LINER LTC1504ACS8 500ma low voltage step-down synchronous Datasheet

LTC1504A
500mA Low Voltage
Step-Down Synchronous
Switching Regulator
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DESCRIPTION
FEATURES
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The LTC®1504A is a self-contained, high efficiency synchronous buck switching regulator. It includes a pair of
on-chip 1.3Ω power switches, enabling it to supply up to
500mA of load current. Efficiency peaks at 92%, minimizing heat and wasted power. The synchronous buck architecture allows the output to source or sink current as
required to keep the output voltage in regulation. 100%
duty cycle operation minimizes dropout voltage.
500mA Output Current at 3.3V Output
Up to 92% Peak Efficiency
100% Maximum Duty Cycle
Internal Reference Trimmed to 1%
Output Can Source or Sink Current
Requires as Few as Four External Components
Input Voltage Range: 4V to 10V
Adjustable Current Limit
Small SO-8 Package
200kHz Switching Frequency Can Typically be
Synchronized Up to 500kHz
The LTC1504A is available in adjustable and fixed 3.3V
output versions. An adjustable current limit circuit provides protection from overloads. The internal 1% reference combined with a sophisticated voltage feedback loop
provides optimum output voltage accuracy and fast load
transient response. The LTC1504A is specified to operate
with input voltages between 4V and 10V. Contact the LTC
factory for guaranteed specifications at 2.7V supply.
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APPLICATIONS
Daisy-Chained Control Outputs
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Small Portable Digital Systems
Active Termination
Auxiliary Output Voltage Supplies
Minimum Part Count/Size Switchers
The LTC1504A is a pin-compatible, functional upgrade to
the LTC1504.
The LTC1504A is available in a plastic SO-8 package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
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TYPICAL APPLICATION
Minimum Part Count 5V to 3.3V Regulator
NC
SHUTDOWN
IMAX
SHDN
5V to 3.3V Efficiency
100
90
+
VCC
CIN
22µF
LTC1504A-3.3
GND
SS
CIN: AVX TPSC226M016R0375
COUT: AVX TAJC476M010
LEXT: COILTRONICS CTX50-1P
80
3.3V AT 500mA
SW
NC
SENSE
COMP
+
70
EFFICIENCY (%)
5V
LEXT
50µH
COUT
47µF
60
50
40
30
1000pF
20
1504A • TA01
10
0
10
100
LOAD CURRENT (mA)
500
1504 • TA02
1
LTC1504A
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PACKAGE/ORDER I FOR ATIO
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(Note 1)
Supply Voltage (VCC to GND) ................................... 10V
Peak Output Current (SW) ....................................... ± 1A
Input Voltage (All Other Pins) ...... – 0.3V to (VCC + 0.3V)
Operating Temperature Range
LTC1504AC ............................................. 0°C to 70°C
LTC1504AI ......................................... – 40°C to 85°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
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ABSOLUTE MAXIMUM RATINGS
ORDER PART
NUMBER
TOP VIEW
IMAX 1
8
COMP
VCC 2
7
SS
SW 3
6
SHDN
GND 4
5
FB/SENSE*
S8 PACKAGE
8-LEAD PLASTIC SO
LTC1504ACS8
LTC1504ACS8-3.3
LTC1504AIS8
LTC1504AIS8-3.3
S8 PART MARKING
*FB FOR LTC1504A, SENSE FOR LTC1504A-3.3
TJMAX = 125°C, θJA = 90°C/W
1504A
1504AI
1504A33 04AI33
Consult factory for Military grade parts.
