IC-Style Modules Shrink Silicon Footprint for Buck Regulators

Dec 12, 2007 4:12 PM
by David Morrison, Editor, Power Electronics Technology

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Other nonmonolithic, IC-style modules also provide opportunities for comparisons. One such device is Linear Technology’s LTC3610 which has a similar current rating to the IR3820. These devices differ in a number of performance specifications, which suggest some of the design tradeoffs that may be required when shrinking POL footprints (see Table 2). Then too, the overall board space required for the complete regulator will depend on the requirements for external inductor and capacitors.

Thermal management becomes an important issue as regulator size is reduced. According to IR, the SupIRBuck’s junction-to-PCB thermal impedance ranges from 2ºC/W to 3ºC/W, depending on the device current rating. Junction to ambient is said to be around 35ºC/W, but the company views this number as less important than the prior parameter because the main heat transfer mechanism is from junction to the pc board. The company does not expect that customers will heatsink the medium- to low-power POL solutions.

The vendor indicates that the current rating given in the datasheet is deemed realistic in typical data center applications and can be achieved with either no airflow or with less than 200 LFM of airflow. Device temperature, which ultimately limits how much current can be delivered, will depend on numerous factors. These include V_IN, V_OUT, V_CC, number of PCB layers, PCB copper thickness, ambient temperature, air flow, and position of the device (is it on the top or bottom of the board?), selection of the inductor, and PCB layout.

The company offers what it describes as “very realistic” demo boards—there are two different types for each device. In addition, IR is providing customized thermal simulation to determine expected thermal performance.

Table 2. SupIRBuck Versus An Existing High-Current Buck Converter

Parameter	Performance
Part number	IR3820	LTC3610
Output current	12 A	12 A
VIN range	2.5-V to 21-V conversion input; 4.5-V to 14-V bias supply (see text)	4 V to 24 V (28 V Max)
VOUT range	0.6 V to 12 V	0.6 V to 0.9 x VIN
Switching Frequency	600 kHz	Up to 2 MHz (min. on-time <100 ns)
Thermal impedance	Theta (junction to PCB) = 2ºC/W to 3ºC/W depending on device current rating Theta (junction-to- ambient) = approx. 35 ºC/W	Theta (junction to PCB) = 0.24ºC/W Theta (junction-to-ambient) = 28ºC/W
Efficiency	Approx. 92% for VIN = 12 V, VOUT = 3.3 V, IOUT = 10 A	92% for VIN =12 V, VOUT =3.3 V, IOUT = 10 A
IQ	Ranges from 20 mA at 5-V VCC to 40 mA at 12-V VCC	900 µA typ. in normal operation 15 µA typ. in shutdown
Protection features	Programmable overcurrent protection, hiccup current limit, programmable soft start with enable, overtemperature protection, pre-bias protection, power good	Programmable current limiting, programmable softstart, output overvoltage protection, optional short-circuit shutdown timer, power good
Package	5-mm x 6-mm QFN	9-mm x 9-mm QFN-64
Device cost	Unit pricing for the SupIRBuck family starts at $2.25 each in lots of 10,000	Unit pricing in 1,000-piece quantities is $8.50/each

As this article goes to press, limited data on the IR38xx is available on the International Rectifier website (www.irf.com). But according to the vendor, the parts are fully characterized and the company has been supporting its alpha customers since April. Furthermore, IR reports that data sheets are being shared with qualified customers only at this time. Those interested in receiving full data sheets, demo boards, and applications support including thermal analysis are encouraged to follow the instruction on the IR website.

The full production release for the majority of devices that have already been designed-in at a lead server customer is expected by the end of this month. Production orders are available immediately. Pricing for the IR38xx devices begins at $2.25 each in 10,000-unit quantities.