LT1616 - 600mA, 1.4MHz Step-Down Switching Regulator in SOT-23

LT1616
600mA, 1.4MHz Step-Down
Switching Regulator
in SOT-23
U
FEATURES
■
■
■
■
■
■
■
■
■
DESCRIPTIO
The LT ®1616 is a current mode PWM step-down DC/DC
converter with internal 0.6A power switch, packaged in a
tiny 6-lead SOT-23. The wide input range of 3.6V to 25V
makes the LT1616 suitable for regulating a wide variety of
power sources, from 4-cell batteries and 5V logic rails to
unregulated wall transformers and lead-acid batteries. Its
high operating frequency allows the use of tiny, low cost
inductors and ceramic capacitors. With its internal compensation eliminating additional components, a complete
400mA step-down regulator fits onto 0.15 square inches
of PC board area.
Wide Input Range: 3.6V to 25V
5V at 400mA from 7V to 25V Input
3.3V at 400mA from 4.7V to 25V Input
Fixed Frequency 1.4MHz Operation
Uses Tiny Capacitors and Inductors
Internally Compensated
Low Shutdown Current: <1µA
Low VCESAT Switch: 220mV at 300mA
Tiny 6-Lead SOT-23 Package
U
APPLICATIO S
■
■
■
■
■
The constant frequency current mode PWM architecture
and stable operation with ceramic capacitors results in
low, predictable output ripple. Current limiting provides
protection against shorted outputs. The low current (<1µA)
shutdown provides complete output disconnect, enabling
easy power management in battery-powered systems.
Wall Transformer Regulation
Local Logic Supply Conversion:
12V to 5V
12V or 5V to 3.3V, 2.5V or 1.8V
Distributed Supply Regulation
Digital Cameras
Battery-Powered Equipment
, LTC and LT are registered trademarks of Linear Technology Corporation.
U
TYPICAL APPLICATIO
3.3V Step-Down Converter
5
VIN
BOOST
LT1616
OFF ON
4
SHDN
GND
2
C1
1µF
SW
FB
3
D2
1
6
R1
16.5k
R2
10k
C1: TAIYO-YUDEN TMK316BJ105
C2: TAIYO-YUDEN JMK316BJ106ML
D1: ON SEMICONDUCTOR MBR0530
D2: 1N4148
L1: SUMIDA CR43-100
C3
0.01µF
D1
100
L1
10µH
90
VOUT
3.3V
300mA: VIN = 4.5V TO 25V
400mA: VIN = 4.7V TO 25V
C2
10µF
VIN = 5V
VIN = 12V
80
EFFICIENCY (%)
VIN
4.5V TO 25V
Efficiency
VIN = 20V
70
60
50
1616 TA01
40
30
0
100
200
300
400
LOAD CURRENT (mA)
500
1616 G02
1
LT1616
W W
W
AXI U
U
ABSOLUTE
RATI GS
U
U
W
PACKAGE/ORDER I FOR ATIO
(Note 1)
Input Voltage (VIN) ................................................. 25V
BOOST Pin Voltage ................................................. 35V
BOOST Pin Above SW Pin ...................................... 25V
SHDN Pin ............................................................... 25V
FB Voltage ................................................................ 6V
Current Into FB Pin ............................................... ±1mA
Operating Temperature Range (Note 2) .. – 40°C to 85°C
Maximum Junction Temperature .......................... 125°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART
NUMBER
TOP VIEW
BOOST 1
6 SW
GND 2
5 VIN
FB 3
LT1616ES6
4 SHDN
S6 PART MARKING
S6 PACKAGE
6-LEAD PLASTIC SOT-23
LTNB
TJMAX = 125°C, θJA = 250°C/ W
Consult factory for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C.
VIN = 10V, VBOOST = 15V, unless otherwise noted. (Note 2)
PARAMETER
CONDITIONS
MIN
Undervoltage Lockout
Feedback Voltage
●
1.225
TYP
MAX
UNITS
3.35
3.6
V
1.25
1.275
V
150
600
nA
FB Pin Bias Current
VFB = Measured VREF + 10mV
Quiescent Current
Not Switching
1.9
2.5
mA
Quiescent Current in Shutdown
VSHDN = 0V
0.01
2
µA
Reference Line Regulation
VIN = 5V to 25V
0.005
Switching Frequency
VFB = 1.1V
Frequency Shift Threshold on FB Pin
fSW = 700kHz
Maximum Duty Cycle
●
●
●
Switch Current Limit
(Note 3)
Switch VCESAT
ISW = 300mA
1
1.4
%/V
1.8
MHz
0.44
V
80
87
%
630
850
220
Switch Leakage Current
mA
350
mV
10
µA
Minimum Boost Voltage Above Switch
ISW = 300mA
1.6
2.5
V
BOOST Pin Current
ISW = 300mA
7
12
mA
SHDN Input Voltage High
1.8
V
SHDN Input Voltage Low
SHDN Bias Current
VSHDN = 3V
VSHDN = 0V
Note 1: Absolute Maximum Ratings are those values beyond which the life
of the device may be impaired.
