LT3667
40V 400mA Step-Down
Switching Regulator with
Dual Fault Protected LDOs
DESCRIPTION
FEATURES
Triple Output Supply from a Single Input Requires
Only One Inductor
n I = 50μA at 12V to 5V, 3.3V and 2.5V with No Load
Q
IN
n Buck Regulator:
Low Ripple ( VOVLO (overvoltage lockout, 42V typical), allowing the output to fall out of regulation.
During start-up, short-circuit, or other overload conditions
the inductor peak current might reach and even exceed the
maximum current limit of the LT3667, especially in those
cases where the switch already operates at minimum ontime. The catch diode current limit circuitry prevents the
switch from turning on again if the inductor valley current
is above 500mA nominal.
Inductor Selection and Maximum Output Current
For a given input and output voltage, the inductor value
and switching frequency will determine the ripple current,
which increases with higher VIN1 or VOUT1 and decreases
with higher inductance and higher switching frequency.
A good first choice for the inductor value is:
L = ( VOUT1 + VD ) •
2.4
fSW
where fSW is the switching frequency in MHz, VOUT1 is the
output voltage, VD is the catch diode drop (~0.5V) and L is
the inductor value in μH. The inductor’s RMS current rating
must be greater than the maximum load current and its
saturation current should be about 30% higher. For robust
operation in fault conditions (start-up or short-circuit) and
high input voltage (>30V), the saturation current should
be above 900mA. To keep the efficiency high, the series
resistance (DCR) should be less than 0.3Ω, and the core
material should be intended for high frequency applications. Table 2 lists several vendors.
Table 2. Inductor Vendors
VENDOR
URL
Coilcraft
www.coilcraft.com
Sumida
www.sumida.com
Toko
www.tokoam.com
Würth Elektronik
www.we-online.com
Coiltronics
www.cooperet.com
Murata
www.murata.com
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LT3667
APPLICATIONS INFORMATION
This simple design guide will not always result in the
optimum inductor selection for a given application. As a
general rule, lower output voltages and higher switching
frequency will require smaller inductor values. If the application requires less than 400mA load current, then a
lesser inductor value may be acceptable. This allows the
use of a physically smaller inductor, or one with a lower
DCR resulting in higher efficiency. However, the inductance
should in general not be smaller than 10µH.
Be aware that if the inductance differs from the simple
rule above, then the maximum load current will depend
on input voltage. In addition, low inductance may result
in discontinuous mode operation, which further reduces
maximum load current. For details of maximum output
current and discontinuous mode operation, see Linear
Technology’s Application Note 44. Finally, for duty cycles
greater than 50% (VOUT1/VIN1 > 0.5), a minimum inductance
is required to avoid sub-harmonic oscillations:
LMIN = ( VOUT1 + VD ) •
2
fSW
where fSW is the switching frequency in MHz, VOUT1 is
the output voltage, VD is the catch diode drop (~0.5V)
and LMIN is the inductor value in µH.
Catch Diode
The catch diode (D1 from block diagram) conducts current
only during switch off-time. Use a 1A Schottky diode for
best performance.
Peak reverse voltage is equal to VIN1 if it is below the
overvoltage protection threshold. This feature keeps the
switch off for VIN1 > OVLO (44V maximum). For inputs up
to the maximum operating voltage of 40V, use a diode with
a reverse voltage rating greater than the input voltage. If
transients at the input of up to 60V are expected, use a diode
with a reverse voltage rating only higher than the maximum
OVLO of 44V. If operating at high ambient temperatures,
consider using a Schottky with low reverse leakage. For
example, Diodes Inc. SBR1U40LP or DFLS160, ON Semi
MBRM140, and Central Semiconductor CMMSH1-60 are
good choices for the catch diode.
Input Capacitor
Bypass the input of the LT3667 circuit with a ceramic
capacitor of X7R or X5R type. Y5V types have poor
performance over temperature and applied voltage, and
should not be used. A 1μF to 4.7μF ceramic capacitor is
adequate to bypass the LT3667 and will easily handle
the ripple current. Note that a larger input capacitance
is required when a lower switching frequency is used
(due to longer on-times). If the input power source has
high impedance, or there is significant inductance due to
long wires or cables, additional bulk capacitance may be
necessary. This can be provided with a low performance
electrolytic capacitor. Step-down regulators draw current
from the input supply in pulses with very fast rise and
fall times. The input capacitor is required to reduce the
resulting voltage ripple at the LT3667 and to force this
very high frequency switching current into a tight local
loop, minimizing EMI. A 1μF capacitor is capable of this
task, but only if it is placed close to the LT3667 (see the
PCB Layout section). A second precaution regarding the
ceramic input capacitor concerns the maximum input
voltage rating of the LT3667. A ceramic input capacitor
combined with trace or cable inductance forms a high
quality (under damped) tank circuit. If the LT3667 circuit
is plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT3667’s
voltage rating. This situation is easily avoided (see the Hot
Plugging Safely section).
Output Capacitor and Output Ripple
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated
by the LT3667 to produce the DC output. In this role it
determines the output ripple, and low impedance at the
switching frequency is important. The second function
is to store energy in order to satisfy transient loads and
stabilize the switching regulator’s control loop. Ceramic
capacitors have very low equivalent series resistance
(ESR) and provide the best ripple performance. A good
starting value is:
18
COUT1 =
50
VOUT1 • fSW
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LT3667
APPLICATIONS INFORMATION
where fSW is in MHz, and COUT1 is the recommended output
capacitance in μF. Use X5R or X7R types. This choice will
provide low output ripple and good transient response.
