A new chipset from STMicroelectronics lets users quickly build reliable and space-efficient powered devices to take advantage of the latest IEEE 802.3bt Power-over-Ethernet (PoE) specification. The PM8804 and PM8805 provide the PoE-converter circuitry for power devices up to class-8, which defines a usable power budget of 71W.
The chipset enhances reliability, saves space, and cuts time to market for next-generation connectivity equipment including 5G “small cells,” WLAN access points, switches, and routers. ST’s new PoE chipset also targets smart-building and smart-office applications including IP cameras, access control systems, lighting, display panels, curtain or shutter controllers, IP phones, video-call systems, and tabletop consoles.
The PM8804 implements a forward converter, including dual low-side gate drivers for high-efficiency forward active-clamp topologies, or a complete PWM controller for a 48V isolated flyback topologies.
Operating frequency is selectable up to 1MHz, allowing the use of small external filter and decoupling components for high power density. Also featuring a high-voltage start-up regulator with 20mA output capability, the PM8804 helps save on board space and bill of materials.
The PM8805 companion chip contains a hot-swap FET, two active bridges, a charge pump for driving high-side MOSFETs, and the IEEE 802.3bt compliant interface. Integrating the active bridges saves the real-estate otherwise occupied by eight discrete MOSFETs and their driving circuitry.
The PM8805 generates a power-good signal for enabling the PM8804 and other circuitry such as an LED driver and supports Maintain Power Signature (MPS) current control that allows the power device to enter power-saving standby without being disconnected.
Features of PM8804
- PWM peak current mode controller
- Input operating voltage up to 75V
- Internal high-voltage start-up regulator with 20mA current capability
- Current sense with programmable blanking time up to 200ns
- Programmable fixed frequency up to 1MHz
- Soft-start up with settable time
- Soft turn off (optionally disabled)
- Dual low-side complementary gate drivers
- Programmable dead time from 10ns to 200ns between GATE1 and GATE2
- Gate drivers have 1A peak capability
- GATE2 optionally turned off for reduced consumption
- 80% maximum duty cycle with internal slope compensation
- Internal pull-up resistor on CTL pin enables the optocoupler connection
- Operating current consumption <1.5mA
- Embedded protections:
- Severe overcurrent protection with latch after 4 events
- Delayed overload protection with automatic restart
- Feedback disconnection (OV) protection on VC
- Thermal shutdown
- High efficiency topologies:
- Flyback, forward
- Flyback with synchronous rectification
- Flyback with active clamp
- Forward with active clamp
Features of PM8805
- Dual active bridge, hot-swap MOSFET and PoE-Power-Device interface in a system-in-package
- 100V N-ch MOSFETs with 0.2Ω total path resistance for each active bridge.
- 100V, 0.1Ω high-side N-ch hot swap MOSFET
- PoE-PD single-signature interface compliant with IEEE 802.3bt / at / af
- Supports 4-pair PoE applications
- Supports 12V auxiliary sources
- Identifies which kind of PSE (standard or legacy) is connected with and provides successful IEEE802.3bt / at / af classification indication as a combination of the T0, T1 and T2 signals (open drain)
- Smart operational modes
- Programmable classification current with
- 3.3ms delay.
- Optional Autoclass feature
- Advanced energy-saving MPS timings
- Two-step hot swap current protection: DC with
- 1ms delay and short-circuit with 10μs delay.
- Controlled pre-charge of the output capacitor
- PGD signal (open drain) to enable an external
- PWM controller
- Thermal shutdown protection
Both devices are now in production. The PM8805 is in an 8mm x 8mm thermally enhanced VFQFPN-43 package featuring exposed pads and is priced at $4.50. The PM8804 is in a 3mm x 3mm, 0.5mm-pitch VFQFPN-16, and priced at $0.60 for orders of 1000 pieces.