LTC1504A
500mA Low Voltage
Step-Down Synchronous
Switching Regulator
FEATURES
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DESCRIPTION
The LTC
®
1504A is a self-contained, high efficiency syn-
chronous 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%, minimiz-
ing heat and wasted power. The synchronous buck archi-
tecture allows the output to source or sink current as
required to keep the output voltage in regulation. 100%
duty cycle operation minimizes dropout voltage.
The LTC1504A is available in adjustable and fixed 3.3V
output versions. An adjustable current limit circuit pro-
vides protection from overloads. The internal 1% refer-
ence 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.
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.
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
APPLICATIONS
Daisy-Chained Control Outputs
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Small Portable Digital Systems
Active Termination
Auxiliary Output Voltage Supplies
Minimum Part Count/Size Switchers
TYPICAL APPLICATION
Minimum Part Count 5V to 3.3V Regulator
NC
I
MAX
5V
SHUTDOWN
SHDN
SW
L
EXT
50µH
3.3V AT 500mA
EFFICIENCY (%)
5V to 3.3V Efficiency
100
90
80
70
60
50
40
30
20
1504A • TA01
+
V
CC
C
IN
22µF
LTC1504A-3.3
GND
SS
SENSE
COMP
1000pF
+
C
OUT
47µF
C
IN
: AVX TPSC226M016R0375
C
OUT
: AVX TAJC476M010
L
EXT
: COILTRONICS CTX50-1P
NC
10
0
10
100
LOAD CURRENT (mA)
500
1504 • TA02
U
U
U
1
LTC1504A
ABSOLUTE
MAXIMUM
RATINGS
(Note 1)
PACKAGE/ORDER I FOR ATIO
TOP VIEW
I
MAX
1
V
CC
2
SW 3
GND 4
8
7
6
5
COMP
SS
SHDN
FB/SENSE*
Supply Voltage (V
CC
to GND) ................................... 10V
Peak Output Current (SW) .......................................
±
1A
Input Voltage (All Other Pins) ...... – 0.3V to (V
CC
+ 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
ORDER PART
NUMBER
LTC1504ACS8
LTC1504ACS8-3.3
LTC1504AIS8
LTC1504AIS8-3.3
S8 PART MARKING
1504A
1504AI
1504A33 04AI33
S8 PACKAGE
8-LEAD PLASTIC SO
*FB FOR LTC1504A, SENSE FOR LTC1504A-3.3
T
JMAX
= 125°C,
θ
JA
= 90°C/W
Consult factory for Military grade parts.
ELECTRICAL CHARACTERISTICS
SYMBOL
V
CC
V
FB
∆V
FB
V
SENSE
∆V
SENSE
I
CC
PARAMETER
Minimum Supply Voltage
Feedback Voltage
Feedback Voltage PSRR
Sense Pin Voltage
Sense Voltage PSRR
Supply Current
CONDITIONS
(Note 7)
LTC1504ACS8
LTC1504AIS8
V
CC
= 5V, T
A
= 25°C unless otherwise specified. (Note 2)
MIN
q
q
q
q
q
q
q
q
q
q
q
q
q
q
q
q
q
TYP
1.265
1.265
1.1
1.1
MAX
1.28
1.29
1.6
1.8
3.40
1.8
2.0
0.6
20
250
300
2.0
2.2
0.8
UNITS
V
V
V
%
%
V
%
%
mA
mA
µA
kHz
kHz
Ω
Ω
V
V
µA
V
V
µA
µmho
µmho
dB
µmho
4
1.25
1.23
Figure 1, 4V
≤
V
CC
≤
10V, LTC1504ACS8
Figure 1, 4V
≤
V
CC
≤
10V, LTC1504AIS8
LTC1504ACS8-3.3
Figure 1, 4V
≤
V
CC
≤
10V, LTC1504ACS8-3.3
Figure 1, 4V
≤
V
CC
≤
10V, LTC1504AIS8-3.3
Figure 1, V
SHDN
= V
CC
, I
OUT
= 0 (Note 4)
Figure 1, V
SHDN
= V
CC
, I
OUT
= 0, V
FB
/V
SENSE
= V
CC
(Note 4)
V
SHDN
= 0V
Commercial
Industrial
Commercial
Industrial
3.20
3.30
1.2
1.2
3
0.3
1.0
f
OSC
R
SW
V
IH
V
IL
I
IN
V
OH
V
OL
I
OH
, I
OL
g
mV
A
V
g
mI
I
MAX
Internal Oscillator Frequency
Internal Switch Resistance
SHDN Input High Voltage
SHDN Input Low Voltage
SHDN Input Current
Error Amplifier Positive Swing
Error Amplifier Negative Swing
Error Amplifier Output Current
Error Amplifier Transconductance
Error Amplifier DC Gain
I
LIM
Amplifier Transconductance
I
MAX
Sink Current
150
130
200
200
1.3
1.3
2.4
±0.1
4.5
±50
350
300
40
8
8
4.95
0.05
±100
600
600
48
2000
12
12
16
17
0.5
±200
1100
1200
±1
Figure 2
Figure 2
Figure 2
Commercial (Note 5)
Industrial (Note 5)
(Note 5)
(Note 6)
V
IMAX
= V
CC
, Commercial
V
IMAX
= V
CC
, Industrial
q
q
q
q
q
q
q
q
2
U
µA
µA
W
U
U
W W
W
LTC1504A
ELECTRICAL CHARACTERISTICS
SYMBOL
I
SS
t
r
, t
f
DC
MAX
PARAMETER
Soft Start Source Current
Output Switch Rise/Fall Time
Maximum Duty Cycle
CONDITIONS
V
SS
= 0V, Commercial
V
SS
= 0V, Industrial
10% to 90% Ouput Swing
V
COMP
= V
CC
q
q
q
q
V
CC
= 5V, T
A
= 25°C unless otherwise specified. (Note 2)
MIN
–8
–8
100
TYP
–12
–12
5
MAX
–16
–17
50
UNITS
µA
µA
ns
%
The
q
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.