Note 2: The LT1616E is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications over the –40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls.
2
8
0.01
0.4
V
15
0.1
µA
µA
Note 3: Current limit guaranteed by design and/or correlation to static test.
Slope compensation reduces current limit at higher duty cycle.
LT1616
U W
TYPICAL PERFOR A CE CHARACTERISTICS
Efficiency, VOUT = 5V
Efficiency, VOUT = 3.3V
100
90
VIN = 12V
VIN = 24V
70
VIN = 12V
400
80
EFFICIENCY (%)
80
VIN = 5V
SWITCH VOLTAGE (mV)
VIN = 8V
90
60
60
50
50
40
40
30
VIN = 20V
70
100
200
500
300
400
LOAD CURRENT (mA)
0
100
200
BOOST Pin Current
500
500
16
L = 10µH
L = 15µH
LOAD CURRENT (mA)
L = 6.8µH
300
OUTPUT LIMITED
BY DISSIPATION
200
600
200
400
SWITCH CURRENT (mA)
1616 G03
Maximum Load Current
at VOUT = 3.3V
L = 10µH
0
1616 G02
Maximum Load Current
at VOUT = 5V
LOAD CURRENT (mA)
500
300
400
LOAD CURRENT (mA)
1616 G01
400
200
0
30
0
300
100
14
BOOST PIN CURRENT (mA)
EFFICIENCY (%)
Switch Voltage Drop
500
100
400
L = 4.7µH
OUTPUT LIMITED
BY DISSIPATION
300
200
12
10
8
6
4
2
100
0
5
15
10
INPUT VOLTAGE (V)
100
20
25
0
5
15
10
INPUT VOLTAGE (V)
20
MINIMUM
400
200
0
20
60
40
DUTY CYCLE (%)
80
100
1616 G07
UNDERVOLTAGE LOCKOUT (V)
FEEDBACK PIN VOLTAGE (V)
SWITCH CURRENT LIMIT (mA)
Undervoltage Lockout
3.7
1.27
600
600
1616 G06
Feedback Pin Voltage
Switch Current Limit
TYPICAL
400
1616 G05
1000
800
200
0
SWITCH CURRENT (mA)
1616 G04
0
0
25
1.26
1.25
1.24
1.23
1.22
–50
–25
0
25
50
TEMPERATURE (°C)
75
100
1616 G08
3.6
3.5
3.4
3.3
3.2
3.1
–50
–25
0
25
50
TEMPERATURE (°C)
75
100
1616 G11
3
LT1616
U W
TYPICAL PERFOR A CE CHARACTERISTICS
Oscillator Frequency
SHDN Pin Current
120
2.00
100
SHDN PIN CURRENT (µA)
SWITCHING FREQUENCY (MHz)
1.75
1.50
1.25
1.00
0.75
0.50
60
40
20
0.25
0
–50
80
0
–25
0
50
25
TEMPERATURE (°C)
75
100
1616 G09
0
5
10
15
20
SHDN PIN VOLTAGE
25
1616 G10
U
U
U
PI FU CTIO S
BOOST (Pin 1): The BOOST pin is used to provide a drive
voltage, higher than the input voltage, to the internal
bipolar NPN power switch.
GND (Pin 2): Tie the GND pin to a local ground plane below
the LT1616 and the circuit components. Return the feedback divider to this pin.
FB (Pin 3): The LT1616 regulates its feedback pin to 1.25V.
Connect the feedback resistor divider tap to this pin. Set
the output voltage according to VOUT = 1.25V (1 + R1/R2).
A good value for R2 is 10k.
4
SHDN (Pin 4): The SHDN pin is used to put the LT1616 in
shutdown mode. Tie to ground to shut down the LT1616.
Tie to 2V or more for normal operation. If the shutdown
feature is not used, tie this pin to the VIN pin.
VIN (Pin 5): The VIN pin supplies current to the LT1616’s
internal regulator and to the internal power switch. This pin
must be locally bypassed.
