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LT3503

3503f

APPLICATIO S

FEATURES

TYPICAL APPLICATIO

DESCRIPTIO

1A, 2.2MHz Step-Down

Switching Regulator in

2mm × 3mm DFN

The LT

3503 is a current mode PWM step-down DC/DC

converter with an internal 1.45A power switch. The wide

operating input range of 3.6V to 20V makes the LT3503

ideal for regulating power from a wide variety of sources.

Its high operating frequency allows the use of tiny, low

cost inductors and ceramic capacitors, resulting in low,

predictable output ripple.

Cycle-by-cycle current limit provides protection against

shorted outputs and soft-start eliminates input current

surge during start-up. The low current (<2µA) shutdown

mode provides output disconnect, enabling easy power

management in battery-powered systems.

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Wide Input Range: 3.6V to 20V

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5V at 1.2A from 11V to 18V Input

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5V at 1A from 7.2V to 18V Input

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3.3V at 1.2A from 8.5V to 12V Input

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3.3V at 1A from 5.5V to 12V Input

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Fixed Frequency Operation: 2.2MHz

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Output Adjustable Down to 780mV

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Short-Circuit Robust

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Uses Tiny Capacitors and Inductors

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Soft-Start

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Internally Compensated

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Low Shutdown Current: <2µA

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Low V

CESAT

Switch: 400mV at 1A

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Thermally Enhanced, 2mm × 3mm 6-Pin DFN

Low Profile Package

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Automotive Battery Regulation

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Industrial Control Supplies

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Wall Transformer Regulation

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Distributed Supply Regulation

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Battery-Powered Equipment

3.3V Step-Down Converter

Efficiency

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

All other trademarks are the property of their respective owners.

4.5V TO 12V

20V MAX

ON OFF

0.1µF

2.5µH

37.4k

10µF

3503 TA01a

120pF

1µF

11.5k

OUT

3.3V

1A, V

> 5.5V

1.2A, V

> 8.5V

BOOST

GND FB

SHDN

LT3503

LOAD CURRENT (A)

0.6 1.0

3503 TA01b

0.2 0.4

0.8 1.2

EFFICIENCY (%)

= 12V

Summary of content (20 pages)