ELECTRICAL CHARACTERISTICS
VCC = 5V, TA = 25°C unless otherwise specified. (Note 2)
SYMBOL
PARAMETER
CONDITIONS
MIN
VCC
Minimum Supply Voltage
(Note 7)
●
4
VFB
Feedback Voltage
LTC1504ACS8
LTC1504AIS8
●
●
1.25
1.23
∆VFB
Feedback Voltage PSRR
Figure 1, 4V ≤ VCC ≤ 10V, LTC1504ACS8
Figure 1, 4V ≤ VCC ≤ 10V, LTC1504AIS8
●
●
VSENSE
Sense Pin Voltage
LTC1504ACS8-3.3
●
∆VSENSE
Sense Voltage PSRR
Figure 1, 4V ≤ VCC ≤ 10V, LTC1504ACS8-3.3
Figure 1, 4V ≤ VCC ≤ 10V, LTC1504AIS8-3.3
ICC
Supply Current
MAX
UNITS
V
1.265
1.265
1.28
1.29
V
V
1.1
1.1
1.6
1.8
%
%
3.30
3.40
V
●
●
1.2
1.2
1.8
2.0
%
%
Figure 1, VSHDN = VCC, IOUT = 0 (Note 4)
Figure 1, VSHDN = VCC, IOUT = 0, VFB/VSENSE = VCC (Note 4)
VSHDN = 0V
●
●
3
0.3
1.0
0.6
20
mA
mA
µA
200
200
250
300
kHz
kHz
1.3
1.3
2.0
2.2
Ω
Ω
fOSC
Internal Oscillator Frequency
Commercial
Industrial
●
●
RSW
Internal Switch Resistance
Commercial
Industrial
●
●
VIH
SHDN Input High Voltage
●
VIL
SHDN Input Low Voltage
●
IIN
SHDN Input Current
VOH
Error Amplifier Positive Swing
Figure 2
●
VOL
Error Amplifier Negative Swing
Figure 2
●
IOH, IOL
Error Amplifier Output Current
Figure 2
●
gmV
Error Amplifier Transconductance
Commercial (Note 5)
Industrial (Note 5)
●
●
AV
Error Amplifier DC Gain
(Note 5)
gmI
ILIM Amplifier Transconductance
(Note 6)
IMAX
IMAX Sink Current
VIMAX = VCC, Commercial
VIMAX = VCC, Industrial
2
TYP
3.20
150
130
2.4
V
±0.1
●
4.5
0.8
V
±1
µA
4.95
V
0.05
0.5
±50
±100
±200
µA
350
300
600
600
1100
1200
µmho
µmho
●
40
48
●
●
8
8
dB
µmho
2000
12
12
V
16
17
µA
µA
LTC1504A
ELECTRICAL CHARACTERISTICS
VCC = 5V, TA = 25°C unless otherwise specified. (Note 2)
SYMBOL
PARAMETER
CONDITIONS
ISS
Soft Start Source Current
VSS = 0V, Commercial
VSS = 0V, Industrial
●
●
tr, tf
Output Switch Rise/Fall Time
10% to 90% Ouput Swing
●
DCMAX
Maximum Duty Cycle
VCOMP = VCC
●
The ● denotes specifications which apply over the full operating
temperature range.
Note 1: Absolute Maximum Ratings are those values beyond which the life
of the device may be impaired.
Note 2: All currents into device pins are positive; all currents out of device
pins are negative. All voltages are referenced to ground unless otherwise
specified.
Note 3: This parameter is guaranteed by correlation and is not tested
directly.
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10
8
VFB = VOUT
6
4
VFB = VOUT
1
VFB = VCC
2
7.5
5
SUPPLY VOLTAGE (V)
0.1
–50
10
–25
0
25
50
75
TEMPERATURE (°C)
100
5
50
ns
100
%
VCC = 3.3V
2.0
1.5
VCC = 5V
1.0
VCC = 10V
0
–50 –25
125
500
400
300
200
100
100k
RIMAX (Ω)
1504A • TPC04
400
RIMAX = 47k
300
250
200
150
RIMAX = 22k
100
50
0
–50 –25
125
4.0
VCC = 5V
450
350
100
Shutdown Threshold vs
Supply Voltage
SHUTDOWN PIN THRESHOLD (V)
CURRENT LIMIT THRESHOLD (mA)
TA = 25°C
VCC = 5V
50
25
75
0
TEMPERATURE (°C)
1504A • TPC03
500
700
CURRENT LIMIT THRESHOLD (mA)
µA
µA
2.5
Current Limit Threshold vs
Temperature
Current Limit Threshold vs RIMAX
0
10k
–16
–17
1504A • TPC02
1504A • TPC01
600
–12
–12
0.5
VFB = VCC
0
2.5
–8
–8
3.0
SWITCH ON-RESISTANCE (Ω)
SUPPLY CURRENT (mA)
SUPPLY CURRENT (mA)
VCC = 5V
IOUT = 0
10
UNITS
3.5
TA = 25°C
IOUT = 0
12
MAX
Switch On-Resistance vs
Temperature
Supply Current vs Temperature
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TYP
Note 4: LTC1504A quiescent current is dominated by the gate drive
current drawn by the onboard power switches. With FB or SENSE pulled to
VCC the output stage will stop switching and the static quiescent current
can be observed. With FB or SENSE hooked up normally, the output stage
will be switching and total dynamic supply current can be measured.
Note 5: Fixed output parts will appear to have gmV and AV values 2.6 times
lower than the specified values, due to the internal divider resistors.
Note 6: The ILIM amplifier can sink but not source current. Under normal
(not current limited) operation, the ILIM output current will be zero.
Note 7: Contact factory for guaranteed specifications at 2.7V supply.
TYPICAL PERFORMANCE CHARACTERISTICS
Supply Current vs Supply Voltage
MIN
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0
25
50
75
TEMPERATURE (°C)
100
125
1504A • TPC05
3
5
7
SUPPLY VOLTAGE (V)
10
1504A • TPC07
3
LTC1504A
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PIN FUNCTIONS
IMAX (Pin 1): Current Limit Set. Connect a resistor from
VCC to IMAX to set the current limit threshold. An internal
12µA current source from IMAX to GND sets the voltage
drop across this resistor. This voltage is compared to the
voltage drop across the internal high-side switch (Q1)
while it is turned on. See the Applications Information
section for more information. To disable current limit,
leave IMAX floating.
voltage. The LTC1504A feedback loop will servo the FB pin
to 1.265V.
