powerZONE Products for the week of August 11, 2003
National Semiconductor Says . . .
LM5007: 80-V Buck Regulator Provides Non-isolated
Bias Power Supply Design
Consolidating its leadership in high-voltage power management circuits, National Semiconductor Corporation announced the industry's smallest high-voltage Buck bias switching regulator to satisfy housekeeping or bias power needs in next-generation communication, automotive, -48 V distributed and battery-powered systems.
"Our new solution will displace a more expensive, less efficient transformer coupled flyback regulator that is currently being used by many designers to produce bias power," said Paul Greenland, Marketing Director of National Semiconductor's power management group. "National Semiconductor's LM5007 effectively eliminates cost and increases efficiency, adopting a simple inductor filter and a couple of external components. Customers will benefit because the LM5007 offers more than 90 percent efficiency at a fraction of the cost and space consumed by today's isolated bias supply solutions."
National's LM5007 is a Buck bias regulator which steps down a high-voltage
(up to 75 V) primary-side power supply and produces a low voltage (10 V
typical) bias supply for secondary-side control devices. Fully integrated
with all functions needed for a high-voltage power supply, the device contains
an 80 V N-Channel power MOSFET rated at 0.7 A peak that can be switched
at high frequencies (up to 500 kHz), allowing the use of a small output
filter to complete a highly efficient cost-effective bias supply solution
that sources up to 0.5 A continuous load current in minimal printed circuit
board (PCB) space
analogZONE Says...
This is just the kind of product direction that we had hoped to see National taking with its recently-announced high-voltage process. The markets that exist -- or are forthcoming -- for higher voltages aren't exactly in the peak of health at the moment, with the 42-V automotive system conversion seemingly being pushed further and further out, and the -48 V telecom market still being hammered from new installations, but the design work going on today will be reflected in excellent sales later on. Buck converters are particularly required in these arenas to power up the lower voltage circuitry and sub-systems.
To get 10 V, for example, you could just throw a 10-V regulator into the circuit but the efficiency would be lousy. The basic content of the LM5007 is, however, a nominal 7-V regulator that provides the rail voltage for the rest of the IC to run off. Because its load varies little with the main load the lack of efficiency is almost irrelevant with a non-switching operating current of typically 500 µA. The regulator is a high-voltage start-up device with the internal current being limited to 10 mA on the application of up to 75 V on the input. The enabled regulator sources current into an external capacitor connected to the Vcc pin and when Vcc reaches 7 V the controller is enabled. An external voltage greater than 7 V, if available, could be connected to Vcc which would shut off the regulator and increase the efficiency.
With the circuit set for a 10-V output the peak efficiency occurs in the range from 100 mA to 500 mA loading with a higher better peak at about 300 mA load. At that point the efficiency with a 12-V input is about 93% which reduces down to a still very respectable 83% with a 75-V input.
The uniqueness of the LM5007 is that it uses a fixed ON time control system where the ON time is inversely varied with the level of the supply voltage (an external resistor sets the ON time ratios connected to Vin.) With 12-V input, for example, the ON time is about 3.0 µs, while at 75 V the ON time is about 350 ns. This gives a fairly constant operating frequency of about 350 kHz (500 kHz max.) The n-channel buck switch drives an LC output filter, giving a low output ripple, and a potential divider -- with ratio set for the desired output voltage -- sets up a feedback voltage to the part which is compared with the internal 2.5-V reference. The output is also bootstrapped with an external capacitor and an internal high-voltage diode.
The typical current limit threshold is about 725 mA provided by a forced OFF time (which is set with a non-resettable OFF timer, with a minimum of 300 ns) inversely proportional to the output voltage -- and over- and under-voltage and thermal protections are also provided. There was no specification for the internal reference in the preliminary data sheet beyond "precision."
The LM5007 is sampling now, with production very close, in MSOP-8 and LLP-8 priced at $1.05 in 1000-piece lots. The data sheet is delayed until about August 25, 2003 when it should be available on line.