Out-Of-Phase Buck Controllers Trim Filtering Requirements For 3-In-1 Chips

By running their dual buck-converter pulse-width modulators (PWMs) 180° out of phase, the ISL6441 and ISL6443 high-performance, triple-output controllers from Intersil reduce input-current ripple. This in turn reduces noise induced by the power supply and allows designers to use smaller input capacitors.

Dual PWMs typically operate in-phase and turn on both upper FETs at the same time. The input capacitor then has to support the instantaneous current requirements of both controllers simultaneously, which forces designers to put more low-ESR capacitors in parallel.

With dual synchronized out-of-phase operation, the instantaneous input current peaks of the regulators do not overlap. Designers can use fewer or less expensive capacitors while reducing the shielding requirements for EMI.

The ISL6441 and ISL6443 provide two synchronous buck-regulated voltage outputs and a linear controller output. The buck PWM controllers use a free-running frequency of either 300 kHz or 1.4 MHz. A current-mode control scheme with an input voltage feed-forward ramp input to the modulator provides simplified loop compensations and excellent rejection of input voltage variations.

The linear regulator controller is a transconductance amplifier with a nominal gain of 2 A/V. The n-channel MOSFET output device can sink a minimum of 50 mA. The reference voltage is 0.8 V. With zero volts differential at its input, the controller sinks 21 mA of current. An external pnp transistor or PFET pass element can be used.

The ISL6443 operates at a lower switching frequency than the ISL6441, 300 kHz versus 1.4 MHz. The higher frequency suits ADSL modems and applications that demand the smallest passive components. The lower frequency is appropriate for satellite and cable set-top boxes, home media servers, digital TVs, and networking infrastructure applications.

Each output has an internal 1%-accurate reference specified from ­40°C to 85°C for regulating any voltage down to a minimum 0.8 V. Both of the ICs also include an integrated phase-lock-loop device that permits bidirectional frequency synchronization of multiple ISL6441s or ISL6443s.

The ISL6441 and ISL6443 are available now in a standard lead-finish or lead-free 28-lead QFN package. Unit pricing in 10,000 unit quantities is $1.60.