SW (Pin 6): The SW pin is the output of the internal power
switch. Connect this pin to the inductor, catch diode and
boost capacitor.
LT1616
W
BLOCK DIAGRA
VIN
5
SHDN
4
1 BOOST
INT REG
AND
UVLO
SLOPE
COMP
Σ
R
Q
S
Q
DRIVER
OSC
Q1
6 SW
FREQUENCY
FOLDBACK
VC
gm
1.25V
U
OPERATIO
2
3
GND
FB
1616BD
(Refer to Block Diagram)
The LT1616 is a constant frequency, current mode Buck
regulator. The 1.4MHz oscillator enables an RS flip-flop,
turning on the internal 600mA power switch Q1. An amplifier and comparator monitor the current flowing between
the VIN and SW pins, turning the switch off when this current
reaches a level determined by the voltage at VC. An error
amplifier measures the output voltage through an external
resistor divider tied to the FB pin. This amplifier servos the
switch current to regulate the FB pin voltage to 1.25V. An
active clamp on the VC node provides current limit.
An internal regulator provides power to the control circuitry. This regulator includes an undervoltage lockout to
prevent switching when VIN is less than ~ 3.5V. The
SHDN pin is used to place the LT1616 in shutdown,
disconnecting the output and reducing the input current
to less than 1µA.
The switch driver operates from either the input or from
the BOOST pin. An external capacitor and diode are used
to generate a voltage at the BOOST pin that is higher than
the input supply. This allows the driver to fully saturate the
internal bipolar NPN power switch for efficient operation.
The oscillator reduces the LT1616’s operating frequency
when the voltage at the FB pin is low. This frequency
foldback helps to control the output current during startup and overload.
5
LT1616
U
W
U U
APPLICATIO S I FOR ATIO
The LT1616 efficiently converts power from an input voltage source to a lower output voltage using an inductor for
energy storage. The LT1616 uses its internal power switch
and an external catch diode (D1 of the application circuit
on the first page of this data sheet) to produce a pulsewidth modulated square wave. Inductor L1 and output
capacitor C2 filter this square wave to produce a DC output
voltage. An error amplifier regulates the output by comparing the output (divided by the feedback resistor string
R1 and R2) to an internal reference. The LT1616 uses
current mode control; instead of directly modulating the
pulse width, the error amplifier controls the peak current
in the switch and inductor. Current mode control has several advantages, including simplified loop compensation
and cycle-by-cycle current limiting.
Figure 1 shows several waveforms of the application circuit on the front page of this data sheet. The circuit is
converting a 12V input to 3.3V at 300mA. The first trace is
the voltage at the SW pin. When the internal switch is on,
the SW pin voltage is near the 12V input. This applies a
voltage across inductor L1, and the current in the switch
At light loads, the inductor current may reach zero on each
pulse. The diode will turn off, and the switch voltage will
ring, as shown in Figure 2. This is discontinuous mode operation, and is normal behavior for the switching regulator. The LT1616 will also skip pulses when the load is light.
VSW
5V/DIV
IL1
0.2A/DIV
VIN = 12V
VOUT = 5V
IOUT = 18mA
VSW
5V/DIV
500ns/DIV
1616 F02
Figure 2. Discontinuous Mode Operation
ISW
0.2A/DIV
200ns/DIV
1616 F01a
IL1
0.2A/DIV
VOUT
5mV/DIV
200ns/DIV
1616 F01b
Figure 1. Operating Waveforms of the LT1616
Converting 12V to 3.3V at 300mA
6
(second trace) and the inductor (third trace) increases.
When the switch turns off, the switch current immediately
drops to zero and the inductor current flows through the
catch diode D1, which clamps the switch node 0.4V below
ground. The voltage across the inductor in this state has
the opposite sense and is equal to the output voltage plus
the catch diode drop, so the inductor current begins to
decrease. The fourth trace shows the output voltage ripple.
If the output is shorted to ground, the output voltage will
collapse and there will be very little voltage to reset the
current in the inductor. The LT1616 can sense this condition at its FB pin. In order to control the current, the LT1616
reduces its operating frequency, allowing more time for
the catch diode to reset the inductor current.
The input and output voltages determine the duty cycle of
the switch. The inductor value combined with these voltages determines the ripple current in the inductor. Along
with the switch current limit, the inductor ripple current
determines the maximum load current that the circuit can
supply. At minimum, the input and output capacitors are
required for stable operation. Specific values are chosen
based on allowable ripple and desired transient performance. The rest of the applications information is mainly
concerned with choosing these and the other components
in an LT1616 application.