Note 4:
LTC1504A quiescent current is dominated by the gate drive
current drawn by the onboard power switches. With FB or SENSE pulled to
V
CC
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 g
mV
and A
V
values 2.6 times
lower than the specified values, due to the internal divider resistors.
Note 6:
The I
LIM
amplifier can sink but not source current. Under normal
(not current limited) operation, the I
LIM
output current will be zero.
Note 7:
Contact factory for guaranteed specifications at 2.7V supply.
TYPICAL PERFORMANCE CHARACTERISTICS
Supply Current vs Supply Voltage
14
12
SUPPLY CURRENT (mA)
10
8
6
4
2
V
FB
= V
CC
0
2.5
7.5
5
SUPPLY VOLTAGE (V)
10
1504A • TPC01
T
A
= 25°C
I
OUT
= 0
SUPPLY CURRENT (mA)
V
FB
= V
OUT
SWITCH ON-RESISTANCE (Ω)
V
FB
= V
OUT
Current Limit Threshold vs R
IMAX
700
CURRENT LIMIT THRESHOLD (mA)
600
500
400
300
200
100
0
10k
R
IMAX
(Ω)
1504A • TPC04
CURRENT LIMIT THRESHOLD (mA)
400
350
300
250
200
150
100
50
0
–50 –25
0
25
50
75
TEMPERATURE (°C)
100
125
R
IMAX
= 22k
R
IMAX
= 47k
SHUTDOWN PIN THRESHOLD (V)
T
A
= 25°C
V
CC
= 5V
U W
Supply Current vs Temperature
10
V
CC
= 5V
I
OUT
= 0
Switch On-Resistance vs
Temperature
3.5
3.0
2.5
2.0
1.5
1.0
0.5
V
CC
= 3.3V
V
CC
= 5V
V
CC
= 10V
1
V
FB
= V
CC
0.1
–50
–25
0
25
50
75
TEMPERATURE (°C)
100
125
0
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
1504A • TPC02
1504A • TPC03
Current Limit Threshold vs
Temperature
500
450
V
CC
= 5V
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
Shutdown Threshold vs
Supply Voltage
100k
3
5
7
SUPPLY VOLTAGE (V)
10
1504A • TPC07
1504A • TPC05
3
LTC1504A
PIN FUNCTIONS
I
MAX
(Pin 1):
Current Limit Set. Connect a resistor from
V
CC
to I
MAX
to set the current limit threshold. An internal
12µA current source from I
MAX
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 I
MAX
floating.
V
CC
(Pin 2):
Power Supply Input. Connect to a power
supply voltage between 4V and 10V. V
CC
requires a low
impedance bypass capacitor to ground, located as close
as possible to the LTC1504A. See the Applications Infor-
mation section for details on capacitor selection and
placement.
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 C
OUT
and becomes the regulated output
voltage. Avoid shorting SW to GND or V
CC
.
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 V
OUT
to GND to set the regulated output
voltage. The LTC1504A feedback loop will servo the FB pin
to 1.265V.
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.
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.
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. C
SS
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.
4
U
U
U
LTC1504A
BLOCK DIAGRAM
SHDN
COMP
12µA
SS
I
MAX
12µA
20.4k
FB
(ADJ ONLY)
SENSE
(–3.3V ONLY)
TEST CIRCUITS
NC
I
MAX
V
CC
SHDN
SW
LTC1504A
GND
SS
FB/SENSE
COMP
L
EXT
47µH
V
OUT
+
V
CC
C
IN
100µF
1µF
C
IN
: AVX TPSE107M016R0125
C
OUT
: SANYO 16CV220GX
L
EXT
: COILCRAFT D03316-473
Figure 1
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
U
W
W
TO INTERNAL BLOCKS
V
CC
Q1
SAW
–
PWM
SW
+
Q2
I
LIM
FB
–
+
+
–
+
–
V
REF
1.265V
12.6k
1504A • BD
Figure 3. Block Diagram
LTC1504A
A
B
C
OUT
220µF
FB/SENSE
–
COMP
+
+
V
REF
0.1µF
7.5k
0.01µF
220pF
A: TEST V
OL
, I
OL
B: TEST V
OH
, I
OH
1504A • TC02
1504A • TC01
Figure 2
U
U
5
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