UCD3138RMHT.A datasheet(20/99 Pages) TI2

20 / 99 page

Error ADC

(Front End)

Filter

Digital

PWM

EAP

EAN

DPWMA

DPWMB

8.8.1.1 Front End

Figure 8-4 shows the block diagram of the front end module. It consists of a differential amplifier, an adjustable

gain error amplifier, a high speed flash analog to digital converter (EADC), digital averaging filters and a

precision high resolution set point DAC reference. The programmable gain amplifier in concert with the EADC

and the adjustable digital gain on the EADC output work together to provide 9 bits of range with 6 bits of

resolution on the EADC output. The output of the Front End module is a 9-bit sign extended result with a gain

of 1 LSB / mV. Depending on the value of AFE selected, the resolution of this output could be either 1, 2, 4 or

8 LSBs. In addition Front End 0 has the ability to automatically select the AFE value such that the minimum

resolution is maintained that still allows the voltage to fit within the range of the measurement. The EADC control

logic receives the sample request from the DPWM module for initiating an EADC conversion. EADC control

circuitry captures the EADC-9-bit-code and strobes the digital compensator for processing of the representative

error. The set point DAC has 10 bits with an additional 4 bits of dithering resulting in an effective resolution of

14 bits. This DAC can be driven from a variety of sources to facilitate things like soft start, nested loops, and

so forth. Some additional features include the ability to change the polarity of the error measurement and an

absolute value mode which automatically adds the DAC value to the error.

It is possible to operate the controller in a peak current mode control configuration; front-end 2 is recommended

for implementing peak current mode control. In this mode topologies like the phase shifted full bridge converter

can be controlled to maintain transformer flux balance. The internal DAC can be ramped at a synchronously

controlled slew rate to achieve a programmable slope compensation. This eliminates the sub-harmonic

oscillation as well as improves input voltage feed-forward performance. A0 is a unity gain buffer used to isolate

the peak current mode comparator. The offset of this buffer is specified in Section 8.5.

IOFFSET

REA

EAP

EAN

AGND

IOFFSET

REA

Front End Differential

Amplifier

Figure 8-4. Input Stage of EADC Module

UCD3138

SLUSAP2J – MARCH 2012 – REVISED NOVEMBER 2021

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