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L6996

The low side driver has been designed to have a low resistance pull-down transistor, around 0.5 ohms. This

prevents the voltage on LGATE pin raises during the fast rise-time of the pin PHASE, due to the Miller effect.

1.8 Digital to analog converter

The built-in digital to analog converter (DAC) allows the adjustment of the output voltage in correspondence to

the Table1 in pag 4: from 0.6V to 1V with 25mV steps, and from 1V to 1.75V with 50mV steps. The DAC can

receive the digital input from the CPU. The programmed voltage is available on VPROG pin, which is capable

of sourcing or sinking up to 250

µA. The internal reference accuracy is ±1%.

1.9 Dynamically changing DAC code

L6996 detects as a transition any change in VID code which duration is larger than 200ns. Then, a timer forces

the chip in a 'transition state' for about 100µs. In such a state, output protections are disabled and OVP pin goes

high.

Current limit threshold can be reduced during the transition state duration by using an external mos shorting part

voltage overshoot/undershoot once the new-programmed voltage has been reached (see waveforms reported

below), especially when the droop is not implemented. Note that the reduced threshold must be however high

enough to allow the output capacitor to charge/discharge within the transition time. During the transition state

duration, zero-cross comparator is disabled and inductor current is allowed to reverse. A negative current limit

is introduced. During OFF time, if inductor current is negative and reaches the threshold, low side MOSFET is

After then, the low side or high side turns ON again, depending on PWM comparator output. This allows switch-

ing frequency to be close to steady state frequency also when the device works in negative current limit protec-

tion.

Dynamically changing the VID code is useful for portable computers, where the CPU is supply at a higher volt-

age when the AC-DC adapter is plugged-in, to increase speed. A lower voltage is instead provided when only

the battery powers the CPU, to save energy.

The dynamic transition is usually made at light load condition, to allow the full current to be available for charg-

ing/discharging the output capacitor:

Iout ~ 300mA

The current limit threshold should be set high enough to charge/discharge the output capacitor within the tran-

sition state duration (see below). If the output voltage changing is higher than 250mV the system can detect an

overvoltage or undervoltage that can shut down the device.