PAGE 1
LT3503 1A, 2.2MHz Step-Down Switching Regulator in 2mm × 3mm DFN U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO The LT®3503 is a current mode PWM step-down DC/DC converter with an internal 1.45A power switch. The wide operating input range of 3.6V to 20V makes the LT3503 ideal for regulating power from a wide variety of sources. Its high operating frequency allows the use of tiny, low cost inductors and ceramic capacitors, resulting in low, predictable output ripple. Wide Input Range: 3.
PAGE 2
LT3503 W W W AXI U U ABSOLUTE RATI GS U U W PACKAGE/ORDER I FOR ATIO (Note 1) Input Voltage (VIN) .................................................. 20V BOOST Pin Voltage .................................................. 40V BOOST Pin Above SW Pin ....................................... 20V SHDN Pin ................................................................ 20V FB Voltage ................................................................. 6V Operating Temperature Range (Note 2) ...
PAGE 3
LT3503 ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, VBOOST = 17V, unless otherwise noted. (Note 2) PARAMETER CONDITIONS MIN TYP MAX Switch Current Limit (Note 3) 1.45 1.75 2.2 Switch VCESAT ISW = 1A 400 UNITS A mV Switch Leakage Current 2 µA Minimum Boost Voltage Above Switch ISW = 1A 2 2.3 V BOOST Pin Current ISW = 1A 25 50 mA SHDN Input Voltage High 2.
PAGE 4
LT3503 U W TYPICAL PERFOR A CE CHARACTERISTICS Maximum Load Current Maximum Load Current 1.8 OUTPUT CURRENT (A) OUTPUT CURRENT (A) MINIMUM 1.0 0.8 0.6 1.4 400 1.2 350 MINIMUM 1.0 0.8 0.6 0.4 0.4 0.2 0.2 0 0 5 6 8 9 7 10 INPUT VOLTAGE (V) 11 450 TYPICAL TYPICAL 1.2 4 VOUT = 5V L = 3.3µH 1.6 1.4 Switch Voltage Drop 500 VCE(SWITCH) (mV) 1.8 VOUT = 3.3V 1.6 L = 2.2µH TA = 25°C unless otherwise noted.
PAGE 5
LT3503 U W TYPICAL PERFOR A CE CHARACTERISTICS TA = 25°C unless otherwise noted. Typical Minimum Input Voltage (VOUT = 3.3V) SHDN Pin Current 50 Typical Minimum Input Voltage (VOUT = 5V) 5.5 7.7 45 40 25 20 15 INPUT VOLTAGE (V) INPUT VOLTAGE (V) 30 4.5 6.7 6.2 4.0 10 5.7 5 0 2 4 6 8 10 12 14 16 18 20 VSHDN (V) 3.5 5.2 1 10 1OO LOAD CURRENT (mA) 3503 G12 Switch Current Limit 10 100 LOAD CURRENT (mA) 1 1000 3503 G14 Switch Current Limit 1.8 2.0 1.9 1.7 1.8 1.7 1.6 1.5 1.4 1.
PAGE 6
LT3503 U U U PI FU CTIO S FB (Pin 1): The LT3503 regulates its feedback pin to 780mV. Connect the feedback resistor divider tap to this pin. Set the output voltage according to: ⎛ V ⎞ R1 = R2 ⎜ OUT – 1⎟ ⎝ 0.78 V ⎠ A good value for R2 is 10.0k. GND (Pin 2): Tie the GND pin to a local ground plane below the LT3503 and the circuit components. Return the feedback divider to this pin.
PAGE 7
LT3503 U OPERATIO (Refer to Block Diagram) The LT3503 is a constant frequency, current mode stepdown regulator. A 2.2MHz oscillator enables an RS flipflop, turning on the internal 1.75A 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 and servos the VC node.
PAGE 8
LT3503 U W U U APPLICATIO S I FOR ATIO FB Resistor Network The output voltage is programmed with a resistor divider between the output and the FB pin. Choose the 1% resistors according to: ⎛ V ⎞ R1 = R2 ⎜ OUT – 1⎟ ⎝ 0.78 V ⎠ R2 should be 20.0k or less to avoid bias current errors. Reference designators refer to the Block Diagram. An optional phase lead capacitor of 22pF between VOUT and FB reduces light-load output ripple.
PAGE 9
LT3503 U W U U APPLICATIO S I FOR ATIO Now the required on time has decreased below the minimum on time of 130ns. Instead of the switch pulse width becoming narrower to accommodate the lower duty cycle requirement, the switch pulse width remains fixed at 130ns. In Figure 2 the inductor current ramps up to a value exceeding the load current and the output ripple increases to ~40mV.
PAGE 10
LT3503 U U W U APPLICATIO S I FOR ATIO Table 1. Inductor Vendors Vendor URL Part Series Inductance Range (µH) Size (mm) Sumida www.sumida.com CDRH4D28 CDRH5D28 CDRH8D28 1.2 to 4.7 2.5 to 10 2.5 to 33 4.5 × 4.5 5.5 × 5.5 8.3 × 8.3 Toko www.toko.com A916CY D585LC 2 to 12 1.1 to 39 6.3 × 6.2 8.1 × 8.0 Würth Elektronik www.we-online.com WE-TPC(M) WE-PD2(M) WE-PD(S) 1 to 10 2.2 to 22 1 to 27 4.8 × 4.8 5.2 × 5.8 7.3 × 7.3 transient response.
PAGE 11
