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LTC1504

500mA Low Voltage

Step-Down Synchronous

Switching Regulator

Final Electrical Specifications

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 represen-

tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

Daisy-Chained Control Outputs

FEATURES

The LTC

1504 is a self-contained, high efficiency syn-

chronous buck switching regulator. It includes a pair of

on-chip 1.5Ω 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.

The LTC1504 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 LTC1504 is specified to operate

with input voltages between 4V and 10V. Contact the LTC

factory for guaranteed specifications at 2.7V supply.

The LTC1504 is available in a plastic SO-8 package.

DESCRIPTION

■

500mA Output Current at 3.3V Output

■

Up to 92% Peak Efficiency

■

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 be

Synchronized Up to 500kHz

TYPICAL APPLICATION

Minimum Part Count 5V to 3.3V Regulator 5V to 3.3V Efficiency

LOAD CURRENT (mA)

EFFICIENCY (%)

100

90

80

70

60

50

40

30

20

10

100 500

1504 • TA02

MAX

SHDN

SS COMP

GND

SENSE

SHUTDOWNNC

1000pF

OUT

3.3V AT 500mA

EXT

: AVX TPSC226M016R0375

OUT

: AVX TAJC476M010

EXT

: COILTRONICS CTX50-1P

1504 • TA01

LTC1504-3.3

, LTC and LT are registered trademarks of Linear Technology Corporation.

APPLICATIONS

■

Small Portable Digital Systems

■

Active Termination

■

Auxiliary Output Voltage Supplies

■

Minimum Part Count/Size Switchers

Summary of content (12 pages)

PAGE 1
Final Electrical Specifications LTC1504 500mA Low Voltage Step-Down Synchronous Switching Regulator U DESCRIPTION FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ 500mA Output Current at 3.
PAGE 2
LTC1504 W U U W W U W ABSOLUTE MAXIMUM RATINGS PACKAGE/ORDER I FOR ATIO (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 ..................... 0°C to 70°C Storage Temperature Range ................. – 65°C to 150°C Lead Temperature (Soldering, 10 sec)..................
PAGE 3
LTC1504 ELECTRICAL CHARACTERISTICS 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.
PAGE 4
LTC1504 U W TYPICAL PERFORMANCE CHARACTERISTICS Shutdown Threshold vs Supply Voltage Current Limit Threshold vs Temperature 4.0 VCC = 5V 450 SHUTDOWN PIN THRESHOLD (V) CURRENT LIMIT THRESHOLD (mA) 500 400 RIMAX = 47k 350 300 250 200 RIMAX = 22k 150 100 50 0 –50 –25 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 0 25 50 75 TEMPERATURE (°C) 100 125 1504 • TPC05 3 5 7 SUPPLY VOLTAGE (V) 10 1504 • TPC07 U U U PIN FUNCTIONS IMAX (Pin 1): Current Limit Set.
PAGE 5
LTC1504 U U U PIN FUNCTIONS during power-up. CSS also compensates the current limit loop, allowing the LTC1504 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.
PAGE 6
LTC1504 U W U U APPLICATIONS INFORMATION OVERVIEW The LTC1504 is a complete synchronous switching regulator controller (see Block Diagram). It includes two on-chip 1.5Ω 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.
PAGE 7
LTC1504 U W U U APPLICATIONS INFORMATION EXTERNAL COMPONENT SELECTION External components required by the LTC1504 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.
PAGE 8
LTC1504 U U W U APPLICATIONS INFORMATION 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 LTC1504 can change the output current, limiting output transient response.
PAGE 9
LTC1504 U U W U APPLICATIONS INFORMATION 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°. 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.
PAGE 10
LTC1504 U W U U APPLICATIONS INFORMATION 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.
PAGE 11
LTC1504 U W U U APPLICATIONS INFORMATION 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.
PAGE 12
LTC1504 U U W U APPLICATIONS INFORMATION tance 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.