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LT3505

3505fc

TYPICAL APPLICATION

FEATURES

APPLICATIONS

DESCRIPTION

1.2A, Step-Down

Switching Regulator in

3mm × 3mm DFN

The LT

3505 is a current mode PWM step-down DC/DC

converter with an internal 1.4A power switch. The wide

operating input range of 3.6V to 36V (40V maximum)

makes the LT3505 ideal for regulating power from a wide

variety of sources, including unregulated wall transform-

ers, 24V industrial supplies and automotive batteries. The

oscillator can be programmed for high frequency operation

allowing the use of tiny, low cost external components or

it can be programmed for lower frequency operation to

maximize efﬁ ciency.

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.

750kHz, 3.3V Step-Down Converter

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

All other trademarks are the property of their respective owners.

Wide Input Range: 3.6V to 36V Operating,

40V Maximum

Up to 1.2A Output Current

Resistor-Programmable Fixed-Frequency Operation

from 200kHz to 3MHz

Output Adjustable Down to 780mV

Short-Circuit Robust

Uses Tiny Capacitors and Inductors

Soft-Start

Low Shutdown Current: <2µA

Low V

CESAT

Switch: 350mV at 1A

Thermally Enhanced, Low Proﬁ le 3mm x 3mm

DFN-8 and MSOP-8 Packages

Automotive Battery Regulation

Industrial Control Supplies

Wall Transformer Regulation

Distributed Supply Regulation

Battery-Powered Equipment

4.2V TO 36V

ON OFF

0.1µF

10µH

36.5k

10µF

3505 TA01

22pF

68pF

1µF

75.0k

11.3k

OUT

3.3V

1.1A, V

> 5V

1.2A, V

> 8V

69.8k

GND

BOOST

LT3505

SHDN

Efﬁ ciency

LOAD CURRENT (A)

EFFICIENCY (%)

0.4

0.8

1.2

0.20

0.6

1.0

= 12V

OUT

= 3.3V

= 750kHz

L = 10 H

Summary of content (24 pages)

