A6984 datasheet(16/47 Pages) STMICROELECTRONICS

16 / 47 page

Device description

A6984

16/47

DocID030396 Rev 1

4.2

Control loop

The A6984 device is based on a constant on-time control loop with frequency feed-forward

correction over the input voltage range. As a consequence the on-time generator

compensates the input voltage variations in order to adapt the duty cycle and so keeping the

switching frequency almost constant over the input voltage range.

The general constraint for converters based on the COT architecture is the selection of the

output capacitor with an ESR high enough to guarantee a proper output voltage ripple for

the noiseless operation of the internal PWM comparator. The A6984 innovative control loop

otherwise supports the output ceramic capacitors with the negligible ESR.

The device generates a TON duration switching pulse as soon as the voltage ripple drops

below the valley voltage threshold.

The A6984 on-time is internally generated as shown in Figure 7.

Figure 7. TON generator

where RTON represents the external resistor connected between the VIN and TON pins, CINT

is the integrated capacitor, CPAR the pin parasitic capacitor of the board trace at the pin 3.

The overall contribution of the CPAR and CINT for the A6984 device soldered on the

STMicroelectronics evaluation board is 7.5 pF typical but the precise value depends on the

parasitic capacitance connected at the pin 3 (TON) that may depend on the implemented

board layouts.

As a consequence, a further fine tune of the RTON value with the direct scope measurement

is required for precise fSW adjustment accordingly with the designed board layout.

The ON time can be calculated as:

Equation 6

The natural feedforward of the generator in Figure 7 corrects the fixed TON time with the

input voltage to achieve almost constant switching frequency over the input voltage range.

On the other hand, the PWM comparator (see Figure 3 on page 12) in the closed loop

operation modulates the TOFF time, given the programmed TON, to compensate conversion

losses (i.e. conduction, switching, inductor losses, etc.) that are proportional to the output

current.

0.9 R

TON



-----------------------------------------------

0.9 R

TON

INT

PAR





-----------------------------------------------------------------------

0.9 R

TON

7.5pF



------------------------------------------------



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