2 July 2012

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One of their more recent additions to their family of low-power converters is the LTC3630 65V 500mA synchronous step-down buck converter.

The LTC3630 can operate with an input voltage range of 4 V to 65 V, and can deliver up to 500 mA of continuous output current. The LTC3630 converter draws only 12µA quiescent current and is about 90% efficient over most of its supply range with a 12 V input.

Very few external components are needed, and external components can be optimized for smaller size because of the programmable peak current limiting features.

The IC comes in a 16 pin 3mm × 5mm DFN or a thermally enhanced MSE16 package with a 0.75mm profile. With such a compact footprint, the device is suitable for portable instruments and small battery-operated devices. Many potential applications for the LTC3630 would be found in the industrial, medical, computing, automotive, and high-end consumer markets.

The LTC3630’s internal synchronous rectification topology and its programmable peak current limit provide a simple method to optimize efficiency while minimizing output ripple and external component size. The LTC3630 utilizes internal high-side and low-side power MOSFETs to act as synchronous power switches. This maintains efficiency, as it draws only about 12μA in its Burst Mode operation while maintaining a regulated output voltage at no load.

In the Burst Mode operation, the inductor current switches directions through the SW pin by the internal high-side and low-side power MOSFETs. The short “burst” cycles are followed by a sleep cycle where both MOSFETs are off, and the load current is supplied by the output capacitor, while the LTC3630 is drawing only 12μA of supply current.

Efficiency is improved at light loads, when the burst cycle period is a small percentage of the total cycle time (lower duty cycle).

The voltage on the VFB pin determines when the sleep mode is activated after a certain voltage is reached. As the output capacitor is discharged in the sleep mode, the, the voltage on the VFB pin decreases. As it goes 5mV below the 800mV reference, the burst cycles are enabled again through a comparator.

At the beginning of the burst cycle, the high side power MOSFET is turned on and the inductor current begins to ramp up. The inductor current increases until either its current exceeds the peak current threshold or the voltage on the VFB pin goes above 800mV. At this time, the high side MOSFET is turned off and the low side power MOSFET is turned on. The inductor current ramps down until the current drops down close to zero, at which point, the high-side MOSFET is turned on again (assuming that voltage on the VFB pin is less than the 800mV reference). This sequence cycles through many times until the sleep mode is activated.

The switching frequency and the number of switching cycles in the “burst” phase is variable, and is a function of the input voltage, output voltage, inductor value, peak current and load current. This architecture offers inherent safety features, such as short-circuit protection.

The VPRG1 and VPRG2 input pins are used to enable fixed outputs of 1.8V, 3.3V or 5V by direct connection (no extra components). When VPRG1 and VPRG2 are tied to ground, the LTC3630’s output voltage can be adjusted by an external resistor divider on the VFB pin to voltages from 0.8V to VIN.

The peak output current is programmed by a resistor tied from ground to the ISET pin. If it is left floating, the peak current is about 1.2 A, and the continuous output current is about 500ma. If ISET is tied to ground, the output current is 50ma (peak current of 0.12 A).

The RUN pin is use to put the device in a shutdown mode, where all internal circuitry is disabled, reducing the DC supply current to 5μA. The FBO pin is used to combine the output current of multiple LTC3630s.The FBO pin of a “master” LTC3630 is connected to the VFB pin of one or more “slave” LTC3630s, t produce combined output currents of much more than 500mA.

The LTC3630 offers a precise run threshold and soft-start feature to ensure that system start-up is well controlled in any environment.

An internal 0.8ms soft-start function limits the ramp rate of the output voltage when powered up to prevent voltage spike on the input supply. A capacitor can be placed from the SS pin to ground for a longer ramp rate. Due to the inherent stability of the LTC3630, no external compensation is required, thereby minimizing the solution footprint.

The device also features Dropout Operation, where a 100% duty cycle is reached as the input voltage decreases toward the output voltage. At his point the high-side power MOSFET is on continuously, and provides output current equal to the peak current, which can be greater than 1A.

As an added safeguard, the LTC3630 features over-temperature shutdown and under-voltage lockout protection, which is set by a resistive divider on the RUN pin.

In summary, the LTC3630’s unique combination of Burst Mode operation, integrated power switches, low quiescent current, and programmable peak current limit provide high efficiency over a wide range of load currents and input voltages.