescent shutdown and
sleep currents low. To pick resistor values, the sum total
of R3 + R4 + R5 (RTOTAL) should be chosen first based
on the allowable DC current that can be drawn from VIN.
The individual values of R3, R4 and R5 can then be calculated from the following equations:
For applications that do not need a precise external OVLO,
the OVLO pin should be tied directly to ground. The RUN
pin in this type of application can be used as an external
UVLO using the previous equations with R5 = 0Ω.
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15
LTC3638
APPLICATIONS INFORMATION
Similarly, for applications that do not require a precise
UVLO, the RUN pin can be tied to VIN. In this configuration,
the UVLO threshold is limited to the internal VIN UVLO
thresholds as shown in the Electrical Characteristics table.
The resistor values for the OVLO can be computed using
the previous equations with R3 = 0Ω.
Be aware that the OVLO pin cannot be allowed to exceed
its absolute maximum rating of 6V. To keep the voltage
on the OVLO pin from exceeding 6V, the following relation
should be satisfied:
R5
VIN(MAX) •
< 6V
R3+R4+R5
If this equation cannot be satisfied in the application,
connect a 4.7V Zener diode between the OVLO pin and
ground to clamp the OVLO pin voltage.
Soft-Start
Soft-start is implemented by ramping the effective reference voltage from 0V to 0.8V. To increase the duration of
the soft-start, place a capacitor from the SS pin to ground.
An internal 5µA pull-up current will charge this capacitor.
The value of the soft-start capacitor can be calculated by
the following equation:
5µA
CSS = Soft-Start Time •
0.8V
The minimum soft-start time is limited to the internal
soft-start timer of 1ms. When the LTC3638 detects a
fault condition (input supply undervoltage/overvoltage or
overtemperature) or when the RUN pin falls below 1.1V,
the SS pin is quickly pulled to ground and the internal
soft-start timer is reset. This ensures an orderly restart
when using an external soft-start capacitor.
Note that the soft-start capacitor may not be the limiting
factor in the output voltage ramp. The maximum output
current, which is equal to half of the peak current, must
charge the output capacitor from 0V to its regulated value.
For small peak currents or large output capacitors, this
16
ramp time can be significant. Therefore, the output voltage
ramp time from 0V to the regulated VOUT value is limited
to a minimum of
Ramp Time ≥
2COUT
V
IPEAK OUT
Optimizing Output Voltage Ripple
After the peak current resistor and inductor have been
selected to meet the load current and frequency requirements, an optional capacitor, CISET can be added in parallel
with RISET to reduce the output voltage ripple dependency
on load current.
At light loads the output voltage ripple will be a maximum.
The peak inductor current is controlled by the voltage on
the ISET pin. The current out of the ISET pin is 5µA while
the LTC3638 is active and is reduced to 1µA during sleep
mode. The ISET current will return to 5µA on the first
switching cycle after sleep mode. Placing a parallel RC
network to ground on the ISET pin filters the ISET voltage
as the LTC3638 enters and exits sleep mode, which in
turn will affect the output voltage ripple, efficiency, and
load step transient performance.
Higher Current Applications
For applications that require more than 250mA, the
LTC3638 provides a feedback comparator output pin
(FBO) for driving additional LTC3638s. When the FBO pin
of a master LTC3638 is connected to the VFB pin of one
or more slave LTC3638s, the master controls the burst
cycle of the slaves.
Figure 11 shows an example of a 5V, 500mA regulator
using two LTC3638s. The master is configured for a 5V
fixed output with external soft-start and VIN UVLO/OVLO
levels set by the RUN and OVLO pins. Since the slave is
directly controlled by the master, its SS pin should be
floating, RUN should be tied to VIN, and OVLO should be
tied to ground. Furthermore, the slave should be configured
for a 1.8V fixed output (VPRG1 = VPRG2 = SS) to set the
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VIN
CIN
R3
D1
LTC3638
(MASTER)
RUN
VFB
SS
R4
VPRG1
OVLO VPRG2
R5
FBO
VIN
VOUT
5V
COUT 500mA
L1
SW
VIN
VFB
RUN
D2
VPRG1
OVLO VPRG2
FBO
As an example, consider the LTC3638 in dropout at an input
voltage of 5V, a load current of 575mA and an ambient
temperature of 85°C. From the Typical Performance graphs
of Switch On-Resistance, the RDS(ON) of the top switch
at VIN = 5V and 100°C is approximately 3.2Ω. Therefore,
the power dissipated by the part is:
L2
SW
SS
TJ = TA + TR
Generally, the worst-case power dissipation is in dropout
at low input voltage. In dropout, the LTC3638 can provide
a DC current as high as the full 575mA peak current to the
output. At low input voltage, this current flows through a
higher resistance MOSFET, which dissipates more power.
