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LT1676

Wide Input Range,

High Efficiency, Step-Down

Switching Regulator

■

Wide Input Range: 7.4V to 60V

■

700mA Peak Switch Current Rating

■

Adaptive Switch Drive Maintains Efficiency at High

Load Without Pulse Skipping at Light Load

■

True Current Mode Control

■

100kHz Fixed Operating Frequency

■

Synchronizable to 250kHz

■

Low Supply Current in Shutdown: 30µA

■

Available in 8-Pin SO and PDIP Packages

■

Automotive DC/DC Converters

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Telecom 48V Step-Down Converters

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Cellular Phone Battery Charger Accessories

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IEEE 1394 Step-Down Converters

The LT

1676 is a wide input range, high efficiency Buck

(step-down) switching regulator. The monolithic die in-

cludes all oscillator, control and protection circuitry. The

part can accept input voltages as high as 60V and contains

an output switch rated at 700mA peak current. Current

mode control offers excellent dynamic input supply rejec-

tion and short-circuit protection.

The LT1676 contains several features to enhance effi-

ciency. The internal control circuitry is normally powered

via the V

pin, thereby minimizing power drawn directly

from the V

supply (see Applications Information). The

action of the LT1676 switch circuitry is also load depen-

dent. At medium to high loads, the output switch circuitry

maintains high rise time for good efficiency. At light loads,

rise time is deliberately reduced to avoid pulse skipping

behavior.

The available SO-8 package and 100kHz switching fre-

quency allow for minimal PC board area requirements.

Figure 1

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

39µF

63V

1676 F01

LT1676

SHDN

SYNC

GND

100µF

10V

MBR160

36.5k

400mA

12.1k

22k

220µH*

2200pF

8V TO 50V

100pF

*65T #30 ON MAGNETICS

MPP #55030

LOAD

(mA)

EFFICIENCY (%)

10 100 1000

1676 TA01

= 12V

= 24V

= 36V

= 48V

Efficiency vs V

and I

LOAD

FEATURES

DESCRIPTIO

APPLICATIO S

TYPICAL APPLICATIO

Summary of content (16 pages)