Transient performance can be improved with a higher value
capacitor if combined with a phase lead capacitor (typically
22pF) between the output and pin FB1. Note that a larger
phase lead capacitor should be used with a large output
capacitor. A lower value of output capacitor can be used to
save space and cost but transient performance will suffer.
When choosing a capacitor, look carefully through the
data sheet to find out what the actual capacitance is under
operating conditions (applied voltage and temperature).
A physically larger capacitor, or one with a higher voltage
rating, may be required. Table 3 lists several capacitor
vendors.
Table 3: Capacitor Vendors
VENDOR
URL
Panasonic
www.panasonic.com
Kemet
www.kemet.com
Sanyo
www.sanyovideo.com
Murata
www.murata.com
AVX
www.avxcorp.com
Taiyo Yuden
www.taiyo-yuden.com
Low Ripple Burst Mode Operation
To enhance efficiency at light loads, the LT3667 operates in low ripple Burst Mode operation which keeps
the output capacitor charged to the proper voltage while
minimizing the input quiescent current. During Burst Mode
operation, the LT3667 delivers single cycle bursts of
current to the output capacitor followed by sleep periods
where the output power is delivered to the load by the
output capacitor. Because the LT3667 delivers power
to the output with single, low current pulses, the output
ripple is kept below 5mV for a typical application. As the
load current decreases towards a no load condition, the
percentage of time that the LT3667 operates in sleep mode
increases and the average input current is greatly reduced
resulting in high efficiency even at very low loads. Note
that during Burst Mode operation, the switching frequency
will be lower than the programmed switching frequency.
At higher output loads (above ~50mA for the front page
application) the LT3667 will be running at the frequency
programmed by the RT resistor, and will be operating in
standard PWM mode. The transition between PWM and
low ripple Burst Mode operation is seamless, and will not
disturb the output voltage.
Audible Noise
Ceramic capacitors are small, robust and have very
low ESR. However, ceramic capacitors can sometimes
cause problems when used with the LT3667 due to their
piezoelectric nature. When in Burst Mode operation, the
LT3667’s switching frequency depends on the load current,
and at very light loads the LT3667 can excite the ceramic
capacitor at audio frequencies, generating audible noise.
Since the LT3667 operates at a lower current limit during
Burst Mode operation, the noise is typically very quiet. If
this is unacceptable, use a high performance tantalum or
electrolytic capacitor at the output.
VSW
5V/DIV
IL
100mA/DIV
VOUT1
5mV/DIV
ILOAD = 10mA
1µs/DIV
FRONT PAGE APPLICATION
3667 F01
Figure 1. Burst Mode Operation
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LT3667
APPLICATIONS INFORMATION
Capacitor C2 and the internal boost Schottky diode (see the
Block Diagram) are used to generate a boost voltage that
is higher than the input voltage. In most cases a 0.22μF
capacitor will work well. Figure 2 shows two ways to arrange the boost circuit. The BOOST pin must be more than
1.9V above the SW pin for best efficiency. For outputs of
2.2V and above, the standard circuit (Figure 2a) is best.
For outputs between 2.2V and 2.5V, use a 0.47μF boost
capacitor. For output voltages below 2.2V, the boost diode
can be tied to the input (Figure 2b), or to another external
supply greater than 2.2V. However, the circuit in Figure 2a
is more efficient because the BOOST pin current and BD
pin quiescent current come from a lower voltage source.
Also, be sure that the maximum voltage ratings of the
BOOST and BD pins are not exceeded.
The minimum operating voltage of an LT3667 application is limited by the minimum input voltage (4.3V) and
by the maximum duty cycle as outlined in a previous
section. For proper start-up, the minimum input voltage
is also limited by the boost circuit. If the input voltage
is ramped slowly, the boost capacitor may not be fully
charged. Because the boost capacitor is charged with the
energy stored in the inductor, the circuit relies on some
minimum load current to get the boost circuit running
properly. This minimum load depends on 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 3 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 VIN1 is ramping very slowly.
For lower start-up voltage, the boost diode can be tied to
VIN1; however, this restricts the input range to one-half of
the absolute maximum rating of the BOOST pin.
5.0
INPUT VOLTAGE VIN1 (V)
BOOST and BD, IN3/BD Pin Considerations
FRONT PAGE APPLICATION
VEN = VIN1, VOUT1 = 3.3V
4.5
TO START
4.0
TO RUN
3.5
3.0
VOUT1
3.5
BD
VIN1
IN1
BOOST
6.5
D1
INPUT VOLTAGE VIN1 (V)
DA
(2a) For VOUT1 ≥ 2.2V
BD
IN1
BOOST
TO START
6.0
5.5
TO RUN
5.0
4.5
C2
LT3667
SW
D1
GND
100 150 200 250 300 350 400
LOAD CURRENT IOUT1 (mA)
3667 F03a
SW
VIN1
50
C2
LT3667
GND
0
VOUT1
4.0
FRONT PAGE APPLICATION
VEN = VIN1, VOUT1 = 5V
0
DA
50
100 150 200 250 300 350 400
LOAD CURRENT IOUT1 (mA)
3667 F03b
3667 F02
(2b) For VOUT1 < 2.2V; VIN1 < 25V
Figure 3. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit
Figure 2. Two Circuits for Generating the Boost Voltage
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APPLICATIONS INFORMATION
Synchronization (QFN Only)
from SYNC to ground which will draw current.