CSS
LTC3638
(SLAVE)
The junction temperature is given by:
3638 F11
Figure 11. 5V, 500mA Regulator
VFB pin threshold at 1.8V. The inductors L1 and L2 do not
necessarily have to be the same, but should both meet
the criteria described in the Inductor Selection section.
Thermal Considerations
In most applications, the LTC3638 does not dissipate much
heat due to its high efficiency. But, in applications where
the LTC3638 is running at high ambient temperature with
low supply voltage and high duty cycles, such as dropout,
the heat dissipated may exceed the maximum junction
temperature of the part.
To prevent the LTC3638 from exceeding the maximum
junction temperature, the user will need to do some thermal
analysis. The goal of the thermal analysis is to determine
whether the power dissipated exceeds the maximum junction temperature of the part. The temperature rise from
ambient to junction is given by:
TR = PD • θJA
Where PD is the power dissipated by the regulator and
θJA is the thermal resistance from the junction of the die
to the ambient temperature.
PD = (ILOAD)2 • RDS(ON) = (575mA)2 • 3.2Ω = 1.06W
For the MSOP package the θJA is 40°C/W. Thus, the junction temperature of the regulator is:
40°C
TJ = 85°C+1.06W • W = 127°C
which is below the maximum junction temperature of
150°C.
Note that the while the LTC3638 is in dropout, it can provide
output current that is equal to the peak current of the part.
This can increase the chip power dissipation dramatically
and may cause the internal overtemperature protection
circuitry to trigger at 180°C and shut down the LTC3638.
Pin Clearance/Creepage Considerations
The LTC3638 MSE package has been uniquely designed to
meet high voltage clearance and creepage requirements.
Pins 2, 4, 13, and 15 are omitted to increase the spacing between adjacent high voltage solder pads (VIN, SW,
and RUN) to a minimum of 0.657mm which is sufficient
for most applications. For more information, refer to the
printed circuit board design standards described in IPC2221 (www.ipc.org).
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17
LTC3638
APPLICATIONS INFORMATION
Design Example
also be rated for an average forward current of at least:
As a design example, consider using the LTC3638 in an
application with the following specifications: VIN = 36V
to 72V (48V nominal), VOUT = 12V, IOUT = 250mA, f =
200kHz, and that switching is enabled when VIN is between
30V and 90V.
First, calculate the inductor value based on the switching
frequency:
12V
12V
L=
• 1–
≅ 78µH
200kHz • 0.575A 48V
Choose a 100µH inductor as a standard value. Next, verify
that this meets the LMIN requirement at the maximum
input voltage:
LMIN =
90V •150ns
•1.2 = 28µH
0.575A
Therefore, the minimum inductor requirement is satisfied
and the 100μH inductor value may be used.
Next, CIN and COUT are selected. For this design, CIN should
be sized for a current rating of at least:
IRMS = 250mA •
12V
36V
•
– 1≅ 118mARMS
36V
12V
The value of CIN is selected to keep the input from drooping less than 360mV (1%) at low line:
CIN >
100µH • 0.575A 2
≅ 1.3µF
2 • 36V • 360mV
Since the capacitance of capacitors decreases with DC
bias, a 2.2µF capacitor should be chosen.
The catch diode should have a reverse voltage rating of
greater than the overvoltage lockout setting of 90V. It should
18
ID(AVG) = 250mA
90V – 12V
= 217mA
90V
During a short-circuit, the average current in the diode
could be as high as IPEAK/2, or 288mA. For margin, select
a catch diode with a reverse breakdown of at least 100V
and an average current of 350mA or higher.
COUT will be selected based on a value large enough to
satisfy the output voltage ripple requirement. For a 1%
output ripple (120mV), the value of the output capacitor
can be calculated from:
0.575A • 2 •10 –6
COUT ≥
≅ 26µF
12V
120mV –
160
COUT also needs an ESR that will satisfy the output voltage
ripple requirement. The required ESR can be calculated
from:
ESR <
120mV
≅ 208mΩ
0.575A
A 33µF ceramic capacitor has significantly less ESR than
208mΩ. The output voltage can now be programmed by
choosing the values of R1 and R2. Since the output voltage is higher than 10V, the LTC3638 should be set for a
5V fixed output with an external divider to divide the 12V
output down to 5V. R2 is chosen to be less than 200k
to keep the output voltage variation to less than 1% due
to the internal 5M resistor tolerance. Set R2 = 196k and
calculate R1 as:
R1=
12V – 5V
• (196kΩ 5MΩ ) = 264kΩ
5V
Choose a standard value of 267k for R1.