PAGE 1
LT1676 Wide Input Range, High Efficiency, Step-Down Switching Regulator U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO Wide Input Range: 7.
PAGE 2
LT1676 W U U U W W W ABSOLUTE MAXIMUM RATINGS PACKAGE/ORDER INFORMATION (Note 1) Supply Voltage ........................................................ 60V Switch Voltage ......................................................... 60V SHDN, SYNC Pin Voltage ........................................... 7V VCC Pin Voltage ....................................................... 30V FB Pin Voltage ...........................................................
PAGE 3
LT1676 ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C. VIN = 48V, VSW open, VCC = 5V, VC = 1.4V unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS 90 85 100 ● 110 115 kHz kHz ● 85 90 % 300 ns VC Pin Boost Threshold 1.35 V dV/dt Below Threshold 0.2 V/ns dV/dt Above Threshold 1.
PAGE 4
LT1676 U W TYPICAL PERFORMANCE CHARACTERISTICS SHDN Pin Shutdown Threshold vs Temperature SHDN Pin Input Current vs Voltage SHDN PIN INPUT CURRENT (µA) 700 600 500 400 300 200 –50 –25 1.30 0 1.28 –5 –10 25°C –55°C 125°C –15 100 125 0 1 3 4 2 SHDN PIN VOLTAGE (V) LT1676 G04 102 100 98 96 50 100 75 1.75 1.50 1.25 1.00 0 TEMPERATURE (°C) 25 50 75 100 1676 G07 1.2 CLAMP VOLTAGE BOOST THRESHOLD SWITCHING THRESHOLD 1.0 0.
PAGE 5
LT1676 U U U PIN FUNCTIONS SHDN (Pin 1): When pulled below the shutdown mode threshold, nominally 0.30V, this pin turns off the regulator and reduces VIN input current to a few tens of microamperes (shutdown mode). When this pin is held above the shutdown mode threshold, but below the lockout threshold, the part will be operational with the exception that output switching action will be inhibited (lockout mode).
PAGE 6
LT1676 W BLOCK DIAGRA VCC 2 R1 5 VIN 3 VSW RSENSE VBG SHDN 1 BIAS VB OSC SYNC 6 LOGIC Q3 I COMP SWDR Q4 SWDR SWON BOOST SWOFF Q2 Q1 D1 GND 4 SWON I BOOST COMP I I VC 8 FB AMP FB 7 gm VTH BOOST I SWOFF Q5 VBG 1676 BD U OPERATIO The LT1676 is a current mode switching regulator IC that has been optimized for high efficiency operation in high input voltage, low output voltage Buck topologies. The Block Diagram shows an overall view of the system.
PAGE 7
LT1676 U OPERATIO Please refer to the High dV/dt Mode Timing Diagram. A typical oscillator cycle is as follows: The logic section first generates an SWDR signal that powers up the current comparator and allows it time to settle. About 1µs later, the SWON signal is asserted and the BOOST signal is pulsed for a few hundred nanoseconds. After a short delay, the VSW pin slews rapidly to VIN.
PAGE 8
LT1676 U W U U APPLICATIONS INFORMATION Once the inductance value is decided, inductor peak current rating and resistance need to be considered. Here, the inductor peak current rating refers to the onset of saturation in the core material, although manufacturers sometimes specify a “peak current rating” which is derived from a worst-case combination of core saturation and self-heating effects.
PAGE 9
LT1676 U W U U APPLICATIONS INFORMATION Minimum Load Considerations As discussed previously, a lightly loaded LT1676 with VC pin control voltage below the boost threshold will operate in low dV/dt mode. This affords greater controllability at light loads, as minimum tON requirements are relaxed.
PAGE 10
LT1676 U W U U APPLICATIONS INFORMATION oscillator frequency during short-circuit conditions can then maintain control with the effective minimum ON time. A further potential problem with short-circuit operation might occur if the user were operating the part with its oscillator slaved to an external frequency source via the SYNC pin. However, the LT1676 has circuitry that automatically disables the sync function when the oscillator is slowed down due to abnormally low FB voltage.
PAGE 11
LT1676 U U W U APPLICATIONS INFORMATION frequency resonance problems, proper layout of the components connected to the IC is essential, especially the power path. B field (magnetic) radiation is minimized by keeping output diode, switch pin and intput bypass capacitor leads as short as possible. E field radiation is kept low by minimizing the length and area of all traces connected to the switch pin (VSW). A ground plane should always be used under the switcher circuitry to prevent interplane coupling.
PAGE 12
LT1676 U TYPICAL APPLICATIONS VIN 12V TO 48V 5 + C1 39µF 63V SHDN VCC VSW C5 100pF 6 2 3 L1 D1 MBRS1100 220µH LT1676 7 FB 8 VC SYNC 4 C2 100µF 10V R1 36.5k 1% 80 R2 12.1k 1% C4 100pF 60 50 40 1676 F04a C1: PANASONIC HFQ C2: AVX D CASE TPSD107M010R0080 C4, C5: X7R OR COG/NPO D1: MOTOROLA 100V, 1A, SMD SCHOTTKY L1: COILCRAFT DO3316P-224 90 70 C3 2200pF X7R R3 22k 5% GND + VOUT 5V 0mA to 500mA EFFICIENCY (%) 1 VIN FOR 3.3V VOUT VERSION: R1: 24.3K, R2: 14.
PAGE 13
LT1676 U TYPICAL APPLICATIONS Micropower Undervoltage Lockout Certain applications may require very low current drain when in undervoltage lockout mode. This can be accomplished with the addition of a few more external components. Figure 6 shows an LTC®1440 micropower comparator/reference added to control the LT1676 via its SHDN pin. The extremely low input bias current of the CMOS comparator allows the impedance of the resistor divider R4/R5 to be increased, thereby minimizing power drain.
PAGE 14
LT1676 U TYPICAL APPLICATIONS to maintain proper short-circuit protection. Transistors Q1, Q2 and resistor R7 form a high VIN, low quiescent current voltage regulator to power U2. Minimum Size Inductor Application Figure 4a employs power path parts that are capable of delivering the full rated output capability of the LT1676. Potential users with low output current requirements may be interested in substituting a physically smaller and less costly power inductor.
PAGE 15
LT1676 U TYPICAL APPLICATIONS VIN 5 6 + R7 10M VIN SYNC VCC C1 VSW 2 L1 3 Q1 PN2484 1 Q2 2N2369 D1 FB SHDN VC VOUT 5V + U1 LT1676 R1 39k C2 7 8 R2 10k GND C3 4 VIN NC V C1: PANASONIC HFQ 39µF AT 63V C2: AVX D CASE 100µF 10V TPSD107M010R0080 D1: MOTOROLA 100V, 1A, SMD SCHOTTKY MBRS1100 (T3) L1: COILCRAFT DO3316-224 5 4 U3 3 7S02 OUTA 1 2 R3 323k 1% R8 6.8M + INA+ INB– U2 LTC1442 REF OUTB HYST V– R5 22k R4 100k 1% R9 240k R6 2.4M 1676 F08 Figure 8.
PAGE 16
LT1676 U TYPICAL APPLICATION Minimum Inductor Size Application VIN 12V TO 48V 5 1 + VIN SHDN VCC C5 100pF VSW C1 12µF 63V 2 3 LT1676 6 L1 D1 220µH + 7 FB 8 VC SYNC C3 2200pF X7R R3 22k 5% GND 4 C2 100µF 10V C4 100pF R1 36.5k 1% VOUT 5V 0mA to 150mA R2 12.1k 1% 1676 TA02 C1: PANASONIC HFQ C2: AVX D CASE TPSD107M010R0080 C4, C5: X7R OR COG/NPO D1: MOTOROLA 100V, 1A, SMD SCHOTTKY MBRS1100 (T3) L1: COILCRAFT DO1608C-224 FOR 3.