Synchronizing the oscillator of the LT3667 to an external
frequency can be done by connecting a digital clock signal
to the SYNC pin. The LT3667 then synchronizes its SW
node to the rising edge of this clock signal, as shown in
Figure 4. The square wave amplitude should have valleys
that are below 0.5V and peaks that are above 1.2V (up to
6V), and its on-time and off-time should not fall below
50ns. There is a time delay of typically 280ns between the
rising edge of SYNC and the rising edge of SW which is
in part caused by the minimum switch off-time. The falling edge of SW is sensitive to the falling edge of SYNC,
it is therefore recommended to adjust the duty cycle of
the SYNC clock signal accordingly to keep its on-time as
short as possible. Alternatively, AC coupling as shown in
Figure 5 can be used to shorten the clock signal's on-time.
The LT3667 may be synchronized over a 300kHz to 2.2MHz
range. The RT resistor should be chosen to set the switching
frequency 20% below the lowest synchronization input.
For example, if the synchronization signal is 360kHz, RT
should be chosen for 300kHz. Since RT also sets the slope
compensation which avoids subharmonic oscillations, the
minimum inductor value must be calculated using the
frequency determined by RT.
FRONT PAGE APPLICATION
UVLO1 Pin (QFN Only)
The switching regulator part of the LT3667 can be independently disabled via the UVLO1 pin. The falling threshold of
the UVLO1 comparator is 1V, with a 75mV hysteresis. The
UVLO1 pin has no effect if VIN1 and VIN2 are below 4.3V,
because then the internal undervoltage lockout keeps the
LT3667 shut down anyway.
Adding a resistive divider from IN1 to UVLO1 as shown
in Figure 6 programs the LT3667 to enable the switching
regulator only when VIN1 is above a certain threshold
voltage VIN(UVLO1), given by:
VSYNC
2V/DIV
RINSING
SYNC
TRIGGERS
SW
VSW
5V/DIV
3667 F04
200ns/DIV
Figure 4. Synchronization Waveforms
VIN(UVLO1) =
Note that due to the comparator’s hysteresis, the switching
regulator will not be enabled until the input rises slightly
above VIN(UVLO1).
VIN1
SYNC
3.3V
10k
IN1
R1
LT3667
10pF
R1+R2
•1V
R2
UVLO1
LT3667
1V
+
–
SWITCHING
REGULATOR
SHUT DOWN
R2
3667 F06
GND
3667 F05
Figure 5. Example of AC Coupling of SYNC Clock Signal
The LT3667 still enters Burst Mode operation at low
output loads while synchronized to an external clock, but
the burst pulses are synchronized to that clock signal. If
synchronization is not needed, the SYNC pin should be
grounded. It may also be tied to a voltage above 1.2V
(logic high), but note that there is an internal 4M resistor
Figure 6. UVLO1 Pin Allows Programmable Undervoltage
Lockout or Independent Disable of the Switching Regulator
Shorted and Reversed Input Protection
If the inductor is chosen so that it won’t saturate excessively, the switching 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
LT3667 is absent. This may occur in battery charging
applications or in battery backup systems where a battery
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LT3667
APPLICATIONS INFORMATION
or some other supply is diode ORed with the switching
regulator’s output. If the IN1 pin is allowed to float and
the EN and UVLO1 pins are held high (either by a logic
signal or because they are tied to IN1), then the LT3667’s
internal circuitry will pull its quiescent current through the
SW pin. This is fine if the system can tolerate a few μA
in this state. If the EN pin or the UVLO1 pin is grounded,
the SW pin current will drop to 0.7μA. However, if the IN1
pin is grounded while the output is held high, regardless
of EN and UVLO1, parasitic diodes inside the LT3667 can
pull current from the output through the SW pin and the
IN1 pin. Figure 7 shows a circuit that will run only when
the input voltage is present and that protects against a
shorted or reversed input. Alternatively, the switching
regulator can be supplied by the LDO at OUT2 as shown
in the Applications Information section of the LDOs.
FB2/FB3 Resistor Networks
Each LDO output voltage of the LT3667 is programmed
with a resistor divider between the output of that LDO and
its FB2/FB3 pin as shown in Figure 8. The pin current, IFB,
(3nA at 25°C, ±40nA at 150°C) of each FB2/FB3 pin flows
out of that pin, which results in
R2
VOUT = 0.8V +1 –IFB •R2
R1
The value of R1 should not exceed 160k to provide a
minimum 5µA load current so that the output voltage
error, caused by the FB2/FB3 pin current, is minimized.
Rearranging for R2 gives:
R2 =
D1
MBRS140
VIN
LDOs
BD
BOOST
IN1
Note that choosing smaller resistors will increase the
quiescent current of the application circuit.
