LINEAR LT3653 User Guide and Manual

L DESIGN FEATURES

1.2A Monolithic Buck Regulator Shrinks Supply Size and Cost with Programmable Output Current Limit

by Tom Sheehan

Introduction Power supplies are often overqualified output limits the maximum power to automotive, industrial, distributed for their job. This is because power dissipation of both the supply and supply, and wall transformer applica- ICs often specify a current limit that the load, thus reducing the potential tions. is more than twice the rated output for localized heating. Monitoring and

current of the device. The power supply controlling the output current also Programmable components are sized to handle the makes for a robust solution, which is Output Current Limit

maximum current that the IC can de- able to withstand harsh overload and Monolithic switching regulators typi- liver, even if loads are unlikely to draw short circuit conditions. cally limit the peak switch current to that current during normal operation. The LT3653 and LT3663 are mono- protect the internal switch from being The components are bigger and more lithic step-down switching regulators damaged during an overload or short expensive than they need to be. that have an accurate output current circuit event. The maximum switch There is, however, an alternative: limit programmable from 400m A to peak current limit is typically more set an accurate maximum output cur- 1.2A. The LT3663 is a general pur- than two times the maximum output rent on the supply once the real world pose high voltage step-down regulator current rating of the part. While the load is known. Accurately setting the while the LT3653 is designed for use peak switch current limit prevents maximum output current reduces the with Linear Technology Bat-Track™ overstressing the IC, it does not keep required current rating of the regula- enabled battery chargers and power the entire application from overheat- tor’s power path components, thus management ICs (PMICs). The maxi- ing during an overload condition. For replacing big, expensive components mum input voltages of 30V (LT3653) example, a regulator with an output with smaller, less expensive ones. or 36V (LT3663) with 60V transient current rating of 1A is typically capable A limit on the regulator’s maximum ride through capability are well suited of providing over 2A at the output.

During an output overload condition,

BOOST L1 the power dissipation of the regula-

HIGH VOLTAGE INPUT 0.1µF

7.5V TO 30V 10V

tor could more than double, making

VIN SW

TRANSIENT C1

LT3653

TO 60V D1 thermal management more difficult.

4.7µF 50V The LT3653 and LT3663 reduce local-

ILIM ISENSE

R1 HVOKGNDVC VOUT ized hot spots by controlling the total

27.4k L2 SYSTEM 3.3µH C2 LOAD power dissipation of the application

WALL VBUS WALLVC SW 22µF

with a programmable, accurate cur-

ADAPTER C4 6.3V 10µF rent limit. 6.3V VOUT OVGATE LTC4098 Conservative design principles call

IDGATE for power path components that are

6.04k (OPTIONAL)

OVSENS

D0–D2 rated for worst-case currents. In the

TO µC BAT

TO µC CHRG

above example, where a 1A part is

NTC BATSENS

PROGCLPROG GND SINGLE-CELL capable of delivering 2A, the power

Li-Ion

path components must be sized for

0.1µF 3.01k 1k greater than 2A, because during an output short circuit or overload the inductor and diode can conduct up

L1 = TDK, VLCF5020T-4R7NIR7-1 L2 = COILCRAFT, LPS4018-332MLC to 2A. In contrast, the Power Path M1 = VISHAY, Si 2333DS

D1 = DIODES INC., DFLS240 components in LT3653 and LT3663 SEE THE LTC4098 DATASHEET FOR MORE INFORMATION applications are sized based on the

ON CONFIGURING THE NTC BATTERY TEMPERATURE

programmed maximum output cur-

QUALIFICATION OR REDUCED IDEAL DIODE IMPEDANCE.

rent limit. Therefore, an application

Figure 1. Charging a single cell Li-ion battery from either a USB input or high voltage input. This

with a 750m A output current limit

solution offers a seamless, highly efficient, low part count approach to dual input charging and

requires only 750m A rated compo-

Power Path™ control of a Li-ion battery-powered application. If additional integration is required for more system supplies, the LT3653 can be used in a similar fashion with the LTC3576 PMIC. nents. This allows for smaller, lower

Linear Technology Magazine • March 2009