VCC (Pin 2): Power Supply Input. Connect to a power
supply voltage between 4V and 10V. VCC requires a low
impedance bypass capacitor to ground, located as close
as possible to the LTC1504A. See the Applications Information section for details on capacitor selection and
placement.
SHDN (Pin 6): Shutdown, Active Low. When SHDN is at a
logic High, the LTC1504A will operate normally. When
SHDN is Low, the LTC1504A ceases all internal operation
and supply current drops below 1µA. In shutdown, the SW
pin is pulled low. This ensures that the output is actively
shut off when SHDN is asserted, but it prevents other
supplies from providing power to the output when the
LTC1504A is inactive. See the Applications Information
section for more details.
SW (Pin 3): Power Switch Output. This is the switched
node of the buck circuit. Connect SW to one end of the
external inductor. The other end of the inductor should be
connected to COUT and becomes the regulated output
voltage. Avoid shorting SW to GND or VCC.
GND (Pin 4): Ground. Connect to a low impedance ground.
The input and output bypass capacitors and the feedback
resistor divider (adjustable parts only) should be grounded
as close to this pin as possible. Pin 4 acts as a heat sink
in the LTC1504A S0-8 package and should be connected
to as large a copper area as possible to improve thermal
dissipation. See the Thermal Considerations section for
more information.
FB (LTC1504A) (Pin 5): Feedback. Connect FB to a resistor
divider from VOUT to GND to set the regulated output
4
SENSE (LTC1504A-3.3) (Pin 5): Output Voltage Sense.
Connect directly to the output voltage node. The
LTC1504A-3.3 feedback loop will servo SENSE to 3.3V.
SENSE is connected to an internal resistor divider which
will load any external dividers. For output voltages other
than 3.3V, use the LTC1504A.
SS (Pin 7): Soft Start. Connect an external capacitor
(usually 0.1µF) from SS to GND to limit the output rise time
during power-up. CSS also compensates the current limit
loop, allowing the LTC1504A to enter and exit current limit
cleanly. See the Applications Information section for more
details.
COMP (Pin 8): External Compensation. An external RC
network should be connected to COMP to compensate the
feedback loop. COMP is connected to the output of the
internal error amplifier.
LTC1504A
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BLOCK DIAGRAM
SHDN
VCC
TO INTERNAL BLOCKS
SAW
Q1
–
PWM
SW
+
COMP
Q2
12µA
SS
ILIM
FB
–
+
+
–
IMAX
FB
(ADJ ONLY)
12µA
20.4k
+
–
VREF
1.265V
SENSE
(–3.3V ONLY)
12.6k
1504A • BD
Figure 3. Block Diagram
TEST CIRCUITS
NC
IMAX
VCC
+
VCC
CIN
100µF
1µF
LEXT
47µH
SHDN
LTC1504A
VOUT
SW
LTC1504A
GND
+
FB/SENSE
SS
COMP
A
FB/SENSE
–
B
COMP
COUT
220µF
+
VREF
0.1µF
CIN: AVX TPSE107M016R0125
COUT: SANYO 16CV220GX
LEXT: COILCRAFT D03316-473
7.5k
220pF
0.01µF
A: TEST VOL, IOL
B: TEST VOH, IOH
1504A • TC02
1504A • TC01
Figure 1
Figure 2
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APPLICATIONS INFORMATION
OVERVIEW
The LTC1504A is a complete synchronous switching
regulator controller (see Block Diagram). It includes two
on-chip 1.3Ω power MOSFETs, eliminating the need for
external power devices and minimizing external parts
count. The internal switches are set up as a synchronous
buck converter with a P-channel device (Q1) from the
input supply to the switching node and an N-channel
device (Q2) as the synchronous rectifier device from the
switching node to ground. An external inductor, input and
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LTC1504A
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APPLICATIONS INFORMATION
output bypass capacitors and a compensation network
complete the control loop. The LTC1504A adjustable
output parts require an additional pair of resistors to set
the output voltage. The LTC1504A-3.3 parts include an
onboard resistor divider preset to a 3.3V output voltage. A
functional 3.3V output regulator can be constructed with
an LTC1504A-3.3 and as few as four external components.
The LTC1504A feedback loop includes a precision reference trimmed to 1% (V REF ), a wide bandwidth
transconductance feedback amplifier (FB) and an onboard
PWM generator (SAW and PWM). The PWM generator is
capable of generating pulse widths from 0% to 100%,
minimizing dropout and maximizing transient response.
The internal sawtooth oscillator typically runs at 200kHz.
Q1 and Q2 are capable of carrying peak currents in excess
of 500mA, with the continuous output power level limited
primarily by the thermal dissipation of the SO-8 package.