EN
LT3667
UVLO1
GND
VOUT
SW
DA
FB1
0.8V – VOUT
IFB – 0.8V/R1
VIN
+
INn
OUTn
LT3667
BACKUP
GND
22
R2
FBn
3667 F07
Figure 7. Diode D1 Prevents a Shorted Input from Discharging
a Backup Battery Tied to the Output. It Also Protects the Circuit
from a Reversed Input, in Which Case the Resistor at the EN Pin
Limits the Current Drawn from That Pin. The LT3667 Runs Only
When the Input Is Present
VOUT
IFB
R1
3667 F08
Figure 8. Setting the Output Voltage of Each LDO
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LT3667
APPLICATIONS INFORMATION
Input Supply
The internal biasing and reference circuitry of the LT3667
is supplied by the IN1 and IN2 pins.This allows connecting
the switching regulator input IN1 to the LDO output OUT2.
This can be used to shield the supply at IN2 from the high
start-up currents of the switching regulator by utilizing the
LDO’s programmable current limit. The Typical Applications section shows an example of such an application,
which also benefits from the reverse voltage protection
of the LDO.
Input Capacitance and Stability
Each LDO is stable with an input capacitor typically between
1μF and 10μF. This input capacitor must be placed as close
as possible to the corresponding input pin. Applications
operating with smaller input to output differential voltages
and that experience large load transients may require a
higher input capacitor value to prevent input voltage droop
and letting the regulator enter dropout.
Very low ESR ceramic capacitors may be used. However,
in cases where long wires connect the power supply to
the LDOs input and ground, use of low value input capacitors may result in instability. The resonant LC tank circuit
formed by the wire inductance and the input capacitor is
the cause and not a result of LDO instability.
The minimum input capacitance needed to stabilize the
application also varies with power supply output impedance variations. Placing additional capacitance on an LDO’s
output also helps. However, this requires an order of
magnitude more capacitance in comparison with additional
input bypassing. Series resistance between the supply and
an LDO’s input also helps stabilize the application; as little
as 0.1Ω to 0.5Ω suffices. This impedance dampens the
LC tank circuit at the expense of dropout voltage. A better
alternative is to use higher ESR tantalum or electrolytic
capacitors at the input in place of ceramic capacitors.
capacitor of 2.2μF to prevent oscillations. Applications with
output voltages of less than 2.5V and applications where
the difference between input and output voltage exceeds
20V require a minimum output capacitor of 10µF. In addition, the ESR of the output capacitor must not exceed 3Ω.
The LT3667 is a micropower device and output load
transient response is a function of output capacitance.
Larger values of output capacitance decrease the peak
deviations and provide improved transient response for
larger load current changes, especially for low output
voltages. Bypass capacitors, used to decouple individual
components powered by the LT3667, increase the effective output capacitor value. For applications with large
load current transients, a low ESR ceramic capacitor in
parallel with a bulk tantalum capacitor often provides an
optimally damped response.
Note that some ceramic capacitors have a piezoelectric
response. A piezoelectric device generates voltage across
its terminals due to mechanical stress, similar to the way
a piezoelectric accelerometer or microphone works. For
a ceramic capacitor, the stress is induced by vibrations in
the system or thermal transients. The resulting voltages
produced cause appreciable amounts of noise. A ceramic
capacitor produced the trace in Figure 9 in response to light
tapping from a pencil. Similar vibration induced behavior
can masquerade as increased output voltage noise.
VOUT2 = 5V
COUT2 = 10µF
VOUT2
1mV/DIV
2ms/DIV
Output Capacitance, Transient Response, Stability
Each LT3667’s LDO is stable with a wide range of output
capacitors. The ESR of the output capacitor affects stability,
most notably with small capacitors. Use a minimum output
3667 F09
Figure 9. Noise Resulting from Tapping On a Ceramic Capacitor
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LT3667
APPLICATIONS INFORMATION
External Programmable Current Limit, Enable
Each EN/ILIM pin (EN2/ILIM2 and EN3/ILIM3) is the collector of a PNP which mirrors the corresponding LDO’s
output at a ratio of 1:799 (see Block Diagram). The EN2/
ILIM2 and EN3/ILIM3 pins are also the inputs to precision current limit amplifiers. If an output load increases
to the point where it causes the corresponding current
limit amplifier input voltage to reach 0.4V, the current
limit amplifier takes control of output regulation so that
its input clamps at 0.4V, regardless of the output voltage.
The current limit threshold (ILIMIT) of an LDO is set by
attaching a resistor (RIMAX) from the corresponding EN/
ILIM pin to ground:
RIMAX =
799 • 0.4V
– 80Ω
ILIM
In order to maintain stability, each EN/ILIM pin requires
a 47nF capacitor from that pin to ground.
In cases where the input to output voltage differential
exceeds 10V, foldback current limit will lower the internal current level limit, possibly causing it to preempt the
external programmable current limit. See the Internal
Current Limit vs Input/Output Differential graph in the
Typical Performance Characteristics section.
If an external current limit is not needed, the corresponding EN/ILIM pin must be connected to ground, in which
case no capacitor is required.
Each LDO can be individually shut down by pulling its EN/
ILIM pin above 1.2V (1V typical). Note that in this case this
pin will draw up to 500µA in certain operating conditions
until the LDO is shut down, which the circuit driving this
pin must be able to deliver. When an EN/ILIM pin is only
used to enable/disable an LDO, no capacitor is required
on this pin.