LT3503 U W U U APPLICATIO S I FOR ATIO Ceramic capacitors have very low equivalent series resistance (ESR) and provide the best ripple performance. A good value is: COUT = 24/VOUT where COUT is in µF. Use X5R or X7R types and keep in mind that a ceramic capacitor biased with VOUT will have less than its nominal capacitance. This choice will provide low output ripple and good transient response.
PAGE 12
LT3503 U U W U APPLICATIO S I FOR ATIO VOUT ILOAD 1A/DIV 36.5k FB 10µF VOUT 50mV/DIV 11.3k VOUT 36.5k 3503 F05a 40µs/DIV 3503 F05b 40µs/DIV 3503 F05c ILOAD 1A/DIV 1nF 10µF FB VOUT 50mV/DIV 11.3k VOUT 36.5k 40µs/DIV ILOAD 1A/DIV 1nF + FB 11.3k 100µF KEMET A700D686M010ATE015 VOUT 50mV/DIV Figure 5. Transient Load Response of the LT3503 with Different Output Capacitors as the Load Current is Stepped from 0.5A to 1.1A. VIN = 12V, VOUT = 3.3V, L = 3.
PAGE 13
LT3503 U U W U APPLICATIO S I FOR ATIO D2 D2 C3 BOOST VIN VIN C3 BOOST LT3503 LT3503 VOUT SW VIN VIN GND VOUT SW GND 3503 F06a 3503 F06b VBOOST – VSW ≅ VOUT MAX VBOOST ≅ VIN + VOUT VBOOST – VSW ≅ VIN MAX VBOOST ≅ 2VIN (6a) (6b) Figure 6. Two Circuits for Generating the Boost Voltage 5.5 7.7 TO START 5.0 INPUT VOLTAGE (V) INPUT VOLTAGE (V) 7.2 TO START 6.7 TO RUN 6.2 4.5 TO RUN 4.0 5.7 3.5 5.
PAGE 14
LT3503 U U W U APPLICATIO S I FOR ATIO Soft-Start The SHDN pin can be used to soft-start the LT3503, reducing the maximum input current during start-up. The SHDN pin is driven through an external RC filter to create a voltage ramp at this pin. Figure 8 shows the start-up waveforms with and without the soft-start circuit. By choosing a large RC time constant, the peak start up current can be reduced to the current that is required to regulate the output, with no overshoot.
PAGE 15
LT3503 U W U U APPLICATIO S I FOR ATIO D4 VIN VIN BOOST LT3503 SHDN GND VOUT SW FB BACKUP 3503 F09 Figure 9. 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 LT3503 Runs Only When the Input is Present Hot Plugging Safely The small size, robustness and low impedance of ceramic capacitors make them an attractive option for the input bypass capacitor of LT3503 circuits.
PAGE 16
LT3503 U W U U APPLICATIO S I FOR ATIO damps the circuit and eliminates the voltage overshoot. The extra capacitor improves low frequency ripple filtering and can slightly improve the efficiency of the circuit, though it is likely to be the largest component in the circuit. An alternative solution is shown in Figure 10c. A 1Ω resistor is added in series with the input to eliminate the voltage overshoot (it also reduces the peak input current). A 0.1µF capacitor improves high frequency filtering.
PAGE 17
LT3503 U U W U APPLICATIO S I FOR ATIO C2 D1 SYSTEM GROUND VOUT C1 VIN SHUTDOWN 3503 F12 : VIAS TO LOCAL GROUND PLANE : OUTLINE OF LOCAL GROUND PLANE Figure 12. A Good PCB Layout Ensures Proper, Low EMI Operation 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 C1. The SW and BOOST nodes should be as small as possible.
PAGE 18
LT3503 U TYPICAL APPLICATIO S 0.78V Step-Down Converter BAT54 VIN 3.6V TO 4V 20V MAX VIN BOOST 0.1µF 1.0µH LT3503 VOUT 0.78V 1.2A SW SHDN ON OFF MBRM140 GND FB 47µF 1µF 3503 TA02 1.8V Step-Down Converter BAT54 VIN 3.6V TO 7.6V 20V MAX VIN BOOST 0.1µF 1.5µH LT3503 ON OFF VOUT 1.8V 1.2A SW SHDN MBRM140 GND 26.1k 120pF FB 22µF 1µF 20k 3503 TA03 2.5V Step-Down Converter VIN 3.6V TO 10V 20V MAX ON OFF BAT54 VIN 0.47µF 1.8µH LT3503 SW SHDN MBRM140 GND 1µF VOUT 2.
PAGE 19
LT3503 U PACKAGE DESCRIPTION DCB Package 6-Lead Plastic DFN (2mm × 3mm) (Reference LTC DWG # 05-08-1715) 0.70 ±0.05 3.55 ±0.05 1.65 ±0.05 (2 SIDES) 2.15 ±0.05 PACKAGE OUTLINE 0.25 ± 0.05 0.50 BSC 1.35 ±0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS R = 0.115 TYP 2.00 ±0.10 (2 SIDES) R = 0.05 TYP 3.00 ±0.10 (2 SIDES) 0.40 ± 0.10 4 6 1.65 ± 0.10 (2 SIDES) PIN 1 NOTCH R0.20 OR 0.25 × 45° CHAMFER PIN 1 BAR TOP MARK (SEE NOTE 6) 3 0.200 REF 0.75 ±0.05 1 (DCB6) DFN 0405 0.25 ± 0.
PAGE 20
LT3503 U TYPICAL APPLICATIO 5V Step-Down Converter VOUT 5V 1A, VIN > 7.2V 1.2A, VIN > 11V 1N4148 VIN 6.7V TO 18V 20V MAX VIN BOOST 0.1µF 3.3µH LT3503 ON OFF SW SHDN MBRM140 GND 61.9k 120pF FB 1µF 10µF 11.3k 3503 TA05 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1766 60V, 1.2A IOUT, 200kHz, High Efficiency Step-Down DC/DC Converter VIN: 5.5V to 60V, VOUT(MIN) = 1.2V, IQ = 2.5mA, ISD = 25µA, TSSOP16/TSSOP16E Packages LT1767 25V, 1.2A IOUT, 1.