Power Electronics

IC-Style Modules Shrink Silicon Footprint for Buck Regulators

International Rectifier’s SupIRBuck family of point-of-load regulators (POLs) co-package controllers and MOSFETs in compact QFN packages, reducing the silicon footprint required to implement single-output, synchronous buck converters. According to the company, these 4-A to 14-A rated devices, also provide higher efficiency than existing monolithic, buck regulators. However, direct comparisons are difficult because of the unusually wide input-voltage range (2.5 V to 21 V) of the SupIRBuck devices.

Targeting high-density, high-performance data center and consumer applications, the IR38xx SupIRBuck family comprises nine devices with varying current ratings, and a two-fold choice of switching frequencies and power management features (see Table 1). But despite the different options, a common footprint enables system designers to migrate from one current-rating to another without changing the pc-board layout.

“Because SupIRBuck has a unique, scalable common footprint, designers can simply ‘cut and paste’ their design to significantly reduce risk and enable faster-time-to market,” says Tim Phillips, vice president of International Rectifier’s (IR’s) Enterprise Power Business Unit. “The addition of the SupIRBuck POL family to IR’s industry-leading XPhase and DirectFET multiphase architecture allows us to provide the most efficient and complete power solution for the data center including enterprise servers,” he adds.
Designed for 4 A, 7 A and 12 A of output load current at a 600-kHz switching frequency, the IR38xx operates from an input voltage range of 2.5 V to 21 V, and generates output voltage as low as 0.6 V. Note that the full 2.5-V to 21-V input range is only valid if a second VCC bias voltage is applied. That bias voltage must be in the 4.5-V to 14-V range. However, IR notes that in most of the target applications a 5-V rail already exists and can be used for the bias voltage. But in those cases where only a single input supply rail is available, the input voltage range is restricted to 4.5 V to 14 V.

Common features include pre-bias start up, fixed 600-kHz switching frequency, hiccup current limit, thermal shutdown, and precise output voltage regulation. Optional features include tracking, programmable power good, and a 300-kHz switching frequency to provide an additional 2 A of output current capability. The SupIRBuck family’s thermally enhanced package with a 0.9-mm profile allows mounting on the backside of the motherboard, making the devices ideally suited for space constrained, high-density server applications.

Table 1. Key specs for International Rectifier’s SupIRBuck family of point-of-load regulators


Part Number

VIN Max/Min
(V)

VOUT Max/Min
(V)

Max Current (A)

FSW
(KHz)

Features

IR3812MPBF

21/2.5

12/0.6

4

600

OCP + OTP + TRACKING

IR3822MPBF

21/2.5

12/0.6

4

600

OCP + OTP + PGOOD

IR3822AMPBF

21/2.5

12/0.6

6

300

OCP + OTP + PGOOD

IR3811MPBF

21/2.5

12/0.6

7

600

OCP + OTP + TRACKING

IR3821MPBF

21/2.5

12/0.6

7

600

OCP + OTP + PGOOD

IR3821AMPBF

21/2.5

12/0.6

9

300

OCP + OTP + PGOOD

IR3810MPBF

21/2.5

12/0.6

12

600

OCP + OTP + TRACKING

IR3820MPBF

21/2.5

12/0.6

12

600

OCP + OTP + PGOOD

IR3820AMPBF

21/2.5

12/0.6

14

300

OCP + OTP + PGOOD

Notes: All devices are offered in 5-mm x 6-mm QFN packages. OCP= overcurrent protection, OTP = overtemperature protection, and PGOOD = power good status pin.

The IR38xx SupIRBuck family of point-of-load (POL) voltage regulators integrates IR’s HEXFET MOSFETs and a synchronous buck control IC in a 5-mm by 6-mm power QFN package. Although the company claims this device reduces the silicon footprint by up to 70% compared to discrete solutions, that comparison assumes the use of a 3-mm x 3-mm MLPD-housed controller plus two S0-8 packages MOSFETs. Also, this discrete approach is assumed to occupy 100 mm2 of board area versus 30 mm2 for the SupIRbuck device. The company notes that if a discrete solution employs a dual S08, the reduction in board area is 35%.

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 VIN, VOUT, VCC, 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.

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