MIC2133 datasheet(23/50 Pages) MICROCHIP

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DS20006653B-page 23

MIC2133

FIGURE 4-7:

MIC2133 Control Loop

Timing in Discontinuous Conduction Mode.

4.5.3

PHASE SHEDDING

To achieve higher efficiency at lighter medium loads,

the Phase 2 is shed off when the DROOP Voltage,

VDROOP, drops below the Phase 2 shed-off threshold,

and the DROOP voltage is equal to eight times the

current sensing voltage in Phase 1. The Phase 2 is

shed on when the DROOP voltage rises above the

Phase 2 shed-on threshold.

The phase shedding thresholds for on and off are

calculated using the following formulas in the equation

below.

EQUATION 4-13:

As shown in Figure 4-8, the PSH pin voltage can be

programmed by an external resistor connected from

the PSH pin to AGND using the equation below.

EQUATION 4-14:

FIGURE 4-8:

Phase Shedding Circuit.

The output load currents at which the secondary phase

will be turned on and off can be calculated from

Equation 5-39.

The reason for this indirect way for setting phase shed-

ding thresholds is the fact that the DROOP pin voltage

has a strong positive temperature coefficient in case the

bottom FETs RDSON are used for sensing current. To

keep the shedding level constant in the current level with

temperature, an NTC resistor can be used to generate a

VPSH voltage with a negative temperature coefficient,

which becomes a positive temperature coefficient identi-

cal to the temperature coefficient of the DROOP voltage

when the RDSON of bottom FETs are used (see

Equation 4-13). Also, the NTC resistor must be placed

close to the Phase 1 bottom FETs to pick up the

temperature of the FET.

EQUATION 4-15:

The equation above is a description of the necessary

temperature coefficient of VPSH, achieved externally

using an NTC resistor on the PSH pin, combined with

a zero temperature coefficient 10 µA current source.

If sensing is done with a sense resistor in series with

the bottom FET, then no NTC resistor is needed on the

PSH pin and sizing the shedding of the secondary

phase (Phase 2) is done using Equation 4-13.

If no phase shedding is desired, then the PSH pin is

floating and will go to VDD, and internally, the level will

be sensed and the secondary shedding will not be

done.

If the PSH pin is externally driven between 0V and 5V,

then an externally controlled action on the shedding

can be done. In that case, the system designers need

to decide when the secondary is shed based on the

information about the load they obtained on their own

at the system level.

Shedding the secondary phase will be an action

conditioned by a hysteresis on the shedding threshold

voltage and a delay of approximately 30 µs.

IL CROSSES 0 AND Vgm > 1.2V

DISCONTINUOUS CONDUCTION MODE STARTS.

Vgm < 1.2V WAKE-UP FROM

DISCONTINUOUS CONDUCTION MODE.

Vgm

VREF_COM

(1.2V)

VDH

VDL

ESTIMATED ON-TIME

VSHED_ON

1.2V VPSH

–

VSHED_OFF

0.8 VSHED_ON



Where:

VPSH = PSH Pin Voltage Programmable by an

External Resistor

VPSH

IPSH RPSH



Where:

IPSH = PSH Current Source (10 µA typical)

RPSH = Resistor Connected from PSH Pin to AGND

VDD

1.2V

30μs DELAY

RISING EDGE

DRIVER

DISABLE

PSH

DROOP

REFSHED

HYS

REFSHED_HIGH

REFSHED_LOW

MIC2133

IPSH

RPSH

dVSHED

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

dVPSH

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

dVDROOP

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

ILOAD dRDSON



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

–

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