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LT1506

4.5A, 500kHz Step-Down

Switching Regulator

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Constant 500kHz Switching Frequency

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Easily Synchronizable

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Operates with Input as Low as 4V

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Uses All Surface Mount Components

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Inductor Size Reduced to 1.8

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Saturating Switch Design: 0.07Ω

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Shutdown Current: 20µA

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Cycle-by-Cycle Current Limiting

The LT

1506 is a 500kHz monolithic buck mode switching

regulator functionally identical to the LT1374 but optimized

for lower input voltage applications. It will operate over a

4V to 15V input range compared with 5.5V to 25V for the

LT1374. A 4.5A switch is included on the die along with all

the necessary oscillator, control and logic circuitry. High

switching frequency allows a considerable reduction in the

size of external components. The topology is current mode

for fast transient response and good loop stability. Both

fixed output voltage and adjustable parts are available.

A special high speed bipolar process and new design tech-

niques achieve high efficiency at high switching frequency.

Efficiency is maintained over a wide output current range

by keeping quiescent supply current to 4mA

and by utiliz-

ing a supply boost

capacitor to saturate the power switch.

The LT1506 fits into standard 7-pin DD and fused lead

SO-8 packages. Full cycle-by-cycle short-circuit protection

and thermal shutdown are provided. Standard surface

mount external parts are used, including the inductor and

capacitors. There is the optional function of shutdown or

synchronization. A shutdown signal reduces supply current

to 20µA. Synchronization allows an external logic level sig-

nal to increase the internal oscillator from 580kHz to 1MHz.

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Portable Computers

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Battery-Powered Systems

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Battery Charger

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Distributed Power

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

LOAD CURRENT (A)

EFFICIENCY (%)

2.0 2.5 3.0 3.5

1506 TA02

0.5 1.0 1.5 4.0

OUT

= 3.3V

= 5V

L = 10µH

Efficiency vs Load Current

5V to 3.3V Down Converter

BOOST

LT1506-3.3

OUTPUT

3.3V

INPUT

1506 TA01

0.68µF

1.5nF

MBRS330T3

100µF, 10V

SOLID

TANTALUM

10µF TO

50µF

CERAMIC

1N914

5µH

SENSESHDN

OPEN

HIGH

= ON

GND

FEATURES

DESCRIPTIO

APPLICATIO S

TYPICAL APPLICATIO

Summary of content (24 pages)