LT1616
U
W
U U
APPLICATIO S I FOR ATIO
Inductor Selection and Maximum Output Current
The duty cycle of the internal switch is:
DC = (VOUT + VD)/(VIN – VSW + VD)
where VD is the forward voltage drop of the catch diode
(D1) and VSW is the voltage drop of the internal switch.
Usually one is interested in DC at full load current, so you
can use VD = VSW = 0.4V. Note that the LT1616 has a
maximum guaranteed duty cycle of 0.8. This will limit the
minimum input voltage for a particular output voltage.
When the switch is off, the inductor sees the output
voltage plus the catch diode drop. This gives the peak-topeak ripple current in the inductor:
∆IL = (1 – DC)(VOUT + VD)/(L • f)
where f is the switching frequency of the LT1616 and L is
the value of the inductor. The average inductor current is
equal to the output current, so the peak inductor current
will be the output current plus one half of the ripple
current:
ILPK = IOUT + ∆IL /2.
To maintain output regulation, this peak current must be
less than the LT1616’s switch current limit ILIM. ILIM is at
least 630mA at low duty cycles, decreasing to 430mA at
80% duty cycle. The maximum output current is a function
of the chosen inductor value:
IOUT(MAX) = ILIM – ∆IL /2.
If the inductor value is chosen so that the ripple current is
small, then the available output current will be near the
switch current limit. A good approach is to choose the
inductor so that the peak-to-peak inductor ripple is equal
to one third of the switch current limit. This leads to:
L = 3(1 – DC)(VOUT + VD)/(ILIM • f)
and
IOUT(MAX) = (5/6)ILIM.
These expressions depend on duty cycle and therefore on
input voltage. Pick a nominal input voltage to calculate L,
then check the maximum available output current at the
minimum and maximum input voltages.
If your application calls for output current less than
400mA, you may be able to relax the value of the inductor
and operate with higher ripple current. This may allow you
to pick a physically smaller inductor or one with a lower DC
resistance. Be aware that these equations assume continuous inductor current. If the inductor value is low or the
load current is light, then the inductor current may become
discontinuous. This occurs when ∆IL = 2IOUT. For details
of discontinuous mode operation, see Linear Technology
Application Note AN44. Also, high duty cycle operation
may require slightly higher inductor values to avoid subharmonic oscillations. See AN19.
The maximum load current as a function of input voltage
is plotted in the Typical Performance Characteristics section of this data sheet. Maximum load current for 3.3V and
5V outputs is shown for several values of L. At the highest
input voltages, the load current is limited by power dissipation in the LT1616.
Choose an inductor that is intended for power applications. Table 1 lists several manufacturers and inductor
series. The saturation current of the inductor should be
above 0.5A. The RMS current rating should be equal to or
greater than output current. For indefinite operation into a
short circuit, the RMS current rating should be greater
than 0.7A. The DC resistance should be less than 0.5Ω in
order maintain circuit efficiency.
Capacitor Selection
A Buck regulator draws from its input a square wave of
current with peak-to-peak amplitude as high as the switch
current limit. The input capacitor (C1) must supply the AC
component of this current. An RMS current rating of
250mA is adequate for LT1616 circuits. The input capacitor must bypass the LT1616 internal control circuitry and
any other circuitry that operates from the input source. A
1µF ceramic capacitor will satisfy both of these requirements. If the impedance of the input source is high (due to
long wires or filter components), additional bulk input
capacitance may be required. In high duty cycle applications (5VIN to 3.3VOUT, for example), increase the input
capacitor to 2.2µF. It may be possible to achieve lower cost
by using an electrolytic capacitor (tantalum or aluminum)
7
LT1616
U
W
U U
APPLICATIO S I FOR ATIO
Table 1. Inductor Vendors
Vendor
Phone
URL
Part Series
Comments
Murata
(404) 426-1300
www.murata.com
LQH3C
Small, Low Cost, 2mm Height
Sumida
(847) 956-0666
www.sumida.com
CR43
CLS62
CLQ61
1:1 Coupled
1.5mm Height
Coilcraft
(847) 639-6400
www.coilcraft.com
DO1607C
DO1608C
DT1608C
Coiltronics
(407) 241-7876
www.coiltronics.com
CTXxx-1
TP1
www.tokoam.com
3DF
D52LC
Toko
1:1 Coupled Toroid
1.8mm Height
Table 2. Capacitor Vendors
Vendor
Phone
URL
Part Series
Comments
Taiyo-Yuden
(408) 573-4150
www.t-yuden.com
Ceramic Caps
X5R Dielectric
AVX
(803) 448-9411
www.avxcorp.com
Ceramic Caps
Tantalum Caps
Murata
(404) 436-1300
www.murata.com
Ceramic Caps
in combination with a 0.1µF ceramic capacitor. However,
input voltage ripple will be higher, and you may want to
include an additional 0.1µF ceramic a short distance away
from the LT1616 circuit in order to filter the high frequency
ripple. The input capacitor should be rated for the maximum input voltage.
