highpowerZONE Products for the week of October 18, 2010

Maxim Integrated Products Says…

MAX5974: Current-Mode Spread-Spectrum PWM Controller
Topology Delivers >90% Efficiency

Maxim Integrated Products introduced the MAX5974, a high-frequency, current-mode PWM controller with an active-clamp architecture and spread-spectrum operation. The device's active-clamp topology delivers better than 90% efficiency, thus reducing power consumption in synchronous forward/flyback power supplies targeting IEEE® 802.3af/at power devices (PDs). The MAX5974A/MAX5974C versions are well suited for universal rectified offline (85V to 265V) or telecom (36V to 72V) input voltages. The MAX5974B/MAX5974D also accommodate input voltages as low as 10.5V (e.g., wall adapters). Targeted applications include PoE PDs such as IP phones, IP cameras, and wireless LAN access nodes. The MAX5974 is also well-suited for universal and telecom input ranges.

The MAX5974A/MAX5974B contain an internal sample-and-hold error amplifier that regulates the output voltage through a coupled inductor. This feedback configuration saves space and cost by eliminating the need for a shunt regulator, optocoupler, and tertiary winding for the bias circuit. Applications requiring optocoupler feedback can use the MAX5974C/MAX5974D, which have a continuously connected feedback input.

EN-Genius Says…

The MAX5974, in its four different configurations, is a PWM controller for a 25 W to 50 W actively clamped synchronous rectification continuous-conduction mode forward converter. The sizing of the main switch gate driver and that of the clamped auxiliary switch driver are optimized for 25 W PoE (802.3af/at) designs.

The A and C versions offer a bootstrap under-voltage wake-up lockout of 20 V with a 13 V hysteresis while the B and D versions have a UVLO rising threshold of 10 V. The A and C versions are ideal for rectified universal ac input supplies (from 65 Vac to 265 Vac) or telecom supplies at a nominal -48 Vdc (with a -36 Vdc to -72 Vdc spread). The low start-up voltage allows for the use of a small storage capacitor.

The B and D versions are ideal for use with 12 Vdc to 24 Vdc supplies such as those obtained from wall worts.

Feedback control differs between the parts with the A and B versions offering a unique sample-and-hold input while the C and D versions have a direct feedback input designed for use with a conventional optocoupler feedback loop. Both devices have a cycle-by-cycle current limit to turn off both drivers, and eight consecutive limits turns on a “hiccup” mode to protect external components by turning off the switching.

For EMI limiting the PWM frequency can be dithered by ±10% spreading switching harmonic frequencies. The center frequency can be externally programmed with a single resistor between 100 kHz and 600 kHz (±8%) but the parts can be externally synchronized up to 1.2 MHz. The main output will drive an external n-channel MOSFET and it can source/sink 650 mA/1000 mA while the auxiliary output will drive a p-channel MOSFET with the aid of a level shifter. Dead time protection between switching is programmable, as is PWM soft-start and current slope compensation, while the duty cycle can be programmed up to 80% of the limit. The error amplifier on-chip has the 1% accuracy of the references that are on-board.

With a +12 V input the current is a typical 1.8 V, which reduces to 100 µA in lock-out.

The innovative output regulation system avoiding a feedback optocoupler will be well received and the four versions cover the majority of supply possibilities for PoE-powered devices as well as portable/quasi-fixed applications such as wireless nodes and security cameras. The parts look to be very efficient and will do very well.

The MAX5974A, MAX5974B, MAX5974C, and MAX5974D are all in production in thermally-enhanced TQFN-16 with pricing from $1.07 in 1000-piece lots.

Data Sheet