Overload Recovery
Each LDO of the LT3667 has a safe operating area protection, which decreases current limit as input-to-output
voltage increases, and keeps the power transistor inside
a safe operating region for all values of input-to-output
voltage. Each LDO provides some output current at all
values of input-to-output voltage up to the device break-
24
down. When power is first applied to an LDO, the input
voltage rises and the output follows the input; allowing the
regulator to start-up into very heavy loads. During start-up,
as the input voltage is rising, the input-to-output voltage
differential is small, allowing the regulator to supply large
output currents. With a high input voltage, a problem can
occur wherein the removal of an output short will not allow
the output to recover. The problem occurs with a heavy
output load when the input voltage is high and the output
voltage is low. Common situations are: immediately after
the removal of a short-circuit or if an LDO is enabled via
its EN/ILIM pin after the input voltage is already turned
on. In such cases, the regulator would have to operate its
power device outside its safe operating are (high voltage
and high current) in order to bring up the output voltage.
Since this is prevented by the safe operating area protection, the output gets stuck at a low voltage. Essentially,
the load line for such a load intersects the output current
curve at two points, resulting in two stable output operating
points for the regulator. With this double intersection, the
input power supply needs to be cycled down to zero and
brought up again to make the output recover.
Protection Features
The LT3667 LDO’s protect against reverse-input voltages, reverse-output voltages and reverse output-to-input
voltages. Current limit protection and thermal overload
protection protect the LDOs against current overload
conditions at their outputs. For normal operation, do not
exceed the maximum operating junction temperature. The
LT3667 IN2 and IN3 (QFN only) pins withstand reverse
voltages of 45V. The device limits current flow to less than
300μA (typically less than 10μA) and no negative voltages
appear at OUT2 or OUT3. The LDOs incur no damage if
their outputs are pulled below ground. If an input is left
open circuit or grounded, the corresponding output can be
pulled below ground by 45V. No current flows through the
pass transistor from the output. However, current flows in
(but is limited by) the corresponding resistor divider that
sets the output voltage. Current flows from the bottom
resistor in the divider and from the FB2/FB3 pin’s internal
clamp through the top resistor in the divider to the external
circuitry pulling OUT2/OUT3 below ground. If the input
is powered by a voltage source, the output sources cur3667fb
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APPLICATIONS INFORMATION
rent equal to its current limit capability and the LT3667
protects itself by thermal limiting. Note that the externally
programmable current limit is less accurate if the output
is pulled below ground.
These protection features can be used to protect the
switching regulator input as shown in the Typical Applications section.
COMMON
PCB Layout
Ceramic Capacitor Characteristics
Give extra consideration to the use of ceramic capacitors.
Manufacturers make ceramic capacitors with a variety of
dielectrics, each with different behavior across temperature and applied voltage. The most common dielectrics
are specified with EIA temperature characteristic codes
of Z5U, Y5V, X5R and X7R. The Z5U and Y5V dielectrics
provide high C-V products in a small package at low cost,
but exhibit strong voltage and temperature coefficients,
as can be seen for Y5V in Figures 10 and 11. When used
with a 5V regulator, a 16V 10μF Y5V capacitor can exhibit an effective value as low as 1μF to 2μF for the DC
bias voltage applied, and over the operating temperature
range. The X5R and X7R dielectrics yield much more
stable characteristics and are more suitable for use as
input and output capacitors. The X7R type works over
a wider temperature range and has better temperature
stability, while the X5R is less expensive and is available
in higher values. Still exercise care when using X5R and
20
The SW and BOOST nodes should be as small as possible.
Keep the FB1, FB2, and FB3 nodes small so that the ground
traces will shield them from the SW and BOOST nodes.
The exposed pad must be soldered such that it can act as a
heat sink. (See High Temperature Considerations section.)
40
20
X5R
CHANGE IN VALUE (%)
CHANGE IN VALUE (%)
For proper operation and minimum EMI, care must be
taken during printed circuit board layout. Figures 12 and
13 show the recommended component placement with
trace, ground plane and via locations. Note that large,
switched currents flow in the LT3667’s IN1, SW, GND
and DA pins, the catch diode and the input capacitor. The
loop formed by these components should be as small as
possible. These components, along with the inductor and
output capacitor, should be placed on the same side of
the circuit board, and their connections should be made
on that layer. Place a local, unbroken ground plane below
these components.
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
0
–20
–40
–60
Y5V
–80
–100
X7R capacitors; the X5R and X7R codes only specify
operating temperature range and maximum capacitance
change over temperature. Capacitance change due to DC
bias with X5R and X7R capacitors is better than Y5V and
Z5U capacitors, but can still be significant enough to drop
capacitor values below appropriate levels. Capacitor DC
bias characteristics tend to improve as component case
size increases, but expected capacitance at operating
voltage should be verified.