3638fa
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LTC3638
APPLICATIONS INFORMATION
The undervoltage and overvoltage lockout requirements
on VIN can be satisfied with a resistive divider from VIN to
the RUN and OVLO pins (refer to Figure 10). Choose R3 +
R4 + R5 = 2.5M to minimize the loading on VIN. Calculate
R3, R4 and R5 as follows:
1.21V • 2.5MΩ
R5 =
= 33.6k
VIN _ OV(RISING)
R4 =
1.21V • 2.5MΩ
–R5 = 67.2k
VIN _ UV(RISING)
R3 = 2.5MΩ –R4 –R5 = 2.4M
Since specific resistor values in the megohm range are
generally less available, it may be necessary to scale R3,
R4, and R5 to a standard value of R3. For this example,
choose R3 = 2.2M and scale R4 and R5 by 2.2M/2.4M.
Then, R4 = 61.6k and R5 = 30.8k. Choose standard values
of R3 = 2.2M, R4 = 62k, and R5 = 30.9k. Note that the falling thresholds for both UVLO and OVLO will be 10% less
than the rising thresholds, or 27V and 81V respectively.
The ISET pin should be left open in this example to select
maximum peak current (575mA). Figure 12 shows a
complete schematic for this design example.
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of
the LTC3638. Check the following in your layout:
1. Large switched currents flow in the power switch, catch
diode, and input capacitor. The loop formed by these
components should be as small as possible. A ground
plane is recommended to minimize ground impedance.
2. Connect the (+) terminal of the input capacitor, CIN, as
close as possible to the VIN pin. This capacitor provides
the AC current into the internal power MOSFET.
3. Keep the switching node, SW, away from all sensitive
small signal nodes. The rapid transitions on the switching
node can couple to high impedance nodes, in particular
VFB, and create increased output ripple.
L1
VIN
VIN
R3
RUN
VFB
OVLO
SS
ISET
R4
CIN
R5
100µH
SW
VIN
2.2M
LTC3638
FBO
D1
VPRG1
VPRG2
GND
COUT
R2
RISET
VOUT
12V
250mA
VFB
RUN
2.2µF
267k
VOUT
R1
LTC3638
CSS
VIN
36V TO 72V
SW
FBO
62k
ISET
L1
33µF
SS
OVLO
VPRG1
30.9k
GND
196k
GND
D1
VPRG2
CIN
3638 F12
VOUT
COUT
Figure 12. 36V to 72V Input to 12V Output, 250mA Regulator
VIN
R4
R2 R1
R3
R5
RISET
CSS
GND
VIAS TO GROUND PLANE
VIAS TO INPUT SUPPLY (VIN)
VIAS TO OUTPUT SUPPLY (VOUT)
OUTLINE OF LOCAL GROUND PLANE
3638 F13
Figure 13. Example PCB Layout
3638fa
For more information www.linear.com/LTC3638
19
LTC3638
TYPICAL APPLICATIONS
Efficiency vs Input Voltage
L1
330µH
SW
VIN
LTC3638
90
VFB
RUN
FBO
CIN
1µF
250V
IOUT = 100mA
95
EFFICIENCY (%)
VIN
4V TO 140V
100
VOUT*
5V
250mA
COUT
22µF
SS
ISET
VPRG1
OVLO
VPRG2
GND
D1
85
VOUT = 5V
80
75
VOUT = 3.3V
70
VOUT = 1.8V
3638 F14
65
*VOUT = VIN FOR VIN < 5V