PAGE 1
LT3505 1.2A, Step-Down Switching Regulator in 3mm × 3mm DFN FEATURES ! ! ! ! ! ! ! ! ! ! DESCRIPTION Wide Input Range: 3.6V to 36V Operating, 40V Maximum Up to 1.
PAGE 2
LT3505 ABSOLUTE MAXIMUM RATINGS (Note 1) Input Voltage (VIN) ....................................................40V BOOST Pin Voltage ..................................................50V BOOST Pin Above SW Pin.........................................25V SHDN Pin ..................................................................40V FB Pin .........................................................................6V VC Pin .........................................................................3V RT Pin ...
PAGE 3
LT3505 ELECTRICAL CHARACTERISTICS The " denotes the 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 VIN Operating Range TYP 3.6 Undervoltage Lockout Feedback Voltage " MAX 36 UNITS V 3.1 3.35 3.6 V 765 780 795 mV 55 150 nA FB Pin Bias Current VFB = Measured VREF (Note 4) Quiescent Current Not Switching, RT = 75.0k 2.0 2.
PAGE 4
LT3505 TYPICAL PERFORMANCE CHARACTERISTICS Efficiency (VOUT = 3.3V, L = 10µH, fSW = 750kHz) Efficiency (VOUT = 5V, L = 10µH, fSW = 750kHz) 95 85 80 80 70 65 0 0.2 0.4 0.6 0.8 LOAD CURRENT (A) 65 50 Efficiency (VOUT = 5V, L = 4.7µH, fSW = 2.2MHz) 95 1.7 0.4 0.6 0.8 LOAD CURRENT (A) OUTPUT CURRENT (A) 80 75 70 65 VIN = 8V VIN = 12V 0 0.2 0.4 0.6 0.8 LOAD CURRENT (A) 0 1.4 *10% DROPOUT 0.9 0.8 TYPICAL, L = 10µH 1.4 1.3 MINIMUM, L = 10µH 1.2 1.1 *10% DROPOUT 1.0 0.
PAGE 5
LT3505 TYPICAL PERFORMANCE CHARACTERISTICS Switching Frequency 3.90 2.20 SWITCHING FREQUENCY (MHz) 2.40 3.80 UVLO (V) 3.70 3.60 3.50 3.40 3.30 Frequency Foldback, RT = 75.0k 0.6 RT = 21k 2.00 1.80 1.60 RT = 30.1k 1.40 1.20 1.00 3.20 3.00 –50 –25 75 0 25 50 TEMPERATURE (°C) 100 0.60 –50 125 50 75 0 25 TEMPERATURE (°C) –25 0.4 0.3 0.2 0.1 RT = 75.0k 0.80 3.10 TA = 25°C 0.5 RT PIN BIAS VOLTAGE (V) Undervoltage Lockout 4.00 0 125 100 0 0.1 0.2 0.3 0.4 0.5 0.
PAGE 6
LT3505 TYPICAL PERFORMANCE CHARACTERISTICS Switch Current Limit Switch Current Limit, RT = 75.0k 1.8 1.8 SWITCH CURRENT LIMIT (A) SWITCH CURRENT LIMIT (A) 1.9 1.7 1.6 1.5 1.4 1.3 1.2 Typical Minimum On Time 160 TA = 25°C 140 1.7 MINIMUM ON TIME (ns) 2.0 1.6 1.5 1.4 1.3 –25 0 25 50 75 TEMPERATURE (°C) 100 1.2 125 100 80 60 40 20 1.1 1.
PAGE 7
LT3505 PIN FUNCTIONS 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. SW (Pin 2): The SW pin is the output of the internal power switch. Connect this pin to the inductor, catch diode and boost capacitor. VIN (Pin 3): The VIN pin supplies current to the LT3505’s internal regulator and to the internal power switch. This pin must be locally bypassed. SHDN (Pin 4): The SHDN pin is used to put the LT3505 in shutdown mode.
PAGE 8
LT3505 BLOCK DIAGRAM 3 VIN VIN C2 INT REG AND UVLO ON OFF SLOPE COMP R3 4 BOOST Σ R Q S Q D2 1 SHDN C4 C3 DRIVER Q1 SW OSC L1 VOUT 2 D1 C1 FREQUENCY FOLDBACK VC gm 780mV 5 GND 8 VC 7 FB 6 RT R1 3505 BD R2 OPERATION (Refer to Block Diagram) The LT3505 is a constant frequency, current mode stepdown regulator. A resistor-programmed oscillator enables an RS flip-flop, turning on the internal 1.4A power switch Q1.
PAGE 9
LT3505 APPLICATIONS INFORMATION 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 20k or less to avoid bias current errors. Reference designators refer to the Block Diagram.
PAGE 10
LT3505 APPLICATIONS INFORMATION 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 ~200mV. The part then remains off until the output voltage dips below 100% of the programmed value before it begins switching again.
PAGE 11
LT3505 APPLICATIONS INFORMATION Finite transition time results in a small amount of power dissipation each time the power switch turns on and off (edge loss). Edge loss increases with frequency, switch current, and input voltage. Input Voltage Frequency Foldback In constant frequency operation (below VIN(PS)) edge loss only reduces the application efficiency.
PAGE 12
LT3505 APPLICATIONS INFORMATION Component Selection for Input Voltage Frequency Foldback Circuit To determine the values of R4, R5, and D3 for a specific application follow the procedure outlined in this section. First select the value of R4 from Table 1. Table 1. RT Pin Resistance RT PIN RESISTANCE (kΩ) 357 237 165 124 100 84.5 71.5 61.9 54.9 48.7 44.2 40.2 37.4 34.0 31.6 29.4 27.4 25.5 23.7 22.6 21.0 20.0 19.1 18.2 16.9 16.2 15.4 14.7 13.7 SWITCHING FREQUENCY (MHz) 0.20 0.30 0.40 0.50 0.60 0.69 0.80 0.
PAGE 13