PAGE 1
LT1506 4.5A, 500kHz Step-Down Switching Regulator U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO The LT ®1506 is a 500kHz monolithic buck mode switching regulator functionally identical to the LT1374 but optimized for lower input voltage applications. It will operate over a 4V to 15V input range compared with 5.5V to 25V for the LT1374. A 4.5A switch is included on the die along with all the necessary oscillator, control and logic circuitry.
PAGE 2
LT1506 U W W W ABSOLUTE MAXIMUM RATINGS (Note 1) Input Voltage .......................................................... 16V BOOST Pin Above Input Voltage ............................. 15V SHDN Pin Voltage ..................................................... 7V FB Pin Voltage (Adjustable Part) ............................ 3.5V FB Pin Current (Adjustable Part) ............................ 1mA Sense Voltage (Fixed 3.3V Part) ............................... 5V SYNC Pin Voltage ........................
PAGE 3
LT1506 ELECTRICAL CHARACTERISTICS TJ = 25°C, VIN = 5V, VC = 1.5V, Boost = VIN + 5V, switch open, unless otherwise noted. PARAMETER Switch On Resistance (Note 7) CONDITIONS ISW = 4.5A Maximum Switch Duty Cycle VFB = 2.1V or VSENSE = 2.9V MIN TYP 0.
PAGE 4
LT1506 U W TYPICAL PERFORMANCE CHARACTERISTICS Feedback Pin Voltage Switch Peak Current Limit 4.7 6.5 4.5 6.0 4.3 4.1 3.9 3.7 3.5 2.430 TYPICAL 5.5 5.0 MINIMUM 4.5 4.0 3.0 1 10 100 LOAD CURRENT (mA) 1000 0 20 60 40 DUTY CYCLE (%) 80 2.420 2.415 2.410 – 50 100 0 25 50 75 1506 G03 Lockout and Shutdown Thresholds –500 125 100 1506 G02 Shutdown Pin Bias Current Shutdown Supply Current 25 2.40 VSHDN = 0V AT 0.37V SHUTDOWN THRESHOLD.
PAGE 5
LT1506 U W TYPICAL PERFORMANCE CHARACTERISTICS Inductor Core Loss for 3.3V Output Switching Frequency 500 550 1.0 540 530 SWITCHING FREQUENCY 300 200 520 CORE LOSS (W) 400 FREQUENCY (kHz) 510 500 490 480 FEEDBACK PIN CURRENT Kool Mµ® 0.01 450 – 50 0 0 0.5 1.5 2.0 1.0 FEEDBACK PIN VOLTAGE (V) 2.5 0.001 –25 0 25 50 75 100 125 0 L= 10µH L= 5µH LOAD CURRENT (A) 3.4 3.2 4.0 L= 3µH 3.8 3.6 3.4 L= 1.8µH L= 1.8µH 3.2 2.
PAGE 6
LT1506 U U U PIN FUNCTIONS FB/SENSE: The feedback pin is used to set output voltage using an external voltage divider that generates 2.42V at the pin with the desired output voltage. The fixed voltage (-3.3) parts have the divider included on the chip and the FB pin is used as a SENSE pin, connected directly to the 3.3V output. Three additional functions are performed by the FB pin. When the pin voltage drops below 1.7V, switch current limit is reduced. Below 1.5V the external sync function is disabled.
PAGE 7
LT1506 W BLOCK DIAGRAM The LT1506 is a constant frequency, current mode buck converter. This means that there is an internal clock and two feedback loops that control the duty cycle of the power switch. In addition to the normal error amplifier, there is a current sense amplifier that monitors switch current on a cycle-by-cycle basis. A switch cycle starts with an oscillator pulse which sets the RS flip-flop to turn the switch on.
PAGE 8
LT1506 U U W U APPLICATIONS INFORMATION FEEDBACK PIN FUNCTIONS More Than Just Voltage Feedback The feedback (FB) pin on the LT1506 is used to set output voltage and provide several overload protection features. The first part of this section deals with selecting resistors to set output voltage and the remaining part talks about foldback frequency and current limiting created by the FB pin. Please read both parts before committing to a final design. The fixed 3.3V LT1506-3.
PAGE 9
LT1506 U W U U APPLICATIONS INFORMATION The internal circuitry which forces reduced switching frequency also causes current to flow out of the feedback pin when output voltage is low. The equivalent circuitry is shown in Figure 2. Q1 is completely off during normal operation. If the FB pin falls below 1V, Q1 begins to conduct current and reduces frequency at the rate of approximately 5kHz/µA.
PAGE 10
LT1506 U W U U APPLICATIONS INFORMATION CHOOSING THE INDUCTOR AND OUTPUT CAPACITOR For most applications the output inductor will fall in the range of 3µH to 20µH. Lower values are chosen to reduce physical size of the inductor. Higher values allow more output current because they reduce peak current seen by the LT1506 switch, which has a 4.5A limit. Higher values also reduce output ripple voltage, and reduce core loss.
PAGE 11
LT1506 U W U U APPLICATIONS INFORMATION Table 2 VENDOR/ PART NO. VALUE DC (µH) (Amps) CORE TYPE SERIES CORE RESIS- MATER- HEIGHT TANCE(Ω) IAL (mm) Coiltronics range for typical LT1506 applications is 0.05Ω to 0.2Ω. A typical output capacitor is an AVX type TPS, 100µF at 10V, with a guaranteed ESR less than 0.1Ω. This is a “D” size surface mount solid tantalum capacitor. TPS capacitors are specially constructed and tested for low ESR, so they give the lowest ESR for a given volume.
PAGE 12
LT1506 U W U U APPLICATIONS INFORMATION (5)(10 − 5) Ceramic Capacitors Higher value, lower cost ceramic capacitors are now available in smaller case sizes. These are ideal for input bypassing because of their high ripple rating and tolerance to turn-on surges. As output capacitors, caution must be used. Solid tantalum capacitor’s ESR generates a loop “zero” at 5kHz to 50kHz that is beneficial in giving acceptable loop phase margin.
PAGE 13
LT1506 U W U U APPLICATIONS INFORMATION and does not fall to less than 1/3 of nominal output voltage, foldback will not take effect. With the overloaded condition, output current will increase to a typical value of 5.7A, determined by peak switch current limit of 6A. With VIN = 15V, VOUT = 4V (5V overloaded) and IOUT = 5.7A: ID(AVG) = ( ) = 4.18A 5.