CIN: TDK C5750X7R2E105K
COUT: TDK C3216X5R0J226MT
L1: COILCRAFT MSS1278T-334KL
D1: DIODES INC PDS3200
60
30
0
120
60
90
VIN INPUT VOLTAGE (V)
150
3638 F14b
Figure 14. High Efficiency 250mA Regulator
L1
68µH
VIN
4V TO 140V
VIN
VOUT*
3.3V
250mA
SW
LTC3638
Soft-Start Waveform
30Ω LOAD
VFB
RUN
FBO
CIN
1µF
250V
ISET
OVLO
220k
D1
SS
VPRG2
VPRG1
220pF
COUT
100µF
470nF
GND
OUTPUT
VOLTAGE
500mV/DIV
3638 F15
10ms/DIV
3638 F15b
CIN: MURATA GRM55DR72E105KW01L
COUT: MURATA GRM43SR60J107ME20
L1: SUMIDA CDRH8D28NP-680NC
D1: VISHAY U1D
Figure 15. Low Output Voltage Ripple 250mA Regulator with 75ms Soft-Start
4V to 125V Input to –15V Output Positive-to-Negative Regulator
CIN
1µF
250V
VIN
250
SW
LTC3638
RUN
200k
VFB
FBO
SS
ISET
VPRG1
OVLO
VPRG2
GND
D1
102k
COUT
10µF
25V
VOUT
–15V
MAXIMUM LOAD CURRENT ≈
VIN
I
• PEAK
VIN + VOUT
2
MAXIMUM LOAD CURRENT (mA)
VIN
4V TO 125V
Maximum Load Current
vs Input Voltage
L1
220µH
200
VOUT = –15V
150
100
3638 TA04a
MAXIMUM INPUT VOLTAGE = 140 –|VOUT|
CIN: KEMET C2225C105KARACTU
COUT: AVX 12103C106MAT
L1: TDK SLF12555-221MR72
D1: ST MICRO STTH102A
20
VOUT = –5V
50
0
30
60
90
120
VIN INPUT VOLTAGE (V)
150
3638 TA04b
3638fa
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LTC3638
TYPICAL APPLICATIONS
4V to 90V Input to 12V/500mA Output Regulator with Overvoltage Lockout
L1
47µH
VIN
4V TO 90V
UP TO 140V
TRANSIENT
VIN
1M
LTC3638
(MASTER)
VFB
RUN
267k
OVLO
CIN1
1µF
200V
13.7k
SS
VPRG1
VPRG2
GND
ISET
FBO
D1
196k
Low Dropout Startup and
Shutdown
VOUT*
12V
500mA
SW
VIN
COUT
47µF
16V
X5R
VIN/VOUT
5V/DIV
VOUT
L1 CURRENT
500mA/DIV
L2 CURRENT
500mA/DIV
1s/DIV
3638 TA05b
L2
47µH
VIN
SW
LTC3638
(SLAVE)
VFB
RUN
CIN2
1µF
200V
OVLO
SS
VPRG2
VPRG1
GND
ISET
FBO
3638 TA05a
CIN1/CIN2: VISHAY VJ2225Y105KXCA
COUT: TAIYO YUDEN EMK325 BJ 476MM-T
L1/L2: WÜRTH 744 778 914 7
D1/D2: CENTRAL SEMI CMSH1-100M-LTN
*VOUT = VIN FOR VIN < 12V
Overvoltage Lockout Operation
D2
VIN
50V/DIV
VOUT
10V/DIV
TRANSIENT TO 140V
72V
L1 CURRENT
500mA/DIV
L2 CURRENT
500mA/DIV
200ms/DIV
3638 TA05c
3638fa
For more information www.linear.com/LTC3638
21
LTC3638
TYPICAL APPLICATIONS
6W LED Driver
Efficiency vs Input Voltage
100
L1
100µH
VIN
FBO
ISET
SS
VDIM
42.2k
PWM
95
VFB
RUN
CIN
1µF
250V
1M
LTC3638
1M
GND
COUT
4.7µF
50V
D1
OVLO
VPRG1
VPRG2
24V LED
250mA
27.4k
M1
90
85
80
3638 TA03a
3.3V
CIN: TDK C5750X7R2E105K
COUT: TDK C4532X7R1H475M
L1: TDK SLF10145T-101M
D1: TOSHIBA CRH01
M1: VISHAY SILICONIX Si2356DS
PWM OPEN
VDIM OPEN
VOUT
SW
EFFICIENCY (%)
VIN
32V TO 140V
30
VDIM = 0.1V TO 1V FOR 10:1 ANALOG DIMMING
PWM = SQUARE WAVE FOR DIGITAL DIMMING
30V OVERVOLTAGE PROTECTION ON VOUT
VIN
SW
LTC3638
R1
470k
CIN
1µF
250V
R2
4.02k
220k