With a 5V input and a 3.3V output, the LTC1504A can
supply 500mA of continuous output current with an
appropriate layout. An on-chip current limit circuit, set
with a single external resistor, can be used to help limit
power dissipation. See the Thermal Considerations section for more information.
Theory of Operation
The LTC1504A primary feedback loop consists of the main
error amplifier FB, the PWM generator, the output drive
logic and the power switches. The loop is closed with the
external inductor and the output bypass capacitor. The
feedback amplifier senses the output voltage directly at the
SENSE pin for fixed output versions or through an external
resistor divider in the adjustable output version. This
feedback voltage is compared to the 1.265V internal
reference voltage by FB and an error signal is generated at
the COMP pin. COMP is a high impedance node that is
brought out to an external pin for optimizing the loop
compensation.
COMP is compared to a 200kHz sawtooth wave by comparator PWM. The output stage takes the PWM signal and
generates nonoverlapping drive for the onboard P- and Nchannel power MOSFETs, which drive the SW pin with a
low impedance image of the PWM waveform. Typical
open-loop output impedance at SW is between 1Ω and
6
3Ω, depending on supply voltage. This high power pulse
train is filtered by the external inductor and capacitor,
providing a steady DC value at the output node. This node
returns to FB or SENSE, closing the loop.
The LTC1504A includes a second feedback loop that
controls operation in current limit. The ILIM amplifier
monitors the voltage at the SW pin while Q1 is on. It
compares this voltage to the voltage at the IMAX pin. As the
peak current through Q1 rises, the voltage drop across it
due to its RON increases proportionally. When SW drops
below IMAX, indicating the current through Q1 has increased beyond the desired value, ILIM starts pulling a
controlled amount of current out of SS, the external soft
start pin. As SS falls, it pulls COMP down with it, limiting
the duty cycle and reducing the output voltage to control
the current. The speed at which the current limit circuit
reacts is set by the value of the external soft start capacitor.
EXTERNAL COMPONENT SELECTION
External components required by the LTC1504A fall into
three categories: input bypass, output filtering and compensation. Additional components to set up soft start and
current limit are usually included as well. A minimum
LTC1504A circuit can be constructed with as few as four
external components; a circuit that utilizes all of the
LTC1504A’s functionality usually includes eight or nine
external components, with two additional feedback resistors required for adjustable parts. See the Typical Applications section for examples of external component hookup.
Input Bypass
The input bypass capacitor is critical to proper LTC1504A
operation. The LTC1504A includes a precision reference
and a pair of high power switches feeding from the same
VCC pin. If VCC does not have adequate bypassing, the
switch pulses introduce enough ripple at VCC to corrupt
the reference voltage and the LTC1504A will not regulate
accurately. Symptoms of inadequate bypassing include
poor load regulation and/or erratic waveforms at the SW
pin. If an oscilloscope won’t trigger cleanly when looking
at the SW pin, the LTC1504A doesn’t have adequate input
bypass.
LTC1504A
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APPLICATIONS INFORMATION
Ideally, the LTC1504A requires a low impedance bypass
right at the chip and a larger reservoir capacitor that can be
located somewhat farther away. This requirement usually
can be met with a ceramic capacitor right next to the
LTC1504A and an electrolytic capacitor (usually 10µF to
100µF, depending on expected load current) located somewhere nearby. In certain cases, the bulk capacitance
requirement can be met by the output bypass of the input
supply. Applications running at very high load currents or
at input supply voltages greater than 6V may require the
local ceramic capacitor to be 1µF or greater. In some
cases, both the low impedance and bulk capacitance
requirements can be met by a single capacitor, mounted
very close to the LTC1504A. Low ESR organic semiconductor (OS-CON) electrolytic capacitors or surge tested
surface mount tantalum capacitors can have low enough
impedance to keep the LTC1504A happy in some circuits.
Often the RMS current capacity of the input bypass capacitors is more important to capacitor selection than value.
Buck converters like the LTC1504A are hard on input
capacitors, since the current flow alternates between the
full load current and near zero during every clock cycle. In
the worst case (50% duty cycle or VOUT = 0.5VIN) the RMS
current flow in the input capacitor is half of the total load
current plus half the ripple current in the inductor—
perhaps 300mA in a typical 500mA load current application. This current flows through the ESR of the input
bypass capacitor, heating it up and shortening its life,
sometimes dramatically. Many ordinary electrolytic capacitors that look OK at first glance are not rated to
withstand such currents—check the RMS current rating
before you specify a device! If the RMS current rating isn’t
specified, it should not be used as an input bypass capacitor. Again, low ESR electrolytic and surge tested tantalums
usually do well in LTC1504A applications and have high
RMS current ratings. The local ceramic bypass capacitor
usually has negligible ESR, allowing it to withstand large
RMS currents without trouble. Table 1 shows typical
surface mount capacitors that make acceptable input
bypass capacitors in LTC1504A applications.