–20
–40
2
4
8
6
10 12
DC BIAS VOLTAGE (V)
14
16
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
–100
–50 –25
3667 F10
Figure 10. Ceramic Capacitor DC Bias Characteristics
Y5V
–60
–80
0
X5R
0
50
25
75
0
TEMPERATURE (°C)
100
125
3667 F11
Figure 11. Ceramic Capacitor Temperature Characteristics
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25
LT3667
APPLICATIONS INFORMATION
OUT1
OUT1
GND
GND
SW
IN1
SW
IN1
24 23 22 21
1
16
2
15
3
14
4
13
5
12
6
11
7
10
8
9
GND
1
20
2
19
3
18
4
17
5
16
6
15
7
14
8
13
9 10 11 12
3667 F13
VIAS TO LOCAL GROUND PLANE
VIAS TO LOCAL GROUND PLANE
Figure 12. Good PCB Layout Ensures
Proper, Low EMI Operation (MSOP)
Figure 13. Good PCB Layout Ensures
Proper, Low EMI Operation (QFN)
Hot Plugging Safely
High Temperature Considerations
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitors of LT3667 circuits. However, these capacitors can cause problems if the LT3667 is plugged into
a live supply. The low loss ceramic capacitor, combined
with stray inductance in series with the power source,
forms an under damped tank circuit, and the voltage at the
input pins of the LT3667 can ring to twice their nominal
input voltage, possibly exceeding the LT3667’s rating and
damaging the part. If the input supply is poorly controlled
or the user will be plugging the LT3667 into an energized
supply, the input network should be designed to prevent
this overshoot. See Linear Technology Application Note 88
for a complete discussion.
The LT3667’s maximum rated junction temperature of
125°C (E- and I-grades) and 150°C (H-grade) respectively
limits its power handling capability.
26
Power dissipation within the switching regulator can be
estimated by calculating the total power loss from an
efficiency measurement and subtracting inductor loss.
Be aware that at high ambient temperatures the external
Schottky diode will have significant leakage current (see
Typical Performance Characteristics), increasing the quiescent current of the switching regulator.
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LT3667
APPLICATIONS INFORMATION
The power dissipation of each LDO is comprised of two
components. Each power device dissipates:
of 12V and a maximum ambient temperature of 85°C, what
will the maximum junction temperature be?
PPASS = (VIN − VOUT) • IOUT
As can be seen from the Typical Performance Characteristics, the switching regulator efficiency approaches 85% at
400mA output current. This leads to a power loss, PLOSS, of:
where PPASS is the power, VIN the input voltage, VOUT
the output voltage, and IOUT the output current. The base
currents of the LDO power PNP transistors flow to ground
internally and are the major component of the ground
current. For each LDO, this causes a power dissipation
PGND of:
PGND = VIN • IGND
where VIN is the input voltage and IGND the ground current
generated by the corresponding power device. GND pin
current is determined by the current gain of the power
PNP, which has a typical value of 40 for the purpose of
this calculation:
IGND =
IOUT
40
1
PLOSS = 5V • 400mA •
– 1 = 353mW
0.85
(For the sake of simplicity and as a conservative estimate
assume that all of this power is dissipated in the LT3667.)
The power dissipations of the LDO power devices are:
PPASS2 = (5V − 2.5V) • 100mA = 250mW
PPASS3 = (5V − 3.3V) • 100mA = 170mW
For 100mA load current a maximum ground current of
2.5mA is to be expected. Thus, the corresponding power
dissipations are:
PGND2 = PGND3 = 5V • 2.5mA = 12.5mW
The total power dissipation equals the sum of the power
loss in the switching regulator and the two LDO components listed above.
The LT3667 has internal thermal limiting that protects
the device during overload conditions. If the junction
temperature reaches the thermal shutdown threshold, the
LT3667 will shut down the LDOs and stop switching to
prevent internal damage due to overheating. For continuous
normal conditions, do not exceed the maximum operating junction temperature. Carefully consider all sources
of thermal resistance from junction-to-ambient including
other nearby heat sources. Both LT3667 packages have
exposed pads that must be soldered to a ground plane to
act as heat sink. To keep thermal resistance low, extend the
ground plane as much as possible, and add thermal vias
under and near the LT3667 to additional ground planes
within the circuit board and on the bottom side.
The die temperature rise is calculated by multiplying the
power dissipation of the LT3667 by the thermal resistance
from junction to ambient. Example: Given the front page
application with maximum output current, an input voltage
Finally, the total power dissipation is:
PTOT = PLOSS + PPASS2 + PPASS3 + PGND2 + PGND3
= 786mW
Using the MSOP package, which has a thermal resistance
of approximately 40°C/W, this total power dissipation
would raise the junction temperature above ambient by:
0.786W • 40°C/W = 32°C
With the assumed maximum ambient temperature of 85°C,
this puts the maximum junction temperature at:
TJMAX = 85°C + 32°C = 117°C
Other Linear Technology Publications
Application Notes 19, 35 and 44 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 318
shows how to generate a bipolar output supply using a
buck regulator.
