US65 / US66

Two-Coil Low Noise Fan Driver

High Output Current

3901090209

Page 6 of 12

Preliminary Datasheet

Rev 006

Jan/07

10 Unique Features

10.1 Soft Switching & Slope setting

The US65/66 provides an efficient solution for low noise application with internal slope control circuit,

controllable via a dedicated pin.

In Two-coil fan driver, the predominant source of electrical noise is when the output driver is switched off.

Due to the electromotive force of the fan coil, the output voltage sharply increases but is internally clamped

by the fan driver. This effect lasts until the energy in the coil is dissipated, then the output voltages decreases

to a normal value, equal to the sum of the fan supply voltage plus the back EMF of the fan in rotation.

The resulting swift change in the coil current increases the overall acoustic noise.

(a) – General view

(b) – Enlarged view on output switching

In the above Fig.1a is observed the US65/66 output voltage with traditional driving technique, also referred as

“hard switching”. In this case, the output driver is directly switched on and off which results in large voltage

spikes, clamped at the output clamping voltage value (Fig.1b).

(a) – General view

(b) – Enlarged view on output switching

When the SLOPE pin is left open (not connected), the US65/66 provides internally defined output slope

duration. In contrast with the “hard switching”, the “soft switching” technique controls the output voltage at the

switching event and a rise/fall time is implemented to the driving signal. On Fig.2b, the output clamping

voltage is even not reached, result of a smoother recirculation of the fan coil current.

(a) – General view

(b) – Enlarged view on output switching

When a simple resistor is connected between the SLOPE pin the ground, the US65/66 modifies the output

that the output voltages spikes from Fig.1 are totally removed on Fig.3.

For most of the application, the internal soft switching mode from the Fig.2 represents the simplest solution

and most adequate balance between low fan acoustic noise and IC power dissipation.