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated by
the LT1616 to produce the DC output. In this role it
determines the output ripple. The second function is to
store energy in order to satisfy transient loads and stabilize the LT1616’s control loop.
In most switching regulators the output ripple is determined by the equivalent series resistance (ESR) of the
output capacitor. Because the LT1616’s control loop doesn’t
depend on the output capacitor’s ESR for stable operation,
you are free to use ceramic capacitors to achieve very low
output ripple and small circuit size. You can estimate
output ripple with the following equations:
VRIPPLE = ∆IL • ESR for electrolytic capacitors (tantalum
and aluminum)
VRIPPLE = ∆IL/(2π • f • COUT) for ceramic capacitors
8
Another constraint on the output capacitor is that it must
have greater energy storage than the inductor; if the stored
energy in the inductor is transferred to the output, you
would like the resulting voltage step to be small compared
to the regulation voltage. For a 5% overshoot, this requirement becomes
COUT > 10 • L(ILIM/VOUT)2
Finally, there must be enough capacitance for good transient performance. The last equation gives a good starting
point. Alternatively, you can start with one of the designs
in this data sheet and experiment to get the desired
performance. Figure 3 illustrates some of the trade-off
between different output capacitors. Figure 4 shows the
test circuit. The lowest trace shows total output current,
which jumps from 100mA to 250mA. The other traces
show the output voltage ripple and transient response
with different output capacitors. The capacitor value, size
and type are listed. Note that the time scale at 50µs per
divison is much larger than the switching period, so you
can’t see the output ripple at the switching frequency. The
output ripple appears as vertical broadening of the trace.
The first trace (COUT = 4.7µF) has peak-to-peak output
ripple of ~ 6mV, while the third trace shows peak-to-peak
ripple of ~15mV.
LT1616
U
W
U U
APPLICATIO S I FOR ATIO
VIN
10V
5
VIN
BOOST
1
LT1616
4
SHDN
GND
2
SW
6
10µH
VOUT
3.3V
FB
22Ω
3
33Ω
COUT
COUT = 4.7µF CERAMIC, CASE SIZE 0805
1616 F04
Figure 4. Circuit Used for Transient Load Test Shown in Figure 3
COUT = 10µF CERAMIC, CASE SIZE 1206
COUT = 47µF, ESR ≅ 0.080Ω (SANYO POSCAP 6TPA47M)
C CASE
COUT = 100µF, ESR ≅ 0.150Ω (TANTALUM AVX
TPSC107M006R0150) C CASE
Regardless of which capacitor or combination of capacitors you choose, you should do transient load tests to
evaluate the circuit’s stability. Avoid capacitors or combinations that result in a ringing response. Problems may
occur if the output capacitance is very low or if a high value
inductor is used in combination with a large value, low
ESR capacitor.
The high performance (low ESR), small size and robustness of ceramic capacitors make them the preferred type
for LT1616 applications. However, all ceramic capacitors
are not the same. Many of the higher value capacitors use
poor dielectrics with high temperature and voltage
coefficients. In particular, Y5V types should be regarded
with suspicion. Stick with X7R and X5R types. Don’t be
afraid to run them at their rated voltage. Table 2 lists
several capacitor manufacturers.
Catch Diode
A 0.5A Schottky diode is recommended for the catch diode
D1. The ON Semiconductor MBR0530 is a good choice; it
is rated for 0.5A forward current and a maximum reverse
voltage of 30V. For circuits with VIN less than 20V, the
MBR0520L can be used. Other suitable diodes are the
Zetex ZHCS500TR and ZHCS750TR, and various versions
of the 1N5818.