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LT1506 U W U U APPLICATIONS INFORMATION RFB LT1506 OUTPUT VSW IN INPUT 2.38V LOCKOUT RHI 3.5µA + SHDN TOTAL SHUTDOWN RLO C1 0.4V GND 1506 F04 Figure 4. Undervoltage Lockout Keep the connections from the resistors to the shutdown pin short and make sure that interplane or surface capacitance to the switching nodes are minimized. If high resistor values are used, the shutdown pin should be bypassed with a 1000pF capacitor to prevent coupling problems from the switch node.
PAGE 15
LT1506 U W U U APPLICATIONS INFORMATION from the switch node. Also note that the high current ground path of the catch diode and input capacitor are kept very short and separate from the analog ground line. The high speed switching current path is shown schematically in Figure 6. Minimum lead length in this path is essential to ensure clean switching and low EMI. The path including the switch, catch diode, and input capacitor is the only one containing nanosecond rise and fall times.
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LT1506 U W U U APPLICATIONS INFORMATION PARASITIC RESONANCE Resonance or “ringing” may sometimes be seen on the switch node (see Figure 7). Very high frequency ringing following switch rise time is caused by switch/diode/input capacitor lead inductance and diode capacitance. Schottky diodes have very high “Q” junction capacitance that can ring for many cycles when excited at high frequency.
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LT1506 U W U U APPLICATIONS INFORMATION The term inside the radical has a maximum value of 0.5 when input voltage is twice output, and stays near 0.5 for a relatively wide range of input voltages. It is common practice therefore to simply use the worst-case value and assume that RMS ripple current is one half of load current. At maximum output current of 4.5A for the LT1506, the input bypass capacitor should be rated at 2.25A ripple current.
PAGE 18
LT1506 U U W U APPLICATIONS INFORMATION formulas show how to calculate each of these losses. These formulas assume continuous mode operation, so they should not be used for calculating efficiency at light load currents. Die temperature is highest at low input voltage, so use lowest continuous input operating voltage for thermal calculations.
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LT1506 U W U U APPLICATIONS INFORMATION 20 40 0 –40 PHASE –20 –80 200 PHASE 2500 150 GAIN 2000 1500 100 ( VC ) 1000 COUT 12pF ROUT 200k VFB 2 × 10–3 50 ERROR AMPLIFIER EQUIVALENT CIRCUIT PHASE (DEG) 0 3000 GAIN (µMho) GAIN VIN = 10V VOUT = 5V IOUT = 2A PHASE: VC PIN TO OUTPUT (DEG) GAIN: VC PIN TO OUTPUT (dB) 40 0 RLOAD = 50Ω –40 10 100 1k 10k FREQUENCY (Hz) 100k 500 100 –120 1M 1k 1505 F10 1506 F11 Figure 10. Response from VC Pin to Output Figure 11.
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LT1506 U U W U APPLICATIONS INFORMATION GMP = Transconductance of power stage = 5.3A/V GMA = Error amplifier transconductance = 2(10–3) ESR = Output capacitor ESR 2.42 = Reference voltage With VOUT = 5V and ESR = 0.03Ω, a value of 6.5k for RC would yield zero gain margin, so this represents an upper limit. There is a second limitation however which has nothing to do with theoretical small signal dynamics.
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LT1506 U W U U APPLICATIONS INFORMATION RIPPLE FILTER 470Ω SWITCHING REGULATOR ADJUSTABLE INPUT SUPPLY + ADJUSTABLE DC LOAD 100µF TO 1000µF 3300pF TO X1 OSCILLOSCOPE PROBE 4.7k 330pF 50Ω TO OSCILLOSCOPE SYNC 100Hz TO 1kHz 100mV TO 1VP-P 1506 F13 Figure 13. Loop Stability Test Circuit VOUT AT IOUT = 500mA BEFORE FILTER VOUT AT IOUT = 500mA AFTER FILTER 10mV/DIV VOUT AT IOUT = 50mA AFTER FILTER LOAD PULSE THROUGH 50Ω f ≈ 780Hz 5A/DIV 0.2ms/DIV 1375/76 F14 Figure 14.
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LT1506 U W U U APPLICATIONS INFORMATION converters 120° out of phase with each other reduces input and output ripple currents. This reduces the ripple rating, size and cost of filter capacitors. Synchronized Ripple Currents A ring counter generates three synchronization signals at 600kHz, 33% duty cycle phased 120° apart. The sync input will operate over a wide range of duty cycles, so no further pulse conditioning is needed. Each device’s maximum input ripple current is a 4A square wave at 600kHz.
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LT1506 U U W U APPLICATIONS INFORMATION Redundant Operation The circuit shown in Figure 15 is fault tolerant when operating at less than 8A of output current. If one device fails, the output will remain in regulation. The feedback loop will compensate by raising the voltage on the VC pin, increasing switch current of the two remaining devices. BUCK CONVERTER WITH ADJUSTABLE SOFT START Large capacitive loads can cause high input currents at start-up.
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LT1506 U PACKAGE DESCRIPTION Dimensions in inches (millimeters) unless otherwise noted. R Package 7-Lead Plastic DD Pak (LTC DWG # 05-08-1462) 0.060 (1.524) TYP 0.060 (1.524) 0.256 (6.502) 0.390 – 0.415 (9.906 – 10.541) 0.165 – 0.180 (4.191 – 4.572) 0.045 – 0.055 (1.143 – 1.397) 15° TYP 0.060 (1.524) 0.183 (4.648) 0.059 (1.499) TYP 0.330 – 0.370 (8.382 – 9.398) ( +0.008 0.004 –0.004 +0.203 0.102 –0.102 ) 0.095 – 0.115 (2.413 – 2.921) 0.075 (1.905) 0.300 (7.620) ( +0.012 0.143 –0.020 +0.