L6918 L6918A

10/35

DEVICES DESCRIPTION

The devices are integrated circuit realized in BCD technology. They provide, in kit, a complete control logic and

protections sets for a high performance four-phases step-down DC-DC converter optimized for microprocessors

supply and High Density DC-DC converters. They are designed to drive N-Channel mosfets in an interleaved

four-phase synchronous-rectified buck topology. Each controller provides a 180 deg phase shift between its two

phases and a 90deg phase-shifted synchronization signal is passed from the master to the slave controller that

locks the signal through a PLL. The resulting four-phases converter synchronized together results in a 90 deg

phase shift on each phase, allowing a consistent reduction of the input capacitors ripple current, minimizing also

the size and the power losses. The output voltage of the converter can be precisely regulated, programming the

master's VID pins, from 1.100V to 1.850V with 25mV binary steps. The reference for the regulation is passed

from the master device to the slave device through apposite pin likewise the synchronization signal. Each device

provides an average current-mode control with fast transient response. They include a 300kHz free-running os-

cillator externally adjustable up to 600kHz, realized in order to multiply by 4 times the equivalent system fre-

quency. The error amplifier features a 15MHz gain-bandwidth product and 10V/

µs slew rate that permits high

converter bandwidth for fast transient performances. Current information is read in all the devices across the

PWM output in order to equalize the average current carried the two phases of each device. Current sharing

between the two phases of each device is then limited at ±10% over static and dynamic conditions. Current

sharing between devices is assured by the droop function. The device protects against over-current, with an

OCP threshold for each phase, entering in constant current mode. Since the current is read across the low side

mosfets, the constant current keeps constant the bottom of the inductors current triangular waveform. When an

under voltage is detected the Slave device latches. The Slave device also perform an over voltage protection

that disable immediately both devices turning ON the lower driver and driving high the FAULT pin. Over Load

condition are transmitted from the Slave device(s) to the master through the SLAVE_OK line.

MASTER - SLAVE INTERACTIONS

Figure 1. Four Phase connection with L6918 family

Master and slave devices are connected together in order to realize four-phase high performance step-down

DC/DC converter. Four-phase converter is implemented using L6918A master and one L6918 slave devices as

shown in figure 1.

A communication bus is implemented among all the controllers involved in the regulation. This bus consists in

the following lines:

– Reference (VPROG_IN / VPROG_OUT pins): Unidirectional line.

The devices share the reference for the regulation. The reference is programmed through the master

device VID pins. It exits from the master through the VPROG_OUT pin and enters the slave device

through the VPROG_IN pin(s). Filter externally with at least 1nF capacitor.

VID 9.0

S YNC_OUT

V PROG_OUT

SLAV E_OK

OS C

SYNC_IN

V PROG_IN

SLAV E_OK

OS C

S YNC_OUT

P GOOD

P GOOD