VOUT
20mV/DIV
ILOAD
100mA/DIV
0
COUT = 100µF TANTALUM AND 2.2µF CERAMIC
Figure 3. Transient Load Response of the LT1616
9
LT1616
U
W
U U
APPLICATIO S I FOR ATIO
Capacitor C3 and diode D2 are used to generate a boost
voltage that is higher than the input voltage. In most cases
a 0.01µF capacitor and fast switching diode (such as the
1N4148 or 1N914) will work well. Figure 5 shows two
ways to arrange the boost circuit. The BOOST pin must be
more than 2.5V above the SW pin for best efficiency. For
outputs of 3.3V and above, the standard circuit (Figure 5a)
is best. For outputs between 2.8V and 3.3V, use a 0.033µF
capacitor and a small Schottky diode (such as the
BAT-54). For lower output voltages the boost diode can be
tied to the input (Figure 5b). The circuit in Figure 5a is more
efficient because the BOOST pin current comes from a
lower voltage source. You must also be sure that the
maximum voltage rating of the BOOST pin is not exceeded.
The minimum operating voltage of an LT1616 application
is limited by the undervoltage lockout (< 3.6V) and by the
maximum duty cycle as outlined above. For proper startup, the minimum input voltage is also limited by the boost
circuit. If the input voltage is ramped slowly, or the LT1616
is turned on with its SHDN pin when the output is already
in regulation, then the boost capacitor may not be fully
charged. Because the boost capacitor is charged with the
energy stored in the inductor, the circuit will rely on some
minimum load current to get the boost circuit running
properly. This minimum load will depend on input and
output voltages, and on the arrangement of the boost
circuit. The minimum load generally goes to zero once the
circuit has started. Figure 6 shows a plot of minimum load
to start and to run as a function of input voltage. In many
cases the discharged output capacitor will present a load
to the switcher which will allow it to start. The plots show
the worst-case situation where VIN is ramping very slowly.
Use a Schottky diode (such as the BAT-54) for the lowest
start-up voltage.
Minimum Input Voltage VOUT = 3.3V
7
BOOST DIODE
TIED TO OUTPUT
6
INPUT VOLTAGE (V)
BOOST Pin Considerations
5
VOUT = 3.3V
DBOOST = BAT54
BOOST DIODE
TIED TO INPUT
V TO START
4
V TO RUN
D2
3
1
C3
BOOST
10
100
LOAD CURRENT (mA)
LT1616
VIN
VIN
1616 F06a
VOUT
SW
Minimum Input Voltage VOUT = 5V
GND
9
1616 F05a
BOOST DIODE
TIED TO OUTPUT
VBOOST – VSW ≅ VOUT
MAX VBOOST ≅ VIN + VOUT
C3
BOOST
LT1616
VIN
VIN
SW
VOUT
INPUT VOLTAGE (V)
8
(5a)
D2
500
7
VOUT = 5V
DBOOST = BAT54
BOOST DIODE
TIED TO INPUT
V TO START
6
5
V TO RUN
GND
1616 F05b
VBOOST – VSW ≅ VIN
MAX VBOOST ≅ 2VIN
1
(5b)
Figure 5. Two Circuits for Generating the Boost Voltage
10
4
10
100
LOAD CURRENT (mA)
500
1616 F06b
Figure 6. The Minimum Input Voltage Depends
on Output Voltage, Load Current and Boost Circuit
LT1616
U
W
U U
APPLICATIO S I FOR ATIO
the SHDN pin, the SW pin current will drop to essentially
zero. However, if the VIN pin is grounded while the output
is held high, then parasitic diodes inside the LT1616 can
pull large currents from the output through the SW pin and
the VIN 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.
Shorted Input Protection
If the inductor is chosen so that it won’t saturate excessively, an LT1616 buck regulator will tolerate a shorted
output. There is another situation to consider in systems
where the output will be held high when the input to the
LT1616 is absent. This may occur in battery charging
applications or in battery backup systems where a battery
or some other supply is diode OR-ed with the LT1616’s
output. If the VIN pin is allowed to float and the SHDN pin
is held high (either by a logic signal or because it is tied to
VIN), then the LT1616’s internal circuitry will pull its
quiescent current through its SW pin. This is fine if your
system can tolerate a few mA in this state. If you ground
PCB Layout
For proper operation and minimum EMI, care must be
taken during printed circuit board layout. Figure 8 shows
the high current paths in the buck regulator circuit. Note
that large, switched currents flow in the power switch, the
D4
5
VIN
VIN
BOOST
1
LT1616
100k 4
SHDN
GND
100k
2
SW
6
VOUT
FB
3
BACKUP
D4: MBR0530
1616 F07
Figure 7. Diode D4 Prevents a Shorted Input from Discharging a
Backup Battery Tied to the Output; It Also Protects the Circuit from a
Reversed Input. The LT1616 Runs Only When the Input is Present
VIN
VIN
SW
GND
SW
GND
(a)
(b)
IC1
VSW
VIN
C1
L1
SW
D1
GND
C2
1616 F08
(c)
Figure 8. Subtracting the Current When the Switch is On (a) from the Current When the Switch is Off (b) Reveals the Path of the High
Frequency Switching Current (c). Keep This Loop Small. The Voltage on the SW and BOOST Nodes Will Also be Switched; Keep These
Nodes as Small as Possible. Finally, Make Sure the Circuit is Shielded with a Local Ground Plane
11
LT1616
U
W
U U
APPLICATIO S I FOR ATIO
catch diode (D1) and the input capacitor (C1). The loop
formed by these components should be as small as
possible. Furthermore, the system ground should be tied
to the regulator ground in only one place; this prevents the
switched current from injecting noise into the system
ground. These components, along with the inductor and
output capacitor, should be placed on the same side of the
circuit board, and their connections should be made on
that layer. Place a local, unbroken ground plane below
these components, and tie this ground plane to system
ground at one location, ideally at the ground terminal of the
output capacitor C2. Additionally, the SW and BOOST
nodes should be kept as small as possible. Finally, keep
the FB node as small as possible so that the ground pin and
ground traces will shield it from the SW and BOOST nodes.
