intersil – ISL8541x Family: New Devices for High Performance DC/DC Conversion with Loads Up to 1A

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Intersil’s ISL8541x family of buck regulators, consisting of the ISL85410, ISL85415 and ISL85418, offers the performance and features required by modern electronic devices; the devices are efficient, small and reliable, and operate across an unusually wide input voltage range.

The ICs integrate PWM control, two power MOSFET, a compensation network, and a protection and monitoring circuit into a tiny 4 x 3mm DFN package, enabling power system designers to implement efficient, compact and cost effective power conversion.

Using a proprietary current mode control scheme, the ICs can precisely regulate the output voltage at input voltages ranging from 3V to 40V, and for a load current up to 1A (ISL85410), 800mA (ISL85418) or 500mA (ISL85415). With output voltage adjustable from 0.6V to 96% of the input voltage, and switching frequency adjustable from 300kHz to 2MHz, the ISL8541x family provides the flexibility to design optimal power solutions.

Applications in which power losses must be kept to a minimum are also well served, with efficiency up to 96% and a quiescent current of just 80μA.

Typical Applications
Figure 1 shows the typical applications of the ISL85410/15/18 devices when used to convert a power source with a wide voltage range to various low output voltages to meet different load conditions with a current of up to 1A.

Figure 1: Typical uses for the ISL8541x family

Figure 1: Typical uses for the ISL8541x family

Highly Efficient Operation at Light-Load
The ISL8541x power management ICs combine a synchronous buck PWM controller with integrated power switches. With compensation circuit and other control functions integrated inside the IC, only a few external components are needed for an efficient DC-DC power supply (see Figure 2).

Figure 2: Typical application circuit using internal compensation

Figure 2: Typical application circuit using internal compensation

To achieve high efficiency under heavy load conditions, the internal high side and low side N-channel MOSFETs of the ISL8541x feature ultra low resistance and low gate charge. In addition, the On/Off control of the switching FETs is optimized, with very little dead time, helping to keep power losses to a minimum.

When supplying light-loads, the converter’s efficiency can be improved by enabling variable frequency operation, in Pulse Frequency Modulation (PFM) mode. Connecting the Sync pin to ground allows the controller to choose such operation automatically when the load current is low; the IC enters the discontinuous current mode of operation when eight consecutive cycles of inductor current crossing zero are detected (see Figure 3).

Figure 3: Transition from PFM to PWM for higher efficiency when supplying light-loads

Figure 3: Transition from PFM to PWM for higher efficiency when supplying light-loads

This means that a DC/DC converter based on an ISL8541x device can achieve superior efficiency over the application’s entire range of operating conditions. Figure 4 shows the efficiency of a voltage regulator based on the ISL85410, with an output voltage of 3.3V over a variety of load and input voltage conditions.

Figure 4: Efficiency at light- and heavy loads

Figure 4: Efficiency at light- and heavy loads

Additional Features for Superior Operation
All three ISL8541x devices also offer various features to help the designer of a DC/DC converter.

Adjustable soft start: to avoid large in-rush currents, the output voltage is slowly increased at start-up to its final regulated value.

Power Good: to indicate the operating condition of the switching regulator, a Power Good (PG) pin is implemented with an open drain output and an internal pull-up resistor.

Adjustable operating frequency: a programmable frequency allows the user to find the best balance of efficiency and external component size. It also provides for low frequency operation at low output voltages when the minimum on-time would otherwise limit operation. The default switching frequency is 500kHz, but the frequency can be programmed in a range between 300kHz and 2MHz.

Synchronization control: to minimize input voltage ripple and to reduce EMI, frequency synchronization is often required in applications with multiple DC/DC converters. The switching frequency of the ISL8541X product family can be synchronized to up to 2MHz by an external signal applied to the Sync pin. This also allows two regulators to be synched together.

Option to use external compensation: the internal compensation parameters are carefully selected to produce stable and dynamic performance with a wide range of output inductor and capacitor combinations. If more improvement in transient performance is needed, an external compensation circuit can be implemented via the Comp pin.

Conclusion
In summary, the ISL8541x regulators provide the designer of DC/DC converters with high efficiency, a rich feature set and the flexibility to optimize the circuit for the needs of the application. These ICs have similar pinout thus they are interchangeable on the system board. In addition to these, two more ICs are added to this family, ISL85412 and ISL85413, that however have lower max nominal output current of 150mA and 300mA respectively, they feature lower quiescent current (50uA), a smaller package (8 lead 3 x 3 TDFN) and they use only default switching frequency. Users who wish to experiment with the devices can do so conveniently with the ISL85410DEMO1Z demonstration board (see Figure 5).

Figure 5: Schematic and image of ISL8541x demo board

Figure 5: Schematic and image of ISL8541x demo board

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