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REVIEW: Power management chip throttles USB source

Switch Mode USB Power Manager & Triple Buck Regulator Provides Faster Charge Time & Minimizes Heat

MILPITAS, CA—Linear Technology Corporation announces the LTC3555, the first in a family of next generation multi-function power management solutions for Li-Ion/Polymer battery applications. The LTC3555 integrates a switching PowerPath manager, a standalone battery charger, an ideal diode, I2C control, three high efficiency synchronous buck regulators plus an always-on LDO, all in a compact, low-profile 4mm x 5mm QFN package. The LTC3555's PowerPath control feature seamlessly manages power flow between an AC wall adapter or USB port, Li-Ion battery and system load while its “instant-ON” operation ensures system load power even with a dead or missing battery. For fast charging, the LTC3555's switching input stage converts nearly all of the 2.5W available from the USB port to charging current, enabling up to 700mA from a 500mA limited USB supply and up to 1.5A when wall powered. An internal 180-milliOhm ideal diode plus optional external ideal diode controller provide a low loss power path, further minimizing heat generation and maximizing efficiency.

The LTC3555's three integrated synchronous buck regulators feature 100% duty cycle operation and are capable of delivering output currents of 1A/400mA/400mA, respectively, with adjustable output voltages down to 0.8V. The internal low RDS(ON) switches enable efficiency as high as 94%, maximizing battery run time. In addition, Burst Mode operation optimizes efficiency at light loads with a quiescent current of only 35uA per regulator (<1uA in shutdown). The high 2.25MHz switching frequency allows the use of tiny low cost capacitors and inductors less than 1mm in height. Furthermore, the regulators are stable with ceramic output capacitors, achieving very low output voltage ripple.

The LTC3555 features USB-compatible programmable current limiting to 100mA/500mA/1A, while its Bat-Track adaptive output control enables high efficiency charging and reduces power dissipation. Standalone autonomous operation simplifies design, eliminating the need for an external microprocessor for charge termination. To preserve battery energy, the LTC3555 draws <23uA from the battery in suspend mode. The charger is compatible with inputs up to 5.5V (7V absolute maximum transient for added robustness). The LTC3555 is available from stock in a compact, low-profile (0.75mm) 4mm x 5mm QFN-28 package. Pricing starts at $4.65 each for 1,000-piece quantities.

Summary of Features: LTC3555

* Complete Multi-Function PMIC: Switching Power Manager, Li-Ion/Polymer Battery Charger, Three Buck Regulators & LDO

* Thermally Enhanced, Low Profile (0.75mm) 28-Lead 4mm x 5mm QFN Package

POWER MANAGER & BATTERY CHARGER

* High Efficiency Switching PowerPath Controller with Bat-Track Adaptive Output Control

* Programmable Input Current Limit (100mA/500mA/1A)

* Maximum Charge Current Programmable up to 1.5A from Wall Adapter

* 180mOhm Internal Ideal Diode Plus Optional External Ideal Diode Controller Provides Low Loss

Power Path from Battery to Load

DC/DCs

* Three High Efficiency 2.25 MHz Synchronous Buck Regulators: 1A, 400mA, 400mA IOUT

* Adjustable Output Voltage Range: 0.8V to VBAT

* Burst Mode Operation with Low IQ: 35uA per Regulator

* Always-On 25mA/3.3V LDO

This QFN -packaged instant-on power management IC from Linear Technology Corporation proves LTC is one of those semiconductor companies that never sleeps. Its LTC3555 next-gen multi-function chip looks like it would be just the ticket for high-end nomadic systems that use the latest in lithium-ion/polymer battery chemistries.

Most high-end portables are now constrained by limited board space, and battery capacities must increase due to more system functions. Of course, the number of system rails also continues to expand as systems run more processors and advanced memories, as well as FPGAs, and DSP cores.

Lots of I/O, dedicated media processors, and RF chips also abound, each with their own power supply requirements. The performance needs for such portables include high efficiency power sources, often running at low output-voltages. These systems usually demand medium to high output-current from buck converters.

The circuits for these DC-to-DC converters ideally would accommodate fast charge-rates, small footprints and Z-heights, and a limited number of external discrete components. Add to the mix the nicety of charging a battery from a system's USB (Universal Serial Bus) port. That would deliver 5-V at up to 500-mA.

Not Clunky Anymore

If you were to poll most designers crafting portable electronics, I think you'd find that the perception is that power management ICs are big, clunky, and low on performance—but relatively cheap. LTC's multi-function LTC3555 changes that picture.