Figure 9 shows component placement with trace, ground
plane and via locations. Include two vias near the GND pin
of the LT1616 to help remove heat from the LT1616 to the
ground plane.
Outputs Greater than 6V
For outputs greater than 6V, connect a diode (such as a
1N4148) from the SW pin to VIN to prevent the SW pin
from ringing above VIN during discontinuous mode operation. The 12V output circuit below shows the location of
this diode. Also note that for outputs above 10V, the input
voltage range will be limited by the maximum rating of the
BOOST pin. The 12V circuit shows how to overcome this
limitation using an additional Zener diode.
Other Linear Technology Publications
Application notes AN19, AN35 and AN44 contain more
detailed descriptions and design information for Buck
regulators and other switching regulators. The LT1376
data sheet has a more extensive discussion of output
ripple, loop compensation and stability testing. Design
Note DN100 shows how to generate a bipolar output
supply using a Buck regulator.
SHUTDOWN
VIN
VOUT
SYSTEM
GROUND
VIAS TO LOCAL GROUND PLANE
OUTLINE OF LOCAL GROUND PLANE
1616 F09
Figure 9. A Good PCB Layout Ensures Proper, Low EMI Operation
12
LT1616
U
TYPICAL APPLICATIO S
12V Output
D4
VIN
16V TO 25V
5
VIN
BOOST
C3
0.01µF
LT1616
OFF ON
4
SHDN
SW
GND
6
FB
2
R1
86.6k
3
C1
1µF
25V
D3
D2
1
L1
33µH
VOUT
12V
300mA
D1
C2
2.2µF
16V
R2
10k
GND
1616 TA03
C1: TAIYO-YUDEN TMK316BJ105ML
C2: TAIYO-YUDEN EMK316BJ225ML
D1: ON SEMICONDUCTOR MBR0530
D2, D4: 1N4148
D3: CMPZ5234B 6.2V ZENER.
D3 LIMITS BOOST PIN VOLTAGE TO VIN + 6V
L1: COILCRAFT DO1608C-333
1.8V Output
D2
VIN
3.6V TO 12V
5
VIN
BOOST
1
LT1616
OFF ON
4
SHDN
GND
2
C1
1µF
16V
SW
FB
3
6
R1
8.87k
R2
20k
C1: TAIYO-YUDEN EMK212BJ105MG
C2: TAIYO-YUDEN JMK316BJ106ML
D1: ON SEMICONDUCTOR MBR0520L
D2: 1N4148 OR EQUIVALENT
L1: MURATA LQH3C4R7M24
C3
0.01µF
L1
4.7µH
VOUT
400mA
D1
C2
10µF
6.3V
GND
1616 TA04
13
LT1616
U
TYPICAL APPLICATIO S
2.5V Output
D2
5
VIN
3.6V TO 16V
VIN
1
BOOST
C3
0.01µF
LT1616
4
OFF ON
SHDN
6
SW
GND
FB
2
R1
10k
3
C1
1µF
16V
L1
4.7µH
VOUT
2.5V
350mA
D1
C2
4.7µF
6.3V
R2
10k
GND
1616 TA05
C1: TAIYO-YUDEN EMK212BJ105MG
C2: TAIYO-YUDEN JMK212BJ475MG
D1: ON SEMICONDUCTOR MBR0520
D2: 1N4148
L1: MURATA LQH3C4R7M24
5V Output
VIN
7V TO 25V
5
VIN
BOOST
LT1616
OFF ON
4
SHDN
GND
2
C1
1µF
25V
SW
FB
3
6
R1
30.1k
R2
10k
C1: TAIYO-YUDEN TMK316BJ105ML
C2: TAIYO-YUDEN JMK316BJ106MG
D1: ON SEMICONDUCTOR MBR0530
D2: 1N4148
L1: TOKO A914BYW-150M
14
D2
1
C3
0.01µF
D1
L1
15µH
VOUT
5V
300mA: VIN = 7V TO 25V
400mA: VIN = 8V TO 25V
C2
10µF
6.3V
1616 TA07
LT1616
U
PACKAGE DESCRIPTION
Dimensions in inches (millimeters) unless otherwise noted.