VFB
RUN
ISET
FBO
OVLO
GND
SS
VPRG1
VPRG2
D1
35.7k
COUT
4.7µF
50V
3638 TA06a
INPUT CURRENT LIMIT =
VOUT
R2 5µA •R1 VOUT
R2
•
• 1+
•
≈
VIN
4 R1+R2
4 R1+R2
*MAXIMUM LOAD CURRENT =
CIN: TDK C5750X7R2E105K
COUT: TDK C4532X7R1H475M
L1: TDK SLF12555T-101M1R1
D1: ROHM RF101L2S
22
VIN
• 75mA ≤ 250mA
36V
3638 TA03b
300
VOUT
36V
250mA*
MAXIMUM CURRENT (mA)
L1
100µH
150
Maximum Load and Input Current
vs Input Voltage
36V to 140V to 36V/250mA with 75mA Input Current Limit
VIN
36V TO 140V
60
90
120
VIN INPUT VOLTAGE (V)
250
MAXIMUM LOAD CURRENT
200
150
100
MAXIMUM INPUT CURRENT
50
0
40 50 60 70 80 90 100 110 120 130 140 150
VIN INPUT VOLTAGE (V)
3638 TA06b
3638fa
For more information www.linear.com/LTC3638
LTC3638
TYPICAL APPLICATIONS
Burst Frequency vs Load Current
100
BURST FREQUENCY (kHz)
WITH BURST FREQUENCY LIMIT
5V to 140V Input to 5V/250mA Output with 20kHz Minimum Burst Frequency
CIN
1µF
250V
VIN
SW
D1
LTC3638
RUN
VFB
ISET
FBO
VPRG2
VPRG1
OVLO
SS
GND
953k
V
LTC6994-1
IN
OUT
DIV
100k
10Ω
+
VOUT
5V
250mA
COUT
22µF
1
WITHOUT BURST FREQUENCY LIMIT
0.1
VIN = 48V
0.01
0.1
1
1000
3638 TA08b
SET
GND
10
100
LOAD CURRENT (mA)
2N7000
Input Current vs Load Current
200k
100
3638 TA08a
CIN: AVX 2225PC105MAT1A
COUT: KEMET C1206C226K9PAC
L1: COILTRONICS DR74-101-R
D1: DIODES INC MURS120-13-F
INPUT CURRENT (mA)
VIN
5V TO 140V
L1
100µH
10
10
VIN = 48V
WITH BURST FREQUENCY LIMIT
1
0.1
0.01
0.1
WITHOUT BURST FREQUENCY LIMIT
1
10
100
LOAD CURRENT (mA)
1000
3638 TA08c
3638fa
For more information www.linear.com/LTC3638
23
LTC3638
PACKAGE DESCRIPTION
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
MSE Package
Variation: MSE16 (12)
16-Lead Plastic MSOP with 4 Pins Removed
Exposed Die Pad
(Reference LTC DWG # 05-08-1871 Rev D)
BOTTOM VIEW OF
EXPOSED PAD OPTION
2.845 ±0.102
(.112 ±.004)
5.10
(.201)
MIN
2.845 ±0.102
(.112 ±.004)
0.889 ±0.127
(.035 ±.005)
8
1
1.651 ±0.102
(.065 ±.004)
1.651 ±0.102 3.20 – 3.45
(.065 ±.004) (.126 – .136)
16
0.305 ±0.038
(.0120 ±.0015)
TYP
0.50
(.0197)
1.0 BSC
(.039)
BSC
RECOMMENDED SOLDER PAD LAYOUT
0.254
(.010)
0.35
REF
4.039 ±0.102
(.159 ±.004)
(NOTE 3)
0.12 REF
DETAIL “B”
CORNER TAIL IS PART OF
DETAIL “B” THE LEADFRAME FEATURE.
FOR REFERENCE ONLY
9
NO MEASUREMENT PURPOSE
0.280 ±0.076
(.011 ±.003)
REF
16 14 121110 9
DETAIL “A”
0° – 6° TYP
3.00 ±0.102
(.118 ±.004)
(NOTE 4)
4.90 ±0.152
(.193 ±.006)
GAUGE PLANE
0.53 ±0.152
(.021 ±.006)
DETAIL “A”
1.10
(.043)
MAX
0.18
(.007)
SEATING
PLANE
0.17 – 0.27
(.007 – .011)
TYP
1
3 567 8
1.0
(.039)
BSC
0.50
(.0197)
BSC
NOTE:
1. DIMENSIONS IN MILLIMETER/(INCH)
2. DRAWING NOT TO SCALE
3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.
MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.
INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
6. EXPOSED PAD DIMENSION DOES INCLUDE MOLD FLASH. MOLD FLASH ON E-PAD SHALL
NOT EXCEED 0.254mm (.010") PER SIDE.
24
0.86
(.034)
REF
0.1016 ±0.0508
(.004 ±.002)
MSOP (MSE16(12)) 0213 REV D
3638fa
For more information www.linear.com/LTC3638
LTC3638
REVISION HISTORY
REV
DATE
DESCRIPTION
A
12/14
Clarified OVLO Pin Function
PAGE NUMBER
6
Clarified Related Parts List
24
3638fa
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection
of its circuits
as described
herein will not infringe on existing patent rights.
For more
information
www.linear.com/LTC3638
25
LTC3638
TYPICAL APPLICATION
12V/250mA Automotive Supply
L1
220µH
VIN
LTC3638
CIN
1µF
250V
X7R
267k
90
80
VFB
FBO
SS
ISET
VPRG1
OVLO
VPRG2
GND
D1
196k
COUT
10µF
16V
X7R
EFFICIENCY
VIN = 24V
VIN = 48V
VIN = 120V
70
60
50
40
100
POWER LOSS
30
20
*VOUT = VIN FOR VIN < 12V
L1: COILCRAFT MSS1246T-224KL
D1: DIODES INC SBR1U200P1-7
1000
POWER LOSS (mW)
RUN
100
VOUT
12V*
250mA
SW
EFFICIENCY (%)
VIN
4V TO 140V
Efficiency and Power Loss vs
Load Current
10
10
3638 TA07
0
0.1
1
1
1000
10
100
LOAD CURRENT (mA)
3638 TA07b
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LTC7138
140V, 400mA Micropower Step-Down Regulator
VIN: 4V to 140V, VOUT(MIN) = 0.8V, IQ = 12μA, ISD = 1.4μA, MSE16 Package
LTC3639
150V, 100mA Synchronous Micropower Step-Down VIN: 4V to 150V, VOUT(MIN) = 0.8V, IQ = 12µA, ISD = 1.4µA, MS16E Package
DC/DC Regulator
LTC3630
65V, 500mA Synchronous Step-Down DC/DC
Regulator
VIN: 4V to 65V, VOUT(MIN) = 0.8V, IQ = 12µA, ISD = 5µA,
3mm × 5mm DFN16, MSOP16E Packages
LTC3637
76V, 1A Synchronous Step-Down DC/DC Regulator
VIN: 4V to 76V, VOUT(MIN) = 0.8V, IQ = 12µA, ISD = 3µA,
3mm × 5mm DFN16, MSOP16E Packages
LTC3630A
76V, 500mA Synchronous Step-Down DC/DC
Regulator
VIN: 4V to 76V, VOUT(MIN) = 0.8V, IQ = 12µA, ISD = 5µA,
3mm × 5mm DFN16, MSOP16E Packages
LTC3810
100V Synchronous Step-Down DC/DC Controller
VIN: 6.4V to 100V, VOUT(MIN) = 0.8V, IQ = 2mA, ISD < 240µA,
SSOP28 Package
LTC3631/LTC36313.3 LTC3631-5
45V (Transient to 60V), 100mA Synchronous StepDown DC/DC Regulator
VIN: 4.5V to 45V, VOUT(MIN) = 0.8V, IQ = 12µA, ISD < 3µA,
3mm × 3mm DFN8, MSOP8 Packages
LTC3642
45V (Transient to 60V), 50mA Synchronous StepDown DC/DC Regulator
VIN: 4.5V to 45V, VOUT(MIN) = 0.8V, IQ = 12µA, ISD < 3µA,
3mm × 3mm DFN8, MSOP8 Packages
LTC3632
50V (Transient to 60V), 20mA Synchronous StepDown DC/DC Regulator
VIN: 4.5V to 45V, VOUT(MIN) = 0.8V, IQ = 12µA, ISD < 3µA,
3mm × 3mm DFN8, MSOP8 Packages
LTC3891
60V Synchronous Step-Down DC/DC Controller with VIN: 4V to 60V, VOUT(MIN) = 0.8V, IQ = 50µA, ISD < 14µA,
3mm × 4mm QFN20, TSSOP20E Packages
Burst Mode Operation
26 Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
For more information www.linear.com/LTC3638
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com/LTC3638
3638fa
LT 1214 REV A • PRINTED IN USA
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