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27
LT3667
TYPICAL APPLICATIONS
5V, 3.3V and 2.5V Step-Down Converter
VIN
6V TO 40V
TRANSIENT
TO 60V
C1
4.7µF
UVLO1 IN1
BOOST
ON OFF
f = 600kHz
2.5V
100mA
C4
4.7µF
RT
174k
R3
340k
R4
158k
EN
SW
PG
DA
FB1
RT
C2
L1
0.22µF 22µH
D1
DFLS160
OUT2
FB2
FB3
EN2/ILIM2 GND EN3/ILIM3
C1-C5: X5R OR X7R
L1: CDRH4D22/HP
R2
294k
C6
22pF
LT3667
BD
IN2
IN3
OUT3
R1
931k
5V
C3 200mA
22µF
3.3V
100mA
R5
499k
C5
2.2µF
R6
158k
3667 TA02
Dual 5V/200mA Step-Down Converter
VIN
7V TO 40V
TRANSIENT
TO 60V
C1
4.7µF
UVLO1 IN1
BOOST
ON OFF
f = 600kHz
5V
200mA
C4
2.2µF
RT
174k
R3
787k
R4
150k
EN
SW
PG
DA
FB1
RT
OUT2
FB2
BD
IN2
IN3
OUT3
FB3
EN2/ILIM2 GND EN3/ILIM3
6V
D1
DFLS160
R1
1020k
R5
787k
R6
150k
C3
22µF
R2
255k
C6
22pF
LT3667
C1-C5: X5R OR X7R
L1: CDRH4D22/HP
28
C2
L1
0.22µF 22µH
5V
200mA
C5
2.2µF
3667 TA03
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LT3667
TYPICAL APPLICATIONS
5V, 3.3V and 2.5V Step-Down Converter with 100mA LDO Current Limits
VIN
8.5V TO 16V
TRANSIENT
TO 60V
C1
4.7µF
UVLO1 IN1
BOOST
ON OFF
RT
37.4k
f = 2MHz
2.5V*
C4
4.7µF
R3
340k
R4
158k
EN
SW
PG
DA
FB1
RT
LT3667
BD
IN2
IN3
OUT3
OUT2
FB2
FB3
EN2/ILIM2 GND EN3/ILIM3
C7
47nF
R7
3.09k
R8
3.09k
C2
L1
0.1µF 10µH
D1
DFLS160
5V
C3 200mA
10µF
R1
931k
R2
294k
C6
22pF
3.3V*
R5
499k
R6
158k
C5
2.2µF
C8
47nF
3667 TA04
*100mA CURRENT LIMIT
C1-C5: X5R OR X7R
L1: CDRH4D22/HP
Programming LDO Current Limits with a Digital/Analog Converter
VDAC
DAC OUTPUT
0V TO 0.8V
3.01k
LT3667
LT3667
EN2/ILIM2
EN2/ILIM2
3.01k
47nF
CURRENT LIMIT = 799
0.8V – VDAC
3.01kΩ + 160Ω
IDAC
DAC OUTPUT
0µA TO 267µA
1.5k
CURRENT LIMIT = 799
47nF
0.4V – IDAC • 1.5kΩ
1.5kΩ + 80Ω
3667 TA05
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29
LT3667
TYPICAL APPLICATIONS
This application allows a small input current to support
a high current pulsed load. The switching regulator is
supplied by the LDO2 at OUT2, which is programmed to
limit its current to 3.5mA. PG2 serves as “READY” signal
to tell a controller (not shown) that C6 is charged to 17V,
the regulation voltage of LDO2. It can then turn on a load
drawing high current out of the switching regulator.
Since LDO2 can only supply 3.5mA, this quickly discharges
C6 and decreases VOUT2 (=VIN1). The switching regulator
will maintain its programmed output voltage until VIN1
drops below the undervoltage lockout threshold of 5.5V
set by R3 and R4.
Pulsed Power Supply for 4mA to 20mA Current Loops.
OUT2 Supplies the Switching Regulator, Which Is Kept Off at Lower Voltages by UVLO1
VIN
18V TO 45V
TRANSIENTS
DOWN TO –28V
LDO2 Input
C1
1µF
ON OFF
BUCK Input
EN
IN2
BOOST
IN1
C4
2.2µF
Threshold
5.5V
Regulation
Voltage 17V
R3
4.5M
R4
1M
RT
174k
f = 600kHz
UVLO1
SW
RT
DA
FB1
C6
1000µF
R8
3010k
R9
150k
D1
DFLS160
BD
IN3
PG2
OUT3
OUT2
FB2
FB3
EN2/ILIM2 GND EN3/ILIM3
C7
47nF
R2
249k
PG2 Indicates
when C6 is
Charged
C5
10µF
R6
158k
R10
150k
READY
1.8V
100mA
R5
200k
3.3V
C3 100mA
22µF
R1
442k
C8
22pF
LT3667
LDO2 Output
Big Capacitor
Here to
Accumulate
Energy
C2
L1
0.22µF 22µH
R7
90.9k
3667 TA06a
ILOAD (mA)
VPG2 (V)
VOUT2 (V)
IIN2 (mA)
3.5mA Current Limit
C1-C5: X5R OR X7R
L1: CDRH4D22/HP
5
4
3
2
1
0
18
16
14
12
10
8
6
4
IIN2 Limited by Current
Limit of LDO2
VOUT2 Rises as C6 is Charged
by the Constant Current IOUT2
C6 is Quickly Discharged
by High Load Current
3
PG2 Signals That
VOUT2 is High Enough
2
1
0
100
80
60
40
20
0
IIN2 Drops as VOUT2
Reaches Programmed Value
Controller Decides
to Activate Load
0
1
2
3
TIME (SECONDS)
30
For more information www.linear.com/LT3667
4
5
3667 TA06b
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LT3667
PACKAGE DESCRIPTION
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
MSE Package
16-Lead Plastic MSOP, Exposed Die Pad
(Reference LTC DWG # 05-08-1667 Rev F)
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)
0.305 ±0.038
(.0120 ±.0015)
TYP
16
0.50
(.0197)
BSC
4.039 ±0.102
(.159 ±.004)
(NOTE 3)
RECOMMENDED SOLDER PAD LAYOUT
0.254
(.010)
0.35
REF
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
16151413121110 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
1234567 8
0.17 – 0.27
(.007 – .011)
TYP
0.50
NOTE:
(.0197)
1. DIMENSIONS IN MILLIMETER/(INCH)
BSC
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.