S6 Package
6-Lead Plastic SOT-23
(LTC DWG # 05-08-1634)
2.80 – 3.00
(0.110 – 0.118)
(NOTE 3)
1.90
(0.074)
REF
2.6 – 3.0
(0.110 – 0.118)
1.50 – 1.75
(0.059 – 0.069)
0.35 – 0.55
(0.014 – 0.022)
0.00 – 0.15
(0.00 – 0.006)
0.09 – 0.20
(0.004 – 0.008)
(NOTE 2)
0.95
(0.037)
REF
0.90 – 1.45
(0.035 – 0.057)
0.35 – 0.50
0.90 – 1.30
(0.014 – 0.020)
(0.035 – 0.051)
SIX PLACES (NOTE 2)
S6 SOT-23 0898
NOTE:
1. DIMENSIONS ARE IN MILLIMETERS
2. DIMENSIONS ARE INCLUSIVE OF PLATING
3. DIMENSIONS ARE EXCLUSIVE OF MOLD FLASH AND METAL BURR
4. MOLD FLASH SHALL NOT EXCEED 0.254mm
5. PACKAGE EIAJ REFERENCE IS SC-74A (EIAJ)
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.
15
LT1616
U
TYPICAL APPLICATION
Bipolar Output DC/DC Converter
VIN
7.5V TO 25V
5
VIN
BOOST
LT1616
OFF ON
4
SHDN
GND
SW
FB
2
C1
1µF
25V
3
D2
1
6
C3
0.01µF
L1A
22µH
•
R1
30.1k
5V
200mA
D1
C2
10µF
6.3V
R2
10k
C1: TAIYO-YUDEN TMK316BJ105ML
C2, C4: TAIYO-YUDEN JMK316BJ106ML
C5: TAIYO-TUDEN JMK107BJ105MA
D1, D3: ON SEMICONDUCTOR MBR0530
D2: 1N4148
L1: 22µH 1:1 SUMIDA CLS62-220 OR
COILTRONICS CTX20-1
–5V LOAD SHOULD BE LESS THAN
1/2 5V LOAD, SEE DESIGN NOTE 100
C5
1µF
6.3V
GND
L1B
22µH
•
D3
C4
10µF
6.3V
1616 TA06
–5V
100mA
RELATED PARTS
PART NUMBER
DESCRIPTION
®
COMMENTS
LTC 1474/LTC1475 0.5A Micropower Step-Down Regulator
10µA IQ, 100% Duty Cycle, 8-Lead MSOP Package
LT1676/LT1776
Wide Input Range Step-Down Switching Regulator
60V Input, 0.7A Internal Switch
LTC1701
0.5A Micropower Step-Down in SOT-23
100% Duty Cycle, High Efficiency: Up to 94%
LT1763
500mA, Low Noise, LDO Micropower Regulator
30µA IQ, 20µVRMS Noise
LT1767
1.5A, 1.4MHz Step-Down DC/DC Converter
Higher Current, 8-Lead MSOP Package
LTC1772
Constant Frequency Step-Down Controller in SOT-23
Higher Current, High Efficiency: Up to 94%
LTC1779
0.25A Micropower Step-Down in SOT-23
Lower Current, 100% Duty Cycle
LTC1877
0.6A Synchronous Step-Down Regulator
High Efficiency, No Diode Required, 8-Lead MSOP
LT1962
300mA, Low Noise, LDO Micropower Regulator
30µA IQ, 20µVRMS Noise, 8-Lead MSOP Package
16
Linear Technology Corporation
sn1616 1616fs LT/TP 0201 4K • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408)432-1900 ● FAX: (408) 434-0507 ● www.linear-tech.com
 LINEAR TECHNOLOGY CORPORATION 2000