Inductor
Table 1. Representative Surface Mount Input Bypass Capacitors
PART
VALUE
ESR
MAX RMS
TYPE
HEIGHT
AVX
TPSC226M016R0375 22µF 0.38Ω
TPSD476M016R0150 47µF 0.15Ω
TPSE107M016R0125 100µF 0.13Ω
1206YC105M
1µF
Low
1210YG106Z
10µF
Low
0.54A
0.86A
1.15A
>1A
>1A
Tantalum
Tantalum
Tantalum
X7R Ceramic
Y5V Ceramic
2.6mm
2.9mm
4.1mm
1.5mm
1.7mm
Sanyo
16SN33M
16SN68M
16CV100GX
16CV220GX
33µF 0.15Ω
68µF 0.1Ω
100µF 0.44Ω
220µF 0.34Ω
1.24A
1.65A
0.23A*
0.28A*
Sprague
593D476X0016D2W
593D107X0016E2W
47µF
100µ
0.93A
1.05A
0.17Ω
0.15Ω
OS-CON
7mm
OS-CON
7mm
Electrolytic 6mm
Electrolytic 7.7mm
Tantalum
Tantalum
2.8mm
4mm
*Note: Use multiple devices in parallel or limit output current to prevent capacitor overload.
where the load is connected. Inductor requirements are
fairly straightforward; it must be rated to handle continuous DC current equal to the maximum load current plus
half the ripple current and its value should be chosen
based on the desired ripple current and/or the output
current transient requirements. Large value inductors
lower ripple current and decrease the required output
capacitance, but limit the speed that the LTC1504A can
change the output current, limiting output transient response. Small value inductors result in higher ripple
currents and increase the demands on the output capacitor, but allow faster output current slew rates and are often
smaller and cheaper for the same DC current rating. A
typical inductor used in an LTC1504A application might
have a maximum current rating between 500mA and 1A
and an inductance between 33µH and 220µH.
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 don’t radiate much energy, but generally cost more than powdered iron rod core inductors with
similar electrical characteristics. The choice of which style
inductor to use often depends more on the price vs size
requirements and any radiated field/EMI requirements
than on what the LTC1504A requires to operate. Table 2
shows some typical surface mount inductors that work
well in LTC1504A applications.
The LTC1504A requires an external inductor to be connected from the switching node SW to the output node
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LTC1504A
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APPLICATIONS INFORMATION
Table 2. Representative Surface Mount Inductors
PART
VALUE
MAX DC
CORE
TYPE
CORE
MATERIAL
HEIGHT
CoilCraft
DT3316-473
DT3316-104
DO1608-473
DO3316-224
47µH
100µH
47µH
220µH
1A
0.8A
0.5A
0.8A
Shielded
Shielded
Open
Open
Ferrite
Ferrite
Ferrite
Ferrite
5.1mm
5.1mm
3.2mm
5.5mm
Coiltronics
CTX50-1
CTX100-2
CTX50-1P
CTX100-2P
TP3-470
TP3-470
50µH
100µH
50µH
100µH
47µH
47µH
0.65A
0.63A
0.66A
0.55A
0.55A
0.72A
Toroid
Toroid
Toroid
Toroid
Toroid
Toroid
KoolMµ ®
KoolMµ
Type 52
Type 52
Ferrite
Ferrite
4.2mm
6mm
4.2mm
6mm
2.2mm
3mm
Sumida
CDRH62-470
CDRH73-101
CD43-470
CD54-101
47µH
100µH
47µH
100µH
0.54A
0.50A
0.54A
0.52A
Shielded
Shielded
Open
Open
Ferrite
Ferrite
Ferrite
Ferrite
3mm
3.4mm
3.2mm
4.5mm
Output Capacitor
The output capacitor affects the performance of the
LTC1504A in a couple of ways: it provides the first line of
defense during a transient load step and it has a large effect
on the compensation required to keep the LTC1504A
feedback loop stable. Transient load response of an
LTC1504A circuit is controlled almost entirely by the
output capacitor and the inductor. In steady load operation, the average current in the inductor will match the load
current. When the load current changes suddenly, the
inductor is suddenly carrying the wrong current and
requires a finite amount of time to correct itself—at least
several switch cycles with typical LTC1504A inductor
values. Even if the LTC1504A had psychic abilities and
could instantly assume the correct duty cycle, the rate of
change of current in the inductor is still related to its value
and cannot change instantaneously.
Until the inductor current adjusts to match the load current, the output capacitor has to make up the difference.
Applications that require exceptional transient response
(2% or better for instantaneous full-load steps) will require relatively large value, low ESR output capacitors.
Applications with more moderate transient load requirements can often get away with traditional standard ESR
Kool Mµ is a registered trademark of Magnetics, Inc..
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electrolytic capacitors at the output and can use larger
valued inductors to minimize the required output capacitor value. Note that the RMS current in the output capacitor
is slightly more than half of the inductor ripple current—
much smaller than the RMS current in the input bypass
capacitor. Output capacitor lifetime is usually not a factor
in typical LTC1504A applications.
