IR3081A

The IR3081A can accept changes in the VID code while operating and vary the DAC voltage accordingly. The

sink/source capability of the VDAC buffer amplifier is programmed by the same external resistor that sets the

oscillator frequency. The slew rate of the voltage at the VDAC pin can be adjusted by an external capacitor between

VDAC pin and the VOSNS- pin. A resistor connected in series with this capacitor is required to compensate the

VDAC buffer amplifier. Digital VID transitions result in a smooth analog transition of the VDAC voltage and

converter output voltage minimizing inrush currents in the input and output capacitors and overshoot of the output

voltage.

It is desirable to prevent negative inductor currents in response to a request for a lower VID code. Negative current

transforms the buck converter into a boost converter and transfers energy from the output capacitors back into the

input voltage. This energy can cause voltage spikes and damage the silver box or other components unless they

are specifically designed to handle it. Furthermore, power is wasted during the transfer of energy from the output

back to the input.

The IR3081A includes circuitry that turns off both control and synchronous MOSFETs in response to a lower VID

code so that the load current instead of the inductor discharges the output capacitors. A lower VID code is detected

by the VID step-down detect comparator which monitors the “fast” output of the DAC (plus 7mV for noise immunity)

compared to the “slow” output of the VDAC pin. If a dynamic VID step down is detected, the body brake latch is set

and the output of the error amplifier is pulled down to 75% of the DAC voltage by the VID body brake clamp. This

triggers the Body Braking

The converter’s output voltage needs to be monitored and compared to the VDAC voltage to determine when to

resume normal operation. Unfortunately, the voltage on the FB pin can be pulled down by its compensation network

during the sudden decrease in the Error Amplifier’s output voltage so an additional pin BBFB is provided. The BBFB

pin is connected to the converter output voltage and VDRP pin with resistors of the same value as on the FB pin

and therefore provides an un-corrupted representation of converter output voltage. The regulation detect

comparator compares the BBFB to the VDAC voltage and resets the body brake latch releasing the error amplifier’s

output and allowing normal operation to resume. Body Braking

disabled by connecting the BBFB pin to ground.

ENABLE

VID = 11111X DETECT

TO ERROR AMP

IROSC (From Current Source Generator)

+

-

+

-

+

-

VOSNS-

VID5

VID3

VID4

VID2

EAOUT

VID1

VID0

VDAC

BBFB

0.6V

VID INPUT

COMPARATORS

(1 OF 6

SHOWN)

-

DIGITAL TO

ANALOG

CONVERTER

+

VDAC BUFFER

AMP

ISOURCE

ISINK

"FAST" VDAC

"SLOW" VDAC

VID DOWN

BB CLAMP

75%

7mV

S

1.7us

BLANKING

REGULATION

DETECT

COMPARATOR

BODY

BRAKE

LATCH

R

VID STEP-DOWN

DETECT

COMPARATOR

800ns

BLANKING

IBBFB

Figure 7. VID Control Block Diagram

Page 13 of 39