0.86
(.034)
REF
0.1016 ±0.0508
(.004 ±.002)
MSOP (MSE16) 0213 REV F
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31
LT3667
PACKAGE DESCRIPTION
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
UDD Package
24-Lead Plastic QFN (3mm × 5mm)
(Reference LTC DWG # 05-08-1833 Rev Ø)
0.70 ±0.05
3.50 ± 0.05
2.10 ± 0.05
3.65 ± 0.05
1.50 REF
1.65 ± 0.05
PACKAGE OUTLINE
0.25 ±0.05
0.50 BSC
3.50 REF
4.10 ± 0.05
5.50 ± 0.05
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
3.00 ± 0.10
0.75 ± 0.05
1.50 REF
23
R = 0.05 TYP
PIN 1 NOTCH
R = 0.20 OR 0.25
× 45° CHAMFER
24
0.40 ± 0.10
PIN 1
TOP MARK
(NOTE 6)
5.00 ± 0.10
1
2
3.65 ± 0.10
3.50 REF
1.65 ± 0.10
(UDD24) QFN 0808 REV Ø
0.200 REF
0.00 – 0.05
R = 0.115
TYP
0.25 ± 0.05
0.50 BSC
BOTTOM VIEW—EXPOSED PAD
NOTE:
1. DRAWING IS NOT A JEDEC PACKAGE OUTLINE
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION
ON THE TOP AND BOTTOM OF PACKAGE
32
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LT3667
REVISION HISTORY
REV
DATE
DESCRIPTION
PAGE NUMBER
A
4/14
Added H-grade in MS16E package to Order Information
B
11/14
3
Clarified Externally Programmable Current Limit specifications
5
Grammatical correction in Setting the Switching Frequency description
16
Clarified Typical Application schematic
29
3667fb
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/LT3667
33
LT3667
TYPICAL APPLICATION
5V, 3.3V and 51mA Regulator
C6
1µF
C7
1µF
IN3
IN2
VIN*
6V TO 40V
IN1
C1
2.2µF
f = 600kHz
BOOST
C2
L1
0.22µF
22µH
UVLO1
RT
174k
EN
SW
RT
DA
FB1
LT3667
D1
DFLS160
51mA*
OUT3
FB2
FB3
EN2/ILIM2 GND EN3/ILIM3
C4
10µF
C9
47nF
R2
249k
C8
22pF
BD
OUT2
R1
931k
R3
499k
R4
158k
5V
C3 400mA
22µF
3.3V*
200mA
C5
10µF
R5
6.19k
3667 TA07
C1-C7: X5R OR X7R
L1: CDRH4D22/HP
*DERATE OUTPUT CURRENT AT HIGHER AMBIENT TEMPERATURES
AND INPUT VOLTAGES TO MAINTAIN JUNCTION TEMPERATURE
BELOW THE ABSOLUTE MAXIMUM
RELATED PARTS
PART NUMBER
DESCRIPTION
LT3500
36V (40VMAX), 2A (IOUT), 2.2MHz Step-Down Switching Regulator VIN: 3V to 36V, VOUT(MIN) = 0.8V, IQ = 2.5mA, ISD < 12µA,
with LDO Controller
3mm × 3mm DFN-10, MSOP-16E
LT1939
25V, 2A (IOUT), 2.2MHz Step-Down Switching Regulator with LDO VIN: 3V to 25V, VOUT(MIN) = 0.8V, IQ = 2.5mA, ISD < 12µA,
Controller
3mm × 3mm DFN-10, MSOP-16E
LT3694
36V (70VMAX), 2.6A (IOUT), 2.5MHz Step-Down Switching
Regulator with Dual LDO Controller
VIN: 4V to 36V, VOUT(MIN) = 0.8V, IQ = 1mA, ISD < 1µA,
4mm × 5mm QFN-28, TSSOP-20E
LT3507/LT3507A
36V, 2.5MHz, Triple (2.4A + 1.5A + 1.5A (IOUT) with LDO
Controller High Efficiency Step-Down DC/DC Converter
VIN: 4V to 36V, VOUT(MIN) = 0.8V, IQ = 7mA, ISD = 1µA,
5mm × 7mm QFN-38
LT3970
40V, 350mA (IOUT), 2.2MHz Step-Down Switching Regulator with VIN: 4.2V to 40V, VOUT(MIN) = 1.2V, IQ = 2.5µA, ISD < 1µA,
IQ = 2.5µA
3mm × 2mm DFN, MSOP-10
LT3502/LT3502A
40V, 500mA (IOUT), 1.1MHz/2.2MHz Step-Down Switching
Regulator
34
COMMENTS
VIN: 3V to 40V, VOUT(MIN) = 0.8V, IQ = 1.5mA, ISD < 1µA,
2mm × 2mm DFN-8, MSOP-10E
3667fb
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
For more information www.linear.com/LT3667
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com/LT3667
LT 1114 REV B • PRINTED IN USA
LINEAR TECHNOLOGY CORPORATION 2014