Large value ceramic capacitors used as output bypass
capacitors provide excellent ESR characteristics but can
cause loop compensation difficulties. See the Loop Compensation section.
Loop Compensation
Loop compensation is strongly affected by the output
capacitor. From a loop stability point of view, the output
inductor and capacitor form a series RLC resonant circuit,
with the L set by the inductor value, the C by the value of
the output capacitor and the R dominated by the output
capacitor’s ESR. The amplitude response and phase shift
due to these components is compensated by a network of
Rs and Cs at the COMP pin to (hopefully) close the
feedback loop in a stable manner. Qualitatively, the L and
C of the output stage form a 2nd order roll-off with 180°
of phase shift; the R due to ESR forms a single zero at a
somewhat higher frequency that reduces the roll-off to
first order and reduces the phase shift to 90°.
If the output capacitor has a relatively high ESR, the zero
comes in well before the initial phase shift gets all the way
to 180° and the loop only requires a single small capacitor
from COMP to GND to remain stable (Figure 4a). If, on the
other hand, the output capacitor is a low ESR type to
maximize transient response, the ESR zero can increase in
frequency by a decade or more and the output stage phase
shift can get awfully close to 180° before it turns around
and comes back to 90°. Large value ceramic, OS-CON
electrolytic and low impedance tantalum capacitors fall
into this category. These loops require an additional zero
to be inserted at the COMP pin; a series RC in parallel with
a smaller C to ground will usually ensure stability. Figure 4b
shows a typical compensation network which will optimize transient response with most output capacitors.
Adjustable output parts can add a feedforward capacitor
across the feedback resistor divider to further improve
LTC1504A
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phase margin. The typical applications in this data sheet
show compensation values that work with several combinations of external components—use them as a starting
point. For complex cases or stubborn oscillations, contact
the LTC Applications Department.
External Schottky Diode
An external Schottky diode can be included across the
internal N-channel switch (Q2) to improve efficiency at
heavy loads. The diode carries the inductor current during
the nonoverlap time while the LTC1504A turns Q1 off and
Q2 on and prevents current from flowing in the intrinsic
body diode in parallel with Q2. This diode will improve
efficiency by a percentage point or two as output current
approaches 500mA and can help minimize erratic behavior at very high peak current levels caused by excessive
parasitic current flow through Q2. A Motorola MBRS0530L
is usually adequate, with the cathode connected to SW and
the anode connected to GND. Note that this diode is not
required for normal operation and has a negligible effect
on efficiency at low (< 250mA) output currents.
VOUT
RFB1*
LTC1504A FB
RFB2*
COMP
CC
*ADJUSTABLE PARTS ONLY
1504A • F04a
Figure 4a. Minimum Compensation Network
VOUT
RFB1*
CFF*
LTC1504A FB
RFB2*
COMP
RC
CF
CC
*ADJUSTABLE PARTS ONLY
1504A • F04b
Figure 4b. Optimum Compensation Network
Soft Start and Current Limit
Soft start and current limit are linked in the LTC1504A. Soft
start works in a straightforward manner. An internal 12µA
current source connected to the SS pin will pull up an
external capacitor connected from SS to GND at a rate
determined by the capacitor value. COMP is clamped to a
voltage one diode drop above SS; as SS rises, COMP will
rise at the same rate. When COMP reaches roughly 2V
below VCC, the duty cycle will slowly begin to increase until
the output comes into regulation. As SS continues to rise,
the feedback amplifier takes over at COMP, the clamp
releases and SS rises to VCC.
Current limit operates by pulling down on the soft start pin
when it senses an overload condition at the output. The
current limit amplifier (ILIM) compares the voltage drop
across the internal P-channel switch (Q1) during its on
time to the voltage at the IMAX pin. IMAX includes an internal
12µA pull-down, allowing the voltage to be set by a single
resistor between VCC and IMAX . When the IR drop across
Q1 exceeds the drop across the IMAX resistor, ILIM pulls
current out of the external soft start capacitor, reducing
the voltage at SS. A soft start capacitor should always be
used if current limit is enabled. SS, in turn, pulls down on
COMP, limiting the output duty cycle and controlling the
output current. When the current overload is removed, the
ILIM amplifier lets go of SS and allows it to rise again as if
it were completing a soft start cycle. The size of the
external soft start capacitor controls both how fast the
current limit responds once an overload is detected and
how fast the output recovers once the overload is removed. The soft start capacitor also compensates the
feedback loop created by the ILIM amplifier. Because the
ILIM loop is a current feedback loop, the additional phase
shift due to the output inductor and capacitor do not come
into play and the loop can be adequately compensated
with a single capacitor. Usually a 0.1µF ceramic capacitor
from SS to GND provides adequate soft start behavior and
acceptable current limit response.