Click to view simplified block diagram

If your circuit operate at less that 5.5-V input, and packs multiple system rails, and needs fast charging, this device may fill the bill. The LTC3555 comprises a highly integrated power management system plus battery charging electronics for USB systems. And, the device can be controlled via an I2 C bus, or simple I/O.

LTC's PowerPath current-limited switching manager (don't you just love that highly descriptive name?) referred to in its press release (on the left), includes automatic load prioritization. The silicon also packs those three general purpose synchronous step-down regulators mentioned in the LTC release.

Unlike linear controllers, the LTC3555's switching input stage can transmit nearly all of the 2.5-W that's available from a USB port to a system's load. Very little power is dissipated in the IC. This alone can ease your worries about thermal budgeting in tight enclosures.

LTC's PowerPath circuitry uses a precision average input-current step-down switcher to optimize USB power. In operation, the LTC3555 actually permits the load current on its VOUT pin to exceed the current drawn by the USB port—without exceeding the USB load specs. The PowerPath switching regulator and battery charger communicate to ensure that input current never violates what USB can deliver. Nifty.

The press release also mentions the so-called ideal diode. This diode, routed from the chip's BAT to VOUT pins ensures that ample power is always available to VOUT , even if there is insufficient power, or no power, at the VBUS pin. The ideal diode is “ideal” because it exhibits about 180-milliohms of forward resistance. This ensures a low-loss power path, minimizing dissipations.

An optional external P-channel MOSFET can also be used to supplement the ideal diode. The external ideal diode would operate in parallel with the internal ideal diode.

Always On LDO

In addition to that low-loss architecture, the always-on LDO (low dropout) regulator provides regulated 3.3-V from available power at VOUT . Drawing low quiescent current, the LDO is on at all times, and can be used to supply up to 25-mA to external circuits.

As mentioned, the IC's three switching regulators can be independently enabled under digital control or using the device's I2 C serial port. Using I2 C , two of the regulators give you adjustable set-points. That lets voltages be reduced when high processor performance isn't needed.

Along with a constant-frequency PWM (pulse width modulation) mode, all three switching regulators also have a low-power burst-only mode setting, as well as automatic burst-mode operation, and LDO modes. These modes contribute to low quiescent current, especially under light loading conditions.

The PowerPath Controller

Whenever VBUS is available and the PowerPath switching regulator is enabled, power is delivered from VBUS to VOUT to drive the combination of the external load (switching regulators 1, 2 and 3) and the battery charger. If the combined load doesn't exceed the PowerPath switching regulator's programmed input-current limit, the output voltage will track 300-mV above the battery level.

By keeping the voltage across the battery charger low, the IC delivers peak efficiency; power lost to the linear battery charger is minimized, and power available to your external load is optimized.

If the combined load at VOUT is large enough to cause the switching power supply to reach the programmed input-current limit, the battery charger will also reduce its charge current by the amount necessary to enable the external load to be satisfied.

As indicated above, even if the battery charge-current is set to exceed the allowable USB current, the USB spec won't be violated. The switching regulator will simply limit the average input-current. The load current at the output will always be prioritized, and only excess available power will be used to charge your system's battery.

If the voltage at the battery terminal is below 3.3-V, or the battery isn't present at all, and the load requirement doesn't cause the switching regulator to exceed the USB spec, the output will be regulated at 3.6-V. If the load exceeds the available power, the output will drop to a voltage between 3.6-V and the battery voltage. finally, if there's no battery present when the load exceeds the available USB power, the output drops toward ground.

Constant-Frequency Switching Regulator

The power is controlled by a 2.25-MHz constant-frequency step-down switching regulator. To meet the USB maximum load spec, the regulator includes a control loop that ensures that the average input-current is below the programmed level. The LTC3555 also includes temperature protection. Battery temperature can be measured using a thermistor mounted close to the battery pack.

The LTC3555 also provides temperature-qualified charging if a grounded thermistor and a bias resistor are connected. By using a bias resistor whose value is equal to the room temperature resistance of the thermistor, upper and lower temperatures are pre-programmed to approximately 40-degreesC and 0degreesC. These temperature thresholds can be adjusted by either choosing a bias resistor value or by adding a second adjustment resistor to the circuit.

Click here for a datasheet (in Adobe Acrobat .PDF format).

For further info, contact Linear Technology Corp., 1630 McCarthy Blvd., Milpitas, Calif. 95035-7417. Phone: 408-432-1900. Fax: 408-434-0507.

Linear Technology , 408-432-1900, www.linear.com

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