LT3505 APPLICATIONS INFORMATION Inductor Selection and Maximum Output Current Catch Diode A good first choice for the inductor value is: Depending on load current, a 1A to 2A Schottky diode is recommended for the catch diode, D1. The diode must have a reverse voltage rating equal to or greater than the maximum input voltage. The ON Semiconductor MBRM140 is a good choice; it is rated for 1A continuous forward current and a maximum reverse voltage of 40V. L = 1.
PAGE 14
LT3505 APPLICATIONS INFORMATION twice its nominal value, possibly exceeding the LT3505’s voltage rating. This situation can be easily avoided; see the Hot Plugging Safely section. Output Capacitor The output capacitor has two essential functions. Along with the inductor, it filters the square wave generated by the LT3505 to produce the DC output. In this role it determines the output ripple so low impedance at the switching frequency is important.
PAGE 15
LT3505 APPLICATIONS INFORMATION ILOAD 1A/DIV VOUT 32.4k 22pF FB 10µF VC VOUT 20mV/DIV AC COUPLED 10.0k 100k 22pF 100k 10µs/DIV 3505 F05b 10µs/DIV 3505 F05c 44pF 10µF ×2 FB VC 3505 F05a ILOAD 1A/DIV VOUT 32.4k 10µs/DIV VOUT 20mV/DIV AC COUPLED 10.0k 22pF ILOAD 1A/DIV VOUT 32.4k 66pF + FB VC 301k 120µF 10.0k KEMET A700D127M006ATE015 22pF VOUT 20mV/DIV AC COUPLED Figure 5.
PAGE 16
LT3505 APPLICATIONS INFORMATION capacitor and a small Schottky diode (such as the BAT-54). For lower output voltages tie a Schottky diode to the input (Figure 6b). The circuit in Figure 6a 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 LT3505 application is limited by the undervoltage lockout (3.6V) and by the maximum duty cycle as outlined above.
PAGE 17
LT3505 APPLICATIONS INFORMATION RUN plications or in battery backup systems where a battery or some other supply is diode OR-ed with the LT3505’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 LT3505’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 the SHDN pin, the SW pin current will drop to essentially zero.
PAGE 18
LT3505 APPLICATIONS INFORMATION damaging the part. If the input supply is poorly controlled or the user will be plugging the LT3505 into an energized supply, the input network should be designed to prevent this overshoot. Figure 10 shows the waveforms that result when an LT3505 circuit is connected to a 24V supply through six feet of 24-gauge twisted pair. The first plot is the response with a 2.2µF ceramic capacitor at the input. The input voltage rings as high as 35V and the input current peaks at 20A.
PAGE 19
LT3505 APPLICATIONS INFORMATION Figure 11 shows an equivalent circuit for the LT3505 control loop. The error amp is a transconductance amplifier with finite output impedance. The power section, consisting of the modulator, power switch and inductor, is modeled as a transconductance amplifier generating an output current proportional to the voltage at the VC node.
PAGE 20
LT3505 APPLICATIONS INFORMATION system ground in only one place. 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 C1. The SW and BOOST nodes should be as small as possible.
PAGE 21
LT3505 TYPICAL APPLICATIONS 2.2MHz, 3.3V Step-Down Converter 1N4148 VIN 0.1µF 3.3µH SW SHDN ON OFF LT3505 10V CMPZ5240B 36.5k FB 698k RT GND MBRM140 VC 20.0k 22pF 10µF 11.3k 100k 1µF 2.50 VOUT 3.3V BOOST Frequency [MHz] / Load Current [A] VIN 6V TO 36V 22pF Switching Frequency 2.25 2.00 Maximum Load Current 1.75 1.50 1.25 1.0 0.75 0.50 0.25 0.00 3505 TA02 5 15 10 25 20 30 Input Voltage [V] 35 40 LTC3505 • TA02b 1.2MHz, 1.8V Step-Down Converter BAT54 BOOST VIN 0.1µF 4.
PAGE 22
LT3505 TYPICAL APPLICATIONS 750kHz, 3.3V Step-Down Converter 1N4148 VIN 4.2V TO 36V VIN VOUT 3.3V 1.1A, VIN > 5V 1.2A, VIN > 8V BOOST 0.1µF 10µH SW SHDN ON OFF LT3505 36.5k 68pF FB RT GND 11.3k VC 10µF MBRM140 75.0k 69.8k 70pF 1µF 3505 TA04 1MHz, 12V Step-Down Converter CMDZ5235B 6V 0.1µF 1k* 0.25W BOOST VIN 13.5V TO 36V ON OFF 15µH SW VIN LT3505 SHDN 54.9k 71.5k 22pF VOUT 12V 1A, VIN > 16.5V 1.1A, VIN > 20.5V FB GND RT 3.3µF 1N4148 MBRM140 VC 4.
PAGE 23
LT3505 PACKAGE DESCRIPTION DD Package 8-Lead Plastic DFN (3mm × 3mm) (Reference LTC DWG # 05-08-1698) R = 0.115 TYP 5 0.38 ± 0.10 8 0.675 ±0.05 3.5 ±0.05 1.65 ±0.05 2.15 ±0.05 (2 SIDES) PACKAGE OUTLINE 0.25 ± 0.05 3.00 ±0.10 (4 SIDES) PIN 1 TOP MARK (NOTE 6) 4 0.25 ± 0.05 0.75 ±0.05 0.200 REF 0.50 BSC 2.38 ±0.05 (2 SIDES) 1.65 ± 0.10 (2 SIDES) 1 (DD) DFN 1203 0.50 BSC 2.38 ±0.10 (2 SIDES) 0.00 – 0.05 BOTTOM VIEW—EXPOSED PAD RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS NOTE: 1.
PAGE 24
LT3505 TYPICAL APPLICATIONS 300kHz, 3.3V Step-Down Converter VIN 4V TO 36V ON OFF 1N4148 VIN VOUT 3.3V 1A, VIN > 5V 1.2A, VIN > 8.5V BOOST 0.47µF 22µH SW SHDN LT3505 36.5k 100pF FB GND RT 11.3k VC MBRM140 226k 2.2µF 68µF KEMET A700D686M010ATE015 100k 150pF 3505 TA06 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.