This type of current limit circuit works well with mild
current overloads and eliminates the need for an external
current sensing resistor, making it attractive for LTC1504A
applications. These same features also handicap the current limit circuit under severe short circuits when the
9
LTC1504A
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output voltage is very close to ground. Under this condition, the LTC1504A must run at extremely narrow duty
cycles (< 5%) to keep the current under control. When the
on-time falls below the time required to sense the current
in Q1, the LTC1504A responds by reducing the oscillator
frequency, increasing the off-time to decrease the duty
cycle and allow it to maintain some control of the output
current. The oscillator frequency may drop by as much as
a factor of 10 under severe current overloads.
Under extreme short circuits (e.g., screwdriver to ground)
the on-time will reduce to the point where the LTC1504A
will lose control of the output current. At this point, output
current will rise until the inductor saturates, and the
current will be limited by the parasitic ESL of the inductor
and the RON of Q2 inside the LTC1504A. This current is
usually nondestructive and dissipates a limited amount of
power since the output voltage is very low. A typical
LTC1504A circuit can withstand such a short for many
seconds without damage. The test circuit in Figure 1 will
typically withstand a direct output short for more than 30
seconds without damage to the LTC1504A. Eventually,
however, a continuous short may cause the die temperature to rise to destructive levels.
Note that the current limit is primarily designed to protect
the LTC1504A from damage and is not intended to be used
to generate an accurate constant-current output. As the
die temperature varies in a current limited condition, the
RON of the internal switches will change and the current
limit threshold will move around. RON will also vary from
part to part due to manufacturing tolerance. The external
IMAX resistor should be chosen to allow enough room to
account for these variations without allowing the current
limit to engage at the maximum expected load current. A
current limit setting roughly double the expected load is
often a good compromise, eliminating unintended current
limit operation while preventing circuit destruction under
actual fault conditions. If desired, current limit can be
disabled by floating the IMAX pin; the internal current source
will pull IMAX to GND and the ILIM amplifier will be disabled.
Shutdown
The LTC1504A includes a micropower shutdown mode
controlled by the logic level at SHDN. A logic High at SHDN
10
allows the part to operate normally. A logic Low at SHDN
stops all internal switching, pulls COMP, SS and SW to
GND and drops quiescent current below 1µA typically.
Note that the internal N-channel power MOSFET from SW
to GND turns on when SHDN is asserted. This ensures that
the output voltage drops to zero when the LTC1504A is
shut down, but prevents other devices from powering the
output when the LTC1504A is disabled.
External Clock Synchronization
The LTC1504A SHDN pin can double as an external clock
input for applications that require a synchronized clock or
a faster switching speed. The SHDN pin terminates the
internal sawtooth wave and resets the oscillator immediately when it goes low, but waits 50µs before shutting
down the rest of the internal circuitry. A clock signal
applied directly to the SHDN pin will force the LTC1504A
internal oscillator to lock to its frequency as long as the
external clock runs faster than the internal oscillator
frequency. Attempting to synchronize to a frequency
lower than the 250kHz maximum internal frequency may
result in inconsistent pulse widths and is not recommended.
Because the sawtooth waveform rises at a fixed rate
internally, terminating it early by synchronizing to a fast
external clock will reduce the amplitude of the sawtooth
wave that the PWM comparator sees, effectively raising
the gain from COMP to SW. 500kHz is the maximum
recommended synchronization frequency; higher frequencies will reduce the sawtooth amplitude to the point that
the LTC1504A may run erratically.
THERMAL CONSIDERATIONS
Each of the LTC1504A internal power switches has approximately 1.3Ω of resistance at room temperature and
will happily carry more than the rated maximum current if
the current limit is set very high or is not connected. Since
the inductor current is always flowing through one or the
other of the internal switches, a typical application supplying 500mA of load current will cause a continuous dissipation of approximately 325mW. The SO-8 package has a
thermal resistance of approximately 90°C/W, meaning
that the die will begin to rise toward 30°C above ambient
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at this power level. The RON of the internal power switches
increases as the die temperature rises, increasing the
power dissipation as the feedback loop continues to keep
the output current at 500mA. At high ambient temperatures, this cycle may continue until the chip melts, since
the LTC1504A does not include any form of thermal
shutdown. Applications can safely draw peak currents
above the 500mA level, but the average power dissipation
should be carefully calculated so that the maximum 125°C
die temperature is not exceeded.
The LTC1504A dissipates the majority of its heat through
its pins, especially GND (Pin 4). Thermal resistance to
ambient can be optimized by connecting GND to a large
copper region on the PCB, which will serve as a heat sink.
Applications which will operate the LTC1504A near maximum power levels or which must withstand short circuits
of extended duration should maximize the copper area at
all pins and ensure that there is some airflow over the part
to carry away excess heat. For layout assistance in situations where power dissipation may be a concern, contact
the LTC Applications Department.
The current limit circuit can be used to limit the power
under mild overloads to a safe level, but severe overloads
where the output is shorted to ground may still cause the die
temperature to rise dangerously. For more information on
current limit behavior, see the Current Limit section.
