UCD3138ARMHT datasheet(62/83 Pages) TI

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Voltage Loop

Filter

Vout

Loop

Mux

Ramp

Module

PCM

Comparator

Ipri

Front

End

Loop

Mux

Fault

Mux

DPWM

Loop

Mux

UCD3138A

SLUSC66B – MARCH 2015 – REVISED MAY 2015

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7.2.6.2

Peak Current Detection

Peak current detection involves all the major modules of the DPPs, the Front End, Filter, Loop Mux, Fault

Mux and the DPWMs. A drawing of the major elements is shown in Figure 7-8.

Figure 7-8. Peak Current Detection Function

The voltage loop is used to select a peak current level. This level is fed to the Ramp module to generate a

compensation ramp. The compensation ramp is compared to the primary current by the PCMC

comparator in the Front End. When the ramp value is greater than the primary current, the APCMC signal

is sent to the DPWM, causing the events described in the previous sections.

The DPWM frame start and output pin signals can be used to trigger the Ramp Module. In this case, each

DPWM frame triggers the start of the ramp. The ramp steps every 32 ns.

The PCM_FILTER_SEL bits in the LoopMux.PCMCTRL register are used to select which filter is

connected to the ramp module:

LoopMuxRegs.PCMCTRL.bit.PCM_FILTER_SEL =0; //select filter0

With Firmware Constant Power/Constant current, Filter 1 and Front End 1 are used as a current control

loop, with the EADCDAC set to high current. If the voltage loop value becomes higher than the current

loop value, then Filter 1 is used to control the PCM ramp start value:

LoopMuxRegs.PCMCTRL.bit.PCM_FILTER_SEL =1;

S P A C E //select filter1 for slope compensation source

In the ramp module, there are 2 bitfields in the RAMPCTRL register which must be configured. The

PCM_START_SEL must be set to a 1 to enable the Filter to be used as a ramp start source. The

RAMP_EN bit must be set, of course.

The DAC_STEP register sets the slope of the compensation ramp. The DAC value is in volts, of course,

so it is necessary to calculate the slope after the current to voltage conversion. Here is the formula for

converting from millivolts per microsecond to DACSTEP.

m = compensation slope in millivolts per microsecond

ACSTEP = 335.5 × M

In C, this can be written:

#define COMPENSATION_SLOPE 150 //compensation slope in millivolts per microsecond

#define DACSTEP_COMP_VALUE ((int) (COMPENSATION_SLOPE*335.5) )

S P A C E //value in DACSTEP for desired compensation slope

S P A C E FeCtrl0Regs.DACSTEP.all = DACSTEP_COMP_VALUE;

It may also be necessary to set a ramp ending value in the RAMPDACEND register.

Application, Implementation, and Layout

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