Electronic Design

Tiny 10-A POL Module Needs Less Bulk Capacitance

A step-down regulator that can hide behind boards enters the point-of-load battleground.

THERE'S A NEW ENTRY IN THE NON-ISOLATED point-of-load (niPOL) sweepstakes. Linear Technology's 10-A (14-A peak) step-down regulator module incorporates everything except input and output capacitors in a lowprofile (2.8 mm), 15- by 15-mm surfacemount land-grid array package.

The module consists of a current-mode controller, ultra-low RDS(ON) FETs with fast switching speed and integrated Schottky diodes, and an inductor. Thanks to its low package profile, it can be mounted on the bottom of a circuit board, saving room on the top surface. Also, its 104 solder pads provide a large enough heat-conduction path to the circuit board, eschewing additional heatsinks and cooling airflow. The base-model LTM4600EV converter operates over an input voltage range of 4.5 to 20 V and supports an output voltage range of 0.6 to 5 V, set by a single resistor. The high-voltage LTM4600HVEV features an operating voltage range from 4.5 to 28 V. Typical switching frequency is 800 kHz at full load. Fault-protection features include integrated overvoltage and short-circuit protection with a defeatable shutdown timer. A built-in soft-start timer is adjustable via a small capacitor.

MODEST OUTPUT-CAPACITANCE REQUIREMENTS Linear showed me the LTM4600 and emphasized how simple it was to design-in, given its all-in-one nature. But when I looked at the design data, I asked about an obvious absence.

Most of the bulk-capacitance typical step-down niPOLs needed to support large, fast current swings in the load appeared to have been left out (see the figure). My question led to a detailed description of the design of the part.

(To be fair, Linear's marketing message was on target. Fewer big tantalum capacitors around the converter's output make it simpler for project engineers to pack the required power into a smaller amount of board space. But what makes this possible?)

Looking at what the data sheet has to say about output capacitance, I learned that the internally optimized loop compensation provides sufficient stability margin for all ceramic capacitor applications. The LTM4600 exhibits low output voltage ripple, allowing a smaller value for COUT, as long as it exhibits low enough effective series resistance (ESR) to meet the output voltage ripple and transient requirements.

COUT can be a low-ESR tantalum capacitor, a low-ESR polymer-capacitor, a ceramic capacitor, or some combination. If all ceramic output capacitors are used, the typical capacitance is 200 µF. The system designer may require additional output filtering.

Circuit designers sometimes need to further reduce output ripple or dynamic transient spike. If so, a table in the data sheet contains an output capacitance matrix that enables designers to relate input and output voltages to the design's required performance in terms of output voltage droop, peakto-peak deviation, and recovery time during a 5-A/µs transient.

Regardless of those conditions, the table values for output capacitance based on ceramic capacitors ranging from 66 to 400 µF with parallel capacitance are achieved with electrolytics ranging from 470 µF to "none." (The value of the ceramics rises as the value of the electrolytics goes down, so the maximum capacitance on the output is never higher than 570 µF.)

MORE DESIGN DETAILS Current-mode control provides cycle-by-cycle fast current limit. Foldback current limiting is provided in an over-current condition while VFB drops.

Transient measurements in the lab show the control loop responding with no clock latency. Linear's engineers say this is unique to the valley-model control architecture of the module. The output voltage typically turns around in 4 to 6 µs, and it recovers fully in 20 to 25 µs.

Pricing for the LTM4600EV starts at $16.50 each and the LTM4600HVEV starts at $19.50 each in 1000-piece quantities. Both are tested and guaranteed to operate from -40°C to 85°C. The device is sampling, and the company is accepting volume orders this quarter.

Linear Technology
www.linear.com

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