LAYOUT CONSIDERATIONS
Like all precision switching regulators, the LTC1504A
requires special care in layout to ensure optimum performance. The large peak currents coupled with significant
DC current flow will conspire to keep the output from
regulating properly if the layout is not carefully planned. A
poorly laid out op amp or data converter circuit will fail to
give the desired performance, but will usually still act like
an op amp or data converter. A poorly laid out LTC1504A
circuit may look nothing at all like a regulator. Wire-wrap
or plug-in prototyping boards are not useful for breadboarding LTC1504A circuits! Open-core inductors located close to the LTC1504A can cause erratic regulation
due to stray flux coupled into PC board traces or the
LTC1504A itself. Changing the orientation of the inductor
or switching to a shielded type will solve the problem.
Perhaps most critical to proper LTC1504A performance is
the layout of the ground node and the location of the input
and output capacitors. The negative terminals of both the
input and output bypass capacitors should come together
at the same point, as close as possible to the LTC1504A
ground pin. The compensation network and soft start
capacitor can be connected together with their own trace,
which should come directly back to this same common
ground point. The input supply ground and the load return
should also connect to this common point. Each ground
line should come to a star connection with Pin 4 at the
center of the star. This node should be a fairly large copper
region to act as a heat sink if required.
Second in importance is the proximity of the low ESR (usually
ceramic) input bypass capacitor. It should be located as close
to the LTC1504A VCC and GND pins as physically possible.
Ideally, the capacitor should be located right next to the
package, straddling the SW pin. High peak current applications or applications with VCC greater than 6V may require a
1µF or larger ceramic capacitor in this position.
One node that isn’t quite so critical is SW. Extra lead length
or narrow traces at this pin will only add parasitic inductance in series with the external inductor, slightly raising
its value. The SW trace need only be wide enough to
support the maximum peak current under short circuit
conditions—perhaps 1A. If a trace needs to be compromised to make the layout work, this is the one. Note that
long traces at the SW node may aggravate EMI considerations—don’t get carried away. If a Schottky diode is used
at the SW node, it should be located at the LTC1504A end
of the trace, close to the device pins.
The LTC Applications Department has constructed literally hundreds of layouts for the LTC1504A and related
parts, many of which worked and some of which are now
archived in the Bad Layout Hall of Fame. If you need layout
assistance or you think you have a candidate layout for the
Hall of Fame, give Applications a call at (408) 954-8400.
Demo boards with properly designed layouts are available
and specialized layouts can be designed if required. The
applications team is also experienced in external component selection for a wide variety of applications, and they
have a never-ending selection of tall tales to tell as well.
When in doubt, give them a call.
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.
11
LTC1504A
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SCSI-2 Active Terminator
High Efficiency 5V to 2.5V Converter with Current Limit
RIMAX*
VCC
5V
IMAX
SHDN
CIN
LEXT
LTC1504A
1µF
GND
SS
MBRS0530L
FB
110Ω
IMAX SHDN
+
TERMPWR
COUT
7.5k
4.7µF
CERAMIC
GND
0.01µF
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PACKAGE DESCRIPTION
110Ω
15k
LTC1504A
FB
SS
CIN: AVX TPSE107M016R0125
COUT: SANYO 16CV220GX
LEXT: COILCRAFT DO3316-473
*SELECT RIMAX VALUE USING CURRENT LIMIT THRESHOLD GRAPH ON PAGE 3
LEXT
110Ω
+
COUT
COMP
NC
1504A • TA03
18
TO
27
LINES
SW
VCC
12.1k
220pF
110Ω
NC
11.8k
COMP
0.1µF
•
•
•
VOUT
2.5V
SW
VCC
+
110Ω
SHDN
12k
7.5k
220pF
0.01µF
COUT: AVX TPSC107M006R0150
LEXT: SUMIDA CD54-470
1504A • TA04
Dimensions is inches (millimeters) unless otherwise noted.
S8 Package
8-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
0.189 – 0.197*
(4.801 – 5.004)
8
0.010 – 0.020
× 45°
(0.254 – 0.508)
0.008 – 0.010
(0.203 – 0.254)
5
0.150 – 0.157**
(3.810 – 3.988)
0.004 – 0.010
(0.101 – 0.254)
0°– 8° TYP
0.014 – 0.019
(0.355 – 0.483)
*DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
**DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD
FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
6
0.228 – 0.244
(5.791 – 6.197)
0.053 – 0.069
(1.346 – 1.752)
0.016 – 0.050
0.406 – 1.270
7
0.050
(1.270)
TYP
1
2
3
4
SO8 0996
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12
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408)432-1900 ● FAX: (408) 434-0507 ● www.linear-tech.com
1504afs, sn1504a LT/TP 1098 4K • PRINTED IN USA
 LINEAR TECHNOLOGY CORPORATION 1997